White Tigers: Current IC and Future Breeding Prospects

White Tigers: Current IC and Future Breeding Prospects

White tigress with cubs. Photo credit: Murtazasmyths [CC BY-SA 4.0]

The current Bengal tiger population of zoos in India has a high-ish mean inbreeding coefficient (IC) of around 0.2054 and this is routinely blamed on white tiger breeding. However, the data does not support that conclusion. The non-white (orange) tigers have only a slightly lower mean IC (0.1934) than the white-gene tigers (0.2168). Further, if the recent breeding of just two zoos is excluded, the mean IC for the white-gene tigers drops to below the non-whites (0.1859), showing how easily the averages can be skewed by the choices of a few.

The mean IC could be lowered significantly in the future while still selectively breeding for the white coat. A table of 698 possible family-line pairings shows that 78% could produce white cubs with IC less than the current overall mean (<0.2), and 39% with IC less than half the current mean (<0.1).

A closer look at the circumstances around the instances of inbreeding suggests a variety of causes unconnected to breeding for the white coat – convenience, animal safety concerns, lack of space, accident, and lack of accurate information to inform breeding choices.

Contents:

  1. The Data Set used
  2. Mean Inbreeding Coefficients (IC)
      2.1 Inbreeding of Non-white (orange) Tigers
      2.2 Inbreeding of White-gene-carrier Tigers
      2.3 Disproportionate effect of the breeding choices of just 2 zoos
      2.4 IC is not a measure of fitness
  3. Potential pairings for Selective Breeding of White coat colour
      3.1 Chart of IC’s for 698 possible pairings between families
      3.2 The results: 78% pairings have IC < mean; 39% have IC < 0.1
      3.3 Outcrossing – reducing the mean IC even further
  4. Need for complete and accurate data
  5. Notes and sources

1. The Data Set used

The data set records 396 tigers alive at around 1 December 2018, from the White Tiger Dataset – ie the current Bengal tiger population of zoos in India.

The 2018 National studbook[1] classifies tigers as “Normal” or “White”, with Normal being orange tigers without the white gene, while White includes both white-colour tigers and orange heterozygous tigers, ie the product of white tiger breeding. [2]  I will not use the term “Normal” as I consider white tigers to be normal and natural also, so have split them into:

  • 193 White-gene-carriers = 123 white-colour tigers (ww) + 70 orange heterozygous tigers (Ow)
  • 203 Non-whites = orange tigers assumed to not carry the white gene (OO and O?).[3] 69 Wild-born tigers and 12 tigers with unknown parents are excluded from calculations, leaving 122 in this group.[4]

2. Mean Inbreeding Coefficients (IC)

Mean inbreeding for the living population, including both white and orange tigers, is given as 0.2054 in the 2018 National studbook (data to 31 March 2018). In the conclusions, the highness of this figure is explained as a consequence of selective breeding for the white coat colour. However, there is no mean IC given for White-gene-carriers versus Non-whites to enable comparison.

Using the data set described above,  I calculate:

  • Overall mean IC = 0.2077
  • Mean IC for the Non-whites = 0.1934
  • Mean IC for White-gene-carriers = 0.2168

That’s not a lot of difference between the mean IC for non-whites and whites! So this does not support the conclusion that white tiger breeding is to blame for high mean IC.

A closer look at the circumstances around individual tigers and pairings suggests some more likely reasons for the high IC…

2.1 Inbreeding of Non-White tigers

Royal Bengal Tiger. Photo by RBan26 [CC BY-SA 4.0]

There have been many instances of orange (non-white tigers) being inbred – IC’s range from 0.0 to 0.48, with a cluster of 32% in the range of 0.30 to 0.39. [5]

Sometimes the inbreeding happens by mistake, when a male cub is left with his mother or sister too long. (A zoo may not have the space to separate them, and zoos that keep tigers in safari-style enclosures tend to keep family groups that get along well, to avoid fighting.) This happened several years ago at one tiger safari where it was found that the father of a litter of cubs was the mother’s son from her first litter. As there had already been inbreeding in the history of this line, the IC of the cubs was very high, around 0.4800. (There is no suggestion of a white gene anywhere in the pedigree of these tigers).

In other instances a zoo gets a pair of tigers and then breeds their descendants together, “making do” with the tigers they already have rather than obtaining new animals with the inherent risks involved in transport and introducing new tigers to each other. (There have been several instances of apparently healthy tigers dying soon after transfer to another zoo, and of tigers killing their newly-introduced mates).

These choices and events have nothing to do with breeding for white colour, although the same can occur with white tigers, for the same reasons.

2.2 Inbreeding of White-Gene-Carrier Tigers

White Bengal Tiger. Photo by Sanu M [CC BY-SA 4.0]

IC’s for White-gene-carriers range from 0.0 to 0.55, with a cluster of 33% in the range of 0.20 to 0.29.

There is no genetic necessity to inbreed white tigers (see 3. Potential Pairings), so why does it happen ?

In addition to those outlined in 2.1, one more reason may be that white tigers are highly treasured and zoos are reluctant to part with them. When available, they are often youngsters from the same litter, so a sibling pair is obtained. The Central Zoo Authority directive to keep animals only in breeding pairs may have unintentionally increased this trend (if a zoo does not find a mate for an unpaired animal within 6 months they must transfer it to another zoo, so safer to obtain two together). [6]

Zoos also seem to do trades within the same group of zoos, causing tigers to be bred within the same family groups over and over. While some groups of zoos clearly keep good records and together have bred low-IC white tigers, I suspect that lack of complete and accurate pedigree information has contributed to some poor breeding choices elsewhere.

2.3 Disproportionate effect of the breeding choices of just 2 zoos

A closer look at the individual white tigers shows an interesting point – the disproportionate effect the choices of just one or two zoos can have on the mean IC.

Two zoos bred a large number of white-colour tigers from the same family line, all highly inbred. The ironic thing here is that the parents have been prolific breeders, with very high survivability of cubs, resulting in 18 white tigers with IC from 0.46 to 0.55

If these 18 tigers are removed from the calculations, the new mean ICs are:

  • Overall mean IC = 0.1890
  • Non-whites  = 0.1934
  • White-gene-carriers = 0.1859

Now the White-gene-carriers have a lower IC than the Non-whites!

Of course, these tigers should not be removed from the calculation, but doing so shows how the choices of just a few zoos can skew the averages.

2.4 IC is not a measure of fitness

Those 18 tigers also illustrate another important point – that the Inbreeding Coefficient is not a measure of health or survivability. It is a measure of risk when planning mating pairs: the IC of the planned cubs is the probability that gene alleles will be identical by descent; ie the risk that if there are deleterious recessive alleles present, that they may be doubled up and show their effects in the cubs. (This can also happen without any  inbreeding). Perhaps this family line of white tigers does not have any deleterious alleles, or perhaps it is simply luck.

3. Potential pairings for Selective Breeding of White coat colour

White Tiger Genetics

Chart 3.1 below shows 698 possible pairings between family groups of White-gene-carrying tigers currently alive, and the resulting IC’s for prospective cubs. Each family group contains from one to several individual tigers with the same parents. Full details of the individuals are held in a separate table (not published).

All these pairings have been selected to produce white-coloured tigers (ww). Pairings between two (ww) parents will produce all white tigers; pairings between one (ww) and one (Ow) we would expect half white and half (Ow); etc.

Chart 3.1 

Chart 3.1 – click to enlarge

3.2 The Results

A whopping 78.2% of these selective pairings for white-colour tigers would result in IC’s less than the current mean IC of the overall population, ie < 0.2077. This means that they would either decrease the mean IC, or have no effect on it.

16.6% of pairings would result in tiny IC’s less than 0.0550 (IC 5%), while a further 22.1% lie between 0.0550 and 0.1049 (IC 6-10%).

Clearly there is a lot of scope for breeding white tigers with very low IC in the current Bengal tiger population !

Of course other factors must be considered apart from IC, such as the logistics of pairing two tigers from distant zoos, and the temperament and health of each individual tiger.

An example on the chart that has already been attempted, is the pairing of a white tigress at Delhi (Family #14) with a heterozygous orange tiger from Mysore (Family #25). The IC of the cubs was a tiny 0.0396 (3.96%). Unfortunately the cubs did not survive, as often happens with a mother tiger’s first litter, but I hope the zoo will try again with this pair.

3.3 Outcrossing – reducing the mean IC even further

Outcrossing: White tigress with her grown heterozygous cubs. Photo by Sivahari [CC BY-SA 4.0 ]

Chart 3.1 does not include the outcrossings (with unrelated orange tigers) that are used to produce heterozygous (Ow) tigers, but these should, and do, occur. The outcrossings will produce White-gene-carriers with IC = 0.0000. Five of the family groups in the table are the direct result of previous outcrossings with wild-born tigers.

(Refer to “Inbreeding and Outcrossing” for explanation and examples of outcrossing).

4. Need for complete and accurate data

Availability of complete and accurate pedigree of tigers bred by all zoos is necessary to enable the best breeding choices to be made, and the information needs to be in a format that is usable by a zoo seeking a partner for their tiger.

Chart 3.1 (along with it’s table of individual tigers) is one method to show where to look for partners for each tiger. It also shows what would happen to the IC if a non-recommended pairing takes place – and thus perhaps discourage such pairings.

An incomplete or inaccurate dataset can give misleading results. Missing records can mean that good pairing opportunities are not identified. An error of just one digit in a studbook number can inadvertently change a tiger’s sire or dam to a completely different animal, and cause a major change in the IC calculation for all of its descendants, and their recommended pairings.[7]

The Indian National Studbook for Bengal Tiger contains records of tigers going back to the 1950s. However, the studbook authors rely on the various zoos to supply the information, and often this information is simply not forthcoming. For example, for the latest national studbook (2018), only about 30% of zoos were noted as having provided updated information.

It would be advantageous for each zoo to be more proactive in ensuring that “their” pedigrees in the studbook are correct and complete – both for current and historic records.

~~~~~~~~~

5. Notes and Sources

* This article may be freely quoted but the source must be credited and cited as: “White Tigers: Current IC and Future Breeding Prospects – whitetigertruths.wordpress.com” OR a link given directly to this article.

* Inbreeding Coefficients were calculated using Wright’s formula to 15 generations, using Breed Mate Pedigree Software.

[1] Wildlife Institute of India (2018) National Studbook of Bengal Tiger (Panthera tigris tigris): IV Edition, Wildlife Institute of India, Dehradun and Central Zoo Authority, New Delhi.TR.No.-2018/34 Pages: 668.

[2] That seems to be the aim, but it is not entirely consistent. In the studbook, some known heterozygous and white tigers are labelled “Normal”, while some not carrying the white gene are labelled “White”.

[3] The use of OO, O?. Ow. and ww is my own convention which I have not seen used elsewhere. The standard method is to use a big W to signify the dominant orange gene with a small w for the recessive white gene. However I find that some online texts make it difficult to distinguish between WW and ww so I use the big O with little w instead to avoid misunderstandings.

Ow = orange-coated tiger carrying the white gene (ie heterozygous)
ww = white-coated tiger
OO = orange-coated tiger known to not carry the white gene
O? = orange-coated tiger not known to carry the white gene.

These O? (O/maybe) tigers have an ancestor that is either Ow or O? so there is a slight chance they do carry the white gene…. see [5]

[4] I read somewhere that one of the regional Species Survival Plans tiger population had a mean IC of 0.01, but that when the wild-born tigers were excluded, this jumped up to 0.10. Sorry, I cannot find the reference, so take this with a grain of salt if you will. It may mean that the Bengal tiger IC is not as high relative to other captive populations as it seems. Wild-born tigers do not belong in the mean IC calculation as they are not a product of captive breeding.

[5] There may be a case for including the O? tigers in with the White-gene-carriers instead, but if done, this does not change the results significantly. It would slightly decrease the mean IC for the White-gene-carriers from 0.2168 to 0.2150; and for the Non-whites from 0.1934 to 0.1903. There would still be a main cluster of 32% in the .20 to .29 group for White-gene-carriers; and still a main cluster of 27% in the .30 to .39 group for the Non-whites.

As these O? tigers probably do not carry the white gene, and are not usually used for white tiger breeding, AND it does not greatly affect the results,  I have grouped them with the OO’s.

[6] Zoos in India 2014 – Legislation, Policy, Guidelines and Strategy
CZA, Edited & Revised by: B.S. Bonal, Inder Dhamija, B.R. Sharma, S.C. Sharma, Brij Kishor Gupta

[7] These errors can and have happened. The IC’s I have calculated for some of the White tiger pairings are higher than those given in the Studbook’s recommended pairings, because of two differences in Sires recorded in the Studbook. Ref  “Proposed Corrections to the 2018 Indian National Studbook of Bengal Tigers”; 14pp; Dec 2018.

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White Tigers – Inbreeding and Outcrossing

White Tigers – Inbreeding and Outcrossing

The early inbreeding in the captive White Tiger population is well-known and often referred to – but it is only half the story. The subsequent corrections by outcrossing are often ignored or misunderstood, resulting in myths such as “all white tigers are inbred”. Here we explain how outcrossing works, not just in theory, but using five real life examples from white tiger captive breeding, in India and America. As a result, several myths are proven false.

In this article:

  • Some myths about white tigers and inbreeding
  • What is inbreeding, and outcrossing ?
  • “But outcrossing doesn’t work…”
  • Examples 1-3 – White tigers with zero or very low inbreeding
  • Examples 4-5 – American white tigers with zero or very low inbreeding
  • More evidence of outcrossing in American white tigers
  • Putting it into perspective
  • Notes and sources

White Bengal  tigress paired with orange Bengal tiger at  Chattbir Zoo.
By Weryah11 (Own work) [CC BY-SA 4.0]

Some myths about white tigers and inbreeding

Inbreeding and outcrossing both occurred in the captive White Tiger population, but the outcrossing is often ignored, leading to white tigers being judged on only a small part of their pedigree. This skewed view leads to misconceptions such as:

  • “All white tigers are descended from just one tiger”
  • “White tigers are only produced by parent to child, brother to sister etc mating”
  • “The early white tigers were inbred, therefore all their descendants must be also”
  • “If a cub’s parents are inbred, the cub must also be inbred”
  • “If it’s a white tiger, it must be inbred, simply because it is a white tiger”
  • “Outcrossing doesn’t work because you still have to backcross to related tigers”

Examples 1-5 below, taken from the actual breeding history, disprove all the above myths.

What are inbreeding and outcrossing ?

Inbreeding is the breeding of closely-related tigers together, eg Father to Daughter, or Brother to Sister. Inbreeding is not “either/or” – it is a matter of degree, which can be measured by the Inbreeding Coefficient (IC). This is the probability that 2 gene alleles in a pair will be identical by descent ie that both alleles in a pair are inherited from the same ancestor. Breeders try to keep the IC as low as possible, with an average of 10-15% generally acceptable (opinions vary, but maintaining an IC of zero is impractical in any captive tiger population).

Outcrossing is the breeding of completely unrelated tigers, [note i] ie the opposite of inbreeding. Outcrossing results in an IC of zero because there is zero probability that any 2 gene alleles in a pair will be identical by descent. Outcrossing introduces new genes from new unrelated tigers (ie founders) into the gene pool, increasing genetic diversity.

Outcrossing a white tiger to an unrelated orange tiger produces heterozygous cubs – ie orange tigers who carry the white gene. These orange tigers may have white cubs when paired with another heterozygous tiger. See White Tiger Genetics – The Basics for details.

“But outcrossing doesn’t work”…

Some critics claim that outcrossing “does not work because you still have to backcross the heterozygous tigers to closely-related tigers in order to produce the white tiger” which again involves inbreeding.  Is this true? If there was only ever one outcross mating possible, this would be true. But in the white tiger population there have been many different outcrosses, resulting in white tigers with zero or low IC, showing that outcrossing does indeed “work”.

Let’s take a look at a few examples and see the effect on the Inbreeding Coefficient [1]. Note that these are not just theoretical examples, but matings that actually happened.

Example 1: Multiple outcrosses producing white tigers with very low IC

Chart 1

Chart 1 – click to enlarge

In Chart 1 the inbreeding % is shown in the blue column. You can see how this increased over the first 20 years of inbreeding (1955-1977) until the 4th generation tigress Seema had a high IC of 50%. But then methods changed. Three new unrelated tigers were bred with the white tigers, ie three outcrosses, and when the distant cousins were backcrossed in 1996 the white tigress Aishwarya was born, with a very low IC of  4.69%. [2]

So far we have been using only Rewa-line tigers, but it gets even better when tigers from the Orissa line were introduced. Aishwarya’s brother Gaurav was bred to Nidhi – a tigress with mixed Rewa-Orissa+outcross ancestry. Their cubs included white tigress Mahasweta, with only 2.44% inbreeding.

Thus Aishwarya is descended from FIVE unique founder tigers (not just one!) – Dhittoo, Moti, Sheroo, Begum and Mohan, while Mahasweta is descended from NINE – the same five, plus Pradeep, Sikha, Rani and Tipu (not shown on chart).

Example 2: Crossing 2 unrelated lines producing white tigers with zero inbreeding

Chart 2

Chart 2 – click to enlarge

In 1980 a new line of white tigers was discovered in the Nandankanan zoo in Orissa. This line traced back to a wild-born heterozygous orange tiger with no relation to the Rewa line. Chart 2 shows what happened when these 2 family lines were crossed: even though the parent tigers were moderately inbred (Debabrata) and heavily inbred (Diana), their white cubs were entirely non-inbred, with IC=0.

Example 3: Crossing two family lines plus one other outcross

Chart 3

Chart 3 – click to enlarge

Chart 3 shows just one outcross in 1981, producing the non-inbred heterozygous tiger Sundar. When he was backcrossed to Rewa-line Shanti, their cubs (including Kiko) had IC of 22.66% – not as low as we’d like, but still far below that of the Rewa line mother. But then in 2001 Laxman, a white tiger born from the Rewa-Orissa line crosses, was bred with Kiko. The resulting white tiger cubs had a low and quite acceptable IC of 4.54%.

American White Tigers

The three examples above are from white tiger breeding in India, but the same principles apply to white tigers anywhere in the world – including the American white tiger population. [3]

Full details of the American pedigree are not publicly available, as much of the breeding has been by private breeders (which begs the question, without knowing the pedigrees how can it be claimed that they are all inbred? ) However, we have some evidence of multiple family lines and outcrossing, and we have more evidence from a recent genetic study.

Example 4: Multiple outcrosses producing white tigers with low IC

Chart 4

Chart 4 – click to enlarge

Similar to Example 1, chart 4 shows part of the early pedigree of the Rewa line white tigers in USA – Mohini, her cubs Ramana and Kesari, and “grand-cubs” Ranjit and Bharat. Three different outcrosses in the 1980s eventually led to white tigress Kitra and her siblings born with low IC of  6.45%.

Note: I do not know Hylton’s ancestry. If he was related to Jack, say his brother, Cherry’s IC would then be 12.5%, but there would be negligible change to Kitra’s IC.

Example 5: Crossing 2 unrelated lines producing white tigers with zero inbreeding

Chart 5

Chart 5 – click to enlarge

In USA there were at least 2 (probably more) unrelated family lines of white tigers – the Rewa line, via Mohini from India, and the Sioux Falls/Circus line via Susie, a heterozygous orange tigress imported from India. [note ii]

Similar to example 2, chart 5 shows how the IC dropped down to zero when Rewa-line Kesari was crossed with Circus line Tony at the Cincinnati zoo  – all their white cubs were non-inbred.

Later, siblings Bhim and Sumita were mated, and their cubs’ IC would be increased. More outcrossing would be needed to lower the IC again. Pedigree charts show that more outcrossing did indeed occur, and genetic analysis gives us even more evidence….

More evidence of outcrossing in American white tigers

A genetic study in 2013 [4] on a group of orange and white tigers found an average heterozygosity (genetic variation) score of 76.1% for white tigers and 77.2% for orange.

This finding has been dismissed by some, who claim that the tigers were all related so of course they will have similar heterozygosity, meaning they are all as inbred as each other. But this claim misses the important point – that a score of 76% means a high level of genetic variation for a tiger, regardless of what colour the tiger is – 76% variation does not indicate an inbred tiger. We should say, they were all as NON-inbred as each other!

Here are a few other heterozygosity scores for comparison, from a different genetic study in 2004. [5] The tigers used in this study were wild-born, or traceable to wild parents.

 

 

Putting it into perspective

The 5 examples plus genetic evidence prove that the myth that “all white tigers are inbred” is false – both in India and America.

They do not prove the opposite, and are not intended to.

The examples do also disprove the myth that “the only way to breed white tigers is to inbreed father to daughter……etc”, and all the other myths listed at the beginning of this article.

There have been many more outcrosses than shown in the examples, and more low-IC and zero-IC white tigers born, and there has been inbreeding too, both of white tigers AND orange tigers (with no white gene). This happens for various reasons  – lack of space for a zoo to keep enough suitable partners; lack of coordination of breeding efforts; larger zoos breeding within their own gene-pool; tigers not cooperating; tendency to obtain brother-sister pairs who already get along well; “accidental” matings, where bro-sis are kept together a little too long (eg Kanpur and Lucknow at MetroZoo Miami); reluctance to risk moving tigers from one zoo to another; maybe ignorance in some cases – let’s hope that it’s not because some actually believe the irresponsible myth that the only way to produce white tigers is to inbreed !

In hindsight, there have been missed opportunities; and there have been cases of attempted breeding that looked good on paper failing because the tigers would not cooperate.

The trend of outcrossing has continued in recent years, creating a new generation of non-inbred heterozygous tigers and new possibilities for healthy mating choices. The future options are beyond the scope of this article, but the examples above show how outcrossing and backcrossing do indeed work to produce white tigers with acceptable low IC, ie non-inbred. These are techniques that continue to be used in the white tiger population.

Notes and Sources

[Note i] Pedigrees can usually only be traced back to the wild-caught ancestors, ie the founders, and we assume that these founders are unrelated. This is not always the case – for example the Amur tiger founders mostly came from a very small population and were quite possibly related, especially as sibling pairs were often captured. In contrast, the founders of the white tiger population came from many different areas of a large population and are not so likely to be related. Please see ..30 founders and ..are they inbred? for further details of the founders.

[Note ii] Susie was a Bengal tigress whose origin is not fully known. It is sometimes assumed that she “must” be related to Mohan, but this assumption seems to be based on the belief that Mohan was the only white tiger, which is incorrect. White tigers occurred in the wild in many districts of India, and where there were white tigers there would also be heterozygous orange tigers – some of which could easily have found their way into captivity in America. Furthermore, as Susie seems to be the source of the gene for ghost-striping, it is unlikely that she was from Mohan’s family. See Golden Tabby and Snow-white Tigers for more information.

Sources

[1] Inbreeding Coefficients were calculated using Wright’s formula to 10 generations using Breed Mate Pedigree Software. (Some were also checked – laboriously – by hand).

[2] Pedigree information for tigers in India, examples 1-3,  is from the International Studbook of Bengal Tigers (2012); Studbook of White Tigers in Indian Zoos 1989; Indian National Studbook of Bengal Tiger 2011, Srivastav, A et al.; and The White Tiger Dataset

[3] Pedigree information for tigers in America, examples 4-5,  is from “Breeding White Tigers” by Sara Iverson, in Zoogoer Vol II No 1, Jan-Feb 1982; “White Tigers and their Conservation” by AK Roychoudhury, in Tigers of the World, Tilson & Seal Ed, Noyes Publications 1987; and pedigree charts from Messybeast.com.

[4] Genetic Diversity of White Tigers and Genetic Factors Related to Coat Color by Sara Elizabeth Carney, May 2013 (PDF).

[5] Phylogeography and Genetic Ancestry of Tigers (Panthera Tigris) by Luo et al, PLoS Bio, Dec 2004 v2 Iss 12 e442

 

White Tigers that Turn Black

White Tigers that Turn Black

Pseudo-melanistic, or “black” tigers: – where did they come from, and why are they suddenly popping up now in the captive white tiger population ?

“Black” Cubs Born in Captivity

Sembian with his mother Anu, at Arignar Anna Zoo, Vandalur.
Photo by Fightingfalcon2005 (CC BY-SA 3.0 )

In June 2010, a white tiger pair (Anu and Bhishmar) at the Arignar Anna Zoo in Vandalur, India, had their 2nd litter of 3 white tiger cubs. One of these cubs was different – his white coat appeared to be turning black! As the cub, Sembian, matured, it became apparent that the blackness was due to an expansion of the normal black stripes, termed “abundism” or more popularly called “pseudo-melanism”. Sembian’s coat had a white background with an over-abundance of blackness – his black stripes were so wide they ran together in places. [1]

Pseudo-melanistic tigers at Nandankanan zoo.
Photo by PALLABI SEN (Own work) (CC BY-SA 4.0)

In July 2014, it happened again, this time at the Nandankanan Zoo in Orissa, India, and this time the litter of 4 was a mixture of white and orange cubs born to Sneha and Manish, a white mother and orange father. Two cubs, 1 white and 1 orange, had the overabundance of black stripes . In May 2016 another pair at the zoo – Renuka and Samrat – had one pseudo-melanistic cub (stillborn). In August 2016 the first pair had another litter of 3 cubs – again, one cub was pseudo-melanistic. [2]

At first, the “black” cubs were thought to be “accidents” of nature, or “mutants” –  but after the 2016 litters the zoo realised they were dealing with normal genetic inheritance.

The Theory

Pseudo-Melanistic white tiger at Nandkanan Zoo.

Pseudo-melanism in tigers appears to be caused by a recessive gene – similar to the white gene, but separate from it as it can affect both white and orange tigers. This would mean that a “normal” orange or white tiger can carry the melanistic gene in a hidden state, as it is masked by the dominant allele.

So assuming it is an inherited trait, a trace of the pseudo-melanistic tigers’ pedigree should give us a clue where it came from. This leads back to 5 wild-born ancestors that all the melanistic cubs have in common: Mohan and Begum of the Rewa line, and Pradeep, Sikha and Rani of the Orissa line. [3] It is highly unlikely that the gene came from the Rewa tigers, as it should have shown up earlier during the inbreeding that occurred in the first few generations of that line. That leaves the 3 from the Orissa line, and a glance at the origin of these 3 tigers shows an obvious candidate : Rani.

Rani, an orange tigress, and one of the founders of the Orissa line of white tigers, was found wild in the Similipal forests in 1967, as a little 7wk old cub. [4]

Location of Similipal Forests.
Source: Google Maps

The Similipal Tiger Reserve had for many years been rumoured to harbour “black” tigers, – rumours that were widely dismissed as myth until 1993 when proof was obtained (the skin of a slain tiger). [5] Since then these pseudo-melanistic orange tigers have been photographed by camera traps in the reserve, and it was estimated that there were 3 of them living there in 2014. The Similipal tiger population is threatened, with only an estimated 26 tigers left there in 2016. [6]

Back in 1975, in the Nandankanan zoo, Rani the Similipal tigress mated with Deepak, an orange male who was later found to carry the white gene. Apparently Rani passed the melanistic trait on to her daughter Ganga (also a white gene carrier), who then passed it on to one or more of her many cubs.

Throughout the next 3 or 4 generations, the recessive melanistic gene gradually spread unnoticed throughout the Nandankanan tiger population, until finally 2 melanistic gene carriers were paired together.

Possible path of pseudo-melanism inherited from tigress Rani. Note: Abbreviated chart.

Meanwhile, in 1999, Laxman, a white male tiger from the Orissa line, was sent to the National Zoological Park in Delhi to breed with their Rewa line white tigers. Laxman was also descended from Rani, and it is possible that he carried the melanistic gene and passed it to some of his descendants, including Anu and Bhishmar, who became the parents of the blackened Sembian. [note i]

There have been too few melanistic births to be certain which of Ganga’s offspring carried the trait, as it is not likely she passed it to all her cubs. A study of the full birth charts shows the most likely option to be Debabrata plus either Pinaki or Jamuna.

This abbreviated chart above shows the possible line of inheritance. Please note that this chart does not show all the generations and tigers involved. For full (and complicated!) details please refer to the ancestry charts in White Tigers Today..

Is it Caused by Inbreeding?

Inbreeding is one way that a recessive trait can show up. However, the expression of a recessive trait does not automatically signify inbreeding, but simply that a trait has had time to be distributed throughout a population unnoticed (because it is masked by the dominant allele).

This late showing of a recessive trait that has been present in the captive population since 1967, illustrates how careful the zoos have been to avoid close inbreeding in the Orissa line of white tigers. [note ii] It has taken all this time for the gene(s) to spread throughout the population – the Nandankanan pseudo-melanistic cubs are 4 generations removed from any common ancestor, and SIX generations removed from the presumed origin, the tigress Rani.

Significance for White Tigers and Tigers in General

Occurrence of White Tigers in the Wild

This concept of a recessive trait spreading unnoticed throughout a population (see above)  is also important in understanding the occurrence of white tigers in the wild. They did not just pop up here and there spontaneously – 1 in 10,000 –  as often suggested, but the hidden white gene seems to have spread from NE India through to Central India, in time becoming so common in some areas that white tigers were born more frequently in these areas –  eg Rewa and Bihar. [7]

Captive Populations Preserving Genetic Diversity

As Rani’s daughter Ganga also carried the white gene (from her father Deepak), she was bred extensively and has many descendants in the captive population today. In the effort to preserve the white gene, the zoos have inadvertantly also preserved other tiger genetic diversity that is endangered in the wild. The pseudo-melanistic trait is a visible example of this, but there is likely much more variation thus preserved that we cannot see with the eye.

“Real” Tigers Come in Many Colours

Throughout history, hunters and naturalists have observed and recorded wild tigers in a startling array of coat colour variations – the common orange with black stripes; white with black stripes; white stripeless; orange stripeless; dark brown with black stripes; heavy black stripes; varying shades of pale to deep orange; black with black stripes; “blue” tigers. [8]

Today only the orange-with-black-stripes variety remains in the wild with any regularity, causing many people in the current generation to believe that they are the only “real” tigers, and that all others are mythical or man-made. The reality is that these beautiful orange-and-black tigers are merely the remnants of the once large and highly diversified tiger population that included many different coat variations.

Conclusion

“Black” tiger in Nandankanan Zoo.
Photo by Jitendraamishra (Own work) (CC BY-SA 4.0) 

The pseudo-melanistic trait arose naturally in the wild, where it apparently survives only in Similipal today. Fortuitously, one tigress from this area was taken into captivity 50 years ago and bred, thus preserving some of the unique genetic diversity of the tigers from that area.

It is highly likely that Rani, the little wild cub from Similipal,  carried the pseudo-melanistic trait and passed it on to her daughter Ganga, who passed it on to one or more of her own cubs.

As the Simlipal population is currently threatened, this is another instance of captive tigers (both white and orange) serving as a reservoir of genetic diversity that is endangered in the wild.

 

Notes

[i] Amongst Anu and Bhishmar’s 13 cubs there were no other reported melanistic cubs. This might indicate that there are other factors influencing or suppressing the trait. 

 [ii] There was only one father-daughter pairing, that of Deepak to Ganga. Offspring were then outcrossed to the unrelated Rewa line, and I have not found any further parent-child or sibling pairings at Nandankanan. In recent years they have also outcrossed to wild tigers, thus strengthening their gene pool further.

The initial breeding of Ganga to her father Deepak would not have produced pseudo-melanistic cubs as we assume that only Rani (not Deepak) carried the gene, which she then passed to her daughter Ganga.

Sources

[1] Times of India, August 30, 2010 – White tiger cub turns black in Chennai Zoo

– Plus many other news reports

[2]

[3] The White Tiger Dataset; and  Bengal Tiger Studbook Dec 2012

[4] Birth of White Tiger Cubs to Normal Coloured Tigers in Captivity, by CH G Mishra, L N Acharjyo, L N Choudhury in JBNHS vol 79 1982.

[5]

  • Born Black – The melanistic Tiger in India by L A K Singh. WWF India. Sept 1999.
  • “Black Tigers” – Reality or Myth by Dr L A K Singh, in WWF Tiger Update, v1 No 4, Oct 1996
  • Black on white or White on Black… by BC Prusty and LAK Singh, in Zoos’ Pring, v XII, No 1, Jan 1997
  • Black Tigers of Similipal Tiger Reserve, Orissa by G H Mishra, in Indian Forester, V122, No 10, Oct 1996.

[6]

[7]

[8]  Born Black – The melanistic Tiger in India by L A K Singh. WWF India. Sept 1999. (Plus many books/articles written by hunters and naturalists in the 19th and 20th centuries)

White Tigers Today – Are They Inbred ?

White Tigers Today – Are They Inbred ?

Ancestry charts indicate that today’s white tigers are genetically diverse and very far removed from the inbreeding of 50 years ago. The tigers have mixed ancestries, from numerous unrelated founder tigers, resulting from 5-6 generations of outcrossing. The proof is in the studbooks. Overall, white tigers today are not inbred and not descended from “only one tiger”.

This study is about the status of white Bengal tigers in India, but has parallels with populations in other countries.

Summary

  1. Condensed Ancestry Charts
    • Mohan-Begum chart, the Rewa line
    • Pradeep-Sikha chart, the Orissa line
  2. So where is all the inbreeding ?
  3. How much inbreeding = “inbred” ?
  4. Significance of outcrossing – adding new founders
    • Graph showing the gene pool increasing over time
  5. Genetic diversity of the founders
    • Map of geographic locations
  6. Are all descended from Mohan ?
  7. Conclusion
  8. A note about the accuracy of the charts
  9. Sources

Definition of Terms
hz = heterozygous for the white gene, ie an orange tiger that carries the white gene

IC = inbreeding coefficient

Outcrossing = mating with an unrelated Bengal tiger

 

1. Condensed Ancestry Charts

The International Bengal Tiger Studbook and the White Tiger Datset contain records of 100’s of tigers born since the 1950s but it is difficult to get a sense of the overall situation without spending many hours studying them. So below are 2 condensed ancestry charts, concerned only with the ancestry of living (or recently alive) white and hz tigers. They use color to show the breeding, outcrossing, and the multitude of different lines, at a glance:

  • Each color shading represents a different line of white/hz tigers with a different mix of founding ancestors – eg pale blue for the line descended from only Mohan and Begum;  light orange for Mohan/Begum/Tipu descendants; etc. Note that tigers of the same line (chart-color) are not identical, they each have a different mix of genes inherited from the various founders in their line.
  • A bright orange bar represents an unrelated outcross, resulting in a new line of tigers, shown by a new chart-color (or stripes). The outcross can be either with a new wild-born founder, or a cross between the Rewa and Orissa lines. Cubs of outcrossings have zero inbreeding.

The charts are read from left (1950s) to right (2016) and show tiger mating partners (in italics), and resulting cubs (in boxes) by generation (in columns). The cubs are grouped by parents, not by individual litter. Family lines that have died out, or that have only  possibly-hz descendants are omitted. For the most part, tigers on the left-hand half of the charts are deceased, those on the right still alive.

  • Left to right = 1950’s to 2016
  • LH side = deceased, RH side = alive
  • Chart-colors = different lines of tigers
  • Bright orange bar = unrelated outcross (IC=0)
  • Click charts to open in new window, click again to enlarge.

Mohan-Begum Chart – The Rewa Line

Rewa Line of Mohan and Begum – Ancestry of current white/hz tigers

The first chart starts from the Rewa line, tracing Mohan and Begum’s descendants.

Following the chart colors from left to right, it is easy to see that:

  • There are NONE of the original pure Rewa line (pale blue) left alive today ie tigers descended ONLY from Mohan and Begum (so clearly no tigers that got all of their genes from Mohan !)
  • There are numerous different lines of white/hz tigers with a kaleidoscope of different founders in their ancestry.
  • There was some inbreeding, but the early inbred lines were corrected by outcrossing to unrelated tigers – (orange bars on the chart).

 

Pradeep-Sikha Chart – The Orissa Line

WhiteTigerAncestryPSC

Orissa Line of Pradeep, Sikha, Rani – Ancestry of current white/hz tigers

The second chart starts from the Orissa line, following Pradeep, Sikha, and Rani’s descendants, and their crosses with the Rewa line. There is less early inbreeding than in the Rewa line, and there is a recent trend towards more outcrossing (eg Sara, Nandan, Jeevan, Cheri).

In the middle of the chart, where tigers of the same line have been paired, matings tend to be between distantly related tigers rather than siblings. For example, with a recent litter (Anini, Krishna, Snehashish, Subhransu) we have to go back 4 generations to find a common ancestor in the pedigree.

There does appear to be some sibling and mother-son inbreeding in recent times (eg Sunder-Tapsi, Sunder-Kamla), which is not good to see, but this involves a small number of zoos and does not affect the rest of the population (see below).

2. So where is all that inbreeding we were told about ?

There was indeed more inbreeding than is shown on these 2 charts, and it wasn’t restricted to white tigers. However, these charts show the ancestry of only the current population of white and hz. The full charts show more instances of inbred lines, and failed inbreeding attempts – but many of those inbred tigers of the past have no living descendants in the white tiger population today.

Past inbreeding of tigers that have no living descendants today has no effect on the current population.*

Inbreeding tends to be self-limiting – after 2-3 generations of continuous inbreeding, the inbred line dies out. (IC>.47). Not a good strategy !

This is likely to happen with the few current cases of inbreeding mentioned above – if not rescued by outcrossing, they will likely die out – hopefully these few zoos will change their practices in line with the majority, the aim should be to breed healthy tigers and avoid inbreeding.

* This is not saying that the inbreeding was OK – it is not necessary and not recommended. However, this article is not about judging the past, it is looking at the present and the status of white tigers today.

3. How much inbreeding = “inbred” ?

Inbreeding is a matter of degree, measured by the Inbreeding Coefficient (IC) on a scale of 0 to 1. For example:

  • IC of 0 = zero inbreeding.
  • IC of 0.25 = 25% inbreeding (eg from a full siblings mating).
  • IC of 0.01 = 1% inbreeding.

At what point do we say an animal, or population, is “inbred” ? If we use the IC>0 criteria, then we’d probably have to say that most white tigers today are inbred BUT we would also then have to say that most captive tigers are inbred – and quite possibly most wild tigers also. (Inbreeding has been observed in wild tigers – and lions – and increases with population fragmentation).

The charts show that there are white/hz tigers with zero inbreeding, and more with a very low level (as their parents were only distantly related).

“Inbred” is popularly understood to mean severely inbred, suffering or close to inbreeding depression. White tigers were at this point 40-50 years ago. Today they are not – they are very far from it.

4. Significance of Outcrossing – Adding new Founders

White tigers are not considered to be a separate subspecies. This puts them in a very strong position: it is not necessary to breed one white tiger to another white tiger. They can be bred to unrelated orange Bengal tigers, adding new founders into the population, and producing hz cubs.

We are 5 to 6 generations onwards from the Rewa line of Mohan-Begum, and the number of founders has increased dramatically from 2 to 19. Each new founder brings his or her unique set of genes into the pool, increasing the genetic variety available to the next generation. The process is ongoing.

  • Each tiger cub gets half of its genes from its mother, and half from its father. When mother and father are unrelated, as in an out-cross, there is zero probability of any of the genes being “identical by descent”, and therefore there is zero inbreeding. This is how inbreeding is corrected.

The graph shows the founders joining the India white tiger gene pool (in blue), 24 in all but 5 have left. (They join the pool when their first white/hz descendant is born, and leave it when their last known white/hz descendant died.)

Look at how that gene pool increased in size!

But numbers are not everything – a small number of founders can hold a large genetic variety if they come from a very diverse population or far-flung locations. Conversely, a large number of founders might have little variation if they all came from one small inbred population.

5. Genetic Diversity of the Founders

WhiteTigerFounderMapThe map shows approximate locations for the capture of the wild-born founders, where known. They are from a wide range of geographic locations and therefore very likely to be a genetically diverse bunch. Add to this that they were collected over a 60 year time period, making it even more likely that they represent very good diversity.

6. Are all descended from Mohan ?

All white tigers in India today are descended from Mohan – AND from Begum, Pradeep, Sikha, and Rani. All 5 of these founder tigers feature in the ancestry of most white/hz tigers. The other 14 founders also feature but in lesser proportions. This is not uncommon:

“In an ideal situation, each founder would be equally represented in the living SSP population. This is seldom the case—typically, a large proportion of the animals have descended from a few prolific founders who have many more living descendants than other founders and are thus more fully represented in the population’s gene pool. ”

from Management and Conservation of Captive Tigers – Ch 8 Intro to a Species Survival Plan (SSP) R. Tilson, K. Traylor-Holzer and G. Brady

After all the outcrossing and generations since Mohan, it is even possible that there are white tigers today that have few of Mohan’s original genes, even though he is an ancestor. Even the white gene itself could have come from the Orissa line (Sikha).

Having Mohan as an ancestor is not a bad thing at all though – he was a fine, big long-lived white tiger (19 years), a fitting patriarch. Begum, Pradeep, Sikha and Rani also lived to good old ages of 16 or 17.

7. Conclusion

Are today’s white tigers “inbred”?

Their ancestry indicates that they are genetically diverse. 5 to 6 generations of outcrossing has increased the gene-pool to that of 19 founders from widely-separated geographic locations, resulting in a mixture of lines and ancestors. Much of the past inbreeding has no effect on the current population (as there are no descendants). While some individual tigers may be inbred today, inbreeding is now the exception not the norm. Taking all this into account, it is reasonable to state that white tigers today are not inbred.

The breeding history is not perfect, and more work would be needed to make the population “text book perfect” – but it is impressive how well the Indian zoos have done over all with the white tiger breeding pool.

8. A note about the accuracy of the charts

The ancestry of some tigers is not clear, and if it looks like there could have been inbreeding, I assume the worst, ie that there was inbreeding. My apologies to the zoos concerned if it is incorrect. I will gladly correct any errors when informed.

The charts are a work in progress and may be updated from time to time.

9. Sources

Studbook of White Tigers in Indian Zoos in ZOO ZEN Vol IV Issue XI June 1989.

International Studbook of Bengal Tigers 2012

Management and Conservation of Captive Tigers – Ch 8 Intro to a Species Survival Plan (SSP) R. Tilson, K. Traylor-Holzer and G. Brady

The White Tiger Dataset

Inbreeding in White Tigers – A K Roychoudhury and K S Sankhala. Proc. Indian Acad. Sci Vol 88B, Part 1, Number 5, Oct 1979.

Genetic Status of White Tigers at Nandankanan Biological park, Orissa – A K Roychoudhury and L N Acharjyo. in JBNHS V.79 1982.

 

 

White Tiger Cub Mortality and Longevity

White Tiger Cub Mortality and Longevity

WhiteTigerCubs

Analysis of the breeding records of approx 1700 Bengal tigers in Indian zoos has shown that white tigers have the same cub mortality and longevity as orange tigers.

 

There are some “statistics” floating around about white tigers that I have not seen backed up with any proof – such as the mythical 80% cub mortality, and a belief that white tigers die younger than orange. The average age for orange tigers in captivity is usually quoted as 16 years, while white tigers were thought to live to only 12 or 14 years on average.

I always want proof for any statements made, so I assembled a dataset of captive Bengal tiger breeding in Indian zoos from 1950’s to 2015, and analyzed it to find out what truth (or not) there was in these figures, and to see if the changes in breeding practices over the decades had had any effect.

I found cub mortality rate to be practically the same for white, orange, and heterozygous tigers, approx 35%, and the average age to be 12-13 years for all captive-born tigers.

As heterozygous orange tigers are the result of the white tiger breeding efforts, I separated known-heterozygous tigers into their own group (Hz) – otherwise they may have skewed the results for the orange tigers.

Details of the dataset, what it includes, and how it was built can be found here: The White Tiger Dataset  –  and tables of the results by decade are at the end of this article.

 

Cub Mortality

Cub mortality was calculated as the number of cubs who died at less than 1 year of age, divided by the total number of cubs born – per decade of breeding, and for the total period.

The average across the entire period, for each of the 3 groups, was practically the same:

  • White tigers:   36%
  • Hz tigers:        37.5%
  • Orange tigers: 34%

These figures are also comparable to the AZA-quoted rate for captive tigers in USA, 35%:

“Historically, the neonatal mortality rate for captive tiger cubs within the first year was about 40%; it has improved to about 35% (data from the AZA Tiger SSP)”

[Management and Conservation of Captive Tigers, M Bush et al]

Cub mortality can be caused by different things – first-time mothers sometimes neglect their litter, or cause death by accidental rough handling. Stressful conditions can cause a mother to reject her young, and differences between zoos may have an impact. Lack of appropriate nutrition during pregnancy can result in unhealthy cubs. Disease can strike down even healthy cubs. Inbreeding depression results in high (or total) cub mortality.

This analysis grouped all cub deaths together regardless of cause, but for white tiger breeding, the probability of inbreeding affecting cub mortality was of special interest.

The graph shows cub mortality rates per decade of breeding – and it reveals an interesting trend. While the rate for orange tigers remained relatively constant, the White and Hz rates peaked in the 1970’s, dropped sharply in the 1980s, then continued their downward trend to the “normal” level in the 1990’s.

This matches the historical evidence that a small period of intensive inbreeding was followed by improved breeding practices and the resulting healthier population.

CubMortalityGraph

Cub Mortality Rates of Captive-born Tigers in Indian Zoos 1950’s to 2015

1970’s: Short Period of Intensive Inbreeding

During the 1970s, all white tigers born in India were descended from only 2 wild-caught founders – Mohan and Begum. The increasing level of inbreeding resulted in inbreeding depression, as shown by the high cub mortality. This was duly noted back then, and the practice ceased by the 1980’s.

1980’s: A New Line of White and More Outcrossing

During the 1980’s, 2 more pivotal changes occurred: – the discovery of a new line of white tigers, the Orissa line, and the introduction of new orange founders into the white tiger breeding pool. During this decade, the number of founders jumped from 2 up to 10 for white tigers, and up to 14 for Hz tigers.

Continuing through the 1990’s, the number of zoos breeding white tigers increased, outcrossing continued, and the cub mortality rate dropped further, down to the “normal” level for orange tigers.

Longevity

The average age (at death) was calculated for adult tigers that reached reproductive maturity, 4 years of age (ie any tigers dying at less than 4 years were excluded). The average age for each of the groups was practically the same:

  • White tigers:   13.2 years
  • Hz tigers:         13.3 years
  • Orange tigers: 12.2 years
  • Wild-caught:   14.3 years

The tables at the end of the article show the age groups per decade of birth for each color type.

The greatest age recorded for a captive-born tiger was 24 years – 1 white and 1 orange tiger reached this age.

The results for wild-born tigers kept in captivity are shown for comparison. Wild-caught tigers appeared to reach a higher average age, and this age seemed to decrease from 17 in the 1950’s down to 14 BUT there is more uncertainty with the wild-born figures because:

  • The birth date of wild-caught tigers is an estimate, and accuracy may have changed over the decades.
  • The wild tigers were captured from a pool that already had the weaker members weeded out, so the survivors do not represent the full spectrum of ages etc.
  • Capture decisions have changed over time, wild tigers are no longer captured at will, but only the problem or rescued tigers are now brought into captivity.

 

Conclusion

White tigers in Indian zoos have the same cub mortality rate and longevity as orange tigers, ie approx 35% and 12-13 years.

There is no indication that the average cub mortality rate is or has ever been 80%.

The cub mortality rate of both white and heterozygous tigers has improved over time, matching the changes in breeding practices.

 

Data Tables

The figures quoted should be taken as indications only, for comparison, not as absolute values. Information on the dataset used can be found here:  The White Tiger Dataset

The tables show the births by decade for white tigers, heterozygous, orange, and wild-caught tigers kept in captivity. The number of tigers that are known to be still alive (at 2015) is shown, then the number of tigers for which no death date is recorded are in the “unk” unknown column (many of these are likely still alive). The % of tigers with unknown death date is in the “A+U” column.

The tigers with known death date are then grouped by age at death. The <1 year group represents the cub mortality group.

The age group %’s are calculated on the total number born. Note that meaningful data is not available for the higher ages from 2000 onward as many tigers born since then are still alive. For these, the total is given to 1999 instead of to 2015.

The final column gives the average age reached by adult tigers. Tigers less than reproductive age (4 years) were excluded from this calculation.

Tigers with unknown date of death (ie still alive or unknown) were also excluded – with the exception of 7 records from 1990-99 of aged tigers that were still alive as at 2015. They were included in the calculations at their current ages, being: Orange = 15 16 18 18 20 White=15 18

Table-ww-ow

Click to enlarge

table-oo

Click to enlarge

More Snow White Tigers in NE India c1800

More Snow White Tigers in NE India c1800

The natives of this district reckon 4 kinds of tiger (Vagh). 1st, Babbra, which is white, has very long hair about the head, and is the largest of all. It is very rare, nor have I met with anyone that has seen it, but I heard that one was killed by Digvijay Raja of Bhewopar some time before the English took possession, and the animal is said to be occasionally seen by the cowkeepers. From the circumstances of the long hair about the head, and of the animal having no spots, I should have imagined that it was the lion; but the people all say that the colour is pure white. It neither attacks  man nor domestic animals. The animal killed in Dinajpoor, and of which the skin was sent by the Marquis Wellesley to Sir Joseph Banks, was probably of this species. 2nd Nongiyachhor or the royal tiger, which lives chiefly among reeds, or in the thickets….

This extract is from the 1838 edition of  “The History, Antiquities, Topography and Statistics of Eastern India; Vol 2: Goruckpoor section, by Robert Montgomery Martin. [1]. The section on Dinajpur gives further information about the tiger killed there: Continue reading