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
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. 
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. 
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.
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.  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. 
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).  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. 
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.
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. 
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. 
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.
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.
[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.
– Plus many other news reports
- May 26, 2016 The Telegraph, Calcutta – Tigress delivers stillborn black cub
- Aug 5, 2016 Times of India – Bhubaneswar zoo gets three tiger cubs
- Dec 29, 2015 DNA Bhubaneswar, Ever seen a melanistic tiger? Visit Nandankanan Zoo to get a glimpse of rare black tiger cubs
- plus many other news reports
 The White Tiger Dataset; and Bengal Tiger Studbook Dec 2012
 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.
- 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.
- New Indian Express September 10, 2014 Similipal Counts 25 Royal Bengal Tigers
- NDTV January 23, 2015 Tiger Numbers Up, But the Black Tiger of Odisha on Verge of Extinction
- The Hindu May 21, 2016 Tigers in Odisha Facing Survival Challenge
 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)