doi:10.1038/nindia.2017.46 Published online 12 April 2017
On 18 January 2017, two litigon cubs were unveiled for public display at a safari zoo in Haikou, China1. The cubs represent an important biological phenomenon, being born of a fertile tigon (a tiger-lion hybrid) and an African lion. They also raise important questions on the biological species concept and the fertility of hybrid individuals.
Earlier, in July 2016, scouring through the archives of the National Library in Kolkata, India, an information scientist* and a librarian** laid their hands upon a rare photograph published in 1980 in the daily newspaper The Statesman2. The photograph, procured and reproduced here (Figure 1) was that of a male litigon. It was described in an accompanying news report as a hybrid of a male Asiatic lion Panthera leo persica and a female tigon (hybrid of a male tiger Panthera tigris and a female African lion P. leo of unknown subspecies) from the Alipore Zoological Gardens in Calcutta (now Kolkata)2. The litigon was named Cubanacan by Jose Lopez Sanchez, the erstwhile Cuban Ambassador to India, and photographed on the cub’s first day of public viewing in the zoo.
The litigon grew up to be one of the world’s largest big cats of the time, weighing around 363 kg, a record 3.5 m long and 1.32 m wide at the shoulders3. However, this second-generation hybrid was forgotten in subsequent literature, although sporadic discussions of tigons and ligers (hybrids of male lions and female tigers) continued in popular media.
Cubanacan was born after 15 years of hybridisation attempts that started in 1964 at the Alipore Zoo4. The zoo reportedly produced its first hybrid cat, a tigon called Rudrani, on 13 October 1972 in the sixth litter of a female African lion Munni and a male tiger Munna. A second tigon, Rangini, was born on 8 March 1974. Both tigons were named by the then Chief Minister of West Bengal, Siddhartha Shankar Ray.
Rudrani was subsequently mated with a male, reportedly Asiatic, lion (but was later genetically established as a hybrid of the African and Asiatic subspecies of the lion5), called Debabrata, and gave birth to Cubanacan, the only surviving litigon in a litter of three.
With the birth of five litigons, Cubanacan being the most famous, Alipore became the only zoo in India to have tigons and the only in the world to have successfully bred litigons6. Cubanacan died on 12 April 1991. Subsequently, the zoo made several attempts to create more novel hybrids of big cats by breeding litigons but they remained unsuccessful6.
In 1985, big cat hybridisation experiments became controversial after the Indian government banned the enterprise7. Although references to big cat breeding experiments at the Alipore Zoo and various pictures of tigons and litigons appeared occasionally in the popular media of the time, efforts were made subsequently to obliterate these references (Figure 2).
The recent finding of the litigon photograph warrants a fresh look into the biological significance and ethical connotations of the classic experiments performed at the Alipore Zoo. This assumes importance following recent suggestions that interspecies hybridisation may be a natural and significant biological phenomenon7. Critical questions have also been raised on the validity of the widely accepted biological species concept that has traditionally defined ‘species’ as reproductively isolated taxa that cannot mate with other taxa to produce viable offspring8.
Conservationists suggest that hybridisation between different species and subspecies is among the many threats to endangered taxa, alongside habitat destruction and degradation, poaching, pollution and illegal wildlife trade9, 10. This view has been endorsed by several international organisations, including the International Union for the Conservation of Nature (IUCN), the World Association of Zoos and Aquariums (WAZA) and many national agencies, leading to widespread condemnation of hybridisation in captivity and in the wild. The extreme abhorrence for hybridisation has also reportedly resulted in euthanasia of hybrid animals in zoos11.
While hybrid populations amidst so-called ‘pure’ populations could potentially threaten their mutual survival, a more fundamental concern is the widespread view which considers inter-species breeding as a form of ‘genetic pollution’. This view fuels the widespread academic notion that species are reproductively isolated units and that all events of hybridisation are undesirable accidents, with hybrids (such as sterile mules) being evolutionary dead-ends. This concept has become a part of standard discourse on biological hybrids. Do these positions, both in the biological sciences as well as in conservation, actually stand up to scrutiny?
The definition of the biological ‘species’, as a fundamental category of biological organisation, has been a long-standing conundrum in biology. The most widely accepted definition, proposed by Ernst Mayr12, suggests that species, at least in a taxonomic sense, are groups of interbreeding natural populations reproductively isolated from other such groups. This definition also explains why members of a species resemble one another closely and differ from other species in biological traits. Although a number of definitions arose during the period of the Modern Evolutionary Synthesis13, 14, including Mayr’s, the most comprehensive and well-subscribed concept emphasised the link between species and gene pools or metapopulations15. These concepts, however, differed in certain properties deemed necessary for an independently evolving metapopulation lineage to be considered a separate species16. These characteristics included specific mate recognition systems, ecological distinctiveness, monophyly, formation of distinctive phenetic classes and, importantly, intrinsic reproductive isolation. These alternative definitions of species demanded that one or more of these characteristics be considered necessary properties of species and this, in turn, led to debates on which metapopulations could be considered as distinct species16.
A historical analysis of speciation processes shows that these so-called properties of species may have originated at different times and followed different developmental paths of evolution. For instance, in the case of reproductive isolation, several pre- and post-zygotic mechanisms (genetic, chromosomal, morphological, anatomical or behavioural) may be responsible for species recognition and/or breeding incompatibility between members of different metapopulation lineages. Importantly, these mechanisms could have arisen by chance — through mutations or chromosomal rearrangements and/or facilitated by migration or genetic drift. They may have also evolved out of necessity, by natural selection, under conditions such as sympatric speciation, the process through which new species evolve from a single ancestral species inhabiting the same geographic region.
Moreover, the evolution of these mechanisms may not be related to other evolving properties of a potential species, such as shared ecological niches, monophyly or fixed qualitative or quantitative traits that distinguish a metapopulation lineage from another as distinct species. Thus, two metapopulation lineages may have differentiated into two distinct species by any of these criteria and yet not have evolved intrinsic reproductive isolation. And this may have happened in the course of the evolutionary histories of the tiger and the lion.
Very few official records of the big cat experiments exist in the Alipore Zoo today, possibly a result of the ban on hybridisation attempts. These experiments, however, establish that hybrid tigons are fertile and can produce viable offspring. This raises a simple, cardinal question: Are evolutionarily closely related species, such as the tiger and the lion, invariably and necessarily reproductively isolated from one another? The Alipore Zoo experiments point to a possible answer: some of these species may have never become reproductively isolated from one another during evolution, after having diverged from their common ancestor and from one another. This would be particularly true of geographically well-separated species like the lion and the tiger, which must have evolved allopatrically, without any opportunity of secondary contact.
If there is no reproductive isolation between tigers and lions, is it hypothetically possible for them to interbreed freely in the wild? One may speculate that ecological and behavioural factors could lower the possibility of such interbreeding in these specialised species, which have other mate recognition factors that ensure greater intra-species reproductive fidelity. Related questions also arise regarding the behavioural and physiological characteristic of a hybrid mammal, such as a tigon or litigon. This is particularly relevant for the litigon Cubanacan, whose parents were both hybrids: the father an Asian-African lion and the mother a tigon.
Why has this first-hand evidence of the breeding viability between different kinds of hybrids failed to attract the attention of biologists? Why have these remarkable biological events not been acknowledged with the scientific importance they deserved? The answers to these questions may constitute classic examples of the way science often progresses and reflect the sociology of scientific discovery and experimentation in particular regimes of dominant scientific ideologies. There’s need for an inquiry into the motivation and the decision-making process that led to the creation of the tigon and the litigon in the Alipore Zoo between 1964 and 1979. Was it plain curiosity or was it a scientific endeavour to better understand mammalian genetics and evolutionary forces at work? An official statement of the time states: “Meanwhile, attempts are afoot to induce companionship between Rudrani and an Indian lion called Devavrata. Signs of mutual acceptability are discernible and should compatibility result perchance, birth of a triple hybrid may not be that improbable as believed so far.”17 Amarendra Nath Guha, then Director of the Alipore Zoo, was also quoted in a newspaper as saying, “(Cubanacan was) an ambitious hybrid of a type believed to be improbable so far”18.
There is now extensive evidence on other hybridisation cases between several distantly related cat species in captivity, including a liger19 from India. Many common domestic cat breeds, such as the Bengal — a hybrid between the domestic cat and the Asian leopard cat20 — are also believed to be of hybrid origin. In fact, feline ancestry appears to have been shaped by natural hybridisation events throughout its evolutionary history20. Increasingly, evidence is being uncovered to show how profoundly hybridisation has influenced the natural evolution of taxa as diverse as butterflies, sharks, finches, parrots, dolphins, bears, wolves, old world primates or even modern humans21, 22, 23, 24.
The traditional belief held by biologists is that hybridisation, which constitutes a breakdown of species-isolation mechanisms, is essentially unnatural, and hence unwelcome. While one can only speculate on the reasons why hybridisation was considered so abhorrent, these views were strongly subscribed to and overemphasised mainly in the early part of the last century, especially when eugenics and Fascism became prominent and the purity of species was considered sacrosanct.
In these hybrid-friendly, gene-editing times, however, it may be worthwhile to reconsider the biological importance of hybridisation, a naturally occurring and ubiquitous process, unsullied by value-laden human prejudices. The time has also perhaps come to conduct a dispassionate and nuanced evaluation of the much reviled and discredited big cat hybridisation experiments of the Alipore Zoo, which culminated in the birth of the litigon Cubanacan. Such an analysis could provide historical insights into closely-held scientific beliefs on species, genetics and evolution. It could also inquire how fallible record keeping of biological data, together with preconceived scientific notions, bureaucratic science management and public attitudes, could perpetuate value-driven belief systems that are anathema to objective, value-neutral science.
[*Payel Biswas, Librarian at the Institute of Urban Transport (India), New Delhi and **Subhodip Bid of the National Library, Kolkata helped locate the only reportedly labelled photograph of the litigon Cubanacan from the Library. Anirban Chaudhuri, Shibaji Mitra and Shibaji Bhattacharya encouraged and supported this study.]
1World Animal Protection in India, New Delhi, India; 2Alipore Zoological Gardens, Kolkata, India; 3National Institute of Advanced Studies, Bangalore, India; 4Nature Conservation Foundation, Mysore, India.
The opinions expressed by the authors are personal and not representative of any organisation.
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4. 100 Years of Calcutta Zoo (1875-1975). The Centenary Celebration Committee, Zoological Garden, Alipore, Calcutta(1975)
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