Directors cut: "How often can big cats be black"

A DSLR camera trap in Thailand's Huai Kha Khaeng Wildlife Sanctuary captured this image of a melanistic leopard.

Melanism in cats

About a month ago, Panthera's Senior Digital Writer contacted Dr. Olutolani Smith and myself (the resident Panthera genetics gurus) about a Facebook follower's questions on melanism in jaguars and leopards, which was made in response to a recent blog post and interview by Dr. Alan Rabinowitz.

Dr. Smith and I each responded separately, and while predictably providing some overlapping information, we also offered some unique insights in our answers, likely as a result of our different backgrounds and interests. Our responses were well received and Panthera decided to turn them into a blog post, which was posted a couple weeks ago, "How often can big cats be black?". The post combined Dr. Smith's and my responses, reduced redundancy and shortened the text to fit with the idea of shorter, more easily consumed online content. It turned out great. However, I got to thinking that for those looking for a bit more detail, Dr. Smith's and my original responses may be of interest. So for those who just can't get enough, please see below for our initial takes on the question posed on Panthera's Facebook page about melanistic cats:

“Why is it that black leopards are more common than black jaguars, when for leopards it is a recessive gene, and for jaguars it is a dominant gene? Wouldn't a dominant gene occur more frequently in the wild than a recessive gene? Does it only appear to be this way because leopards have been more easily bred in captivity to produce them, [and] jaguars don’t breed as well in captivity? “Black jags, may also be harder to find in the dense forests and so appear rarer in the wild than wild black leopards. Does a mutation like melanism occur frequently in the right environment, or are the only occurring instances of the gene come from being inherited/passed down?”

From Dr. Byron Weckworth, Regional Scientist and China Program Director of our Snow Leopard Program:

In the wild, surveys of confirmed melanism in both jaguars and leopards show that the frequency is about 10% for each species. You could be right that the perception of frequency is impacted by how melanistic leopards are more prevalent in popular media, etc. But the reality is that, in the wild, the frequency is about the same.

Yet, as you observed, melanism in jaguars is determined by the dominant allele of a gene, vs. recessive in leopards. So why aren't black jaguars everywhere, or at least more common? In a similar scenario in humans, polydactyly (having more than five fingers/toes) is also usually a dominant trait, so why aren't there more people with six fingers? To better understand this question, we need to take the next step in our understanding of genetic inheritance of traits.

It is true that in a randomly mating population, absent any evolutionary mechanisms (e.g. genetic drift, mutation, natural selection), that we would expect a dominant trait to proliferate to higher frequency than a recessive trait. This is called Mendelian inheritance, which forms the basis of your excellent question.

However, populations are rarely mating randomly (i.e., inheritance of traits is not random), and when we add in evolutionary forces like mutation and natural selection, models of Mendelian inheritance can break down. Also consider that even if a trait is dominant, it may be so rare in the population that its frequency will always be limited (such is the case in polydactyly), which is one reason that might explain why even if melanism is a dominantly inherited trait in jaguars, it can also be rare. Dominant in inheritance doesn't mean dominant in function, which relates to natural selection (see below).

The mutation that causes melanism in leopards and jaguars is different in each species. For leopards, melanism is caused by a recessively inherited mutation that inhibits a gene function related to coloration. This happens via a complicated biochemical pathway coded by that gene that usually results in a "normal" colored cat, but is disrupted by the mutation and results in a melanistic animal. But in jaguars, the mutation is on a different gene, is dominantly inherited, and results in a gain of function in that gene, activating a different biochemical pathway that produces the same melanistic trait in the cat.

As mentioned above, if we map all the confirmed occurrences of jaguars and leopards across their global distributions, we see that about 10% of individuals in both species are melanistic. However, the cases of melanism are not evenly distributed across their ranges. In fact, if you use complex statistical models of these occurrences with environmental factors, the results show there are much higher incidences of melanism—in both species—associated with more moist habitats (which correlates with dense forests).

For leopards, there are no confirmed sightings of melanistic cats in Central Asia or far east Asia, places where the habitat tends to be more temperate and open. The same is true for melanism and habitat types for jaguars in the Americas. The conclusion here, when we see non-random distribution of a trait, is that there is likely some sort of adaptive selection occurring where melanism is either advantageous (or disadvantageous) in certain environmental conditions. That melanistic cats are more common in moist forests suggests an adaptive advantage to the trait in these environments, perhaps related to thermoregulation or some other functional characteristic.

From Dr.Olutolani Smith,  Consultant Geneticist working with the Tigers Forever Program:

I agree with Dr. Weckworth’s comments—it’s a brilliant question and one I hadn’t really thought about before. 

A couple of papers by Eizirik et al.  and Schneider et al. explain the genetic basis of melanism in Felidae. They conclude that melanism is present in about 13 of the 37 felid species—so, not just jaguars and leopards—and that there are different color phases in some of the other species, such as Geoffroy’s cat, oncilla, Pampas cat, and Asian Golden Cat.

Melanism is strongly related to two particular genes in mice—ASIP and MC1R. ASIP (agouti signalling protein) is involved in hair follicles producing a color pigment. MC1R (melanocortin 1 receptor) is a gene that codes for the receptors that control pigment formation.

When they looked for similar mutations in domestic cats and other wild cats, the Eizirik group found that melanism arose at least four different times in Felidae, and each mutation is slightly different in each of the cat species affected. 

For example, the jaguar mutation is a 15 bp deletion in MC1R, and the jaguarundi mutation is a 24 bp deletion at a different site of the MC1R gene. Affected jaguars are homozygous (two copies of the mutation) or heterozygous (one copy of the mutation), and wild-type or normal jaguars are homozygous for the full gene. This implies that the mutation has a dominant effect.

In leopards, the melanism is thought to be caused by a small mutation in the ASIP gene, but this is inherited as a recessive trait, meaning that only homozygous individuals with two copies of the mutation are melanistic.

It was initially assumed that these melanistic mutations have an adaptive advantage under certain ecological circumstances, which is why they have persisted. And perhaps what happened is that they appeared spontaneously, as most mutations do, and persisted because of this advantage. Cats with the melanism gene, whether it is a dominant or recessive trait, will be positively selected for where it is advantageous, and hence you will find more melanism in certain regions.

It seems that melanistic forms for both jaguars and leopards are more likely to be found in moist forests (for example, more melanistic leopards are found in Southeast Asia, and none have been documented in regions like the Russian Far East and China). And, as Dr. Weckworth pointed out, although the mutations are inherited in a slightly different way, they affect different genes, and the frequency of the phenotype (melanism) in jaguars and leopards is essentially the same.