“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,” Minna mused, “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?”
Two of our top geneticists took a stab at Minna’s question.
The perception of frequency does seem impacted by how melanistic leopards are more prevalent in popular media, camera trap photos from the field, etc. But in the wild, the reality is that the frequency is about the same for both species: around 10%.
As Minna observed, melanism in jaguars is determined by the dominant form of a gene (MC1R), vs. a recessive form of the gene ASIP 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, in the absence of any evolutionary mechanisms like genetic drift or natural selection, we would expect a dominant trait to proliferate to a higher frequency than a recessive trait under the laws of Mendelian inheritance. However, individuals within a population rarely mate randomly, and the prevalence of melanism in big cats seems to be affected by their environment, i.e. it is positively selected for in certain habitats.
Melanism has been reported in up to 13 of the 37 felid species—so, not unique to just jaguars and leopards—with different color phases in species like Geoffroy’s cat, oncilla, Pampas cat, and Asian Golden Cat. So far, ASIP (agouti signalling protein) and MC1R (melanocortin 1 receptor) have been identified as the genes affecting pigment formation in cats.
And interestingly, research by Eizirik et al. and Schneider et al., shows that the mutation that causes melanism affects a different gene (e.g. MC1R or ASIP), and/or a different part of the gene in each species. 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 same gene.
Recent research by da Silva confirms that these melanistic mutations might have an adaptive advantage under certain ecological situations, which is why both dominant and recessive forms are prevalent in the wild. But cases of melanism are not evenly distributed across their ranges.
Melanistic forms for both jaguars and leopards are more likely to be found in moist forests. For example, ‘black’ leopards have been documented in Southeast Asia, but not in Central Asia or China. Similar sightings have been recorded for jaguars, with more melanistic forms observed in dense forest habitat, but not in the more open or temperate forest of the Americas.
So in effect, Minna’s thoughts that the mutations that cause melanism occur more frequently in the right environment is correct. The fact that melanistic cats are more common in moist forests suggests an adaptive advantage of the trait in these environments, perhaps related to thermoregulation or some other functional characteristic that is yet to be determined.
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