Of all the possible topics, behaviours and areas I have encountered throughout my studies, melanism has remained perhaps the most fascinating and intriguing topics I have ever studied. Whilst from the outset it may not seem like something you can really sink your teeth into, I’m going to use today’s post to try and sell you the wonders of the topic.
I want to keep the range of topics I blog about as broad as possible (within the realm of big cats), and I’m aware I haven’t written a “biology” themed post for a while, however I’m also conscious of wanting to keep this blog as accessible as possible, whether you have a scientific background or not so if you do have any questions left unanswered by the end of my post please don’t hesitate to get in touch, and I’ll either do my best to answer them myself or point you in the direction of someone who can!
So what is melanism? The official dictionary definition of melanism is a darkening of body tissues caused by excessive production of melanin, especially as a form of colour variation in animals. For those of you who haven’t come across melanin before, it is a polymer derived from tyrosine (an amino acid and the building blocks of all proteins) which is responsible for determining skin and hair colour. Historically the amount of melanin in the skin depends on the population’s exposure to the sun. This is why for instance, people from colder climates such as Northern Europe and Iceland have a different skin colour to people from climates closer to the equator.
You’ve probably come across the term albino, or albinism before, a condition characterized by a complete or partial lack of pigment in the skin. In simple terms you can think of melanism as the opposite of albinism.
*IMPORTANT* I think it’s important to clarify at this stage (as I was nearly in danger of making this mistake), that White Tigers are not albino, they are simply pigmentation variants of the Bengal tigers. White tigers lack the pigment pheomelanin which is found in red and orange hair – which explains the Bengal tiger’s orange fur. From the outset I set out for this blog to be a learning curve for both the wider internet and myself, and I guess I’ve just learnt my first lesson!
Anyway, I digress. I’m sure you’re all aware of black panthers – they’re perhaps the animals most commonly associated with melanism. Black panthers are in fact any species of big cat (Panthera) who display the melanistic colour variant (i.e black, or near black in colour). In Asia and Africa black panthers are more correctly known as black leopards, whilst over the ocean in the Americas, black jaguar could be considered the more correct name.
The next few paragraphs are where melanism in cats (and more importantly big cats) begins to get even more interesting. It might get a little scientific/wordy at times but I’ll do my best to keep everything easy to read and understand!
Melanistic coat colourations occur as a common polymorphism in 11 out of 37 felid species. A polymorphism is a condition in biology and zoology where there are two or more different morphs (forms) in the population of a species. These morphs which make up an animal’s phenotype can occur in one of three different ways (i.e the colour of the cat’s coat can be determined in one of three ways).
- Phenotype is genetically determined
- Phenotype is set by environmental cues (remember this one, I’ll come back to it later)
- Phenotype is randomly assigned
*A phenotype is a set of observable characteristics of an individual resulting from the interaction of it’s genotype with the environment. In other words, a phenotype could be classed as everything you see on an organism whereas the genotype is everything you don’t see, which determines the organism’s phenotype*
Whilst melanism can reach an extremely high population frequency in some cases, such as the black leopards of Malaya, and the ones found on the slopes of Mount Kenya, it has never achieved fixation. For fixation to happen, the gene for non black coats would need to be lost completely, and this explains why there are no populations of leopards or jaguars with solely black coats. As you can see from the images I’ve included in this post, melanism doesn’t rid leopards and jaguars of their characteristic coat patterns, the excess pigment (melanin) merely masks, or disguises them to a certain degree. The extent to which the cat will appear black is reflected on how “strongly” each animal expresses that particular gene.
Now, to explain the final part of this post, and to reveal why I find melanism so interesting I first need to include one more definition which will hopefully make my reasoning more clear.
- Allele : One, of a number of alternative forms of the same gene. Sometimes, different alleles result in different observable phenotypic traits – such as pigmentation in the skin.
There are two types of alleles, dominant and recessive, and the combination of these types in an organism will determine if the cat will inherit a black coat or not.
So to get right to the bottom of why I find this topic so interesting I’ll serve you up some basic genetics just to spice up your Friday night!
Chromosomes come in pairs, and all our genetic information is stored on them, including our, and big cat’s skin colour. The characteristic is controlled by a dominant allele if the allele (for skin pigmentation) is present on one, or both chromosomes in the pair. The characteristic controlled by a recessive allele develops only if the allele is on both chromosomes in the pair.
What’s so fascinating about melanism in big cats, is that in jaguars, melanism is conferred by a dominant allele, whilst in leopards the trait is recessive! Now, whilst no official studies have been completed yet, there is a general consensus from the scientific community that melanism in leopards in the Malay Peninsula may be evolving to give the cats a selective advantage when hunting their prey. A camera trap study undertaken between 1996 and 2009 found that the percentage of black leopards compared to regular coloured leopards could be as high as 90%, suggesting that the “dark” allele has nearly been fixated in the region.
In theory, this means that eventually all of the leopards in the Malay Peninsula region would be born with black, or near black coats whereas many years ago this may not have been the case, many years ago. This gradual increase in melanistic leopards is an example of genetic drift (which is the variation in the relative frequency of genotypes in a population), and genetic drift happens to be one of the base mechanisms for evolution.
So, we’ve finally arrived at why I find melanism in big cats so interesting. We spend much of our lives learning about how animals have evolved over time into the creatures we see on Earth today. And whilst these events do take hundreds of thousands to millions of years, this gradual change (or fixation) in the leopards of the Malay Peninsula could well be evolution happening before our eyes, although there’s a (very) strong possibility that none of us reading this will be around to see it!
I hope you’ve enjoyed reading this post as much as I’ve enjoyed writing it. I know it has taken a long time to arrive at my point, but I hope you have at least managed to learn one or two things along the way, and maybe even picked up some of my enthusiasm for this fascinating topic!