Natasha Howell of Bristol University unearths the mystery of striking colouration in mammals and what skunks and bees have in common. This blog is part of our colourful countdown to the holiday season where we’re celebrating the diversity and beauty of the natural world. Click here to read the rest of the colour countdown series.
The vast majority of mammal species are not known for their striking colouration. Compared to other groups such as birds, insects, and amphibians, mammals are rather drab. Their colouration is often comprised of dull browns and grey tones. This does serve an important purpose, helping them to camouflage in their environments, therefore offering protection from predators by making them difficult to detect. However, not all mammals are dull. There are many instances in which species have evolved to be contrastingly black-and-white in their colouration. Think porcupines, giant pandas, and zebras. These striking colour patterns may make these animals highly conspicuous in their environments – a problem for most mammals, as flashy colouration comes with a higher risk of being spotted by a predator. So why, then, have a number of mammals developed these highly contrasting black-and-white coat patterns?
We know that the black-and-white stripes along the body of the striped skunk act as a signal – they advertise the skunk’s ability to spray noxious secretions from its anal glands, which makes them off-putting to predators. This is an example of aposematism, where an animal’s colouration signals the possession of a defence mechanism that will harm potential predators, thereby making the animal an unprofitable prey item. But what of other black and white mammal species, such as porcupines, the honey badger, and the yellow-throated marten? Could they be using their colouration in a similar manner; namely, to advertise defences that make them unprofitable prey in the eyes of a predator?
To begin figuring this out, my co-authors and I called upon the work of pioneer naturalist Alfred Russel Wallace. In recent years, there has been an informal consensus that aposematism refers to colouration that signals the possession of noxious chemicals, as in many brightly-coloured species of frogs and caterpillars. However, both Wallace’s original writings on aposematism, and evolutionary biologist Edward Bagnall Poulton’s subsequent definition of the word, were a lot broader than how aposematism is thought of today. They included not just toxicity or noxiousness in their explanations, but any defence mechanism that could leave a predator worse-off for having tried to take down the defended prey item. Such defence mechanisms include morphological defences like hard or spiny exoskeletons, and behavioural defences like rolling into a ball when frightened or having unpredictable flight paths, as well as the well-known example of a physiological defence: emitting disagreeable odours and fluids.
To account for this broader definition of aposematism, my co-authors Manisha Koneru, Kasey Brockelsby, Konatsu Ono, and Dr. Tim Caro, collected data for seven types of defence in mammals, split into three categories: morphological (weaponry, spines, and long and/or thick fur), behavioural (pugnacious behaviour and gliding), and physiological (noxious anal secretions, and toxicity and/or distastefulness). They also collected colouration scores for contrasting black-and-white head and body patterns that appear in hundreds of mammal species. Myself and co-author Dr. Catherine Sheard then used Bayesian phylogenetic comparative models to seek out any correlations between colour patterns and anti-predator defences.
Our macroevolutionary analyses discovered a number of correlations between conspicuous head and body patterns, and many of the various types of defence mechanism. Here are some examples of the exciting discoveries we made:
Mammals with contrasting blocks of black-and-white fur upon the head tend to behave in a pugnacious manner when approached by predators. This is reflected in the tenacity of species such as the yellow-throated marten, which possesses beautifully-contrasting black-and-white head colouration. Yellow-throated martens are well-known for being fearless and attacking animals multiple times their own size when threatened.
Mammal species with irregular blocks of black-and-white fur on the body were also found to be more likely to exhibit pugnacious behaviour. The wolverine is a famous example of a mammal with this coat pattern that is known for its rather belligerent attitude. In addition, we found that ‘salt and pepper’-like splotches of black-and-white colouration are very strongly correlated with possession of defensive spines. This is perhaps an obvious relationship if you look at species such as the African crested porcupine or short-beaked echidna. Nevertheless, this was a relationship that had never been systematically supported across mammals until now.
Right: A captive greater grison at Hamerton Zoo Park in Cambridgeshire, showcasing the species’ ‘reverse countershading’ in which the dorsum is much lighter than the ventrum; a colour pattern we found to be associated with the presence of noxious compounds that can be emitted from the anal glands. Photograph by Catherine Evans (used with photographer’s permission).
Finally, we revealed that mammals with a white dorsum and a black ventrum (an unusual colour pattern referred to as ‘reverse countershading’) tend to possess defensive anal secretions. So, this particular defence may not only be advertised by body striping as in the striped skunk. Reverse countershading is exemplified by animals like the honey badger and both species of grison. All three species are known to possess noxious secretions which they can release from specialised anal glands when threatened.
Other research in the colouration field has shown that certain black-and-white coat patterns in mammals may not have an aposematic function. For example, Dr. Tim Caro and Dr. Martin How have found convincing evidence that the stripes of zebra species act to confuse tabanid horse flies, thereby preventing the flies from biting. Additionally, Dr. Caro and colleagues’ work investigating the functions of colouration in the giant panda have revealed that its black-and-white contrasting coat may function as a form of ‘compromise crypsis’, where the white patches of fur match against a snowy background, and the black patches help to camouflage against the darkness of forest shade.
It is also worth noting that these broad-scale correlations do not necessarily mean that every species with a certain colour pattern is advertising a certain defence. Detailed behavioural observations at the species level are vital to ascertain the mechanisms that are driving these correlations.
This research was conducted as part of my MSc in Palaeobiology with the University of Bristol’s School of Earth Sciences. Our recently-published paper discusses our findings in greater detail, which can be found here. I have since transferred to the University’s School of Biological Sciences, where I continue to work with Dr. Tim Caro to investigate the evolution of mammal colouration for my PhD research. Currently, I am working to expand the comparative dataset used in this study to include all terrestrial mammal species. We intend to continue using phylogenetic comparative methods to test evolutionary predictions about the relationships between colouration and various social and habitat characteristics across the mammal class.