It has been well documented that animals on islands end up developing interesting traits; Darwin's famous finches with huge variations in beak size is possibly the most obvious example. Sometimes large animals like elephants and mammoths can shrink in size, whereas smaller creatures like rodents become huge. One of these characteristics that can change is colouration, particularly becoming all-black (known as melanism). New data published on one species of bird found on the Solomon Islands has found that the size of the island can predict the frequency that melanism occurs.
Scientists from the University of Miami used the chestnut-bellied flycatcher to investigate the connection between melanistic individuals and island size. The species is endemic to the islands - it is not found anywhere else in the world - so it is a good indicator of island effects on a species. 13 islands of differing sizes were visited to look for the birds. The chestnut-bellied flycatcher usually has a brown underside (as the name indicates), but there are also several melanistic individuals that can be found within the populations.
Results of the study found that there was a strong correlation between island size and black birds; the smaller the island, the more melanistic individuals found. Sometimes the black varieties of birds contributed to nearly a third of birds of that species found on the smaller islands. The authors put forward the explanation that because this pattern of melanistic animals is reflected throughout other taxa (animals other than birds), there must be some sort of advantage to have a darker colouration on the smaller islands, instead of it merely being coincidence.
Previous studies have suggested that melanism and aggression have a genetic link, especially in mammals and fish. For example; lions with blacker manes have higher testosterone (meaning they are more aggressive to outsiders), resulting in fending off more rivals and being able to reproduce with more females, passing on more genes. This prompted the authors to conclude that smaller islands - which in turn have smaller breeding territories - could result in more aggressive individuals as they are more likely to be successful in competitions for mates, resulting in more melanistic individuals in the population.
Studies of island populations of species have always been important in understanding the basic principals of evolution and ecology, such as the origins of species and the adaptations they undergo. Results like this add to the significance of islands as natural experiments for learning about biodiversity and speciation.
The original paper can be found here.