Animals that find themselves on islands are known to evolve odd traits, some rodents and birds are known to get bigger, while mammoths and elephants shrink in size. But there are lots of other little oddities found in island populations of animals, and one of them is melanism, or having an all-black coloration. New research, looking at one species of bird found across many of the Solomon Islands, has found that the size of the island predicts the frequency of melanistic birds present.
Using the chestnut-bellied monarch, a bird endemic to the archipelago – meaning it’s found nowhere else on Earth – as a species in which to investigate the link between island size and melanism, scientists from the University of Miami set about visiting 13 islands of varying size looking for the bird. The chestnut-bellied monarch normally has, as the name suggests, a brown belly, but there are also a few all-black individuals living within the populations.
“I thought this would be the perfect species to explore these questions about the ecology of plumage diversification and the origin of species, as the variable populations of the chestnut-bellied flycatcher may be at different stages of the speciation process,” explained J. Albert Uy, who co-authored the paper published in The Auk: Ornithological Advances. “It took me over a decade to finally manage to get to the Solomon’s, and I've been working on these flycatchers now for nearly 10 years.”
They found that the smaller the island, the higher the frequency of the black variety of the bird, with the coloration accounting for almost a third of the birds on some of the smaller islands. The researchers suggest that because this pattern of melanism is repeated through different taxa – from insects to reptiles – that developing a dark coloration on small islands must confer some sort of advantage, rather than simply being an odd quirk.
Some studies have hinted at a possible genetic link between melanism and aggression in both mammals and fish. This made the researchers speculate that the smaller islands – which therefore have reduced breeding territories – might be favoring those birds which are also the most hostile when it comes to competing for a mate, which happen to be the black ones.
“Patterns of biodiversity on islands have always been important for understanding fundamental principals in ecology and evolution… Uy and Vargas-Castro reveal fascinating patterns of melanism and island size,” says Rebecca Safran from the University of Colorado, who was not involved in the study. “These patterns add to the fundamental importance of islands as natural experiments for studies in biodiversity.”
