Animals living in islands tend to develop weird traits to acclamatize themselves with the environment over time, becoming big like Galapagos tortoises or small like extinct dwarf elephants or losing the ability to fly like the flightless parrots of New Zealand. But some animals have a tendency to develop “melanism” or, dark or black coloration, cites a new study.
J. Albert Uy and Luis Vargas-Castro of the University of Miami found an ideal species to study this phenomenon in the Chestnut-bellied Monarch or Monarcha castaneiventris, a bird found in the Solomon Islands, which is subspecies found in the Russell Islands.
After visiting 13 islands of varying sizes to survey their Chestnut-bellied Monarch populations, Uy and Vargas-Castro confirm that island size predicts the frequency of melanic birds, with populations on smaller islands including more dark individuals.
Because the pattern is repeated on island after island, it is unlikely to have developed by random, instead, dark coloration must provide some sort of benefit to birds on small islands, said the researchers.
Since earlier studies on mammals and fish have found a genetic link between melanism and aggressive behavior, Uy and Vargas-Castro speculate that the limited space available on smaller islands makes it more competitive for breeding territories, giving an advantage to the most aggressive individuals.
Previous experiments with other Monarcha castaneiventris subspecies using taxidermied birds and recorded songs have shown that melanic birds react more aggressively than their chestnut-bellied counterparts when they perceive a threat to their territory.
Uy, who had been fascinated by Chestnut-bellied Monarchs ever since he was a graduate student, said: “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. It took me over a decade to finally manage to get to the Solomons, and I’ve been working on these flycatchers now for nearly 10 years.”
Rebecca Safran of the University of Colorado, an expert on divergence between bird populations who was not involved in the study, said, “Using the same archipelago that enchanted Ernst Mayr decades ago, Uy and Vargas-Castro reveal fascinating patterns of melanism and island size. These patterns add to the fundamental importance of islands as natural experiments for studies in biodiversity.”
Their paper was published in The Auk: Ornithological Advances.