Sexual Dimorphism Ensures Survival and Prosperity

Sexual dimorphism means that males look different than females. Sometimes, the differences can be so substantial that, rather than talking about different sexes, we might be inclined to see them as two different species: elephant seal males are 8 times larger than females; or, in some loris, the female can be green and the male purple.

Sexual selection through male combat for the purpose of getting the mating right or females choosing amongst the males have been seen as the main factor for sexual dimorphism.

In other cases, the sexual dimorphism is determined by physiological needs: human females must have larger pelvises to be able to deliver large offspring. "Another explanation is that sexual differences arise so that the sexes can utilize different resources and not compete with each other. In some hummingbirds, for example, the sexes differ in the length of their beaks, allowing each to drink nectar from different flowers. By diverging in their resource use, the population as a whole can reach higher levels", said Jonathan B. Losos, professor of organismic and evolutionary biology at Harvard's Faculty of Arts and Sciences and curator in herpetology at the Harvard Museum of Comparative Zoology.

He investigated this phenomenon on Anolis lizards from the Caribbean. His team found out that sexual dimorphism permitted the lizards to colonize much larger niches and habitats than if the sexes were similar, and to avoid male-female competition for resources - a fact which led to an increase of the population.

Sexual dimorphism could have boosted evolution in a way that till now has escaped the researchers' understanding. The Anolis lizards are a classic case of adaptive radiation, diverging in many species after reaching Cuba, Hispaniola, Jamaica, and Puerto Rico. They have adapted to local environments: anoles on open habitats evolved longer legs, better suited for running, while tree species display shorter legs, adapted to movement on narrow and irregular twigs.

But the anole lizards also vary a lot in their sexual dimorphism, from species where adult males and females are very similar to those where adult males are thrice heavier than females are.

But sexual dimorphism was linked to differences in microhabitat use, diet, and behavior. "Adaptive radiations are well-known for the fine-tuning of organismal traits to match their niches," said co-author Marguerite A. Butler, assistant professor of zoology at the University of Hawaii. "We noticed that every well-known adaptive radiation has sexual dimorphism -- such as Hawaiian honeycreepers, Galapagos finches, and African cichlids -- but these radiations are studied in terms of one sex only, usually males. So we asked a simple question: If animals are adapting to their environment, but the sexes are different, does the sexual variation add to diversity? In other words, is it important to consider both sexes? The answer is yes."

The team assessed the "morphospaces" (ecological niches linked to limb length, body length, and mass) of 15 different anole species. Just 14 % of niches were common for both males and females of the same species, 45 % just for the females and 36 % for the males. "Clearly, sexual dimorphism significantly increases the ecological niche occupied by a species. Among West Indian anoles, sexual dimorphism results in a 59 % increase over the morphospace occupied by females alone, and an 88 % increase over the morphospace occupied by males alone", Butler said.