Check mate

Females of some insect species dominate males. Why?

By Ritu Gupta
Published: Thursday 15 July 2004

Check mate

-- (Credit: USDA)Ants are intriguing creatures. Their complex behaviour has baffled us for long and it's some irony that humankind -- amongst whom male-dominated societies abound -- mindlessly typecasts female ants as robust, overbearing and egg-laying machines. The males on the contrary are projected as gentle, even weak-willed creatures forever relegated to inferior roles.

To the layperson, there is more than a grain of truth in this impression. For, most ant colonies are female-dominated with queens controlling almost every activity, including the number of males that can survive in the congregations. And to date, this tiniest struggle of power has confounded entomologists with only novices attributing it to the female ants' domineering disposition. In other words, they have no genetic variability. This makes hymenoptera n males vulnerable to disease pathogens. The 'sick-male' theory proposes that female-based insect societies are a response to higher risks of infection to males. "Disease and infections are a very powerful and on-going force in natural selection, which favours individuals that possess forms of genes that make them more resistant to infection," says O'Donnell. "In some cases, an individual that has more than one form of a gene can ward off more parasites. In humans, for example, there are different forms of a blood gene that can help ward off the malaria parasites. People with two different alleles (variants) for this gene are more resistant to malaria," he adds. But since insects have no such individual means, they have created a set of social rules to survive and diminish the risks imposed by susceptible males.

No exposure, please
Among behaviours possibly affected are the division of labour between males and females. While colonies vary widely in the total number of individuals, females vastly outnumber males and seem to do most of the work. "Males typically do not do very much work for their colonies," says O'Donnell. "In some instances males do small amounts of labour inside the nest. But rarely are they allowed outside of the nest and they do not forage for food or building materials -- activities that might expose them to disease pathogens," adds the entomologist. However, males of some hymenoptera n species contribute to in-nest labour such as thermoregulation. Male bumble-bees are the among the most prominent of such creatures.

The new theory also concludes that when some males in a colony are exposed to a disease pathogen, their counterparts also suffer social ostracism. Sometimes, females even attack and kill the diseased males.

Its a model, after all
According to the researchers, although the haploid susceptibility model predicts general differences between males and females, not all species are expected to exhibit the same behavioural pattern. For instance, males of some paper wasps are dominant over females. In some bumble-bee species, such as the Bombus bifarius and B huntii, males are tolerated in their natal nests and are permitted to return after leaving on mating flights. As per the paper, such diversity in responses is caused by the scale of parasite pressures that each species faces, as well as the genetic variability of the species.

O'Donnell believes that much more research needs to be done to test whether males are less resistant to disease than females and to substantiate the sick-male model. "We hope our study will inspire scientists to test these ideas," he avers. Field studies showing higher rates of infection or diseases in males would provide a supporting but non-rigorous test. A stronger approach would be to expose males and females with pathogens during controlled experiments.

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