WHY do some creatures appear to give up on reproduction and put all their effort into raising the offspring of others? The answer may be that they have no choice, having been sterilised by parasites.
It has long been a mystery why some insects鈥攕uch as ants, bees, wasps and termites鈥攈ave evolved to form 鈥渆usocial鈥� societies, in which many animals help the colony by foraging for food, for instance, rather than reproducing. But in next month鈥檚 issue of Proceedings of the Royal Society B (vol 264, p 689), Sean O鈥橠onnell of the University of Washington in Seattle suggests that the neutering of females by parasites is at least part of the reason.
O鈥橠onnell was struck by the idea when studying the wasp Mischocyttarus mastigophorus in Costa Rica. 鈥淚t appeared that those females with higher parasite loads were less likely to have well-developed ovaries,鈥� he says. He suggests that bacterial and protozoan parasites that live inside the insects sterilise or reduce the fertility of their hosts, for instance by altering their hormonal balance, but do not shorten their life span.
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The best way for an affected insect to pass on its genetic make-up to the following generations would then be to help raise the offspring of unaffected relatives, says O鈥橠onnell. He adds that earlier research showed that infection of the bumblebee Bombus terrestris by the parasitical protozoan Crithidia bombi also reduces ovary development, and that infected bees work for their colonies longer than noninfected workers.
O鈥橠onnell looks forward to testing the idea by experiment. 鈥淭he possibility of experimental manipulations, such as altering parasite loads and measuring effects on host behaviour, is very exciting,鈥� he says. But first, researchers will have to find out more about the interactions between the parasites and the insects. 鈥淎t present, we know almost nothing about these,鈥� he warns.
Bill Hamilton of Oxford University, who has played a leading role in understanding eusocial behaviour, thinks the theory is 鈥渃ertainly not a crazy one鈥�. He suggests that it may tie in with the idea that colonies are 鈥渂eing run by their microbial symbionts鈥濃�攁t least in the case of termites, whose gut organisms make up about half of their biomass.