Hermaphroditic animals are unique because they have both male and female reproductive parts. But according to a report published online this week by the Proceedings of the National Academy of Sciences, outside stresses can favor one sex over the other. Scientists report that under times of strain hermaphroditic animals become more male than female.

Previous research had indicated that hermaphroditic plants tend to produce more pollen, a male characteristic, under stressful conditions. Roger N. Hughes of the University of Wales and his colleagues investigated whether or not animals exhibit a similar response. They established colonies of Celleporella hyalina (see image), a hermaphroditic marine invertebrate, and subjected them to varying conditions. Those groups exposed to environmental stresses such as food deprivation, temperature shock, dessication or physical damage grew to include a greater percentage of male members than did the control colonies.

The authors suggest that the animals most likely respond in this way to improve their chances of reproduction. Localized environmental stress increases the risk of mortality within a particular colony. But because sperm can travel, increasing the number of males in an endangered colony improves the chances that these afflicted individuals will be able to access nonstressed potential partners and leave descendants.