Playboy males live fast, die young

Alex Jordan
Friday, 3 September, 2010
Bob Beale

Promiscuous males are so intent on pursuing sexual partners that they can neglect even essential tasks such as eating, says a new study published in the Journal of Evolutionary Biology.

The finding suggests that male promiscuity is not more common – despite its potential evolutionary advantages – because it is subject to natural limitations: playboy males have stunted growth and go to an early grave.

"Perhaps it's nature's way of telling males to be more faithful to their sexual partners," says Alex Jordan, a doctoral student, who conducted the study with Professor Rob Brooks, director of the UNSW Evolution and Ecology Research Centre.

"We wondered why the natural world is not a more promiscuous place. For males, especially, mating with a high number of partners results in the greatest reproductive success, so you would think that the rule should be the more the better.

"In fact, our research revealed that males pay a significant cost of promiscuity that places an upper limit on the number of sexual partners they can have throughout their lifetime."

The researchers conducted behavioural trials with tropical fish, as well as examining the lifetime costs of male reproduction, the first such study involving vertebrate animals. Males of many species increase their reproductive effort with unfamiliar mates – a phenomenon known as the Coolidge effect.

When the male fish were regularly supplied with new unfamiliar females throughout their life, they spent less time looking for food and more time pursuing the females. Males living with unfamiliar females also grew more slowly and to a smaller adult size, and tended to die sooner.

In contrast, males living with a single partner ate regularly, grew steadily throughout their lives and lived longer.

"The considerable costs of promiscuity to the individuals involved reveal a natural limitation on promiscuous behaviour, previously undescribed in vertebrates," says Jordan.  "Perhaps those who wish for a more promiscuous existence will see this as a warning.

"What it tells us scientifically is that the evolution of extreme promiscuity seems to be curtailed by the physiological costs involved: although promiscuity has advantages, the trade-off might be too great in the long-term."

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