Female fish have become better swimmers in order to escape male sexual harassment, new research claims.
The female guppies have learnt to shake off the unwanted male attention by changing their physiology to swim faster and more efficiently.
The scientists behind the research likened it to athletes who train to become better at their sports.
The study, led by academics at the universities of Glasgow and Exeter, exposed female Trinidadian guppies for several months to varying levels of male harassment.
After five months, females exposed to higher levels of harassment were able to swim much more efficiently, using less energy to swim at a given speed compared to those exposed to lower levels of harassment.
"In the wild, male guppies spend most of their time courting and coercing females in an attempt to mate with them," said Dr Safi Darden of the University of Exeter.
"Most of this male attention is unwanted and females attempt to avoid males by rapidly swimming away from them during pursuits.
"It seems that prolonged increases in high-intensity swimming in females, caused by male harassment, leads to changes in the physiology or swimming mechanics of individual fish, which reduces the energy costs of swimming and could allow female guppies to reduce the burden of this coercive behaviour."
Dr Shaun Killen, of the University of Glasgow, said that sexual coercion of females by males was widespread across the animal kingdom.
"Typically, male reproductive success is limited by access to females, and males of many species will try to overcome this using a number of behaviours, such as chasing and even attacking females in an attempt to gain a mating," he said.
"These types of behaviours are considered sexually harassing as males are attempting to coerce females into mating with them. Females can spend a lot of energy avoiding males in these situations and can even be injured.
"To reduce these costs, one possibility is that females may be able to change their own behaviour or physiology in ways that reduce the negative energetic consequences of harassment or allows them to more easily escape male coercion.
"An important factor appears to be swimming technique, and female guppies that experienced lower levels of harassment spent more time swimming with their pectoral fins extended, an indicator of an inefficient swimming technique.
"This change is very similar to that seen in human athletes who train to become better at their sports."
The study was funded by the Natural Environment Research Council and the Leverhulme Trust.
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