What Females Want: Just what do fish find sexy?

26 May 2001

in 2001

Scientists agree: some people are universally gorgeous. Studies in evolutionary biology show that few things are more advantageous to success than being attractive, since good looking individuals leave more offspring than their unattractive contemporaries.

However, studies of mate choice have been criticised for assuming there is one standard of attractiveness. If this were the case, everyone would look similar – representing the “universally attractive” archetype – because only one ‘type’ would mate. But we know that it’s not just the gorgeous who mate; in humans, at least, mate choice is dictated by individual and often idiosyncratic preferences.

But what about fish – what do they find sexy? Dr Rob Brooks at The University of New South Wales may have some answers. And his discovery is to be published in the journal, Evolution.

The 31-year-old lecturer in biological science has been studying the guppy, the small, brightly coloured freshwater fish popular in home aquariums. Among guppies, males do all the displaying, and females do all the choosing. Male guppies incessantly display their colour patterns to females in the hope of being chosen to mate. Dr Brooks wanted to test if all female guppies like the same type of male guppy, and whether these differences were genetic.

Working as Postdoctoral Fellow in the lab of Professor John A. Endler at James Cook University in Townsville, he undertook a year-long experiment, peering into the mating activities of guppies in an aquarium. He first selected 44 male guppies to mate with three females each, creating 132 families of fishes of known parentage.  These matings produced 300 female offspring, whom he then separated for study.

Dr Brooks then observed the mate choice behaviour of these females over the next year, documenting how sexually responsive, or ‘choosy’, they were, and which male attributes they preferred. He found that most of the differences among females in mate choice were in responsiveness to males in general, and in willingness to mate. About one quarter of this variation was genetic; that is, sisters showed a stronger similarity in how sexually responsive they were in comparison to unrelated female guppies. This suggests that sexual responsiveness can be an inherited trait.

But there appeared to be no inheritance when it came to what the females found ‘sexy’; that is, what females found attractive was seemingly not influenced by whether they were sisters. Females generally considered males with big tails and high-contrast colour patterns (especially those with lots of orange and iridescent colouration) the most attractive. But they disagreed on whether they found large areas of black colouration and the overall brightness of a male’s colour pattern attractive or unattractive, even among sisters. Some of the females spurned these males, while others considered them attractive.

Together, the findings that there is genetic variation in sexual responsiveness, and that there are individual differences among females in what they find attractive, have profound evolutionary and philosophical consequences. While certain traits make individuals more likely to be attractive to the majority of females, there is no universal standard of sexual attractiveness. Differences among the females in responsiveness and in mating preference may help maintain variation in the ornaments and displays that males use to attract mates, reinforcing rather than eroding individual differences among males.

Adapting Dr Brooks’ methods to study other organisms, potentially even humans, will help us better understand not only evolution by sexual selection, but also the biological basis of individuality.

Future research: Dr Brooks and his research group at the University of New South Wales continue to study the genetic basis of sexual attractiveness and attraction. They are currently planning to study how the evolutionary forces unleashed by female mate choice influence the evolution of the X and Y chromosomes (which determine sex), once again using guppies. This work will involve field work in Trinidad and Venezuela, where guppies occur naturally, and north Queensland where they appear as a feral species.

E-mail: rob.brooks@unsw.edu.au

Funding sources: The work was funded by an Australian Postdoctoral Fellowship and a large grant from the Australian Research Council. Brooks currently holds a UNSW Vice Chancellor’s Teaching and Research award: an new initiative of the Pro Vice-Chancellor (Education) to help new staff develop their teaching and research careers.

Biographical notes: Dr Robert Brooks, 31, was born in South Africa and came to Australia four years ago, where he is now an Australian permanent resident. He received his Bachelors degree and PhD from the University of the Witwatersrand, Johannesburg. In 1997, he began postdoctoral work at James Cook University, Townsville, Qld, and it was here that the research described was conducted. In January he joined the School of Biological Science at UNSW where he now lectures in Evolution, Ecology and Genetics.

Collaborator: His postdoctoral advisor and collaborator is the renowned evolutionary biologist Professor John A. Endler of the School of Tropical Biology, James Cook University, Townsville as well as the Department of Ecology, Evolution and Marine Biology, University of California at Santa Barbara.

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