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PUBLIC RELEASE DATE:
24-Mar-2014

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Contact: Hannah Postles
h.postles@sheffield.ac.uk
01-142-221-046
University of Sheffield
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Biased sex ratios predict more promiscuity, polygamy and divorce in birds

Birds in female-dominated populations are more likely to ditch and 'divorce' their mates while promiscuity increases in predominantly male environments, according to new research.

A joint study by the University of Sheffield and the University of Bath gives the first conclusive proof that rates of divorce and infidelity in birds are affected by the adult sex ratio of the population they live in – a theory previously discounted by biologists.

The study, which examined the pair bonding and mating behaviour of 197 different species of bird, found the divorce rate was higher in species with a female-biased sex ratio.

However, when the number of males outweighed females, the frequency of extra-pair mating increased in socially monogamous birds.

The Marie Curie funded research, published in the journal Current Biology, also found both sexes were more polygamous when the ratio was skewed towards the opposite sex.

Professor András Liker, Marie Curie Research Fellow at the University of Sheffield during the study, said: "If there is more of one sex than another, members of the rarer sex have a higher chance of getting a new partner for breeding than members of the common sex.

"Basically, the rarer sex has more opportunity to 'play the field' and either cheat on the partner or leave in favour of a new mate.

"Many animals form pairs to mate and then look after offspring. However, in animals as well as in humans, couples may not be permanent – divorce and infidelity are remarkably common."

Professor Tamás Székely, of Biodiversity Lab at the University of Bath, said researchers were interested to observe that both sexes 'play the game'.

"If there are lots of unpaired members of the opposite sex, there is more opportunity for the rarer sex to attract several partners," he added.

"We found that a female bias in sex ratio destabilises the pair bond system, although it is unclear as to whether this is due to lots of unpaired females luring males away from their mate or if the male, given a greater choice of partners, decides to switch partners more frequently.

"We also found that extra-pair mating was more common when there were more males than females - this could be due to females 'shopping around' when they have more males to choose from, or it could be that there is more rape by males unable to secure a permanent mate.

"Biased sex ratios may destabilise pair-bonds and increase marital infidelity, although relevance of these findings for other species need to be thoroughly tested."

Professor Robert Freckleton, of the University of Sheffield's Department of Animal and Plant Sciences, said the role that sex ratio plays in human mating behaviour should be studied further.

He added: "Surprisingly, some of our results in birds have striking parallels in human societies – for example, the frequency of divorce is related to sex ratio in some human societies in the same way.

"These results suggest that adult sex ratio may have an important role in shaping mating behaviour in a wide range of organisms, and deserve further investigations both in humans and non-human animals."

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Additional information

The University of Sheffield

With almost 25,000 of the brightest students from around 120 countries, learning alongside over 1,200 of the best academics from across the globe, the University of Sheffield is one of the world's leading universities.

A member of the UK's prestigious Russell Group of leading research-led institutions, Sheffield offers world-class teaching and research excellence across a wide range of disciplines.

Unified by the power of discovery and understanding, staff and students at the university are committed to finding new ways to transform the world we live in.

In 2011 it was named University of the Year in the Times Higher Education Awards and in the last decade has won four Queen's Anniversary Prizes in recognition of the outstanding contribution to the United Kingdom's intellectual, economic, cultural and social life.

Sheffield has five Nobel Prize winners among former staff and students and its alumni go on to hold positions of great responsibility and influence all over the world, making significant contributions in their chosen fields.

Global research partners and clients include Boeing, Rolls-Royce, Unilever, AstraZeneca, Glaxo SmithKline and Siemens, as well as many UK and overseas government agencies and charitable foundations.

For further information, please visit http://www.sheffield.ac.uk

University of Bath

We are one of the UK's leading universities, ranked number one in UK for student satisfaction in the 2013 National Student Survey (NSS) and in the top ten of all national league tables, including being named 'Best Campus University' in the Sunday Times Good University Guide 2014.

Our Mission is to deliver world class research and teaching, educating our graduates to become future leaders and innovators, and benefiting the wider population through our research, enterprise and influence. Our courses are innovative and interdisciplinary and we have an outstanding record of graduate employment.

View a full list of the University's press releases: http://www.bath.ac.uk/news/ or follow the University's latest news on Twitter @Uniofbath

For further information please contact:
Hannah Postles
Media Relations Officer
The University of Sheffield
0114 222 1046
h.postles@sheffield.ac.uk



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