The Difference between the Sexes - From Biology to Behaviour

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Tim Birkhead

University of Sheffield, United Kingdom

Biography

Like many of my generation of professional biologists, I was a fanatical naturalist as child. Encouraged by my father, a keen birdwatcher, and my mother who was an accomplished artist, I was indulged and allowed to keep birds in my bedroom whose walls I painted in my own designs. My mother gave me a sense of enthusiasm and my father instilled in me a strong work ethic. We lived in a rural area outside Leeds, northern England and with a freedom almost unknown today, I spent countless wonderful days bird watching. My life revolved around birds: I raised young magpies, rooks, tawny owls and starlings and I had an aviary in the garden that I filled with foreign birds. During a family holiday in North Wales when I was twelve my father took me to Bardsey Island for the day. I remember being thrilled by the thrift-covered cliffs, the azure seas and clear skies full of cheering choughs. As we walked back towards the boat at the end of the day we saw a young man sitting with a telescope and a notebook studying birds. My father casually said to me: ‘You could do something like that’, little realising how significant his comment was. I went to Newcastle University to read zoology and I loved it. Then in the spring of 1972, I travelled from Newcastle to attend a student conference on birds at the Edward Grey Institute, Oxford. During the conference David Lack, the director (and arguably the greatest ornithologist ever), asked if anyone was interested in undertaking a D.Phil. there. Of course I was! Preferring to walk rather than sit, the ‘interview’ took place in light rain as we walked up and down outside St Hugh’s College. I babbled on about my passion for birds and much to my surprise by the time I returned to Newcastle an offer of a D.Phil. place was waiting and with it, the opportunity spend three summers on another island, Skomer, off the coast of Wales studying guillemots. The guillemot was a fortuitous choice. Although I had no idea when I started that despite being socially monogamous, this species was sexually rather promiscuous. One thing lead to another and utilising the avicultural skills I had acquired as a teenager, made the study promiscuity in birds the main focus of my research. In addition though, the Edward Grey Institute in Oxford also houses one of the best ornithological libraries in the world and it was here during the winter evenings that I started to discover the extraordinary history of birds, culminating – some thirty years later - in ‘The Wisdom of Birds’. My main interests are: science, art, history of science, teaching undergraduates, birdwatching and guitars.

Abstract

Darwin and Postcopulatory Sexual Selection

The recognition in the past twenty years that the females of most animal species are promiscuous has changed the way we think about reproduction. The two most fundamental questions about female promiscuity are these: (i) What is its functional significance? In other words, what evolutionary benefits do females obtain from copulating with more than one male? And (ii) What are the underlying mechanisms that dictate which of a female’s several partners will fertilise her eggs? In other words, are there any rules that dictate the outcome of sperm competition? These two questions require different types of thinking; the first focuses on the evolution of traits, the second is concerned with mechanisms. The two approaches are complimentary and together provide greater insight into reproductive processes. There is no consensus about the benefits females obtain from copulating with more than one male. We used a quantitative genetics approach to address this question in order to establish the heritability of sexually selected traits, but more revealingly, the genetic correlations between different traits in the zebra finch. This showed unexpected associations between a sexually selected trait and immune function, and this result provides a basis for why females prefer particular males. We also compared the genetic basis for different personality traits in the zebra finch, again with some unexpected results. In terms of mechanisms, I will focus on female processes. It is well established that a variety of male attributes, including sperm numbers and sperm quality, influence the outcome of sperm competition. The influence that females have over the fertilisation of their ova is much less clear. The notion of cryptic female choice - defined as any female-mediated process that influences the outcome of sperm competition - is controversial, partly because cryptic female choice is difficult to demonstrate unequivocally. I present two examples, using the feral fowl as a model study species; one in which females base their choice of sperm on the male’s phenotype. In the second, male phenotype was excluded as a cue (by using artificial insemination), yet we still obtained evidence that females can discriminate between the sperm of different males. Finally, I will consider some of the evolutionary adaptations to sperm competition. Reproductive traits are among the most rapidly evolving traits, especially when females are promiscuous. We used our quantitative genetics study of the zebra finch to examine the evolution of reproductive traits associated with sperm competition and female promiscuity. We show that genetic correlations can act as constraints on the evolution of optimal traits.