A common trend in size differences between males and females is a long-standing puzzle. A study of shorebirds shows that the type and strength of competition for mates may explain much of the pattern.
Five decades ago, the German evolutionary biologist Bernhard Rensch1 presented an intriguing rule for the differing sizes of male and female animals. He found that in several groups (clades) that each contain related species, male size relative to female size increases with the body size of the species. Rensch's rule has since been verified in animals as diverse as arthropods, reptiles, birds and mammals, including primates2. The causes behind the rule, however, have remained unclear2,3,4,5. Why are males much larger than females in many animals with large body size? And why, in the same clade, are males similar or even smaller than females in species with small body size1,2?
As they describe in Proceedings of the National Academy of Sciences, Székely et al.6 have carried out a comparative analysis of shorebirds that show such trends, and have come up with some thought-provoking conclusions. They show that the trend in sexual size difference (SSD) can be explained by two aspects of sexual selection (which arises from competition over mates), and the interaction between them. One aspect is the strength of sexual selection involved; the other is the agility of the male's display. The outcome of the analysis is similar whether it is based on body mass or wing length as a measure of size.
Males of many animals compete in fights for mates, and such contests favour large body size (Fig. 1). Through genetic correlations between the sexes2,4,5, such competition may also lead to some increase in female body size, though less so than in males. In consequence, the mean size of each sex will increase, and so will the relative size difference between them. Increased male-biased SSD therefore tends to become associated with large body size. On the other hand, there are several smaller species in which sex roles are reversed and females compete strongly for males. Such species tend to have female-biased SSDs5.
Some forms of male competition can favour smaller males. For example, in species where males compete by acrobatic aerial displays, there may be strong sexual selection for small male body size. For geometrically similar animals, agility increases with reduced body size7, and this might lead to higher mating success of smaller males in certain birds8,9. Through genetic correlations it may also lead to some reduction in female body size. This in turn can help to explain why female-biased SSD increases with reduced body size in some birds and other animals with agile male display5.
Székely et al.6 tested the relative role of these mechanisms in producing SSD among 102 species of shorebirds and their allies (a clade named Charadriides). This is an ideal group for the purpose as it encompasses great variation in the traits of interest6,8,10. The analyses confirm Rensch's rule, showing that males are bigger than females in most large species, and that male-biased SSD increases with body size. In contrast, females are bigger than males in many small species.
The social mating system of a species can be used as a proxy for the strength of sexual selection in males. The strength increases from polyandry (where females compete to have several mates), through monogamy, to polygyny (where males compete to have several mates). As predicted, Székely et al. find that stronger sexual selection is associated with increasing male-biased SSD, and more agile aerial display is associated with increasing female-biased SSD. Notably, there are strong interactions between agility and strength of sexual selection. In polygynous species, the female is larger if male displays involve agility (Fig. 2); if they don't, males are the larger sex. Forms with less male competition (monogamy or polyandry) tend to have larger females regardless of male agility (Fig. 2). Such interactions may help to explain why some groups with intense male competition show a range of SSD patterns, from female bias to male bias.
Sex difference in choice of habitat or food might also influence SSD. Székely et al. find that sexual difference in bill length — a trait of crucial importance for foraging — does not follow Rensch's rule. And when male and female body sizes are controlled statistically, sex difference in bill length is not associated with display agility or strength of sexual selection. So, unlike body size, which depends strongly on sexual selection, SSD in bill length is influenced mainly by other (probably ecological) selection pressures.
The generality of these conclusions can be explored by deriving predictions from them for tests in other animals. One prediction is that large males are favoured in species with large body size that compete for females in contests requiring strength or endurance5 rather than agility. Moreover, among small species in the same clades, sex-role reversal and stronger competition among females, or male competition by agile8 and energy-demanding5 display, are expected to favour reduced male size, sometimes leading to female-biased SSD.
A review2 of SSD in 40 groups shows that exceptions to Rensch's rule are rare and mostly occur in animals where females are the larger sex. There is much scope for testing the predictions in these and other groups. Advances in comparative and other analyses of SSD2,6 open possibilities for clarifying the striking patterns in sexual size difference that have long puzzled biologists. Analyses of the form and strength of sexual selection are likely to be crucial in such work, as shown by Székely and colleagues' study of shorebirds. There is plenty of interesting work ahead — for instance, it is still not clear why, in most clades where some species show stronger sexual selection in females than males, this occurs mainly in its smaller forms.
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Journal of Zoology (2007)
Biologie in unserer Zeit (2005)