Across the animal kingdom, males have evolved fancy physical ornaments, songs and courtship rituals, all in an attempt to attract the opposite sex. Most of the male ornaments and sexually-selected traits biologists tend to study are large, elaborate and flamboyant. But mathematical models predict that sexual selection is just as likely to make an ornament smaller or more modest as it is to make it more elaborate. Recent research by Dr Sam Tazzyman and Prof Andrew Pomiankowski from UCL’s Department of Genetics, Evolution and Environment, in collaboration with Prof Yoh Iwasa at Kyushu University, investigates why male ornaments tend to get bigger rather than smaller.
Sexual selection is the process whereby traits are favoured because they increase an individual’s success at obtaining mates, often at the expense of survival or condition. Sexual selection is a special case of natural selection, where natural selection is concerned with increasing the overall fitness of an organism. Sexual selection may act in opposition to natural selection, when traits that make you more attractive to the opposite sex also make you less fit in other ways. In these cases, the form a trait takes may be somewhere between the most attractive (sexual selection optimum) and the most fit (natural selection optimum). Sexual selection has been the focus of a great deal of evolutionary research, both experimental and theoretical, because it has the power to generate extreme physical and behavioural adaptations: huge antlers, complicated courtship displays, brightly coloured plumage, etc. Most research has focussed on bright, bold, exaggerated traits like these. But theory suggests that sexual selection should be just as likely to drive traits to be less extreme (than the natural selection optimum) as it is to make them more extreme. So why don’t we see sexually reduced traits much in nature?
First, Tazzyman and collegues searched the literature on mate choice and sexual selection for examples of reduced sexual traits – that is, cases in which females prefer males with a trait smaller, duller or less elaborate than the natural selection optimum. They found that for many types of trait, reduction simply isn’t possible, or is extremely difficult to define. For example, when a sexually-selected trait is a particular colour of a patch of plumage, how can we define exaggeration or reduction in this trait? Is the size of the patch most relevant, or the hue or saturation of colour? Similarly, for many traits, the natural selection optimum might be zero – no trait at all. For example, in many species the male is brightly coloured while the female is dull, here the dull colouration can be considered no trait and is assumed to be the natural selection optimum. Likewise, in many species the males carry physical adornments such as the red crests (or combs) of many gamefowl, which are totally absent in females.
Certainly, these issues with definition occur most frequently for colour, pheromone and behavioural traits. Morphological traits tend to lend themselves more readily to being classified on a simple scale, in which both exaggeration and reduction of that trait is possible. There are a few cases of females showing a preference for a smaller trait, but these examples are few and far between. Of 40 sexual traits for which both elaboration and reduction could be defined, 34 were found to be subject to sexual selection for exaggeration.
This imbalance may be partly explained by our own observation bias – smaller traits may be more difficult to detect and so tend not to become the subject of study. But, it is unlikely this is the full explanation. However, it seems that females may suffer a similar problem; if biologists aren’t noticing small ornaments, maybe the females aren’t either. This is one of three possible hypotheses that Tazzyman and colleagues tested to explain the apparent asymmetry in the direction of sexual selection. Male ornaments are signals, aimed at attracting a female – if that trait cannot easily be seen or detected by the female, then it cannot serve it’s purpose. Consistent with this, a mathematical model of sexual selection assuming asymmetrical signalling efficacy (where smaller traits are less effective at conveying their message) showed that exaggerated traits were more likely to undergo the ‘runaway’ selection characteristic of sexually-selected ornaments.
Their models also ruled out two other possible explanations – that it is more costly for a female to prefer a small trait than a large one, and that is it more costly for a male to carry a small trait than a large one. Neither of these models resulted in a bias towards exaggeration. Only models including an asymmetry in the efficacy of signalling produced results that mirror what we observe in nature.
Sexual selection acts upon traits that make one sex more attractive to the other, and can favour characteristics that are otherwise detrimental to survival or condition. Sexual selection has the power to generate the bright, flamboyant, exaggerated characteristics such as antlers that we see in many animals. Although many theoretical models predict both exaggeration and reduction in sexual traits, in wild populations, we rarely see this – almost all documented sexual traits are more extreme than their natural selection optimum. Sexual traits act as signals to the opposite sex, and this may explain why in the wild, sexual selection tends to exaggerate and elaborate traits which are more visible to females and so more effective at communicating their message.
This research was made possible by funding from the Natural Environment Research Council (NERC), and the Engineering and Physical Sciences Research Council (EPSRC)