This month saw the long-awaited publication in PLoS ONE of a paper describing the courtship of the peacock spider Maratus volans, a tiny arthropod whose displays have helped it achieve the heady heights of internet fame over the past couple of years (well, at least in those parts of the internet where people like to watch videos of little animals dancing around). Girard and her fellow researchers used high-speed video recordings and laser vibrometry to show that male spiders use vibratory signals in addition to ornamentation and motion displays in order to attract a mate.
I have written previously on how males and females invest different amounts of resources in their gametes (sperm and eggs), and how this imbalance creates the conditions for sexual selection – Darwin’s proposed mechanism for the evolution of different body shapes and sizes across the sexes. Sexual selection covers both female choice and male competition, scenarios that have led to the development of exaggerated male ornaments and weaponry respectively (consider, for example, the beautiful train of the peacock, or the fierce antlers of stags).
While weaponry is used to fight or simply intimidate opponents (as well as the rather ungentlemanly acts of prising rival males from females mid-copulation, and trapping females in mating burrows, as is the wont of some beetles), ornaments serve to impress and seduce the watching female. Highly-ornamented species are often those in which both sexes mate with multiple partners, with males offering nothing more than their sperm – not for them the worries of caring for offspring, or providing food and territory for their mate*. The displays that males engage in often serve to highlight their ornaments – male greater sage grouse Centrocercus urophasianus are a prime example:
This type of behaviour is especially evident in ‘lekking’ species, where males gather on a display ground (the lek) and parade their wares to potential partners. Only those males with the very best ornaments are deemed good enough by the choosy females, and each will likely mate with multiple partners – meaning that the genes of a select few sires are making it into the next generation. This leads us to the essence of the ‘lek paradox’: if females are selecting males on the basis of certain trait values, this should erode genetic variation in these traits, meaning that all traits should converge to similar values. If all traits were the same, females would be unable to choose between males, and – more importantly – there would be no point in trying to do so. I like to remember this paradox through the reappropriation of the lyrics to a popular song:
However, there is plenty of evidence to show that female choice on the basis of sexual traits persists. So, how can we explain the maintenance of genetic variation for sexual traits? One proposed mechanism is that ornaments develop a strong relationship with an individual’s ability to acquire resources from its environment and convert them internally to usable forms – a relationship known as ‘condition-dependence’. This ability includes factors such as fighting disease, catching prey, foraging, and metabolising nutrients. All the genes underlying these factors are associated with the sexual trait due to condition-dependence, and so the trait serves as an indicator of how the vast majority of an individual’s genome is performing in its current environment. Rather than eroding the variation in a few genes that encode a trait, selection is now based on the vast variation of virtually the entire genome. Not only that, but changes in the environment will alter which genotypes perform best, and mutations in any area of the genome will have some effect on mating success.
While the paradox is named after lekking species, which often provide the most extreme examples of ornamentation, the problem extends to all those species where males do not give their partners direct (or ‘material’) benefits. Research in this field helps us to figure out the wider effects of sexual selection – for example, can it help to prevent the build-up of deleterious mutations in a population? On a different level, it is interesting to ask why such behaviour exists – is sex really worth the male making himself quite so obvious to predators? How does a female ‘know’ which male is ‘good enough’? This paper gives us a nice description of the courtship behaviour that we see in this video, and provides a basis for further study of these charismatic little animals (and others in the genus Maratus) – this is especially intriguing as the ‘multi-modal’ nature of their courtship is ripe for further investigation. Each facet is a drain on resources, whether it be the development of the colourful abdominal flaps and ornamented third legs, or the waving and dancing itself – to say nothing of the vibrational drumming, wonderfully described as ‘rumble-rumps’ by the paper’s authors. Why have the males evolved these multiple signals? Do they represent different features of his quality, and can females discriminate between them? Is one signal more important than all the rest? I’m sure I’m not the only one who’s excited about what else this colourful spider can inform us about evolution.
Get the paper here.
See more videos from the Elias lab at Berkeley here.
Check out Jurgen Otto’s fantastic photographs here.
*Note: I could not find much detail in terms of the mating system in Salticidae, much less this particular species, so it may indeed be that males are providing females with direct benefits. In which case, ignore me.