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(I did wonder about using all Ks for that title…)

In studying the animal behavior part of the curriculum, you may well have read about courtship & mating systems. In many cases it’s the male that initiates courtship, & sometimes they use very elaborate displays to catch the female’s eye. Think of birds-of-paradise and bower birds as examples. This elaborate behaviour, & the physical features that often go with it (such as brightly coloured plumage or massive racks of antlers) are viewed as the result of sexual selection. If an individual has a feature that enhances their chances of mating successfully, and that feature has a genetic underpinning, then it may be passed to some of their offspring & over time spread through the population. This tends to be a bit one-sided, in that females tend to be the ‘selective’ sex & so sexual selection tends to affect males more than females. In fact, ‘sexual selection theory predicts hat females… invest less in courtship signals than males’ (Berry & Breithaupt, 2010)

Now, visual displays & sounds are frequently part of courtship behaviour, but chemical signals also play a significant role: for example, in around 1600 species of moth, the female releases a pheromone that guides males to her position. An individual male detecting such a signal gains information about the sex of the ‘sender’ and also of their physiological state. Chemical signals aren’t cheap to produce: the often-complex molecules involved can incur quite an energy cost in their manufacture. So this rather contradicts the aforementioned prediction by sexual selection theory: it’s called the ‘female pheromone fallacy’ (aren’t we into alliteration tonight?) It could be that the pheromones originally evolved for some other function & have subsequently been co-opted to attract males. In an attempt to clarify this conundrum, Fiona Berry & Breithaup (2010) studied chemical signalling by female signal crayfish (Pacifastacus leniusculus). Apparently female crays release urine in the presence of males – what the researchers wanted to know, was whether the pee of female crayfish has any effect on the males’ behaviour.

In order to do this they played the role of matchmaker, staging pairings between male & female crayfish during the reproductive season. Male crayfish can produce a series of spermatophores & thus can potentially mate with & inseminate many female partners. Having mated with a female they wander off to seek another partner, and don’t play any part in looking after any offspring that might eventuate – this is the role of the females alone. In other words, there’s a big imbalance between male & female crays in terms of their investment in producing little crayfish, & so females might benefit by discouraging males. In this context, you’d predict that females wouldn’t produce courtship signals as they’d have entirely the reverse effect. 

The researchers knew that in another crayfish species, both sexes release urine, & urine contains a whole range of chemicals that could act as signals. They started out with blindfolded crayfish pairs (the control condition) – the blindfolding was to remove any visual cues that might influence behaviour. (This was important, because Berry & Breithaupt used a fluorescent dye to allow them to visualise urine release, & they’d hardly want the swirling patterns of ‘visible’ urine to distract their study animals.) Seven of 15 males made attempts to mate with their partners. When the females were prevented from releasing urine, there were no mating attempts & all behaviour by the males was aggressive. If female pee was added to the water by the researchers, in 6 of the 15 pairs the males again attempted to mate.

Two crayfish fight in a cloud of visualized urine. Credit: Fiona Berry, BMC Biology.

It turned out that both sexes released urine, but they differed in the context & also the duration of release. Mating males produced less than males engaged in aggressive interactions, while females released urine for roughly the same amount of time regardless of whether they were fighting or courting. In fact, 1/3 of the males didn’t pee at all during courtship, which rather suggests that male urinary pheromones don’t act as an indicator of male quality. However, all the females in this study released urine before mating, mostly during precopulatory aggression; in fact, the researchers noted that “female urine release coincides with aggressive rather than reproductive or submissive behaviours”, and that male pee production is reduced, or stopped altogether, as they change from fighting to courting. Berry & Breithaupt (2010) concluded that during social interactions both male & female crays release urine as an aggressive display rather than an aphrodisiac. However, female urine also acted as a trigger to change the males’ behaviour, so that they became more likely to try to mate. It could well be that female urination sets up conditions that allow them to assess mate quality: perhaps only the stronger males will persist with courtship in the face of the female’s aggression.

F.C.Berry & T.Breithaupt (2010) To signal or not to signal? Chemical communication by urine-borne signals mirrors sexual conflict in crayfish. BMC Biology 8: 25 doi:10.1186/1741-7007-8-25 http://www.biomedcentral.com/1741-7007/8/25

 This story obviously captured bloggers’ imaginations (& I was rather late out of the blocks), & you can read more about it at (for example) Live Science, Not Exactly Rocket Science, Pharyngula,  & the University of Hull’s news site