In most animals, females are generally the ones that choose the males. This is a massive generalisation (for example, it doesn't apply in this case), but I hope people who work on this topic will forgive me for it. Generally speaking, it’s the females that get to size up the males, check out whatever trait it is that’s attractive to them (be it weight, head feather colour, ability to sing, or muscle size) and then choose who they want to mate with.
However, how animals (even insects) behave when choosing mates is by no means governed by fixed rules, and is influenced by many different things. I’ve previously written about fish that will change how they court females depending on who’s watching and male crickets that will change their victory displays after fighting with another male depending on their audience. Similarly, what a female chooses in a male mate isn’t totally free from influences outside the quality of the male in question.
In some species, such as the field cricket, wolf spider and cowbirds, females with more experience choose differently to nave females. But what other things might affect what females choose?
Pretty much all animals come into contact and may be infected by parasites at some point in their life. Amazingly, parasites seem to affect the mating behaviour of animals in some unusual and unexpected ways. Some parasites castrate their hosts, or change who the host wants to mate with. Others can even cause sex-role reversals, such as in the bush cricket.
One recent study by Beckers & Wagner has found that female field crickets that are infected with a parasite are less choosy in who they mate with compared with uninfected females. In this species, males produce song to attract females. Females like males who can chirp the fastest, and males that chirp really slow generally aren’t attractive at all. Unfortunately for male field crickets, their song also attracts a parasitoid fly that drops its babies (‘larvae’) all over and around the cricket, that eat away at it and kill it in around a week. Females getting near these males can also get infected in a similar manner. One somewhat sad upshot of being infested is that those males also sing less (I guess I would too if I had parasites eating away at my body).
To look into how females differed in choosing males to mate with once they had become infested, the researchers took females that either were infested with the parasite larvae and some that weren’t. They then looked to see what the female crickets would choose when they were played the recordings of males singing at them at various chirp rates from two loudspeakers. By watching which loudspeaker the female walked towards, the researchers could see which ‘male’ the female would have chosen. The infested females weren’t any more or less likely to walk towards the speakers than the uninfested females – they were equally as responsive to the recorded male singing. However, infested females were less choosy in which male songs they headed towards: the uninfested females would choose males that sung at intermediate speeds over the slow-singing males, whereas the infested females were happy with either song-rate.
Why might the infested females be less choosy? One likely reason is that the parasite stops them producing eggs within 3 days of becoming infested, so they only have a short window in which to mate. A second possibility is that females that have become infested in the wild are likely to have become so by getting too close to an infested male. If this is the case, it is likely that a lot of the males around them are infested and therefore not singing as much, so the female’s best bet is not to worry too much about the singing quality and just get the business of mating over and done with. Finally, females that are infested are likely to be low on energy, using what they have to deal with the side-effects of infestation, and being too choosy with males (and searching for the sexiest male) uses up energy they just don’t have to spare.
The main finding from this study also has implications for the evolution of this species, as one result of the infested females being less choosy is that males of lower quality manage to get matings (and thus spread their genes) even though they wouldn’t have if the female wasn’t infested with a parasite. This is interesting from an evolutionary perspective, as it shows how one animal (the parasite) can totally change the ‘survival of the fittest’ in another species.
Beckers, O. M., & Wagner Jr, W. E. (2013). Parasitoid infestation changes female mating preferences. Animal Behaviour, 85: 791-796.