Having organised an evolutionary biologist from Plymouth University to speak to the biology society, today I was introduced to the concept of sperm competition in males of various species. The topic of evolutionary biology is rapidly becoming a growing interest of mine, and today we were also enlightened to the fact species, such as the peacock, have evolved to prioritise attraction of mates over increased survival chance. This idea was explored in Dawkin’s ‘The selfish gene’, as propagation of genes is the ultimate priority of any organism. Perhaps a more humorous development of this idea is expressed the stalk eyes fly. The impracticality of having its eyes situated on relatively huge stalks vs the increased chance of mating with ludicrous head extensions, is balanced in favour of the attraction to females. The result of the course of natural selection for this aptly named fly is revealed in the picture below.
Returning to the original premise of this article; what is sperm competition? Sperm competition is the physical competition between the sperm of two separate males to fertilize the eggs of a lone female. A male’s fitness is usually measured as a function of the number of females inseminated, however in many animal species fertile females mate with many male partners. When this happens, whose sperm will fertilise her eggs? This actually improves the genes inherited by the females offspring thus increasing the probability of the female’s genes being propagated in future generations. Males in many species have evolved mechanisms to give their own sperm a special advantage after deposition in the female reproductive regions. In addition to super powered sperm, some males of species have developed rather more brutal mechanisms of ensuring their sperm fertilises the egg. Spiked penises of many species dissuade females from copulating more than once, thus are only inseminated once.
Some males, instead of or in addition to their own mechanisms of sperm competition, will guard their female partners from more copulation even after sperm deposition. These postcopulatory interactions do produce fertilization benefits for the guarding male. Mate guarding exists in a variety of forms including prolonged copulations, mating plugs, mate grasping, and mate mounting. In the parasitic wasp Cotesia rubecula competition for mates is intense and there is a short window of time immediately following copulation in which a second male may induce a mated female to copulate again. In order to distract rival males, a recently mated male will mimic a female long enough for the mated female to become unreceptive. Female mimicry in this species acts as a post-copulatory mate guarding tactic employed by males to increase paternal reproductive success.
Interestingly, in primates, there is a direct correlation between the prevalence of sperm competition in the species and testes size. Bearing large testes is a significant burden due to the fact mitotic division is so frequent in testes; large amounts of energy are consequently expended in maintaining large testes. Chimpanzees experience a considerable level of sperm competition due to their promiscuous nature. In accordance with the correlation aforementioned, Chimps have testes around 150g. This can be compared to the 20g testes of Gorillas, a species to which sperm competition isn’t factor.
Having received some recommended reading from today’s speaker, hopefully I can further my knowledge into natural selection.