Death by Sex : why would an animal mate itself to death?

A study published last month described several species of small mammals partaking in “suicidal reproduction”. That is, mating which is so energetic and stressful that animals die as a result. In these mammals, mating sessions average 9.4 hours but can last up to 14, leading to wildly increasing stress hormones and death by immune system failure. It’s a pretty big sacrifice for some romance. Even more bizarrely, deaths were synchronised – there was a sudden die off of loved-up males. This was all down to fatal stress hormones, which increase throughout a breeding season.

So how and why on earth would something like this evolve?

Suicidal reproduction, also known as semelparity is seen more widely in nature, but usually in cases where lots of young are produced, such as insects or fish. The species  in this study were all marsupials – mammals whose young develop in external pouches. They don’t spawn vast numbers of offspring. Putting all your eggs (or sperm) in one basket for a single frantic period of mating is unusual in mammals. So how could this be?

The answer seems to lie in the sperm. Sperm competition, where instead of competing for access to mates, males compete through their sperm, where the fittest sperm will be most likely to fertilise the egg. Species that showed suicidal reproduction had bigger testes in proportion to their body size, and the females tended to be promiscuous, both indicators of sperm competition. If males have to compete intensely with the sperm of others, huge investment in mating is a necessity to ensure they father offspring. This investment is so high that males end up unable to fight off disease.

So why is there so much sperm competition in these mammals? It appears to be all to do seasonal environment and food. Females will do best if they coincide energy-draining breeding seasons with the time when food is most abundant. When females can reasonably predict when there will a lot of prey around, they tend to have shorter breeding seasons. Shorter breeding seasons mean less time for males to mate, which leads to more intense competition between males. It turns out that the mammal species that engage in semelparity are all from areas where the abundance of their prey is more predictable, year on year. Yes, mating yourself to death does seem like a bizarre strategy. But for these male marsupials at least, it may be the only way to get those all important genes into the next generation.

If you want to know more about sperm competition, you can do so through the medium of interpretive dance! This video just won Dance Your PhD, and is all about sperm competition between brothers in chickens.

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