How crazy would it be if right after a man had sex, he just dropped dead? This is the case for many marsupials such as antechinus, who go on a sex frenzy for twelve to fourteen hours and then die immediately after. The cause of death is a cascade of hormones, triggered by high testosterone levels, leading to the collapse of their immune system.

The term for animals that have one reproductive episode before death is semelparous. In the case of antechinus, males reach sexual maturity at approximately eleven months old, just before breeding season, meaning they only live to the ripe old age of one.

There has been much speculation over why semelparity happens in marsupials. The leading hypothesis for a long time was they died so that more food resources would be available for their offspring. However, Australian scientists have just discovered the real reason behind this phenomenon: sperm competition.

Sperm competition is a form of sexual selection in males. If a female mates with two or more males, the male whose sperm wins the race to the eggs has higher reproductive success. Males who are able to fertilize more eggs and therefore produce a lot offspring are more ‘fit’ than other males because their genes are passed on to the next generation at a higher rate. Basically, instead of physically fighting, male sperm duke it out in the female’s reproductive tract to ensure that the best males sire the offspring.

What causes sperm competition in many marsupials to be so vigorous that it kills them? It’s the female’s fault, really.

All females in a population are fertile at the exact same time of year, they are fertile for a very short period of time and they are promiscuous. Therefore, males must exert maximum effort to ensure that their sperm is successful under these very stressful circumstances. One way they do this is by having extremely large testes, allowing them to build up high volumes of sperm to fertilize as many females as possible.

Semelparity is seen in plants and some fish, but is thought to be quite rare in mammals. One classic example is the Pacific salmon, which mature in the ocean, swim into freshwater to spawn and then die shortly after.

Sperm competition, on the other hand, is quite pronounced in the animal world. Many tactics exist, such as males guarding their mates, applying pheromones to the female which reduces her attractiveness to other males and secreting a copulatory plug. This plug is a gelatinous excretion which clogs the female’s genital tract so that even if she mates with another male, his sperm is unlikely to get into her reproductive tract.

Another ploy is to physically remove the sperm of another male. For example, a male Dunnock bird will peck at the female’s reproductive tract before mating with her to remove any sperm that was deposited from a previous male suitor.

Doesn’t human sex seem boring now, compared to the wacky animal world?