Australasian Science: Australia's authority on science since 1938

Suicidal Sex Explained

By Magdeline Lum

Sex is suicidal for some marsupials, and termites communicate by headbaging.

August in Australia marks the mating season for the antechinus, a mouse-like marsupial. Males go into a frenzy mating with as many females as they can, with some encounters lasting up to 14 hours. By the end of the mating season, the males are literally falling apart and dying.

This death toll occurs in 20% of all known species of insectivorous marsupials, including all 12 species of antechinus, three species of phascogale, and kalutas. Some populations of northern quolls and dibblers also lose their males after the breeding season.

Research conducted by Dr Diana Fisher of the University of Queensland has overturned previous explanations for this die-off. A popular explanation has been that the deaths were altruistic as they prevented competition with offspring for resources.

However, Fisher found that a combination of extreme stress, immune system collapse, infections and internal bleeding led to the death of males. These males have spent the majority of their lives as juveniles and reach sexual maturity at 11 months just before breeding season begins.

In the month before breeding, their testes are shut down so that all the sperm they have stored is all that the males have. There will be no more, so even if a male survived the breeding season it wouldl be infertile. This injects urgency into mating.

When the life history of 52 species of insectivorous marsupials from Australia, Papua New Guinea and South America was examined, the seasonal availability of food was related to the timing and length of the breeding season. Males experiencing a shorter breeding cycle had a lower survival.

The synchronisation of food availability with the breeding season did not explain the die-off. It was also found that females of species that had the highest numbers of males dying had breeding seasons much shorter than the period of time for peak availability of food. This creates a time pressure on males and leads to competition.

In response, males do not resort to physical violence but rather let the sperm do the fighting. They possess testes that are large relative to their body size. This allows them to fertilise as many females as possible during the breeding season and to let the sperm battle it out within the female reproductive system.

Headbanging Termites Warn of Danger

Communication over long distances can be challenging. The sound of our voices can be difficult to understand at a distance of 100 metres, but this did not stop people from communicating over distances of hundreds or thousands of kilometres for millennia. Soldiers along the Great Wall of China warned of approaching enemies by using smoke signals.

Another method of communication over long distances is drumming, but this is not exclusive to humans. Termites have also mastered the skill of drumming by hitting their heads on the ground to raise the alarm.

Two termite species of the southern African savannah, Macrotermes natalensis and an Odontotermes species, produce signals within their nests that travel throughout their networks. Mr Felix Hager and Prof Wolfgang Kirchner of the University of Bochum in Germany used high-speed cameras to capture termites bashing their heads into the ground around 11 times per second, sending out vibrations.

These vibrations travelled up to 40 cm before they became undetectable by termites. Yet Hager and Kirchner observed worker termites beyond this distance return to the nest. This was because soldier termites would respond to vibrations by drumming, thereby propagating the signal further – much like the game of Chinese Whispers.

In the laboratory, termites were exposed to a synthetic range of frequencies but the termites only stopped what they were doing when the frequency recorded in the wild was recreated.