Some higher mammals are literally dying for the chance to have sex, a behaviour usually only seen in animals like spiders.
Theses tiny mammals, like the antechinus, seem to adapt this behaviour of literally mating themselves to death, because they only live through one breeding season. Because the females of the species all go into “heat” at once, they need to have sex as much as possible within that small window of time to ensure they pass on their genes.
Reproductive suicide has been observed in plants, invertebrate species, and even some fish, but has only evolved in four different groups of mammals.
The mammal’s rare form of reproductive suicide — called semalparity — means the male’s stress hormones skyrocket, causing the immune system to collapse, and after mating they get sick and die.
Four different insect-eating marsupials (in the genuses Antechinus, Phascogale, and Dasykaluta) from Australia, Papua New Guinea, and South America use this mating strategy.
During breeding season they mate most of their day, and one reproductive encounter can last up to 12 hours with these mammals. Almost every ounce of metabolic energy is put toward mating, and they die very shortly after the breeding season ends. To pull off this super-sex feat, their Testosterone skyrockets, ultimately disabling their immune systems.
Why do they do this? One theory suggests that reproductive suicide evolved because when the father dies off they are no longer competing for food resources against the mother and children. That ensures more food will be available for the surviving females and babies.
But after reviewing previous research, the scientists found that food availability had little to do with the development of reproductive suicide in these marsupial species.
Their new study, published in the journal Proceedings of the National Academy of Sciences Oct. 7., suggests that it could be the females themselves that caused the behaviour to develop.
The energy-consuming breeding season seems to come at the same time the insect population is at its peak and food is widely available. This means that the breeding season for these animals is very short — usually only a few weeks — and male competition is intense.
When peak food arrives, all of the females go into heat at the same time. Because of their short mating period, males don’t have the luxury of taking their time with a group of females over a long period. Since they never get a second mating season, they have adapted to make their one shot really count, killing themselves for sex.
The males don’t compete by fighting with each other. The goal is to produce the most sperm. Generally the males with the larger testis size to body size are more successful because they generate more sperm and can therefore spend more time with a female — decreasing the chances she will mate with another.
This reproductive suicide strategy is beneficial in some species, like these small marsupials, because one reproductive bout can actually produce more offspring than other closely related species do from a series of reproductive episodes. And the more testosterone, the longer the sex.
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