The curious case of the antechinus

Semelparity is one of those words you choose when you’re playing Balderdash. When you see it you immediately know that no one at the table knows the definition or even has a clue about its place in the English language. That is unless you have an absurd number of animal behaviorist friends. 

Semelparous organisms are those that have one chance to mate before they die. There are more of these types of organisms that I would have expected (salmon, some spiders, grain plants) but I never would have guessed that a mammal would be semelparous. If you know anything about evolution then semelparity sounds completely ridiculous! Reproductive success is the key to species’ survival. So then why would evolution favor an animal which dies after a single mating event? 

This is the question many behaviorists have been asking about animals like the brown antechinus (an-tee-kine-us). The antechinus is a marsupial mammal found in Australia whose male counterparts have only one opportunity to go forth and multiply.

The way it works is this. Females have a single fertile period for about two weeks between May and September, depending on the weather. Shortly before this time males will begin preparing for the mating jamboree. This entails seeking out females, ceasing food foraging to conserve energy, and even slowing sperm production in a last ditch attempt at “quality, not quantity.” During the two week period males will mate with as many females as they can, neglecting sleep and food in order to better their chances at successful reproduction. You can imagine this requires a great deal of energy; since the males are not eating their bodies resort to drastic measures. 

The antechinus’ body begins to break apart proteins in a process called gluconeogenesis. Essentially the animals must “make” glucose from molecules they already have to fuel their bodies for the final hurrah. This process stimulates the release of cortisol, a hormone that tells your body that you are starving. Sustaining high levels of cortisol in the blood leads to immunosuppression, brain damage and failure of the inflammatory response, all contributing to the phenomenon of male die-off.

But we still haven’t addressed why this trait has survived the rigorous guidelines that nature provides. One approach to answering this question is a cost-benefit analysis. This approach compares how much energy a male antechinus uses to mate a single time, to how much energy he would use to continue living and mate several times. If the first amount is smaller, you can see how male die-off would be favored. Another theory is that multiple mating events would decrease the likelihood that a given offspring would survive due to competition for attention and resources from its mother.

While neither of these theories is completely accepted in the field there are several observations that most behaviorists would agree favor the natural selection of semelparity.

Firstly, semelparous organisms tend to have larger litter sizes than iteroparous organisms. Second, semelparous species tend to live or breed in areas with predictable and seasonal changes. And thirdly, the chance of offspring survival is greater than the chance of adult survival. That is to say that were a male antechinus to live past the first mating season its chances of survival to the next mating season is so low that it is silly to live at all. Who knew that nature was such a cynic?

Sources and further reading

Bates, J. 2002. "Antechinus swainsonii" (On-line), Animal Diversity Web. Accessed October 29, 2013 at http://animaldiversity.ummz.umich.edu/accounts/Antechinus_swainsonii/

Meri Oakwood, Adrian J. Bradley, and Andrew Cockburn (2001) Semelparity in a large marsupial. Proc. Roy. Soc. Lond. 268, 407-411.

http://www.nature.com/scitable/knowledge/library/semelparity-and-iteroparity-13260334

http://www.huffingtonpost.com/2013/10/08/suicidal-reproduction_n_4063001.html

Source of background photo: http://animal.discovery.com/wild-animals/10-animal-dads.htm