Tuesday, 24 March 2015

The Strange Story of Antechinus

 Or Why It Doesn't Always Pay to be Too Macho

    A few decades ago, when my mother was still alive, we both happened to take a short stroll through the rainforest at Mount Glorious, west of Brisbane, when suddenly we noticed an animal like a big mouse come scurrying up the trunk of a tree. Gazing at its pointed, foxy, most un-mouselike face, I suddenly exclaimed, "Good heavens! It's an antechinus! We are lucky to see such a thing during the daytime." A short time later, the penny dropped. We had been incredibly lucky, for we had arrived during the only two weeks of the year when it would have been active by day: the mating season. It had only been while I was at university that the remarkable life cycle of these mysterious creatures had begun to be unraveled - in fact, not far from where we had seen it.
     Antechinus (an-tee-ky-nus) is a large genus of an even larger assemblage of mouse -, rat-, or
Antechinus subtropicus (Queensland Museum)
occasionally cat-sized marsupial predators of which the average person knows nothing. Those who have heard of them usually call them as marsupial mice, but marsupial shrews would be a better term, for they occupy the niche belonging to shrews in the rest of the world: small predators scurrying around, feeding on insects, grubs, lizards, and the like. The exact number of species keeps changing all the time with the growth our knowledge. The little rascal you see at left is the species my mother and I saw on Mount Glorious, but it wasn't the name I knew at the time, and least of all the name under which it was originally studied. Not to worry; they all have the same bizarre lifestyle.
     The breeding season, as pointed out, is highly synchronized, and lasts only two or three weeks. The exact dates vary with the species and the locality, but are ultimately set by changing day length - which is typical of breeding seasons in general. In the lead-up to it, the male puts on a lot of extra weight. He needs to, because once the "season" strikes, he goes into a frenzy. He is hardly interested in eating or sleeping; all he wants to do is fight and mate. And he goes about both with a whole-heartedness bordering on obsession. Once a colleague of mine discovered one of them when he has doing his dawn check of his box-trap line. While he was attempting to weigh and measure the little monster, out of the undergrowth stormed a rival male intent on doing battle. My colleague picked up the newcomer by his tail and held him there, but the newcomer took no notice. All he was interested in was fighting the other male. As for mating, once he's got the female, he bear-hugs her with his forepaws, grasping her in his jaws by the scruff of the neck like a tom cat, and stays joined to her at the groin, pumping away, for anything from nine to twelve hours. He has her where he wants her, and he's certainly not going to let any other male get his sperm in.
     Three weeks after the end of the mating season, all the females are pregnant, and all the males are dead. Yes, dead. The stress of fighting and mating has been just too much. Their bodies have been flooded with corticosteroid stress hormones, gnawing away nasty ulcers in their stomachs, and leaving their bodies wide open to infections. You can sometimes find them flaked out on the floor of the forest, unable to move, easy pickings for any passing predator. Ultimately, their brave hearts can take it no longer.
     This male die-off has been found in every species of Antechinus investigated, along with some species not closely related to Antechinus: the rat-sized phascogales, a Western Australian animal called a kaluta, and the northern quoll (but not, apparently, the other species of quolls). Why is it so?
     Ultimately, it is the result of positive feedback in a process involving a short mating season and short-lived animals. Mating seasons are timed to allow births at the most opportune period for the raising of the young. Why it should be so short is far from clear, but once it becomes closely synchronized, certain things follow. In species where the father is not necessary for the raising of the young, there is a selective advantage in a male mating with as many females as possible, and thus fathering a maximum number of children. This is a zero sum game. The more children A fathers, the fewer are fathered by B. In practice, the biggest and toughest males monopolize the females, while a high proportion of males never get a chance to pass on their genes at all.
     Thus, one in every ten bull moose are killed every year in mating fights. Think about it! As a reward for his fighting ability, a prime bull moose gets to dominate a harem, which must be reconstituted every year until, after a few short years, some young blade knocks him off his pedestal. After that, worn out and alone, he becomes wolf bait. Life is never easy if you're  male. The corollary, of course, is that if you are an up-and-coming bull moose who finds yourself outclassed, it is best to back off, and try again next year. He who fights and runs away may live to fight another day.
     The trouble is, Mr Antechinus doesn't have this option. His life is too short. Even among the females, only about half survive for a second breeding season. The male is in a quandary. If he doesn't pass on his genes this time around, he will probably never get another chance. He will have lost his place in the evolutionary scheme of things. His life will have been a failure. He's got to pass on his genes now even if it kills him. And it does.
     Of course, once such a process is in place, it makes just plain common sense for the female to come into breeding condition while all the males are still alive - otherwise, she too will have lived in vain. Thus, the mating season becomes even tighter and more synchronized. It's a vicious circle.
     And, fellows, it doesn't always pay to be too macho.