Scientists settle 30 year old argument, blame females and sperm competition.
by John Timmer
by John Timmer
Male survival after mating is pretty dismal for this species
Many animal species consist of members that will only mate once before dying. This reproductive strategy, often seen in fish and insects, can make evolutionary sense when the species is able to produce a lot of offspring from that single mating. Given that salmon can release thousands of eggs when they spawn, a single mating can produce a lifetime's worth of offspring.
That's not true for mammals, though. Raising young internally limits the number you can produce from a single mating, while the extensive post-natal care required by mammalian young ensures that the female has to stick around for a while after giving birth. But males don't always participate in postnatal care, so it probably shouldn't be a surprise to learn that there are mammals out there that engage in what researchers are terming "suicidal reproduction." The problem is that the behavior only occurs in a small number of marsupial species, and researchers have been arguing for 30 years about why that is the case. Now, some Australian researchers have come up with an answer: a combination of sperm competition and promiscuous females.
The marsupials that engage in this "one strike and you're out" approach to mating all die off because of a general immune failure that happens shortly after mating. This has nothing to do with the process of mating itself; in fact, it starts well in advance of mating, as the males build up a store of sperm and then permanently shut their gonads down. The question wasn't so much how the males' death takes place, but why. What sort of evolutionary advantage could this provide?
One hint came from the fact that the species that have this life cycle (technically called semelparity) are all small insect eaters. The possibility that they were all related species led to the suggestion that there might be something that genetically predisposed them to this lifestyle. Another idea was that the species might live in an environment where the availability of food was severely limited; thus, the males dying would free up resources to make their progeny more successful.
But a bit of biogeography suggested something else might be at play. The suicidal reproduction was mostly found in species at the far southern end of the range of marsupials. In fact, the new paper notes that male survival after mating drops the further you get from the equator. In these cooler environments, most insect populations experience a boom in the summer and are pretty sparse the rest of the year.
This in turn causes the females to time their reproduction to the availability of the calories needed to support it. The net result is that the entire female population in these species is ready to mate within a narrow window of time. With mate availability at a premium, they mate with pretty much any male available (the authors refer to the females as "mating promiscuously").
The authors hypothesized that the males' evolutionary response to this situation is to engage in what's called sperm competition. To confirm this, they measured the relative testes size of these and several related species. As male survival went down, the testes size relative to their body went up. The males that engaged in suicidal reproduction also mated for twice as long (an average of 9.4 hours!) as their less-competitive relatives. These are signs, researchers conclude, that the males are trying to ensure that their sperm is what does the fertilizing, even if the female goes on to mate with others.
The huge amount of resources expended on mating, combined with the fact that survival in small mammals is pretty low to begin with, means that the males are unlikely to survive to mate a second time anyway. So instead, they simply tune their bodies to make the most out of the one chance they get. And at an average of 9.4 hours, it's hard to say that they don't accomplish this.
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