Female red flour beetles (Tribolium castaneum) shouldn’t want to mate more than once. They get enough sperm from a single male to fertilize all their eggs, and mating with multiple males can actually harm them. So why do many red flour beetle females mate multiple times?
New research published this week in Science provides one answer to this question. Red flour beetles often go through population ‘bottlenecks’: the population can get nearly wiped out, so only a few individuals survive to produce the next generation. This leads to lots of inbreeding. And inbreeding can lead to lots of problems.
The solution? Mate more.
The researchers forced 33 different populations of beetles to inbreed by brother-sister mating for 8 generations. They also had a large population of beetles that didn’t inbreed during this time. After the 8 generations, inbred females that only mated with one male had only half the number of offspring as females that weren’t inbred. But if you let them mate with 5 males, they were back to cranking out babies.
So, they showed that being promiscuous was a good thing for inbred females. But, were they actually more likely to be promiscuous than normal females?
The next experiment tested just that: give inbred females and noninbred females the chance to mate with 10 healthy males and see who takes advantage most of this wonderful opportunity. It’s easy to tell when female red flour beetles don’t want to mate. They run away, roll over on their sides or back, and generally just make it really difficult for even the most ambitious male to work his charm.
By the way, this experiment was done 15 generations after inbreeding stopped. Females from the populations that had been inbred were way more likely to mate with lots of males than the females whose ancestors weren’t inbred. They started mating faster, mated longer, and had more total matings than the never-been-inbred ladies. In fact, they spent almost 40% of their time having sex, compared to less than 20% for the control females.
The authors of the paper suggest that inbreeding makes natural selection work in overtime, because the changes in mating behavior were very dramatic and happened very quickly (in evolutionary time). Living in an inbred population means that females are more likely to mate with a close relative who carries the same bad mutations as she does. Females who mate with many males have a better chance of finding a guy with compatible genes that she can pass on to healthy offspring.
Who knew that keeping it all in the family could be such an aphrodisiac?
Michalczyk, L., Millard, A., Martin, O., Lumley, A., Emerson, B., Chapman, T., & Gage, M. (2011). Inbreeding Promotes Female Promiscuity Science, 333 (6050), 1739-1742 DOI: 10.1126/science.1207314