Within the animal kingdom, getting your leg over first does not necessarily end the competition over mates. Females of many species will copulate with multiple males, meaning that males need to develop other strategies that will boost their chances of reproductive success. For example, new research suggests that if a male flour beetle sniffs competition on a prospective female, he not only cranks up his courting efforts, but he also starts producing more sperm for the lucky lady.
Interestingly, it also transpires that the male won’t go through all of this effort if he thinks the competition is too stiff; if he whiffs the perfume of too many other males on a potential mate, then it’s not worth his while to go the extra distance, so he gives up and searches for an alternative. And they say romance is dead. The findings have been published in the open access journal Behavioral Ecology.
When a female mates with more than one male, the sperm compete within her genital tract to fertilize her egg. And it seems to be the case that the more sperm a male deposits, the more likely they are to be successful. But given the fact that males can’t produce an unlimited supply of sperm on demand, and that making sperm can be a costly activity, males need to adjust their sperm investment based on the risk and intensity of competition. This means that the males need to gather information to assess the potential risk accurately, such as using visual or acoustic cues to determine whether the female may have already mated with another male.
There is also evidence that, in insects, males can use smelly chemicals transferred from other males to females to judge the risk of sperm competition and to adjust their behavior accordingly. However, scientists didn’t know whether males can perceive the risk and level of competition when other males are not around. To find out more, researchers from the University of Exeter decided to study broad-horned flour beetles, since the females are known to mate with several different males.
During courtship, the male mounts the female and stimulates her, drumming his leg on her abdomen until she decides whether to mate with him or not. This highly tactile ritual can last up to ten minutes, providing the male with a decent window to assess the competition situation.
For their experiment, the researchers exposed virgin females to different numbers of males in a vibrating tube that transferred their scent, but kept them separate so they could not mate. The researchers then assessed their courtship efforts to see whether the number of males the female had been exposed to affected their efforts. They found that males will work extra hard to woo the female if she had been in contact with fewer than three competitors, but wouldn’t put in the effort if she bore the scent of more than three.
Taking this one step further, the researchers counted the sperm in the female’s reproductive tract after they were allowed to mate to see whether her contact with others affected the amount he invested in her. They found that the males would allocate significantly more sperm to females bearing the scent of competitors than control females that had not been exposed to other males.
Taken together, the researchers conclude that, rather than indicating the female’s mating status, the smelly chemicals present on her from other males actually provide information on the presence and maybe density of other rivals in the environment, which is used to adjust reproductive investment.