The literature on sexual selection theory abounds with theories related to why females mate multiply when the sperm from one male is usually sufficient to fertilize all of her eggs. Not to mention that fact that sex in the animal world is often rough, resulting in costly injuries. In some systems where males offer large nuptial gifts (often big blobs of nutrients in otherwise nutrient-poor environments), the answer is pretty straightforward. In these cases, the costs the female incurs while having sex are offset by the resources she gains when she mates with him. In Callosobruchus maculates, a seed beetle, the benefit that a female gets from mating is surprisingly subtle.
Seed beetles spend the first part of their lives living in dried beans before emerging as adults. Their adult lives last about 8 days which they devote almost entirely to mating and laying eggs. They often live in dry environments and are able to survive without any food or water for the duration of their adult lives, but will drink and eat if these resources are available.
A group of researchers studying this system thought to themselves, “Hey, what if parched females are able to absorb water from male ejaculates? Wouldn’t that be a great way to obtain water in a dry environment?” If this were indeed the case, then we would expect to see thirsty females mate with lots of males while females who were supplemented with water should be less promiscuous (since sex in this system carries costs to females, including genital tract injury).
Surprisingly, this is exactly what they found! Females who were provided with water no longer had to mate to obtain this resource. These females mated with fewer males, lived longer and were able to produce more eggs than their thirsty, promiscuous counterparts.
My favorite part of this article was the description of when they decided the seed beetles were indeed mating. They described a successful copulation as one in which, “…the male had inserted his aedeagus into the female and was leaning back, motionless.” Sounds like the seed beetle females are getting a raw deal.
I wonder if this information could be used to control pests. For example, seed beetles often achieve pest status by laying their eggs in legumes. Could making an extra effort to keep legume storage areas extremely dry control seed beetle populations? Thirstier females mate with more males, but produce fewer successful eggs. Perhaps long-term drying could reduce population numbers. Understanding the subtle mechanisms underlying mating decisions may in some cases provide us with useful information for controlling the population densities.
In conclusion, it seems as though female seed beetles mate with multiple males in order to obtain “ejaculatory hydration benefits”. Who would have thought? What other subtle benefits that we have yet to imagine are luring animals into multiple promiscuity?
Ursprung, C., den Hollander, M., and Gwynne, D.T. (2009). Female seed beetles, Callosobruchus maculatus, remate for male-supplied water rather than ejaculate nutrition. Behavioral Ecology and Sociobiology 63: 781-788.