Female beetles get water from the males' ejaculate

That is so gross, yet also very cool.

The cowpea weevil or Callosobruchus maculatus has an arms race that is going between the males and females. This beetle species are promiscuous, and there is a lot of advantage for the males to be the last one to have mated with a particular female in terms of reproductive success.

This issue has spawned a variety of weird behaviors and adaptations. For example, the males have spines on their intromittent organs (read: insect penises) that puncture the females insides. This is to discourage them from mating with other males. In response the females kick the males during mating to limit the damage done by the spines.

That is weird, but this is just crazy.

Dr Martin Edvardsson, publishing in Animal Behavior, has shown that the females get water and nutrients from the males' over-sized ejaculates.

Here is our friend the cowpea weevil, courtesy of the Canadian Agriculture Department:

Edvardsson research circulates around the concept of a nuptial gift. A nuptial gift is an expensive -- in terms of energy -- gift to the mother to increase the father's reproductive success. (How cute.)

From a male's point of view, nuptial gifts containing material resources may function either as paternal investment, increasing female offspring production, or as mating effort, increasing the proportion of eggs fathered by the gift-giving male. These functions are not mutually exclusive and their relative importance has been the subject of much debate. Male uncertainty over parentage will diminish the payoff from paternal investment. Most female insects are promiscuous and last-male sperm precedence is often high. Much of a male's investment is therefore likely to be wasted on production of offspring fathered by other males. Hence, mating effort is currently viewed as the most prevalent function of nuptial gifts. (Citations removed.)

In this case, he has shown the nuptial gift the male gives to the female in terms of a large ejaculate ensures that she does not need to go get water in the future. Further, she does not need to mate in the future to get more water and nutrients. The incremental benefit of mating for her in terms of greater water decreases because of previous mating, meaning she will do so less in the future.

Because I have probably butchered this concept, here is the press release:

Female bruchid beetles can absorb the water in the seminal fluid through their reproductive tracts and need to mate less frequently the more water they take from each mating. This is to a male's advantage because the longer the female goes without mating with another male, the greater his chance of successful fertilization. By transferring a large amount of water with the sperm, a male can help ensure his sperm has more time to fertilize the eggs without having to compete with the sperm from future matings. Dr Martin Edvardsson of the University of Exeter says: 'The large ejaculates may have evolved because males can make it less beneficial for females to remate by providing them with a large amount of water.'

From morsels of food to less useful offerings like dried leaves or balls of silk, insects' nuptial gifts are thought to perform the role of enticing a female to mate or investing in the resulting offspring. However, this study shows that males can also prevent females from mating with other males by giving them a valuable nuptial gift. Dr Edvardsson says: 'This research offers an alternative theory on the function of 'nuptial gifts', which are an important part of insect courtship and mating.'

Dr Edvardsson argues that the trade-off between the costs and benefits of mating is essential to the mating behaviour of female bruchid beetles. The males have spines on their genitalia that puncture the females' reproductive tract as they mate. Because of the damage this causes, females must carefully trade off the costs and benefits of mating, and limit the number of times they mate depending on their need for water and sperm.

Because there are always costs as well as benefits associated with mating, similar trade-offs are likely to be important in many species where males provide their mates with material resources. 'The key thing' says Dr Edvardsson 'is that the resource provided by males is less beneficial to females the more of it they already have, like water or food for example.' (Emphasis mine.)

The nuptial gift in part makes up for the reproductive cost of mating to the females, which is high in this case, but Edvardsson argues that this is probably not how it evolved. Instead, the large ejaculate probably evolved first so that the male would have more sperm to compete with other males. Then, the female evolved a way to utilize the water and nutrients in that already present sperm. "Well, hey...it's here."

And so the female-male arms race continues in the beetle...

(I am sure that you are all deeply disappointed that I have not been able to find a picture or video of beetle copulation or exemplary large ejaculate, but sadly there are things that even YouTube doesn't have. You could help me if you like. Just type, "ejaculation" into google and see if you can find something. :)