Female Callosobruchus maculatus mate when they are thirsty: resource-rich ejaculates as mating effort in a beetle

Because male uncertainty over parentage limits the value of paternal investment in offspring, mate attraction and facilitation of ejaculate transfer are thought to be important functions of nuptial gifts. However, these are unlikely functions for valuable resources in ejaculates delivered inside the female. Instead, ejaculates containing costly nuptial gifts may be maintained because females alter their mating behaviour in response to the trade-off between the costs and benefits of mating. The value of receiving an additional gift should decrease with improved female physiological condition. Providing a female with a substantial gift will therefore make it less profitable for her to remate and reduce the risk of future sperm competition. Females of the bruchid beetle Callosobruchus maculatus are harmed by the spiny male genitalia during copulation but also appear to derive material benefits from the large ejaculates. I kept female C. maculatus with access to water and other females without access to water. All females were given the opportunity to mate with a new male every day. Females without access to water mated more frequently than females with access to water. I suggest that female C. maculatus mate more frequently to obtain water when dehydrated and that this may select for ejaculates containing large amounts of water in males. By providing their mates with a large amount of water, males can delay female remating and reduce the risk of future sperm competition.