For many males in the animal kingdom, sex is a one-time thing, and they leave the females with DNA packaged in sperm and little else. But according to a new Trends in Ecology & Evolution article, these brief encounters may help supply females with beneficial resources. Ejaculate, even in the tiniest amounts, also contain potentially helpful proteins and molecules, and some females may have evolved ways to exploit these resources.
Recent studies found that seminal fluid (the liquid part of semen outside of sperm) can affect females and their offspring independent of fertilization. Chock-full of proteins, sugars, salts, and RNA, seminal fluid might be a little-explored perk of sex, especially for species that lack obvious “nuptial gifts” like nutrients or defensive compounds. In roundworms and mice, these paternal RNAs appear to play a role in the development of the embryo, though exactly what effects these molecules have are largely unknown.
A couple years ago, Russell Bonduriansky from the University of New South Wales and colleagues examined the offspring of female neriid flies (Telostylinus angusticollis) who had mated with multiple males of varying sizes. If she mated with a male before she was fertile, the immature eggs wouldn’t be fertilized – even though sperm was transferred. But if she mated with a second male two weeks later when her eggs were mature, his sperm would fertilize the eggs – yet the size of the offspring would be closer to that of the first male. Chemicals in the first male’s seminal fluid seem to have lingering effects, even if he isn’t the father.
The team reasoned that if seminal fluid – regardless of actual paternity – played an important role in the development of offspring, then females should have evolved ways to seek out this beneficial resource from males. "Females might be choosy even when they don't have eggs ready to be fertilized," Bonduriansky says in a statement. "They might be getting something for future offspring that will be fertilized later on, or they might be getting something for themselves."
Some females are known to store semen from multiple males, and this system may have more advantages than just giving her a way to hold out for better genes. The team predicts that – in the absence of long-term sperm storage and first-male sperm precedence – selection will favor females who prefer males with beneficial seminal fluid when fertilization is unlikely. Females may be able to assess seminal fluid quality before mating if signals of male condition (such as features of his courtship display) are linked somehow with seminal fluid quality, Bonduriansky explains to IFLScience.
Once the eggs are ready to be fertilized, females might shift their preferences towards males that also provide sperm-borne benefits – like good, compatible genes. This could explain why females of some species choose mates and copulate before they’re mature or outside of their fertile window. However, if that’s the case, the first male who didn’t sire any offspring may have wasted his investment – which may drive the evolution of male counterstrategies.