It sounds completely counter-intuitive, but some male animals may get such a benefit from having, and transmitting, sexually transmitted infections (STIs) their immune systems are dialed down to increase their chances of catching something. In case any readers are reconsidering their sex lives, it's worth stressing this phenomenon has yet to be confirmed and is unlikely to occur in humans.
According to one evolutionary model, males maximize their chances of passing on genes through indiscriminate sex. “For females, it often pays to be a bit more choosey,” Dr Meghan Head of the Australian National University said in a statement. “Producing eggs can be quite taxing, and they have to invest a lot more time and effort after they’re fertilized as well.”
We know from widespread observations that this promiscuous male/picky female model doesn't apply to all species. Among the many where it does, however, the males often resort to dirty tricks, like barbed genitals, to maximize the chances they will father children, irrespective of the consequences for their mate.
In Evolutionary Ecology, Head models the possibility that this includes reduced immunity so they can pass on STIs to females they mate with. "When animals become infected with a virus or bacteria they usually respond in one of two ways," Head said. One option delays reproduction to focus on fighting the infection. In Head's words, the other has them respond, "Oh no I'm sick, I better have lots of offspring now, because I'm not going to survive into the future to have them.” Likely future sterility can have the same effect.
Males who anticipate so-called terminal investment have an incentive to make their partner sick shortly after mating. If the female is unlikely to have had the chance to meet any other males, her body will respond to the illness by maximizing use of available sperm.
Head told IFLScience that even after allowing for the risk infections pose to males it could still be to the evolutionary advantage of males of some species to let an STI infect them provided they pass it on.
The only evidence outside calculations this phenomenon occurs is the observation males of certain species have lower immunity than females. We can't yet be sure this is the reason.
Head is trying to correct this by studying eucalyptus beetles sometimes infected with a sexually transmitted mite. She told IFLScience the beetle was chosen because its size makes it easy to keep in larger numbers and its short time to sexual maturity means it will be possible to observe changes over many generations. Moreover, the mites, which live under the beetle's wings, are much easier to detect than bacteria or viruses.
Head has made a career out of studying sexual conflict, where partners' interests collide. She told IFLScience females usually adapt to the weapons males develop, e.g. by evolving protective pads to counter blunt spiky genitalia. “In this case, there is a third player, the STI, which also wants to be transmitted, so females are at a disadvantage,” Head said. The infectious agent will evolve to make it hard for females to detect if a male is infected.
Although she agreed that parthenogenesis looks increasingly appealing, Head noted human males are probably off the hook. The model only works for species that don't form long-term pair bonds, and where females don't engage in long-term parental care.
Alas, some men find other ways to make themselves objectionable. Head was co-author of a paper infamously rejected by a leading journal because it lacked a male co-author. She told IFLScience that work was in her secondary field, and the experience did not inspire her interest in sexual conflict.