Evolution should select for individuals that employ ways of discerning members of their own species from any others. Reproductive success of those engaing in sex with organisms of another species should therefore be drastically reduced.
The “lock-and-key” hypothesis holds that there is only one specific key (male genitals) that fits inside a certain lock (female genitals). While this theory isn’t without its own controversies, there are many examples where it holds true.
In the case of two parapatric (some overlap in geographic niche) carabid beetle species, males copulate with females of both species indiscriminately. Ladies that are fertilized by the wrong gentlemen can lay these fertilized eggs, but the rate of successful fertilization is low, and the survival of the offspring is negligible.
More often, females suffer fatal injuries from copulations with heterospecific males due to rupturing of their vaginal membranes.
Males don’t fare much better, as they often experience broken genitals following mismatched copulations. This effectively prevents them from achieving successful fertilization in the future.
Wounds, Infection, and Death
In addition to the wounds sustained by females from being stabbed with the wrong machinery, they are also subject to increased risk of infection from said wounds.
A detailed study of the consequences of heterospecific copulation in two species of fruit fly (Drosophila santomea and D. yakuba) used fluorescently labelled microbeads strategically placed on the male genitalia.
When sex is between species, the haemocoel (body cavity) of the female is more likely to be invaded by foreign substances. This was brilliantly demonstrated by the presence of the fluorescent beads that had been transferred via their partner’s genitals.
It’s bad enough to have a morphologically incorrect penis stabbing through your body without being subjected to an increased rate of infection as well.
Even worse, imagine having an entire dead male of the wrong species attached to your nether regions.
This is the unfortunate circumstance for female reptile ticks (Aponomma species) that happen to come into contact with the wrong males in areas where their geographic distributions overlap.
In these situations, it is possible for two (or even three) different species to be lodged on the same reptile host.
Males are generally indiscriminate, and will undertake forced copulations with females of the wrong species. This results in a mess of genital confusion such that they cannot remove themselves from her after the act.
Thus, their entire body becomes the ultimate mating plug, though sadly it is of no biological consequence.
Engaging in cross species sex is clearly bad news for both males and females, but females are most likely to suffer increased mortality as a result.
This post is an adapted excerpt from my book “Wild Sex: The Science Behind Mating In The Animal Kingdom“