Male marine gastropods (snails) rarely provide any kind of post-sperm transfer investment in offspring.
After copulation is complete, males tend to go their separate way while females lay their eggs on any number of benthic substrates (i.e. rocks or shells on the ocean floor).
However, the snail Solenosteira macrospira is a major exception to this rule. In this species, males provide a substantial effort in terms of parental care.
Females deposit their fertilized eggs directly on to the shells of males, transforming them into brooding grounds for developing offspring. Their shells become completely covered with embryos, which carries a significant energetic cost to the dutiful dads.
But here’s the kicker: the male is genetically responsible for something in the region of 25 per cent of the embryos he is carrying.
Females of this species exhibit one of the highest levels of polyandry seen in an internally fertilizing organism, utilizing the sperm of several male donors to fertilize their eggs.
On average, the load of babies being carried around by a single male will have been sired by between six and fifteen males. So in this instance females are making use of the genetic benefits of multiple matings to the direct detriment of the males carrying the embryonic load.
This post is an adapted excerpt from my book “Wild Sex: The Science Behind Mating in the Animal Kingdom“