Male secondary sexual characters may have evolved to advertise males that carry large numbers of beneficial sexually transmitted microbes highly potent against pathogens

Reinoculation of Ideas about the Benefits of Copulation: Reply to Rowe et al. Michael P. Lombardo, Patrick A. Thorpe, Harry W. Power. Trends in Ecology & Evolution, August 13 2020. https://doi.org/10.1016/j.tree.2020.07.008

Our bSTM (beneficial sexually transmitted microbes) hypothesis of copulation may explain why female birds either copulate repeatedly with a single mate or with multiple partners when only a single copulation may be sufficient to fertilize an entire clutch. We hypothesized that females may directly benefit from high frequencies of copulation and multiple copulation partners if they are inoculated with bSTMs that function as therapy against current infection or as ‘vaccination’ against future infection. Our hypothesis focused on birds but may apply to other animals.

The bSTM hypothesis requires that beneficial microbes are acquired by females during copulation. Moran and Dunbar [8] experimentally showed that female pea aphids (Acyrthosiphon pisum) acquire bSTMs that increase their resistance to parasitoids and fungal infections, increase their tolerance to heat stress, and facilitate their expansion to different host plants. Smith and Mueller [9] reviewed additional examples of bSTMs in humans and mosquitoes.

We predict that careful examination of STMs and their effects on their hosts is likely to identify more bSTMs in wild populations. The bSTM hypothesis assumes that the probability that a female becomes colonized by STMs increases with the number of her copulations with a single partner or with multiple partners. Observations that copulations with multiple partners increases the diversity of the reproductive microbiomes of female lizards [10] and mice [11] satisfy this assumption.

We predicted that male secondary sexual characters may have evolved to advertise males that carry large numbers of bSTMs highly potent against pSTMs, and Rowe et al. [2] reiterated this prediction. By showing a positive correlation between bill coloration, a sexually selected trait in male mallards (Anas platyrhynchos), and the bacteria-killing ability of their semen, Rowe et al. [12] demonstrated the possibility that sexual selection can mold signals associated with bSTMs. We expect that future research will discover male secondary sexual characteristics to be associated with a male’s ability to deliver bSTMs to females.