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| Buring beetles caught in the act. Photo by Jena Johnson. |
Burying beetles are unusual among insects in that they provide parental care and are often monogamous. When burying beetle pairs find a small dead bird or rodent, they pluck it bald, coat it in antibacterial and antifungal body secretions, and dig a hole around it. The female lays her eggs around the carcass-ball, so that their newly hatched babies can feast on it, as well as additional food that the devoted the parents bring them. In this system, mothers are capable of taking care of their babies by themselves, but they do better if fathers stick around. This has led to sexual conflict in these species: after mating, many males will climb to a high perch to release pheromones to attract additional females… but if his partner catches him, she’ll knock him off his perch.
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| A nurturing burying beetle mom feeding her young. Photo by Paul Hopwood. |
Male and female burying beetles also differ over how much sex to have. Males are better off when they have lots of sex, because this increases their chances of fathering more young. However, females are better off when they have less sex, because they don’t need a lot of sex to fertilize their eggs and too much sex reduces their ability to provide maternal care. In this battle of the sexes, genitals can become a specialized weapon.
A research team from the University of Exeter in the United Kingdom, including Paul Hopwood, Megan Head, Eleanor Jordan, Mauricio Carter, Emma Davey, Allen Moore, and Nick Royle, hypothesized that selectively breeding burying beetles who have more or less sex could lead to changes in genital structures over multiple generations. They randomly assigned burying beetles a monogamous partner and measured how much sex they had. They then bred the offspring of the high-sex parents together to create high-sex genetic lines, they bred the offspring of the low-sex parents together to create low-sex genetic lines, and they bred offspring from random parents together to create control genetic lines. They continued these high-sex, low-sex and control lines for ten generations. They then measured and compared the genitals of these 10th generation offspring.
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| Porno pictures of burying beetle genitals. Images A and B are male genitals from the top and side views. Images C and D are female genitals from the top and bottom views. The blue and pink dots are the landmarks the researchers used for measuring genital size and shape. Image from the Hopwood et al. 2016 paper in Evolution. |
The selective breeding program changed genital shape in both males and females after only ten generations. Males from the high-sex genetic lines had longer and straighter penis-like structures with shorter sensory hairs, whereas males from the low-sex genetic lines had shorter penis-like structures with longer sensory hairs. Perhaps these long sword-like penis-like structures help males overcome females that resist male sexual advances by wrestling, kicking and curling their abdomens away. Females, on the other hand, had shorter vulvas and thicker genital claws (yes, these girls have developed genital claws for this sex war) in both the high-sex and the low-sex genetic lines compared to the control genetic lines. Furthermore, the genetic lines in which males developed longer penis-like structures had females that developed more narrow-set genital claws, showing that these traits were changing together from generation to generation.
There is a tremendous diversity of genital shapes across the animal kingdom. We often think of this genital diversity as coming from the compatibility of the sexes, like a lock and key, when the truth is that love is a battlefield and genitals are weapons of war.
Want to know more? Check this out:
Hopwood, P., Head, M., Jordan, E., Carter, M., Davey, E., Moore, A., & Royle, N. (2016). Selection on an antagonistic behavioral trait can drive rapid genital coevolution in the burying beetle, Evolution DOI: 10.1111/evo.12938
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