Love, War and Genital Shape

The size and shape of your junk may depend on how much sex your ancestors had… that is, at least, if you are a burying beetle.

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.

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.

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

The Harm of Verbal Promiscuity

Eastern chimpanzees don't want to be judged. Image by Ikiwaner at Wikimedia.com.

Whether they have one true love for life, multiple partners, or are free-loving, animals have many different mating systems. We have different scientific terms for these different mating systems, and most of these terms have very specific meanings. An animal is socially monogamous when it has one exclusive mating relationship, but maybe has sex with others outside of that relationship. It is sexually monogamous when it has one exclusive sexual relationship and is sexually faithful to that partner. Animals are polygamous when they have multiple sexual relationships. Polygamous animals can be polygynous (when one male has a mating relationship with multiple females), polyandrous (when one female has a mating relationship with multiple males) or polygynandrous (when multiple males and multiple females all have a mating relationship). However, one mating system term has been used much more loosely: promiscuous. In some scientific papers, promiscuous is used to describe animals that aren’t choosy about whom they mate with. Others use promiscuous to describe animals that don’t form mating relationships. But promiscuous is also misused by many people, including scientists, to refer to polygamous animals. This loose use of terminology can be damaging to both our scientific understanding and our society.

Scientific terms generally come from common language, but are then are given more specific definitions for their scientific use. When we confuse scientific terms for their common-use meanings, society can be harmed. For example, the Merriam-Webster Dictionary includes a dozen different definitions of “theory” that all include strong elements of uncertainty (such as “an unproved assumption” and “an idea that is suggested or presented as possibly true but that is not known or proven to be true”). In contrast, the scientific term theory refers only to scientific explanations that have been substantiated through such a large amount of rigorous scientific testing and evidence that we are almost certain they are true (because scientists are supposed to never be completely certain). When the scientific term theory is confused with the common word “theory”, then concepts regarded essentially as fact among informed scientists are disregarded by politicians and many of the general public as “just a theory”.

Promiscuity is one of those scientific terms that was originally borrowed from common language and is now confused with its common-word counterpart. The Merriam-Webster Dictionary defines “promiscuous” as “having or involving many sexual partners”, which is almost exactly the scientific definition of polygamous. Thus, “promiscuous” is often misused, even by scientific researchers, when polygamous, polygynous, polyandrous, or polygynandrous are more accurate. Misidentifying the mating system of a species can obscure meaningful connections between behavior and ecology and can negatively impact conservation, captive breeding efforts, and medical and psychological advances.

Image from freedigitalphotos.net

Mark Elgar and Therésa Jones from the University of Melbourne and Kathryn McNamara from The University of Western Australia found that when “promiscuous” is misused in research, scientists are much more likely to use it to refer to polyandrous females than to polygynous males. This biased misuse of the word reflects our moral judgments and causes us additional harm as a society. The common word “promiscuous” has pejorative connotations and evokes negative emotions, especially when applied to women. Our human cultures generally have expectations that women will be faithful to one partner, while we are more understanding of the infidelities of men. When applied to animals, and especially primates, promiscuity has an anthropomorphic nature that places our human expectations and interpretations on other species. It’s bad enough when we judge each other – let’s try not to judge animals too.


Want to know more? Check this out:

Elgar, M., Jones, T., & McNamara, K. (2013). Promiscuous words Frontiers in Zoology, 10 (1) DOI: 10.1186/1742-9994-10-66