Wild Sex: The Science Behind Mating in the Animal Kingdom by Carin Bondar

Author:Carin Bondar [Bondar, Carin]
Language: eng
Format: epub
Publisher: Pegasus Books
Published: 2016-08-01T22:00:00+00:00

Transvestites

Outside of the extreme examples of females with male-like genitalia, there is no shortage of cross-dressing in the animal kingdom. Girls posing as boys, and boys posing as girls. At first thought transvestitism may seem counterintuitive, but ecological complexities sometimes mean that cross-dressing is not only advantageous, it’s critical in maximizing biological fitness. One of the best-studied examples of animal transvestites comes from the damselfly (Ischnura species) world, where it is exhibited by over a hundred different species. Female damselflies come in two very distinct morphologies: gynomorphs are the ‘girlie girls’, the true females of the population. These ladies range in colour from orange to brown, and are very distinct from males. Andromorph females are bright blue and green, and bear a striking resemblance to males. Now, why would a female want to appear as a male? As with many invertebrates, female damselflies have a sperm storage organ (spermatheca) and they can obtain all the sperm that they need for their entire reproductive lifespan in one or two copulations. The sexual behaviours between males and females are therefore disturbingly different. Males copulate daily, and almost all these copulations are forced upon unwilling females. The ‘male mimicry hypothesis’ posits that the andromorph female form evolved in response to the extremely high level of sexual harassment in many of these species. Indeed, gynomorph females experience a higher level of harassment than andro-morphs, and this could have direct negative impacts on their health and biological fitness. So you may be thinking, why not move towards a monomorphic (one physical appearance only) morphology overall? Why should the gynomorph morphology be evolutionarily sustained if these ladies receive a higher level of harassment and a lower level of fitness? As with any biological system, there are more factors to be considered.

In low-density populations where harassment levels are low, gynomorph females do just fine. It’s in the high-density populations that we see a substantial level of harassment. In addition, gynomorph females are more cryptically coloured against their natural grass backgrounds, meaning that they enjoy a lower level of detection by predators. Male damselflies primarily use their vision to find females, so cryptically coloured gynomorph females may, in some low-density scenarios, be more difficult to find. Gynomorph females undergo a drastic colour-change during development – their dorsal side goes from red (immature) to dull brown or orange (mature). Andromorph females do not experience any colour change of this nature, nor do males. This ontogenetic (developmental) change in coloration seems to allow males to identify when gynomorphs are immature. They receive a significantly lower level of harassment as juveniles than do andromorph females, whose colour remains constant throughout their lives. Overall, it seems that in varying ecological circumstances there are advantages to being either gynomorph or andromorph, which explains the persistence of both morphologies in damselfly populations.

There are other cases where transvestite females may be disadvantaged by appearing as males. It’s fairly common in sexually dimorphic creatures (with highly ornamented males) for females to display rudimentary versions of the males’ sexual ornaments.

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