The sexually confusing lives of male insects

Two male mallard ducks enjoying male-male bonding time. Mallards have been known to have high levels of same-sex coupling within populations. Photo by Norber Nagel: Wikimedia Commons

It’s likely that you’ve heard of it. Some of you may have even witnessed this behavior yourself . From dogs in the park, to mallard ducks at the pond, there are many examples of animals known to exhibit homosexual tendencies. In the world of zoology, these are often referred to in the catch-all category ‘same-sex-sexual (SSS) behaviours’. These behaviours are of high interest to scientists, as they allow an opportunity to study the wonderful world of animal sexuality. An animal that engages in an SSS behaviour is paying high energetic costs, and the positive effects upon relative fitness remain unclear for many species. This is indeed a mystery, and has many people asking “Why?”.

Scientists have a number of theories suggesting why animals engage in SSS behaviours. Due to the diverse behaviours and life histories of different animals, it’s likely that SSS behaviour has become present in different groups for a wide variety of reasons. One hypothesis suggests that SSS helps to establish dominance within social relationships between primates (Wickler, 1967). In garter snakes, solitary males are hypothesized to mimic females to gain access to communal hibernation  sites (Shine et al., 2003). Another theory suggests SSS behaviour exists for the purpose of purging, and subsequent production of new male gametes – this act is referred to charmingly as ‘sperm-dumping’ (Levan et al., 2009).

This fantastic creature known as the red flour beetle (Tribolium castaneum) practices the gallant behaviour of ‘sperm dumping’ to maximize reproductive success with receptive females (Levan et al. 2009). Photo by Peggy Greb: Wikimedia Commons

One particular group of animals known for their tendencies to engage in SSS behaviours are the insects. A quick search of Wikipedia provides a list of more than 60 insect species that have been observed practicing some sort of this behaviour. With an estimated 10 quintillion insects alive on Earth at any given time (May, 1988), it would seem highly possible that somewhere this very moment, insects are actively engaged in same-sex-sexual activities – though again, the burning question is “Why?”. This is precisely the question two scientists recently set out to answer.

Scharf and Martin (2013) recently reviewed 110 species of arthropods known to engage in male SSS behavior. Within these species, SSS behavior can be rampant; when populations get excited, often more than 85% of males can participate! The team found that the available evidence explains very few examples of insects that gain improved fitness from adapting this behaviour. Based on their findings, the team suggests that confusion is a more likely explanation to this interesting phenomenon.

In the insect world, being more discriminatory in mate choice has a negative cost to individual fitness. The underlying reason? The cost of missing an opportunity to mate with a female is greater than the cost associated with attempting to mate with a less fit or completely unsuitable host (like a male). When a male insect is provided a chance to mate, and there is reasonable likelihood that the recipient is a viable female, the male will take a chance. Unfortunately for other unexpecting males, for males and females of similar-looking species, and for terribly confusing inanimate objects, the enthusiasm isn’t always mutual. In fact, one Australian beetle has become so enamoured with discarded beer bottles (Gwynne & Rentz, 1983) that local industry redesigned their packaging to keep male beetles from ignoring females, to attempt mating with a shapely piece of glass.

This rather amusing video demonstrates that ‘big and shiny’ can go a long way in the insect world. If a male can mistake a beer bottle for a receptive female, the likelihood of a male accidentally mounting another male doesn’t seem so far-fetched.

Often, insects are unable to discern sexual mates from others. Despite frequent visual likeness between male and female insects, other factors contribute to this interesting conundrum. Most insects communicate their desire to copulate through releasing olfactory cues called ‘pheromones’ into the environment. In the insect world, it’s generally the females who decide when it’s time to mate. The release of pheromones by females is a perfect indication to potential male mates for a chance to ‘get busy’. Interestingly, these attractive chemicals can be transferred accidentally from a female to a male during copulation. This means that after copulation, a mated-male could potentially smell identically like a mate-seeking-female. If a second male chases the scent of the pheromone, and finds this recently-mated-sexy-smelling-male he will make a move. Even if there is some doubt, a failed mating attempt with a male is far less energetically expensive than trying to find another receptive female. The result? A male insect attempting to mount another male insect; same-sex-sexual behavior resulting from confusion.

To continue to support their theory of ‘insect sexual confusion’, Scharf and Martin plan to study conditions that govern the likelihood of same-sex-sexual behaviours within insect populations.  Stay tuned for their further developments in trying to better understand the fascinating world of same-sex-sexual behaviours in our diverse, and ever-abundant, six-legged friends.

References:

Gwynne, D. T., & Rentz, D. C. F. (1983). Beetles on the bottle: male buprestids mistake stubbies for females (Coleoptera). Australian Journal of Entomology 22:79-80.

Levan KE, Fedina TY, Lewis SM (2009) Testing multiple hypotheses for the maintenance of male homosexual copulatory behaviour in flour beetles. J Evol Biol 22:60–70.

May, R. M. (1988). How many species are there on earth?. Science 24: 1441-1449.

Scharf, I., & Martin, O. Y. (2013). Same-sex sexual behavior in insects and arachnids: prevalence, causes, and consequences. Behavioral Ecology and Sociobiology 67:1719-1730.

Shine, R., Langkilde, T., & Mason, R. T. (2003) Confusion within ‘mating balls’ of garter snakes: does misdirected courtship impose selection on male tactics? Animal Behaviour  66: 1011–1017.

Wickler, W. (1967). Socio-sexual signals and their intra-specific limitation among primates. Primate Ethology. Weidenfield and Nicolson, London, pp. 69-147.