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TERMITE PARTNER SELECTION

We know that termites pair up during swarming events, but what makes a termite chose one partner over another? Steve Broadbent, Regional Director for Ensystex, reviews the research.

 

I think everyone is well aware of the selection procedures that occur in higher life forms. We humans will select a partner who we find physically attractive, whose personality we enjoy and perhaps we’ll also consider other factors, such as how they will provide or care for us, their integrity, or honesty. The same is true for many animals and birds, where the males in particular might put on a showy courtship display to attract a female partner.

Courtship is a social behaviour in which there is a relationship between males and females of the same species, leading to mating and reproduction. Courtship evolved as a result of males competing with one another in order to ‘win over’ a female and fertilise her eggs. Eggs are generally a more scarce resource than sperm, so it is up to the males to drive the mating process. This has led to sexual selection among males and females, with male-to-male competition and female choice.

Courtship is an extension of this competitive process. So what do we know about partner attraction in lower life forms, such as insects?

Courtship rituals have been observed in some insect species, such as vinegar flies (Drosophila melanogaster). Over 60 years ago, Bastock and Manning1 described how male and female flies approach within 2 mm of each other, then the male circles around the female. Males vibrate one wing during their circling, which stimulates the female. Females are very discriminatory and, if the males do not approach in the correct manner, they are kicked off!

In monogamous animal species, which also exhibit biparental care of offspring, we expect to see courtship rituals and selectivity in mate selection – for example, the singing and attraction displays of birds. However, monogamy and biparental care are uncommon in insects, with the exception of the infraorder Isoptera (termites), which is the second largest animal taxon with monogamous biparental care, after birds (Class: Aves).

Most termite reproductives are monogamous during the initial founding of the colony, meaning they mate for life. Both the queen and king are involved in raising the offspring, displaying biparental care. The question arises, then: are termites selective in their choice of mate?

The details of how termites locate a partner are uncertain; however, we do know that the dealated termites join to form ‘tandem pairs’, with the male following behind the female, striving to keep contact with the posterior segment of the female abdomen with his antenna (main picture above; credit: Thomas Chouvenc/University of Florida). Some tandem pairs separate, but partner switching is rarely observed after a tandem pair has remained together for 30 seconds or more. If they find a suitable nest site, the pair will move underground to mate.

In the founding of a termite colony, the parental investment is enormous, since the first generation of larvae are entirely dependent upon the founding pair.2 For several weeks, during the founding of the colony, it is sustained solely by the fat reserves of the founding pair, until foragers emerge from the first eggs to provide nutrition. It seems probable therefore that size and/or weight may play a role in mate selection in termites, as it does in most invertebrate species.

Examining tandem pairs of Coptotermes formosanus, Husseneder and Simms3 reported that males collected from tandem pairs had larger heads than single males, indicating either a competitive advantage or a female preference for larger males. Also, males with large heads paired mostly with females with large heads, and heavy males paired mostly with heavy females. This suggests that mate choice may be based on size, which is likely related to body mass and lipid mass, with the latter providing an indication of the fat reserves available for starting a new colony (the bigger the better).

It would also be expected that genetic quality is important in the colony formation process, since parents with ‘good genes’ will enhance the vigour and quality of the future colony. In turn this leads to the concept that good genes will be those from heterozygous termites (heterozygous essentially meaning having two different genes that create a physical trait). Heterozygous animals typically sire offspring with increased fitness because of their increased genetic diversity (e.g. greater disease resistance) or overdominance (e.g. recessive deleterious genes are not expressed in heterozygotes). Heterozygous offspring can only occur if the king and queen come from different termite colonies.

There are two potential mechanisms that could achieve this outcome; the males and females are able to recognise whether they are related and select their mate accordingly and/or they rely on mass swarming from multiple colonies in the area to maximise the chance of encountering a reproductive from a different colony.

In their study on C. formosanus, using genetic analysis, Husseneder and Simms found no evidence that reproductives actively select mates from different nests. However, they did observe that females involved in tandem pairs had a higher level of genetic diversity (higher degree of heterozygosity) than females that did not pair up. This higher level of heterozygosity was not correlated with size, so the cue that drives the preferred selection of heterozygous females remains unknown. Nevertheless, choosing females with higher levels of heterozygosity will increase the genetic fitness and potential success of the colony.

If alates are not actively selecting mates from different nests, then swarming distance and the synchronisation of swarming events appears to offer the greatest opportunity to increase the chances that alates from different colonies pair together to increase genetic diversity.

In summary, size-related physical traits and inbred genetic traits appear likely to influence partner selection and competitive advantage of both male and female alates in Coptotermes spp. Males with increased head width, and females with increased heterozygosity, are more likely to join to form new colonies with increased colony vigour. Large males prefer to choose large females, which provides for greater resources in the early stages of colony development.

 

Steve Broadbent, Regional Director, Ensystex Australasia

 

1 Bastock, M. & Manning, A. 1955. The courtship of Drosophila melanogaster. Behaviour, 8, 85-111.

2 Shellman-Reeve JS. 1999. Courting strategies and conflicts in a monogamous, biparental termite. Proc. R. Soc. Lond. B. Biol. Sci. 266:137–144.

3 Husseneder, C. & Simms, D.M. 2008. Size and heterozygosity influence partner selection in the Formosan subterranean termite. Behav. Ecol. 19:764–773.

 

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