The love life of the burying beetle is really quite charming. Two beetles meet, they fall in love, they bury a rotting animal together and they raise their little beetle family on the flesh of the carcass. Romantic, right? 

Male burying beetles are equipped with large antennae, which are themselves equipped with extremely sensitive scent detectors. As soon as they catch a whiff of a rotting carcass, usually a small bird or rodent, the male scampers over to guard it. Once he's in control of a carcass, he emits pheromones meant to attract females. The problem is that those pheromones also attract other males. And when multiple males are in the presence of a good-looking dead animal, there's bound to be a fight. 

And for burying beetles, size matters. It's typically the larger of two males who wins a competition. The prize is the carcass. And dominion over the rotting flesh of a dead creature means that the true prize is that the champion starts to look real sexy. To female burying beetles, at least.

The females have a similar process. By the end, the carcass is controlled by a single dominant pair.

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By the end, the carcass is controlled by a single dominant pair … at least that's how it's supposed to work. Image: Tab Tannery, Flickr

The honeymoon works like this. First, the pair excavates a hole under the carcass. Then they strip it of hair or feathers, using the soft material to line the hole. The bare carcass is then coated in a layer of oral and anal fluids, which have antimicrobial and antifungal properties. This liquid application has a dual purpose: it slows down the decay process, while preventing other beetles from smelling it. The carcass is then pushed into the pit and buried, but not before the female deposits her fertilised eggs nearby. Once the larvae hatch, they have a delicious meal of decomposing vertebrate just waiting to be devoured. 

At least, that's how it's supposed to work.

In reality, the carcass isn't home to a single mated pair of beetles. It's the site of a big beetle sex party. Subordinate males and females, who lose fights and don't get to control the carcass, still stick around. The females lay their eggs near the carcass, taking advantage of the dominant pair's hard work, much like cuckoos lay their eggs in the nests of other birds. Meanwhile, subordinate males work hard to fertilise those eggs, mating with whatever females they can. As a result, nobody knows who the father of each larva is.

“In reality, the carcass isn't home to a single mated pair of beetles. It's the site of a big beetle sex party.”

In an experiment, University of Exeter researcher Mauricio J. Carter and his colleagues divided male burying beetles into groups that had either high mating rates or low mating rates. A high mating rate offers a male a reasonably good guarantee of paternity: the more he mates, the more likely he is to be the father of any particular beetle baby.

Over several generations, the researchers artificially shaped two genetic lineages. The males of one family were more likely to show a high rate of mating, while the males of the other had a lower mating rate. In other words, the beetles' sex drives were encoded into their genes. The results of the study were published this week in the journal Evolution.

The team found that those beetles selected for a high mating rate were more insecure than those selected for a low mating rate. That is, when put into a competition over access to a carcass, the more virile males were more likely to quickly enter into a dominance display – but only if they were bigger than their opponents. If they were smaller, they quickly scaled down their dominance displays, surrendering to the challengers.

The study reveals how genetics interacts with the social environment to produce behaviour. Despite each beetle's genetic tendency towards a high or low sex drive, they were able to modulate their behaviour based on the social context in which they found themselves. "Such flexibility of behaviour in response to a change in social context [is] common, but relatively poorly understood," explained Carter in an official statement. "Our research increases our understanding of this important process that helps organisms adapt to changes in their environment."

You might think that the males who have the most sex have nothing to worry about. After all, they're the kings of their castles (and here 'castle' means rotting, hairless, anal secretion-covered rodent carcass). But sexual supremacy introduces a kind of social stress that the more laid-back males don't have to contend with. "Males that have more sex really are more insecure about their social status," added study co-author Nick J. Royle. They have more to lose. 

Top header image: Larissa, Flickr