Heat stress may change male burying beetle mounting behaviour

The behaviour of burying beetles is helping scientists understand that hot weather can affect far more than an animal’s survival.

As temperatures climb, heat can alter how these ectothermic insects function and communicate, as well as how they recognise potential mates.

New research suggests that male burying beetles mount other males more frequently when exposed to heat stress. The study also found, however, that the behaviour was common at normal temperatures.

The findings prompt questions about whether these encounters are merely mistakes or part of a more complicated social pattern.

A chemical message on the body

A thin coating of waxy chemicals called cuticular hydrocarbons, or CHCs, covers the outer surface of insects.

These substances help to prevent water loss while also conveying information insects use to recognise mates, identify sex and respond to rivals.

Solène Morelle, a PhD student at the University of St Andrews in the United Kingdom, is investigating whether heat alters these chemical signals in ways that affect beetles’ sexual recognition and behaviour.

“Evidence suggests that there is a trade-off between the signalling and waterproofing functions of CHCs,” she said.

“This indicates that heat-induced changes in CHC profiles may alter behavioural and reproductive outcomes.”

Balancing survival and communication

In hot, dry conditions, a beetle needs effective protection against dehydration.

Longer-chain CHCs are generally better at reducing water loss because they are less prone to evaporation. Shorter-chain CHCs are more volatile, potentially making them easier for other beetles to detect.

This creates a potential conflict: a chemical coating that provides better protection in the heat may communicate social information less clearly.

As a result, a beetle may be harder to identify as male or female, or as a partner or rival, influencing beetle behaviour.

“If there is a functional shift, I would expect to see a move toward longer-chain CHCs that are better at limiting water loss versus shorter-chain which are more volatile and therefore more effective as detectable signals to potential mates,” said Morelle.

Burying beetles and complex parental behaviour

The researchers examined Nicrophorus vespilloides, a burying beetle recognised for its unusually involved parental behaviour.

Male and female beetles bury the bodies of small animals, including rodents and birds. Their young then feed on the carcass.

The parents may cooperate to feed the larvae, defend the nest and drive competitors away.

“Coordinating all of that depends on effective chemical communication, so if heat stress disrupts their olfactory pathway, that could translate into reduced reproductive success,” explained Morelle.

How a heatwave changes beetle behaviour

The team compared beetles kept in control conditions at 20°C with beetles exposed to a simulated three-day heatwave at 26°C.

Morelle recorded the frequency with which males mounted other males and the duration of those encounters.

The researchers then extracted CHCs from the beetles’ bodies and analysed them using gas chromatography-mass spectrometry, a technique that separates and identifies chemical compounds.

“I was surprised to find out how much same-sex mounting the beetles showed, even under normal conditions,” said Morelle.

“And what I did not expect was the increase in reciprocal mounting under heat stress – we don’t yet know what this means!”

A mistake may have a low cost

When a beetle mistakes another male for a female, same-sex mounting can use up time and energy. Even so, a brief unsuccessful attempt may be less costly than hesitating and missing a real mating opportunity.

“So, the physiological cost of a single unsuccessful mating attempt probably isn’t very high on its own and does not outweigh the risk of losing a chance to mate with a female,” noted Morelle.

There may also be reasons for the behaviour that have nothing to do with sex recognition. Mounting can sometimes happen during competition, social conflict or periods of heightened activity.

The results so far do not establish why the beetles behave this way.

Mixed signals may influence beetle behaviour

Problems with chemical recognition could have consequences beyond mating. Burying beetles need to tell a cooperative partner apart from an intruder.

A rival can take over the buried carcass and kill the larvae already there before producing its own offspring.

A confused response at the wrong time could therefore affect an entire brood.

Chemical communication is consequently central not just to courtship, but also to parental defence and long-term reproductive success.

Another consequence of climate change

Climate change is already exposing many ectotherms, including insects, fish and reptiles, to temperatures close to their physical limits.

Scientists understand many direct effects of heat, including dehydration, slower growth and reduced fertility. Changes in social behaviour are still more difficult to predict.

The team continues to collect and analyse its chemical data. Its next task is to establish whether heat-related increases in same-sex mounting reduce offspring numbers or survival.

The results may reveal whether the behaviour is a harmless side effect of hot conditions or another way in which heat can quietly affect the health of animal populations.

The findings from this ongoing research were presented at the Society for Experimental Biology (SEB) conference in Florence, Italy.