New research suggests that heat stress increases the occurrence of same-sex sexual interactions between male burying beetles – but also that a surprising amount of male-male encounters occur under control conditions. This ongoing project is currently investigating whether a possible trade-off between heat-protection and social communication cuticular hydrocarbons may be involved in possible sex recognition errors, and if these behaviours may result in any costs to reproductive success.
Cuticular hydrocarbons (CHCs) provide two crucially important functions for insects; waterproofing their cuticle to prevent water loss and prevent dehydration in hot and dry environments, and chemical communication between individuals that is used to identify mates and distinguish between sexes.
"Evidence suggests that there is a trade-off between the signalling and waterproofing functions of CHCs," says Solène Morelle, a PhD student at the University of St Andrews, UK. "This indicates that heat-induced changes in CHC profiles may alter behavioural and reproductive outcomes."
Temperature shifts caused by climate change are already pushing some wild animal species to their physiological limits, and the effects are especially impactful on ectotherms, or cold-blooded animals, such as fish, reptiles and most invertebrates. As well as physiology, heat stress is also affecting social and reproductive behaviours, but these impacts are not as well understood.
This project, presented at the Society for Experimental Biology conference in Florence, Italy, aims to reveal how heat stress is influencing same-sex mountings in cold-blooded animals, and whether this behaviour is linked to sexual recognition errors or not.
The team chose Nicrophorus vespilloides, a species of burying beetle, as their study organism for this project as they have a very elaborate system of parental care that involves burying the bodies of small vertebrates like birds and rodents underground and cooperating to feed their larvae and defend the brood against rival beetles.
"Coordinating all of that depends on effective chemical communication, so if heat stress disrupts their olfactory pathway, that could translate into reduced reproductive success," says Ms Morelle.
To assess the prevalence of male-male mountings under both control conditions (20°C) and under a simulated 3-day heatwave (26°C), Ms Morelle observed how often and for how long the beetles mounted within a set time, before extracting CHCs from their cuticles and analysing them using gas chromatography-mass spectrometry.
Preliminary results appear to confirm Ms Morelle's hypothesis that same-sex sexual behaviour increased under heat stress. "But I was surprised to find out how much same-sex mounting the beetles showed even under normal conditions," says Ms Morelle. "And what I did not expect was the increase in reciprocal mounting under heat stress – we don't yet know what this means!"
Sex recognition errors could result in energy wasted on same-sex mating, assuming that same-sex sexual behaviour is not adaptive. "However, burying beetles seem to engage in same-sex sexual behaviour even under control conditions," says Ms Morelle. "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."
As well as sex recognition, CHC-based communication can also be important for discerning a beetle's partner from an intruding rival that might be trying to take over the carcass and kill the offspring, making misidentification potentially disastrous for long-term reproductive success.
Ms Morelle and her team are still collecting and analysing the CHC data, so the role of CHCs in the same-sex interactions is still under investigation. "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," says Ms Morelle.
The next step in Ms Morelle's study is to assess whether the increase in same-sex mounting under heat stress carries any measurable reproductive costs and whether these could affect population health.
