The research, published in the journal Animal Behaviour and led by Dr Sophie Mowles of Anglia Ruskin University (ARU), studied aggressive interactions between male wide-horned hissing cockroaches (Gromphadorhina oblongonota).
Animal contests are usually won by the larger opponent and physical fighting is often avoided if clear differences exist between competitors. However, during a series of laboratory contests, the researchers closely matched the cockroaches for size so there were no visible differences in their fighting capabilities, known as their resource holding potential.
Heavily weaponised male wide-horned hissing cockroaches use their pronotal horns as they compete for females through vigorous contests, often butting and flipping their male opponents onto their backs.
Encounters also involve ‘low aggression’ behaviour including repeated approaches towards the opponent, which may retreat or adopt a low posture to guard against being overturned. During the laboratory contests, actions reflecting these dominant and submissive behaviours were scored for each animal.
A CT scan of each cockroach was then carried out allowing the researchers to study their whole body, including the size of their respiratory system.
Crucially, they found significant differences in the respiratory volumes of the cockroaches, and these were directly associated with their fighting prowess. The dominant individuals were found to have larger respiratory volumes compared to their similarly sized submissive opponents.
Dr Sophie Mowles, Senior Lecturer in Animal and Environmental Biology at Anglia Ruskin University (ARU), said: “When studying contest behaviour it is important to consider not just the physical weaponry used by species, or the combative behaviours they employ, but also the underlying physiology that allows this energetically costly behaviour to take place.
“When visible differences are removed by size-matching opponents, fights between male cockroaches are likely to escalate into trials of strength, and our study found that some cockroaches have much larger respiratory capacities than others, allowing them to dominate these contests. The increased ability to effectively deliver oxygen to their body tissue may enhance the fighting ability of these dominant males.
“Adaptations for prolonging aerobic respiration in these cockroaches have probably evolved as a way of maximising oxygen exchange when burrowing through leaf litter, and we have shown that these adaptations also play a crucial role in physical contests between males, and therefore sexual selection.”