Lithobates catesbeianus. Credit: Randy Dzenkiw (CC BY-SA)
Fighting is a common occurrence in the animal kingdom. Lions chase down zebras and elephant seals wrestle over mates, but how do animals decide when to fight?
Research has traditionally focused on the short-term costs of animal conflict, but behavioural ecologists are now calling for a shift to the long-term impacts.
“This connection would allow us to gain deeper insights into the evolutionary dynamics of animal contests and the trade-offs individuals face,” says Professor Paulo Enrique Cardoso Peixoto, an ecologist at Universidade Federal de Minas Gerais in Brazil and co-author of an opinion paper published in Trends in Ecology and Evolution.
According to the theory of natural selection, survival of the fittest is not just about brute strength but also an animal’s overall ability to raise offspring successfully and reproduce.
The authors argue that this element of reproduction is often overlooked in animal conflict research.
“If snapping shrimp lose a claw during a fight, it can regrow, so it’s not a total loss because they can recover and fight subsequent contests,” says Peixoto.
“But if a beetle breaks a horn during a fight, it will not regrow. So that individual will be unable to fight again. Since they often fight for female access this means they will be unable to reproduce.”
To get a better understanding of how the field has been researched, the authors conducted a systematic review of 73 articles focussing on 62 animal species. They identified 24 different costs and characterised these into 6 categories: increased metabolism, increased stress and decreased immune response, increased risk of injury and mortality, decreased foraging opportunities, decreased predator awareness and decreased investment in parental care.
The authors found it difficult to compare findings across the papers because there were so many ways the cost of conflict has been measured.
“There is huge variation in the measurements researchers take,” says Peixoto.
For example, while research into insects usually measures direct injuries, studies in fish and crustacean tend to measure metabolic costs.
“Variation is not a bad thing, but the lack of standardisation in the way we do this precludes us from estimating whether there is an average cost between different species or investigating the variation among species.”
Philaeus chrysops. Credit: Anatoly Ozernoy (CC BY-SA)
The authors also argue that the costs measured are not often the most relevant and centre on isolated incidents.
“We need to link the average cost in a single contest to the individual’s longevity or lifetime reproductive success,” says Peixoto.
“For example, are there contexts that favour individuals that always fight and are more aggressive, and other situations that favour more cautious individuals that only fight weaker rivals to increase their chances of winning?”
To resolve this gap in the literature, the authors suggest a 3-step process.
Future researchers looking into animal conflict should start by identifying the most important cost for the species in focus.
Then, the researchers should measure how this cost accumulates during a single contest, relative to the animal’s baseline.
The last step is to link this data to how frequently individuals fight and then compare how many offspring frequent and non-frequent fighters produce.
“By linking individual contests to lifetime reproductive success, we can understand how different contexts and environmental situations could favour the evolution of decision strategies in different species,” says Peixoto.
