How do animals decide when to fight and when to walk, fly, slither, or swim away? Most research on animal conflict has focused on the short-term costs of single interactions, but a pair of behavioral ecologists argue that these one-time events might paint an incomplete picture. In an opinion paper publishing July 16 in the Cell Press journal Trends in Ecology & Evolution, the researchers say that to really understand the consequences of animal conflict, we need to also consider its long-term and cumulative impact on an individual's longevity and reproduction.
"By linking individual contests to lifetime reproductive success, we can understand how different contexts and environmental situations could favor the evolution of decision strategies in different species," says author and ecologist Paulo Enrique Cardoso Peixoto of Universidade Federal de Minas Gerais in Brazil.
Contests are ubiquitous across the animal kingdom, note the authors. In many cases, these fights are symbolic and may or may not involve physical contact, but they can deplete energy reserves, and in some cases result in physical injury and death. Usually, animals engage in contests when competing for a resource such as food, shelter, territory, or mating partners. Since contests have the potential to result in both benefits and costs, scientists have long thought that different species must have evolved strategies for deciding whether to fight, and if they do fight, when to quit.
The researchers say that the optimal conflict strategy would vary depending on the species and situation. For example, the potential benefits might outweigh the risks for very valuable resources, but for less important resources, individuals might be better off abstaining, especially if they are unlikely to win. They also note that some costs carry more weight than others.
"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, and since they often fight for female access, this means they will be unable to reproduce anymore."
To examine how costs have been characterized and quantified previously, the researchers conducted a systematic review of field and laboratory studies. From 73 articles spanning 62 animal species, they identified 24 different types of cost, which they grouped into six 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.
They found that researchers studying different types of animal tend to measure different types of cost, which makes it difficult to compare findings. For example, studies in crustaceans and fishes were more likely to measure metabolic costs, whereas studies in insects usually measured direct injuries.
"There is huge variation in the measurements researchers take," says Peixoto. "Variation is not a bad thing, but the lack of standardization 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."
In many cases, the researchers argue, the costs measured are not the most relevant for understanding the consequences of contests. They also rarely extend beyond measuring the short-term consequences of 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 favor individuals that always fight and are more aggressive, and other situations that favor more cautious individuals that only fight weaker rivals to increase their chances of winning?"
To link short- and long-term costs, the researchers propose a three-step process, starting with identifying the most important cost for the species they're studying. The next step is measuring how this cost accumulates during a single contest, relative to the animal's baseline. Finally, the team recommends linking this single-contest data to long-term data on how frequently different individuals fight, and how many offspring frequent and non-frequent fighters produce.
"By knowing the average number of fights that different individuals are involved in and their lifespans, we can estimate whether individuals who fight more or less have better lifetime reproductive success," says Peixoto. "This connection would allow us to gain deeper insights into the evolutionary dynamics of animal contests and the trade-offs individuals face."
