Proximate costs of fighting in male cichlid fish: The role of injuries and energy metabolism

Abstract

While much is known about the functional significance of strategic decisions in animal fighting, relatively little is understood about the mechanisms that underlie the making of those decisions. In mechanistic terms, strategic decisions, such as either escalating a fight or giving up, are made in relation to the proximate costs that opponents inflict (or can potentially inflict) upon one another. These costs include physical injury and also the physiological consequences of engaging in an energetically demanding activity. We studied the role of injury and energy metabolism during fights between male cichlid fish, Tilapia zillii. In relation to injuries incurred during fights, scale loss differed depending on whether the winner was smaller or larger than its opponent; smaller winners inflicted significantly more damage on their opponents than they received, whereas this difference was not apparent in those fights won by the larger fish. In relation to energy metabolism, escalated fighting resulted in a significant depletion of total sugar reserves in the muscle and the liver. It appears that the muscle energy reserves are respired anaerobically, as was evident from the accumulation of lactate in the muscle. Losers had significantly higher levels of muscle lactate than winners. Together, the injury data and the metabolic data suggest that escalated fighting is costly for both winners and losers, but especially so for losers. These data are discussed in relation to models of animal decision making and we conclude that the difference between opponents in the proximate costs incurred during fighting is likely to underlie the making of decisions such as continuing, giving up or escalating the fight.