To flee, or not to flee? The answer to that question may lie in physiology. Whether an animal adopts an active or passive defense strategy when faced with danger is related to its behavioral phenotype, or ‘personality’. For example, bold individuals typically behave actively and will fight off or flee from predators, whereas shy individuals try to avoid detection in the first place by remaining perfectly still. The fact that such divergent defense strategies persist within populations suggests that both confer a fitness advantage, but what are the physiological traits driving these personalities that selection acts upon? Given that active and passive defense strategies require markedly different energetic costs, Weiqun Lu and his student Emmanuel Rupia from Shanghai Ocean University, China, teamed up with visiting scientists Sandra Binning and Dominique Roche from University of Neuchâtel, Switzerland, to test whether different metabolic profiles underscore bold and shy behavioral phenotypes.
The team began by rigorously testing the behavior of olive flounders to figure out which individuals were bold (e.g. quickly investigated a novel food object and repeatedly tried to escape from a net), and which were shy (e.g. didn't swim away when touched and were slow to respond to food). Next, the team measured oxygen consumption rates during a series of tests to determine each fish's metabolic profile. First, they continuously chased and air-exposed a fish for several minutes and then measured the exhausted fish's oxygen consumption to determine its maximum metabolic rate (MR). The scientists then left the fish undisturbed for 24 h while measuring their oxygen consumption to find the animal's lowest metabolic rate while at rest (standard MR). The difference between maximum and standard MR is known as the aerobic scope (capacity for oxygen-fueled metabolism) and individuals with higher aerobic scopes can have more active lifestyles. At the end of the 24 h rest period, one member of the team startled the fish by waving their hand over the tank to determine the fish's MR during acute stress.
The team found that bold and shy flounders have very different metabolic profiles. Compared with bold fish, shy individuals had lower standard and maximum MRs as well as lower aerobic scopes, which is consistent with a passive and less energetically expensive behavioral phenotype. The fish's metabolic responses to an acute stress were also reversed; bold fish quickly increased their MR in response to the unexpected hand movement and this spike in oxygen consumption would help them to fight off or swim away from the supposed threat. Conversely, shy fish rapidly decreased their MR as the hand waved threateningly above them; this drop in oxygen consumption would enable them to remain perfectly still and undetected until the danger had passed. The strong correlations between metabolic profiles, stress-induced changes in MR, and behavioral phenotype are a clear indication that physiology can dictate personality.