Predators affect the metabolic rate of prey

Posted on September 24, 2012

Metabolic processes are responsible for all biological activities, but what determines their rate?  The connection between respiration rate (R ) and the mass of an animal’s body (M) is typically expressed by the equation R = aMb, where a is a scaling constant and b is an exponent describing the slope of the respiration – mass  relationship on a log scale.  It’s commonly assumed that the exponent tends to be close to 3/4 due to physical constraints such as the surface area : volume ratio.  However, it’s now clear that b can vary widely between about 2/3 to 1, and close compliance with 3/4 scaling seems to be the exception rather than the rule.  Recent studies show that b varies with habitat or temperature conditions, even among populations of the same species, but very little is known about how ecology affects metabolic scaling.  Using a freshwater crustacean (the amphipod Gammarus minus) as a model species, the metabolism – mass relationships of animals from five small springs in Pennsylvania, USA  were compared.  Data on resting metabolism were obtained by measuring the respiration rates of 526 individual amphipods held in glass syringes filled with spring water and fitted with oxygen electrodes.  The slopes of the respiration – mass relationship varied between populations, being significantly lower (b = 0.54-0.62) for amphipods living in springs that contained predatory fish (the sculpin Cottus cognatus) than those from springs where sculpin were absent (b = 0.76-0.77).  Field experiments showed that amphipod growth, like metabolism, varied with the presence or absence of predators.  It could be that the metabolic demand of young amphipods was relatively high to ensure fast growth and early reproduction, while the metabolic needs of larger, mature amphipods were lower as a result of selective pressure for reduced rates of swimming, foraging, growth, and thus lower conspicuousness to predators.  The results suggest that metabolic scaling isn’t decided simply by internal physical constraints but is much more likely to be the result of interactions between physiology and ecology.

Reference:  Glazier, D.S. et al.  2011.  Ecological effects on metabolic scaling: amphipod responses to fish predators in freshwater springs.  Ecological Monographs 81(4), 599–618.