COS 67-5
The energetic cost of foraging explains growth anomalies in tadpoles exposed to predators
Theoretical models predict that predator-induced defences should be costly. Tadpoles respond to invertebrate predators by reducing activity and changing their body proportions. While some studies have reported that exposure to predators leads to lower growth rates, others have found no effect. The aim of this study was to measure the effects of predator cues on energy expenditure in tadpoles of the Arabian toad (Bufo arabicus). Tadpoles were exposed to caged Anax imperator larvae or predator-free conditions for 20 days, after which the respiration rate of fed and starved individuals were measured. In a second set of experiments swimming activity and respiration rate were measured simultaneously. Energy expenditure was partitioned into basal metabolism, specific dynamic action, and the costs of foraging or movement.
Results/Conclusions
In agreement with previous studies predator-exposure lowered the overall metabolic rate by 19%. Specific dynamic action (the cost of digestion) increased resting metabolism by 11%. Control tadpoles moved significantly more (93.6±3.9 cm/min) than predator-exposed animals (50.1±7.5 cm/min), and there was a positive correlation between the distance moved and metabolic rate. The metabolic rate of individuals who moved the furthest was 400% higher than stationary tadpoles, indicating that activity can be energetically expensive and can consume as much as 37% of assimilated energy. I could not detect an effect of predator-exposure on basal metabolism. These finding suggest that energy savings from reduced activity may partially offset the cost of lost foraging opportunities and explain contradictory results of previous growth studies.