Energetic consequences of antipredator behaviour in small mammal prey

A project undertaken at The Hawkesbury Institute for the Environment, Western Sydney University, and supervised by Dr Christopher Turbill

This study investigated the behavioural and energetic responses of small mammals to variation in perceived risk of predation. Predation causes direct mortalities but also has important indirect effects on the behaviour, morphology, physiology and life-history of prey species. It has been found that non-lethal effects of predation risk can be even more important than direct mortalities in causing long-term impacts on prey population dynamics. Reductions to food energy intake and changes in energy allocation are central mechanisms of the non-lethal predation risk effects. Experimental studies are required to determine how small mammals adjust their energy budgets in response to perceived predation risk.

Foraging effort must be sufficient to balance total energy requirements, which comprise both active and resting components. At a minimum, rate of food intake is determined by resting energy expenditure and energy costs of foraging activity. For small mammals, metabolic heat production makes up a large proportion of resting energy expenditure. The energetic cost of keeping warm can be greatly reduced, however, by a physiological capacity to employ a temporary reduction in body temperature while resting. Torpor is often used in response to reductions in food intake. Joining the dots, we hypothesised that energy savings from torpor might provide a mechanism allowing small mammals to mitigate the energy deficit caused by a reduction in foraging activity in response to predation risk.

To test this hypothesis, we conducted a series of experiments to measure the effect of perceived predation risk on the foraging effort and thermoregulatory energy expenditure of   small mammals (dunnarts Sminthopsis crassicaudata, and wild-caught house mice Mus musculus domesiticus). We manipulated levels of ground cover as a reliable proxy for perceived predation risk and measured the within-individual responses of animals foraging in semi-outdoor enclosures. Prior to the foraging experiments, we characterised a number of behavioural and metabolic traits using standard laboratory tests. We tested whether these measurements could explain variation among individuals in their response to levels of ground cover.

Our experiments quantified the interacting effects of predation risk and starvation on foraging effort, and its link with thermoregulatory metabolism during resting. As expected, foraging effort and hence food intake decreased under low levels of ground cover. A higher perceived risk of predation (i.e. low cover) was also associated with a lower body temperature during resting. The extent of reduction in body temperature during resting was correlated with the reduction in food intake during activity. A low intrinsic energy state caused mice to increase their foraging effort and food intake, but this increase was curtailed under low cover levels (high perceived predation risk). Again, reductions in foraging effort were compensated by reduced body temperature and hence metabolic energy expenditure during resting.

This study provided experimental results to demonstrate the linkages among predation risk, foraging behaviour and thermoregulatory physiology and energy expenditure in small mammals. In support of our hypothesis, we found that energy savings from reductions in thermoregulatory energy costs during resting can facilitate reductions in foraging activity and hence exposure to the mortality risks from predation. By enhancing resistance to starvation, torpor also reduces the risk of predation. Moreover, torpor use during resting provides a mechanism to compensate for reductions in food energy intake because of anti-predator behaviours. Energy savings during resting, such as by using torpor, allows a limited energy budget to be allocated to non-resting functions, which should moderate the non-lethal impacts of predation risk on population dynamics of prey. A low energetic state, not surprisingly, resulted in greater foraging activity. Our experimental design showed how this effect is moderated under conditions of high perceived predation risk. Again, reductions in resting metabolism (and hence body temperature) were linked with the reduction in foraging effort under low cover. Our study has provided experimental evidence that thermoregulatory energetics have important effects on the response of small mammals to the interacting challenges of starvation and predation risk.

Figure 1. The study measured the activity, body temperature and daily energy budgets of small mammals foraging under different levels of ground cover in semi-outdoor enclosures.

 


Figure 2. Mice foraged for seeds in a tray of sand. Daily seed consumption, and hence foraging effort, was reduced under conditions of low ground cover (see Fig. 1), which simulated high predation risk.

Figure 3. Mice compensated energetically for a decrease in foraging effort when exposed to low ground cover (high perceived predation risk) by reducing their thermoregulatory energy expenditure, as indicated by temporary reductions in body temperature during their rest phase (blue and red symbols). Black bar represents the night phase. ‘Ta’ (green line) indicates air temperature.