Making do with less: The effects of chronic predation stress on immune function
Exposure to predators shifts many animals into an emergency life-history stage. The resulting behavioural and physiological changes increase the likelihood that an individual will be able to evade a predator. On the other hand, long-term (i.e. chronic) exposure to predators is thought to induce a ‘chronic stress’ state that is maladaptive. However, another perspective suggests that the changes that occur during chronic exposure to predators allow the animal to optimize function (and fitness) given sub-optimal conditions. For example, the effect of chronic predator stress on disease resistance may reflect physiological trade-offs required to maintain an animal’s flight-or-fight capacity. We examined the effects of chronic predator (i.e. praying mantid) exposure on reproductive output, anti-predator behaviour, and immune function in the cricket, Gryllus texensis. Crickets were housed in one of 3 conditions: with a praying mantid (Mantis religiosa), with a stick insect (Carausius morosus)(non-predator) or without another species. Measurements were made before and after each 3-week trial.
Preliminary results show that crickets housed with a mantid laid fewer eggs compared with controls and spent more time in shelter in an anti-predator behavioural trial. Total phenoloxidase, an enzyme important for immune function, did not differ in activity across groups. Induction of phenoloxidase activity, however, was greater in the mantid-exposed group. Glutathione levels were also lower in mantid-exposed crickets than in controls. The data from this study will be combined with data previously collected in this laboratory and incorporated into a physiological network model. The model will be used to examine whether physiological changes leading to a reallocation of resources to flight-or-fight behaviours correlate with those that reduce the negative effects of this reallocation on immune function. Instead of being ‘maladaptive’, changes that occur during chronic exposure to predators may reflect a physiological system optimized for sub-optimal conditions.