The presence of a predator, in captivity or in the wild, triggers a fight or flight reaction in most animals. As an animal's stress levels rise, so do the levels of stress hormones, or corticosteroids, circulating in the blood. This physiological stress response is caused by an interaction between the brain and the adrenal glands known as the hypothalamo-pituitary-adrenal, or HPA axis. Most animals' behavioural responses to predators are also influenced by their past experiences. If animals are not faced with predators these natural responses might be reduced or even lost during evolution and the animals will appear tame. The central question that Thomas Rödl and co-workers wanted to answer is: when animals' physiological and behavioural responses towards a possible predator have been reduced or even lost over the course of time, are they capable of re-activating them?
An isolated island is the perfect location to look for a predator-free habitat, except that not many islands are totally isolated any more. The researchers set out to investigate the Galápagos marine iguana(Amblyrhynchus cristatus), because this animal lived without any land predators for millions of years until dogs and cats were introduced onto the islands a century ago. The authors picked out three different study areas in which the marine iguana lives: one with no predators, one with infrequent predators and finally one near a town with high predation pressure, mostly from dogs.
At each study site, the authors either left the animals undisturbed,restrained them, or chased them and then re-captured them 2–3 weeks later. To assess the animals' stress response the team approached them at a walking speed of 0.5 m s–1 and measured the flight initiation distance, which is how close they could get to the iguanas before they started to move away. This distance was 4 m in animals used to predators compared to only 1 m for the `predator-free' animals. When the team recaptured iguanas from all three sites, however, the flight initiation distance increased in all the animals, showing that a once-in-a-lifetime capture was enough to change their behaviour significantly.
To further categorise the iguana's stress levels, the team also measured the blood plasma level of corticosteroids. The major finding of the experiment was that isolated iguanas from the predator-free site did not produce stress hormones when chased, while their relatives living in areas with a high predation pressure did. Restraining the iguanas caused a huge increase in corticosteroids in all groups, indicating that their ability to produce corticosteroids is intact even if they live on a predator-free site. This suggests that the predator-induced activation of the HPA axis is dormant in animals isolated from predators but can quickly reactivate when predation resumes. Even isolated animals from the predator-free site started to produce corticosteroids after a few re-captures, showing that they are fast learners.
Unfortunately, the changes observed in the stress responses are most likely not enough to actually escape the newly introduced threats, such as dogs or scientists. For this reason, the Galápagos marine iguana has nearly been driven to extinction by dogs in some areas of the Galápagos Islands. The authors speculate that the iguanas' potential for behavioural change is simply not big enough to deal with a mammalian predator, even though they found the physiological stress response to novel predators to be highly flexible.