— Even a human with a dog leaves this Galapagos Islands iguana unflustered (Image: Thomas Rödl)
Up close and personal – the iguanas have ruled the 'roost' for so long, they have forgotten what it is to fear (Image: Thomas Rödl)
In the absence of predators, marine iguanas on the Galapagos Islands have evolved an excessive tameness. It has left them with a blunted physiological response to what should be a stressful situation, a new study shows.
Although these iguanas can move quickly under certain circumstances, they fail to ramp up their speed to escape danger. This inadequate response could mean that certain conservation measures are ultimately doomed, say researchers.
On some Galapagos Islands, the marine iguana (Amblyrhynchus cristatus) has no predators beyond hawks, which attack them infrequently. Scientists believe this is the reason the iguanas remain unwary of possible threats.
Thomas Rödl of Princeton University in New Jersey, US, and colleagues experimented by seeing how near they were able to approach the animals before the reptiles took fright. Many times, the iguanas waited until the researchers were only 2 metres away from them before they started slowly moving away.
The scientists continued trailing the iguanas for 15 minutes in a low-speed pursuit, at the end of which they captured the animals and took blood samples.
Facing the sack
The blood samples showed that levels of a stress hormone called corticosterone had remained low throughout the slow pursuit.
Theyre clearly not interpreting this as a stressful situation, says Michael Romero of Tufts University in Medford, Massachusetts, US, who helped conduct the study.
The iguanas were next placed in sacks for a brief period of time, after which their stress hormone levels had quadrupled. The next time researchers pursued the same iguanas, the animals stress hormones quadrupled from baseline levels during the low-speed chase itself.
Although the boost in stress hormone suggests that the iguanas learned to recognise humans as a threat, the animals did not dramatically change their behaviour. They maintained slightly more distance from the scientists and moved away slightly faster than before but not fast enough to get away. The animals are actually capable of significant speeds of up to 10 kilometres per hour.
We do know that they can learn what a predator is but they cannot learn how to mount a flight response, Romero explains. He adds that it remains unclear why the animals do not flee appropriately.
The findings suggest why certain conservation practices such as reintroducing a tame species into an area with an aggressive predator may not work. Some animals might simply never learn to escape this type of danger. Theres a good chance that they may not be able to adapt to it, says Romero.
Journal reference: Proceedings of the Royal Society B (DOI: 10.1098/rspb.2006.3755)