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Too Mellow For Our Predatory World: Flight Behavior Of Marine Iguanas

Science Daily — Marine iguanas on the Galapagos Islands live without predators - at least this was the case up until 150 years ago. Since then they have been confronted with cats and dogs on some islands of the Archipelago. For scientists, they are therefore a suitable model of study in order to discover if such generally tame animals are capable of adapting their behavior and endocrine stress response to novel predation threats.

Scientists from the Max Planck Institute for Ornithology, the University of Ulm Tufts University and Princeton University were able to show that the stress response induced by corticosterone (CORT) is absent in predator-naïve animals but can be fully restored with experience. However, as the researchers found out, the flight distance of the reptiles does not sufficiently increase, which limits their ability to successfully escape from newly introduced predators (Proceedings of the Royal Society B, FirstCite Early Online Publishing December 2006).

Who of us has not dreamt of living on an island? Apparently, island life has certain advantages. This is also true for the marine iguanas. For millions of years they have lived without natural predators. In the course of evolution they have become excessively tame. Hundreds of reptiles doze, spread-eagled, on black lava rocks, soaking up the sun - behavior that would be unthinkable in an environment with predators, where reptiles are persistently exposed to the threat of being devoured by others.

The absence of predators may lead to adaptations in the behavior of insular animals. Over a longer evolutionary period, birds for example can lose their ability to fly. This would greatly reduce their ability to escape should new predators appear. In contrast to such "hard wired" traits, however, behavioral patterns should be significantly more flexible. In program  introducing tame species into the wild, animals are trained to recognize and cope with predators. But in most cases it is very difficult to predict the flexibility in behavior because little is known about the underlying physiological mechanisms that control behavior such as flight.

The scientist around Thomas Rödl from the Max Planck Institute for Ornithology visited the Galapagos Islands for field studies from December 2003 until January 2004 and again in March 2005, using the research platform of the Max Planck Society. The islands in the Pacific Ocean continue to be a magnet for evolutionary biologists. Nowhere else is it possible to observe such a variety of different adaptation strategies in such a relatively small number of species. However, it is not only scientists who are drawn to the island; increasingly tourists have also discovered this unique archipelago. In 2005, approx. 126,000 people descended on the island, and the trend is rising. And this causes problems. Not only do tourists disturb the many animals living on the Galapagos Islands; they also introduce alien animal and plant species which are causing great damage to local flora and fauna.

Rödl and his colleagues Silke Berger as well as Michael Romero and Martin Wikelski wanted to find out to what extent marine iguanas from various populations and with different predator-experiences would differ in terms of stress response and behavior. To this end the researchers conducted so-called "harassment experiments". At first, they recorded the original flight initiation distance of "naïve" animals. The researchers then experimentally chased the animals for 15 minutes, always approaching up to the point where the reptiles moved away and fled a short distance. At the end of the experiment, the scientists captured the animals and collected blood samples in order to determine CORT levels. When animals interpreted the situation as threatening, the concentration of corticosteroid hormones in the blood plasma increased within just a few minutes.

There were significant differences in the various island populations examined by the scientists: marine iguanas with no experience of predator-pressure tolerated a human approach up to one or two meters and failed to mount a stress response even during sustained chasing.
 

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