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Our research topics

Charles Darwin was puzzled about the naivety of a marine iguana (Amblyrhynchus cristatus), which he repeatedly tossed into the sea and which did return to shore again and again, next to where Darwin was standing and watching the animal. He knew that marine iguanas can swim perfectly [watch Video], and thus he expected the animal to escape ["...and as often as I threw it in, it returned in the manner above described...Perhaps this singular piece of apparent stupidity may be accounted for by the circumstance, that this reptile has no enemy whatever on shore...", Charles Darwin in Journal of Researches]. The observation that animals on islands do not run away from people became legendary as "tame behavior" and has been attributed to the lack of terrestrial predators. Unfortunately, times changed. Domestic cats and dogs had been introduced on some of the islands in the archipelago, which grew wild and built feral populations of novel predators, killing numerous native animals. As potential additional disturbance, over 100.000 tourists annually flock the colonies of marine iguanas. When a predator or human being approaches an animal, it usually flees as soon as the threat gets too close. This flight distance is very short in island animals.

The perception of an imminent threat causes an immediate physiological stress response in animals at the continent, i.e. those, which always experienced predators during their phylogenetic development. The heart beat accelerates, the blood pressure increases. If the threat is not over quickly, or if the predator continues its pursuit, additional stress hormones (corticoids) are secreted. which persist in the body for a longer time. Stress hormones regulate the mobilization of energy and prepare the body for increasing physical demands.

Animals native to Galápagos, which do not flee or do flee too late from approaching humans or dogs, don't seem to correspond with this general pattern of anti-predator behavior, as outlined above. Therefore, we ask whether these island animals demonstrate reduced physiological stress responses and to what degree they can regain the ability to respond adequately to the novel threat of introduced cats and dogs.

As part of our project we quantify flight initiation distances, i.e. the distance at which animals start to run away from an approaching human being. We also measure the physiological parameters of a stress response and compare sites at which animals are prone to predation by cats and dots with sites at which none of these predators are present. In February 2005, one stretch of coast at the island of San Cristobal has become tragically known for dog aggression which virtually wiped out a population of marine iguanas (see photos). We took blood samples for hormone analysis from a certain area, where dead bodies and live marine iguanas lie alternatingly on the coastal rocks, and predation threat could not have been more imminent.

Stress hormones, when elevated for a continued time period, can suppress the immune system. This relationship is another topic in of our studies, where we try to find answers to questions from basic research as well as applied conservation biology.

Causes and consequences of tame behavior in marine iguanas

tame iguanas at the beachGalápagos is famous for the "tameness" of its wildlife what is generally attributed to the lack of large terrestrial predators.  Choosing the marine iguana (Amblyrhynchus cristatus) we want to understand the relationship between the perception of a potential threat, the activation of a HPA stress response, and the initiation of or, the lack of flight behavior. When animals demonstrate low wariness, is human approach not perceived as a potential threat, i.e. a physiological stress response is not mounted? Or, alternatively, is an increase in stress hormones not translated into a flight response? A comparative approach serves to reveal differences between populations with different impact by tourism and/or introduced predators. In addition, an experimental approach will more details about the underlying mechanisms that are responsible for such differences. This project shall also contribute to applied conservation biology by offering a better understanding of the impact that tourism and introduced predators might have on the fitness of Galápagos marine iguanas.
First results can be seen in publications listed below.

Stress and fitness in marine iguanas

big male on its territoryThe stimulation of the HPA-axis during potentially stressful situations has evolved to bring the animal into a physiological state of emergency that is helping to cope with the situation and to increase survival. The marine iguana is a good model to investigate the relationship between the stress response and potential fitness benefits, because individuals vary largely in body size, body condition, and social status. A study of our collaborators Prof. Michael Romero, Tufts University, MA, and Prof. Martin Wikelski, Princeton University, investigates fitness differences between individuals that can be related to differences in the functionality of their HPA-axis.

Reproduction, stress hormones and the immune system of marine iguanas

a group of bachelorsBaseline and stress-induced elevations of the stress-hormone corticosterone are expected to influence the animals' responsiveness to immune challenges. We are testing these relationships in free-ranging marine iguanas applying harmless experimental challenges with a plant antigen and using blood cell counts to determine hematological indices. We are especially interested in factors explaining individual differences in the immune response due to social status, body condition, sex, and age.

In a comparison between males with different reproductive strategy (territorial, satellite, non-reproducing bachelor) we are investigating the interactions between reproductive steroid hormones, stress hormones, and the immune system. Marine iguanas are especially suitable for this investigtion because the different reproductive strategies allow predictions about hormone concentrations and body condition. We can make use of a natural experiment to investigate questions on the relationships between the different physiological parameters.
First results can be seen in publications listed below.

Collaborations and acknowledgments

Our project enjoys lots of support and cooperations. This study would not have been possible without the collaboration with  Prof. Martin Wikelski, Princeton University, NJ, and Prof. Michael Romero, Tufts University, MA. We also benefit essentially from scientific cooperations with Dr. Lynn B. Martin, Princeton University, NJ, Prof. Elisabeth Kalko, University of Ulm, Germany, Dr. Marylin Cruz, and Dr. Virna Cedeno, both at the Galápagos Genetics Epidemiology and Pathology Laboratory, Prof. Gabriele Gentile, Tor Vergata University Rome, Italy, Dr. Francesco Origgi, University of Milano, Italy, Prof. Mark Mitchell and Dr. Javier Nevarez, Louisiana State University, LA. The success of our project also depended and depends greatly on the helpful support from the staff at the Charles Darwin Research Station, most of all Patricia Robayo, Ximena Naranja, Susana Cardenas, Monika Andrate, and Don Ramos, as well as from the staff at the National Park Service, most of all Victor Carrion, René Valle, Washington Tapia, Dani Rueda and the staff at the San Cristobal office. We are very grateful to everybody who helped in the field, Ricardo Avellan, Louis Carron, Andrea Coloma, Veit Eitner, Paola Espinoza, Christian Martinéz, Martina Wagner, and Andrea Wittenzellner. To study wildlife responses towards dogs we cooperated with Shelley Thomas, Dr. Godfrey Merlen who work with WildAid, and Sarah Darling at Puerto Ayora. Many thanks to all who helped and contributed to our studies!

Scientific publications and conference contributions:

Public outreach and reports in the media:

(i) print media

(ii) radio & television
(iii) online media (selected links)

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