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
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
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
general pattern of anti-predator behavior, as outlined above.
Therefore, we ask whether
these island animals demonstrate reduced physiological stress responses
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.
consequences of tame behavior in marine iguanas
Galápagos 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
First results can be seen in publications listed below.
Stress and fitness in marine iguanas
The 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
Baseline 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
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!
publications and conference contributions:
S., Romero, L. M., Kalko, E. K. V., Wikelski, M., Rödl, T.
(2007). Behavioral and physiological adjustments to
new predation threat in an endemic island species, the Galápagos
marine iguana. Hormones
& Behavior 52: 653-663.
- Rödl, T., Berger, S., Romero, L. Wikelski,
M. (2007). Behavioural and physiological characteristics of
predator-naivety in an island iguana. 6th
International symposium on physiology,
behavior, and conservation of wildlife, Berlin, Germany.
- Rödl, T.,
Berger, S., Romero, L. M., Wikelski, M. (2007). Tameness and stress
physiology in a predator-naive island species confronted with novel
predation threat. Proceedings Royal Society London B 274: 577-582.
- Berger, S., Martin, L.B., Wikelski, M., Romero, M., Kalko,
E. K. V., Vitousek, M. & Rödl, T. (2005) Hormones and Behavior
47/4: 419-429. Corticosterone suppresses immune activity in territorial
Galápagos marine iguanas during reproduction. Hormones and
- Rödl, T., Berger, S. & Wikelski, M.
(2004). Testing the stress response to human disturbance in a "tame"
island reptile species. 5th International symposium on physiology,
behavior, and conservation of wildlife, Berlin, Germany. Advances in
Ethology 38: p.44.
- Berger, S., Rödl, T., Kalko, E. K. V. &
Wikelski, M. (2004). Influence of introduced predators on flight
behaviour and corticosterone concentrations in Galápagos marine
iguanas (Amblyrhynchus cristatus). 5th International symposium on
physiology, behavior, and conservation of
wildlife, Berlin, Germany. Advances in Ethology 38: p.22.
- Rödl, T., Berger, S., Romero, L. M. &
Wikelski, M. (2004). Tameness and stress response in Galápagos
marine iguanas. 2nd European Conference on Behavioral Biology (ECBB),
Groningen, Aug. 2004.
- Berger, S., Wikelski, M., Kalko, E. &
Rödl, R. (2004). Immune activity in male marine iguanas
(Amblyrhynchus cristatus) during reproduction. 2nd European Conference
on Behavioral Biology (ECBB), Groningen, Aug. 2004.
- Rödl, T., Berger, S. & Wikelski, M.
2004. Trouble in paradise: About stress, Galápagos iguanas, and
introduced predators. Alexander von Humboldt American Association, 5th
National meeting, Washington, DC.
- Berger, S., Martin, L. B., Rödl, T.,
Vitousek-Bemis, M. N., Romero, L. M., Wikelski, M. & Kalko, E. K.
V. 2003. Immune function, steroid hormones, and male reproductive
strategies in the Galápagos marine iguana (Amblyrhynchus
cristatus). Animal Behavior Society annual meeting, Boise, ID, USA.
- Rödl, T., Berger, S., Romero, L. M. &
Wikelski, M. 2003. "Harassment" fails to induce a stress response in
marine iguanas (Amblyrhynchus cristatus). Animal Behavior Society
annual meeting, Boise, ID, USA.
- Mitchell, M. A., Nevarez, J., Dae-Young, K.,
Rödl, R., Berger, S. & Wikelski, M. 2003. Evaluating the
long-term health effects of an unnatural oil spill on marine iguanas
(Amblyrhynchus cristatus). Association of reptilian and amphibian
veterinarians annual conference, Minneapolis, MN.
outreach and reports in the media:
- Journal of
Experimental Biology 210, p. iv: "Iguanas are too tame for their own
good" (J. B. Andersen)
- Frankfurter Allgemeine Zeitung, 18.4.2007
Feuilleton (Diemut Klärner)
- Max Planck Research 1/2007, p. 12: "Too tame for this world"
- Der Spiegel (major German periodical) No. 52,
22.12.2006, p. 109: "Too lazy to flee" (german)
- NATURE 444, 7.12.2006, p. 657, research
highlights "To flee or not to flee"
- Landshuter Zeitung
daily newspaper, Aug. 20, 2005,
p.48: “A paradise in danger” (german by Gisela Dürselen)
- Ein Herz für
Tiere [Europe’s largest animal
magazin] 9/05, pp.18-19: “Curious land iguanas” (german by Alexander
- Münchner Merkur, daily newspaper 119,
24./25.5.2003, p.4: "A tame Iguana" [german]
- Max Planck Forschung 2/2003, p.54-65: "Excursion
to the imps of darkness" (by Christina Beck).
- Rödl, T. 2003. "Tame dragons on
Galápagos" [german]. Promotion text for the company ZARGES,
sponsoring our research http://www.zarges.de/news.php?chid=97&ac2=det&1st=0
- Weilheimer Kreisbote regional newspaper,
26.6.2002. "People from the council" [german].
(ii) radio & television
- Austrian radio station Ö1 (2005):
5x5 minutes of interview on our marine iguana project
- Local Galápagos television (2004):
Feature of the organization "Wild Aid"
(iii) online media (selected links)