• Causes and consequences of low wariness
• Stress response and fitness
• Stress hormones and the immune system
• Birds and mistletoes
• Latitudinal differences in testosterone concentrations
  
  

Galápagos is famous for the “tameness” of its wildlife what is generally attributed to the lack of large terrestrial predators. Choosing the marine iguana (Amblyrhyhnchus 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. An experimental approach will reveal 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.

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. We are trying to characterize fitness differences between individuals that can be related to differences in the functionality of their HPA-axis.

in collaboration with Prof. Michael Romero, Tufts
University, and Silke Berger, University Ulm, Germany

Stress hormones and the immune system in Marine Iguanas

Baseline and stress-induced elevations of the "stress-hormone" corticosterone are expected to influence the animals' responsiveness to experimental immune challenges. We are testing these relationships in free-ranging marine iguanas applying PHA and KLH challenges and using blood cell counts to demonstrate immune cell trafficking and 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 collaboration with Silke Berger, University Ulm, Germany; Lynn B. Martin, Princeton University; Prof. Mark Mitchell, Louisiana State University

in collaboration with Prof. D. Ward, Stellenbosch University, Matieland, S-Africa; Adam Green,
University of Wisconsin, Madison; Prof. A. Golan-Goldhirsch, Dr. O. Barazani & Dr. S. Volis all Ben Gurion University, Israel.

This is a multidisciplinary study on a system composed of a desert mistletoe (Plicosephalus acaciae), its main bird vector, the Yellow-vented bulbul (Pycnonotus xanthopygos) and human settlement activity in the Arava valley between Israel & Jordan, which favors bulbul populations. We are characterizing the mechanisms of fruit dispersal in this system to explain the patterns of distribution of the plant through the birds' behavior as a function of human settlement activities. Past work consisted of behavioral studies on bird activity ranges, of physiological experiments on the birds’ gut passage time as well as the mistletoe establishment capacity, of analysis of movement data for the determination of a seed shadow, and of spatially explicit geographical data on the species' distribution. We want to add information on the genetic distances between populations.

Temperate zone bird species usually face a rigid annual cycle that requires tight timing of reproductive events. In addition, many birds in the temperate zones are migratory and face strong competition for territories and sexual partners during the relatively short breeding season. Therefore, it is generally assumed that temperate zone birds demonstrate higher testosterone concentrations during the reproductive period than tropical species. We are testing this prediction in captive European Stonechats (Saxicola torquata ssp.) from different populations of tropical and temperate origin, held in a common garden situation, using a recently established non-invasive method of quantification of fecal steroid metabolite concentrations.

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Selected Publications
Gwinner, E., T. Rödl, H. Schwabl, 1994. Pair territoriality of wintering stonechats: behaviour, function and
hormones. Beh. Ecol. Sociobiol. 34: 321-327.

Rödl, T. 1994. The wintering of territorial Stonechat pairs Saxicola torquata in Israel. J. Ornithol. 136: 423-433.

Rödl, T. & H. Flinks, 1996. Nutrition of Stonechats (Saxicola torquata) and Mourning Wheatears (Oenanthe
lugens) wintering sympatrically in Israel. Ecology of Birds 18:107-126.

Rödl, T. 1999. Environmental factors determine numbers of overwintering European Stonechats Saxicola rubicola
- a long term study. Ardea 87(2): 247-259.

Rödl, T. & D. Ward (2002). Host recognition in a desert mistletoe: early stages of development are influenced by
substrate and host origin. Funct. Ecol. 16(1): 128-134.

Rödl, T. & E. Gwinner (subm.) State-dependent territorial aggression in wintering European Stonechats (Saxicola
torquata). Behav. Ecol. Sociobiol.

Räß, M., Rödl, T. Van’t Hof, T. & Gwinner, E. (subm.) The function and control of song in overwintering stonechats.

Published abstracts
Rödl, T. 1997. Relationship between status and aggression in wintering stonechats, Saxicola torquata, in Israel.
in: M. & B. Taborsky (eds.). Advances in Ethology 32, suppl. to Ethology: p.147. Proc. of the International
Ethological Conference, Vienna.

Räß, M., T. Rödl, V. Canoine & T. Van’t Hof 1998. Is singing in wintering Common Stonechats Saxicola torquata
associated with territory density? In: Adams, N.J. & R.H. Slotow (eds) Proc. 22 Int. Ornithol. Congr., Durban:
BirdLife South Africa.

Rödl, T. & Gwinner, E. 2002. "Assessing the likelihood of cross-seasonal pair associations in a wintering
migrant", Smithonian Symposium "Birds of Two Worlds", Sheperdstown, WV.

Popularization of my work
Max-Planck-Research 2/2002: 50-53. Christina Beck: “Stonechats in Dummy Test“.

Süddeutsche Zeitung [major German newspaper] 10. Juli 2002: R5. Bernhard Granier: “Birds have no jetlag“.

National Geographic Germany Oct. 2002: 16. Margit Enders: “Platonic love guarantees survival“.

Max-Planck-Research 4/2003: Christina Beck: ... (in press).

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