http://www.mpg.de/english/illustrationsDocumentation/documentation/
pressReleases/2006/pressRelease20060901/
"Stress and the city": Urban birds keep cool
Ornithologists of the Max-Planck-Society demonstrate that urban birds
are more resistant to acute stress than forest dwelling birds
Animals colonizing cities are exposed to many novel and potentially
stressful situations. Chronic stress, however, can cause deleterious
effects. Hence, wild animals would suffer from city life unless they
adjusted their stress response to the conditions in a city. Jesko
Partecke, Ingrid Schwabl and Eberhard Gwinner of the Max Planck
Institute for Ornithology Andechs/Seewiesen in Germany have now shown
that European blackbirds born in a city have a lower stress response
than their forest counterparts. This reduced reactivity probably has a
genetic basis and could be the result of the urban-specific selection
pressures to which urban blackbirds are exposed (Ecology 87(8) 2006).
Many species have developed a symbiotic relationship with humans. For
example, European blackbirds, European starlings and house sparrows
thrive in concrete habitats around the globe. The sparrow is now so
closely associated with man that its original niche is unknown, whereas
the European blackbird was - as little as 200 years ago - a reclusive
forest dweller. These species seem to profit, for instance, by the
warmer microclimate and additional anthropogenic food supply in cities.
However, they are also confronted with many novel and potentially
stressful anthropogenic disturbances, such as the permanent presence of
humans, higher densities of cats and dogs, noise and light pollution as
well as traffic.
Among the physiological coping mechanisms used by vertebrates -
including humans - to ensure survival under adverse environmental
conditions is the acute stress response, characterised by the release
of glucocorticoid steroid hormones. The acute short-term secretion of
these hormones is considered beneficial in that it helps to mediate
adaptive behavioural and physiological responses. In prolonged stress
situations, however, chronically elevated levels of circulating
glucocorticoids can impair reproductive, immune, and brain functions.
Thus, wild animals would suffer from city life unless they adjusted
their stress response to the conditions in a city.
Whereas changes in the behaviour of urban birds have been frequently
documented - e.g. urban blackbirds are often tamer than their relatives
from ‘natural’ habitats - it was not previously known whether the
physiological stress response also changes to suit the urban life
style. If so, then it should be tested whether this adjustment is based
on the flexibility of individuals experiencing different environmental
conditions or is the result of micro-evolutionary adaptations that have
evolved during the urbanisation process.
Scientists at the Max Planck Institute for Ornithology
Andechs/Seewiesen in Germany, originally under the leadership of the
late Max-Planck Director Dr. Eberhard Gwinner, answered these questions
in a recent experiment. They hand-raised European blackbird nestlings
collected from the centre of Munich, Germany and a nearby forest area
(beeline distance 40 kilometres) and kept both groups in the same bird
room for one year. These two groups therefore lived under exactly the
same controlled environmental conditions, both during their
developmental phase and later during the experiment.
During their first autumn, winter and spring, Jesko Partecke applied a
standardised capture and handling stressor protocol to all urban and
forest blackbirds and simultaneously collected blood samples, to
determine the concentration of corticosterone, the stress hormone in
birds. Under normal conditions, i.e. undisturbed, urban and forest
blackbirds did not differ in their corticosterone secretion. In
addition both groups showed a similar acute hormonal stress response
during their first fall. This, however, changed considerably during
their first winter and spring: Then the stress response of urban
blackbirds was distinctly blunted in comparison to their forest
counterparts.
‘These results show, for the first time, that city life changes
physiological coping mechanisms in wild animals, which are necessary
for survival’, said Jesko Partecke. Such a reduced hormonal stress
response could be ubiquitous and, presumably, necessary for all animals
that thrive in ecosystems exposed to frequent anthropogenic
disturbances such as those in urban areas. The scientists suggest that
the difference in the hormonal stress response between urban and forest
blackbirds is genetically determined and probably the result of the
extreme selective forces in the city; as a result, those individuals
that get along better with the ‘urban stressors’ enhance their
prospects of survival in a city.
Why the two groups of birds did not differ in their stress response
during their first autumn is still an unanswered question. ‘One
explanation is that the reduced sensitivity of urban blackbirds to
stressors occurs later in their life’, speculated Partecke. Data
obtained from free-ranging birds in their native urban and forest
habitats are also inevitably necessary to verify the observed pattern
under natural conditions. In addition, future studies are needed to
assess the possible adaptive value of an attenuated stress response in
urban habitats, by relating survival and fitness data to the trait
variation.
Original work:
Jesko Partecke, Ingrid Schwabl and Eberhard Gwinner
Stress and the city: Urbanization and its effects on the stress
physiology in European blackbirds Ecology, 87(8), 1945-1952 (2006)
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