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Research
We seek to understand the ecological and
evolutionary processes by which temperature affects the behaviors,
physiologies, and life histories of ectothermic animals. Such an
understanding has become critical because global climate change
continues to challenge these organisms. Although biologists have
expended great effort to understand the ecological consequences of
climate change, the evolutionary consequences remain less clear.
Yet, evolutionary responses will determine the ecological
interactions within future ecosystems. A theory of thermal
adaptation could offer major advantages to both basic and applied
biologists.
Current members of the lab are focused on
three major questions:
1)
How do environmental conditions influence
strategies of thermoregulation?
2)
How do body temperatures affect the
evolution of thermal physiology?
3)
How does thermal physiology affect the
evolution of the life history?
To answer these questions, we are engaged in
mathematical modeling of evolutionary processes, spatial analyses of
thermal heterogeneity, field studies of thermoregulatory behavior,
lab studies of thermal physiology, and comparative analyses of life
histories. A brief sampling of this work is described below.
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Developing a spatially
explicit theory of thermoregulation
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Current theory suggests that the cost of thermoregulation
should drive the evolution of thermal physiology (Angilletta
et al. 2006). Yet three decades since the
development of a theory of thermoregulation (Huey
& Slatkin 1976), we still cannot predict the body
temperatures of animals in natural environments. For
example, a recent comparative analysis indicated that
lizards thermoregulate less accurately when the cost of
thermoregulation is low (Blouin-Demers
& Nadeau 2005). But optimality theory predicts
the exact opposite! The failure of current theory likely
results from models and experiments that do not account for
the spatial distribution of temperatures. Our computer
simulations revealed that the energetic cost of
thermoregulation depends on the patchiness of the
environment (Sears
& Angilletta 2008). By accounting for
spatial structure, we can quantitatively predict
the movements and temperatures of animals in real
environments.
In collaboration with the
Mike Sears' lab at Southern
Illinois University, we are developing a spatially explicit
theory of behavioral thermoregulation. The predictions
generated by computer simulations are being tested in
outdoor arenas at the
Sevilleta LTER Site in New
Mexico. These arenas enable us to manipulate the thermal
environment and bound the movements of animals. We control
the temperatures in these arenas by placing shade cloth in
specific configurations (see photos to the right). During
the past two summers, we monitored the thermoregulation of lizards in
the enclosures to test the predictions of our model. Future experiments will document the
influence of competition and predation on thermoregulation. This research
has been supported by the
National Science Foundation.
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See photos of the
arena construction.
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See photos of the
field experiment. |
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Above: An aerial photo of our experimental arenas
(400 m2 each). Patches of
shade cloth were used to create a unique thermal environment
in each arena.
Below: A male lizard (Sceloporus jarrovi)
shuttles between patches of sun and shade in one of our
enclosures.
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Understanding
physiological acclimation in stochastic environments
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When body temperatures vary over space and time, natural
selection should shape the thermal physiology of organisms.
Two distinct strategies enable an organism to deal with thermal
variation. An individual can develop the physiological
capacity to perform over a wide range of temperature
(thermal generalization). Alternatively, an individual can
repeatedly alter its cellular structure in response to
changes in temperature (thermal acclimation). Thermal
acclimation provides a superior benefit because the organism can match its thermal physiology to its current
environment. However, the selective advantage of thermal
acclimation depends on the magnitude and predictability of
thermal change within and among generations (Angilletta
2009). During the past few
years, we have evaluated models of optimal acclimation
through comparative and |
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experimental studies of geographically widespread ectotherms.
Our current research focuses on a model system: the fruit
fly (Drosophila melanogaster). We are using
ecologically relevant measures of physiological performance,
such as the rate of egg production (daily fecundity) and
survival at extreme temperatures. Our comparative studies
have revealed patterns of acclimation between populations
that contradict the predictions of theory (see plots to the
right). To follow up on these comparative studies, we are
working with selection lines of flies that evolved under
different levels of thermal heterogeneity and gene flow for
more than three years (a collaboration with Sam Yeaman of
the University of British Columbia). By comparing thermal sensitivities
and acclimation capacities among selection lines, we will
determine the degree of support for competing models of
acclimation. This project constitutes the first experimental
test of quantitative models of physiological acclimation,
which should build substantially on the insights generated
from our comparative studies.
Finally, we are
working with other researchers to
determine the molecular and genetic basis of variation in
acclimation capacity among selection lines, including the
regulation of membrane fluidity, the induction of heat-shock
proteins, and the allocation of cellular resources. These
mechanistic studies will help us to either validate or
improve our assumptions about the constraints on
physiological adaptation. |
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Above:
Performance curves of fruit flies (D. melanogaster)
acclimated during development, but the pattern did not match
the prediction made by the beneficial acclimation hypothesis
(Leroi et al. 1994). Flies were raised at two stochastic
treatments (upper left panel) and one constant treatment
(25°C, not shown); these treatments mimicked air
temperatures in Marlton, NJ, and Miami, FL (National Climate
Data Center, Stations 284229 and 83163). Although flies from
NJ were predicted to acclimate such that the thermal optimum
of performance differed between developmental environments
(upper right panel), a difference in the mean performance
was observed instead (lower right panel). The same pattern
was observed in flies from FL (lower left panel). Data are
means for 17 and 14 isofemale lines from NJ and FL,
respectively (Cooper et al., in prep.). Error bars are 95%
confidence intervals. |
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Predicting biological
responses to climate change
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Predicting
biological responses to climate change requires knowledge of
behavior, physiology, life history, genetics, ecology, and
biogeography. In short, the problem ranks among the most
complex problems that biologists could pursue. At the same
time, the unprecedented changes in climate on local,
regional and global scales demand our attention if we are to
preserve biodiversity in the coming years. Currently, we are
applying ecological and evolutionary theory to assess the
biological impacts of climate change on several scales.
Does urban warming affect the
ecology and evolution of organisms within major cities?
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We have been working with geographers
from the
Center for Urban and Environmental Change to
quantify the ecological and evolutionary impacts of urban
warming. Cities are hotter than their surroundings for many
reasons, including the greater |
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Above: Maps of Indianapolis depict the impact of
land use (left-hand map) on surface temperature (right-hand
map). These maps enabled us to randomly sample animals
inside and outside of the urban heat island (reproduced from
Weng et al. 2004). |
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radiation from surfaces, the greater
emission of heat, the thermal mass of buildings, the reduced
evapotranspiration from soil, and the unusual pattern of
convection. In fact, rates of warming within some cities
have even exceeded rates of global warming. Therefore, urban
environments serve as natural experiments for quantifying
the biological consequences of climate change. To date, we
have documented divergence in the thermal tolerances of fungi
and insects distributed along urban-rural gradients (McLean
et al. 2005;
Angilletta et al. 2007). Using a
thermal map derived from remote sensing (see photo above),
we are currently comparing the thermal tolerances of
arthropods throughout Indianapolis (Cooper
et al 2008). In the future, we hope to define the
generality of these physiological responses and create
evolutionary models tailored to urban environments.
Can mechanistic models predict
shifts in species’ ranges during global climate change?
During the past few years, we have collaborated with
Lauren Buckley of the
University of North Carolina to parameterize mechanistic
models of range shifts during climate change (Ehrenberger
et al. 2008). Mechanistic models—based on
behavior, physiology, and life history—offer ecologists a
means to overcome the limitations of models based on
climate envelopes. With
Buckley and other researchers, we are determining the
accuracy and generality of competing mechanistic models.
This work builds on the lab's studies of physiological
diversity in the eastern fence lizard (Sceloporus
undulatus). The research is supported by grants from the
National Center for Ecological
Analysis and Synthesis and the
National Evolutionary Synthesis Center.
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Below: Predicted ranges of the eastern fence
lizard (Sceloporus undulatus) based on the
contemporary climate and a plausible warming scenario (a
uniform increase of 3°C).
Reproduced
from Ehrenberger et al. (2008). |
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