PhD‐theses - Ethologische Gesellschaft

PhD‐theses
EVOLUTION OF PLASTIC LIFE‐HISTORIES
Barbara Fischer
barbara.fischer@bio.uio.no
Research Update
PhD Thesis, 2010, Supervisors: PD Dr. Barbara Taborsky, PD Dr. Ulf Dieckmann, Dept. Behavioural
Ecology, University of Bern, Switzerland and Evolution & Ecology Program, IIASA, Laxenburg, Austria
Most environments fluctuate unpredictably at least to some degree, but surprisingly most life history
models ignore stochastic environmental changes. By theoretical modelling we investigated the
conditions favouring phenotypic plasticity of life histories, influences on the extent of plasticity in a
trait and why and how much plasticity should change with age in stochastic environments. We
analyzed how organisms should optimally allocate energy to reproduction vs. maintenance and how
plastic they should be in their allocation decisions in environments differing by their degree of
variability and predictability. It turned out that optimal reproductive investment should not increase
monotonically with growing energy availability as one would intuitively assume. Instead, the reaction
norm describing reproductive investment has a U‐shape, with high reproductive investment when
energy availability is close to zero ('terminal investment') and when energy availability is high. At an
intermediate level of energy availability, reproductive investment decreases to a minimum or even to
zero. This indicates that in intermediate environments it can be optimal to skip reproductive events.
Organisms may buffer the impact of environmental fluctuations by storing energy. So far, general life
history models analysed optimal storage strategies in deterministic environments only. When
incorporating stochastic changes of conditions, our model results suggest that when environments
are almost constant, it is not optimal to store. Allocation to storage is favoured when environments
become more variable and/or more predictable. Our results suggest that the simultaneous allocation
of energy to reproduction, maintenance and storage can be optimal, which contrasts earlier findings
from models in deterministic environments. Finally, we asked why plasticity should vary with age and
whether there are windows of plasticity over life time. Many organisms possess 'plasticity windows',
during which they are responsive towards external cues. When exposed to an environmental cue at a
time outside the plasticity window, no adjustment occurs. We hypothesized that information gain
through environmental sampling is a potential mechanism that can lead to the evolution of age‐
dependent plasticity. Our results suggest that this mechanism may indeed give rise to a remarkable
diversity of age‐dependent plasticity patterns, ranging from lifelong plasticity to narrow plasticity
windows. As a specific example of the evolution of plasticity we modelled how mothers should solve
the offspring size‐number trade‐off when environmental cues are unreliable. Our model results
suggest that plastic offspring size strategies are superior to fixed strategies when environmental cues
are at least moderately reliable. The plasticity threshold depends on plasticity costs and the diffe‐
rence of resources available to mothers. Plastic strategies are increasingly favoured the more
offspring survival varies between environmental states. The occurring switches between small and
large offspring are predicted to be substantial, which is confirmed by those empirical studies that did
find plasticity in offspring size to exist.
Reference: Fischer, B., Taborsky, B. and Dieckmann, U. 2009. Unexpected patterns of plastic energy
allocation in stochastic environments. American Naturalist, 173: 108‐120.
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