PhDâ€theses - Ethologische Gesellschaft

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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. 7
THE INFLUENCE OF EARLY ENVIRONMENT ON LATER LIFE ‐ PHYSIOLOGY, BEHAVIOUR AND EVOLUTION Alexander Kotrschal alexander.kotrschal@ebc.uu.se Research Update PhD Thesis, 2010, supervised by PD Dr. Barbara Taborsky, Behavioural Ecology, Institute of Ecology and Evolution, University of Bern, Switzerland Current affiliation: Animal Ecology, Evolutionary Biology Centre, Uppsala University, Sweden. Phenotypic plasticity is a universal feature of organisms. Environmentally induced parental effects represent a form of phenotypic plasticity spanning generations. There is rapidly growing evidence that the environmental conditions experienced by females around reproduction can influence offspring phenotype in plants and animals. However, even environmental conditions experienced long before reproduction during the juvenile period of females can mediate crucial life‐history traits. Females of the cichlid fish Simochromis pleurospilus are known adjust offspring size in response to own juvenile nutrition. We tested whether these condition enable them to predict the postnatal conditions of their offspring better than the current conditions do, which requires certain conditions of habitat variability, distribution of juveniles and adults and gene flow between populations to hold. By combining detailed ecological and population surveys with fine‐scaled population genetics we confirmed that these requirements hold for our study species in their natural environment. Next we investigated the physiological consequences for juveniles of growing up in poor or rich environments and found a persistent alteration in metabolic rate and food conversion efficiency. In preparation of this study we developed a non‐invasive method to determine fat storage to be used in small, live fish. This technique estimates fat based on measures of swim bladder size, and predicts visceral fat content better than available morphometric condition indices. Besides physiology, we detected that the early nutritional conditions also affect learning abilities. Unexpectedly, the absolute amount of food did not affect learning. Instead we found that a single ration switch experienced by juveniles enhanced their learning abilities both as juveniles and as adults (Kotrschal & Taborsky, 2010a). This appears to represent an adaptive strategy, as higher cognitive abilities may confer a survival advantage in variable environments. Overly fast growth during a specific, short time window during the development negatively influenced adult learning abilities, however. Although it was not possible to test mating preferences in dependence of early nutrition, we detected that our study species has a very peculiar mating system which posses features of both resource defence mating systems and exploded leks: The distribution of males, females and resources clearly resembles leks. Male territory defence, however, seems to enhance the quality of resources and thus generates critical resources for females, which attracts females in the vicinity of courting males (Kotrschal & Taborsky, 2010b). Kotrschal, A. & Taborsky, B. 2010a. Environmental change enhances cognitive abilities in fish. PLoS Biology 8: e1000351. Kotrschal, A. & Taborsky, B. 2010b. Resource Defence or Exploded Lek ‐ A Question of Perspective. Ethology 116: 1‐10. 8
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