Influence of past and present environment on life history

Early environmental experience may shape phenotypes for life, and hence developmental plasticity is a key factor in life history evolution. This is increasingly acknowledged among evolutionary biologists. Building on my earlier research in this field, this project pursues two major aims. (i) Identify evolutionary and proximate mechanisms causing significant "early-environment effects" on life histories. (ii) Develop a general framework for understanding the interplay of effects caused by past and present environments and internal states in shaping the trade-off between growth and reproduction in different ecological contexts. To achieve these goals the project will pursue an integrative approach of theoretical and empirical research. The empirical part will use African mouthbrooding cichlids as model systems for life history evolution in iteroparous, long-lived vertebrates. Early- environment effects may influence the trade-off between growth and reproduction in two principle ways. (i) A feedback loop may be triggered by size-dependent mortality, causing diverging optima for individuals exposed to good or bad growth conditions early in life, irrespective of environmental conditions during adulthood. (ii) Alternatively, individuals exposed to bad growth conditions during the juvenile phase may maximize fitness by responding with compensatory growth when conditions improve. I shall study which of these mechanisms would maximize genetic fitness in dependence of mortality and growth conditions. With help of a mathematical model I shall analyse optimal life history strategies of the African cichlid fish Simochromis pleurospilus taking juvenile and adult environment, and a range of natural levels of mortality risk and competition into account. A particularly interesting early-environment effect on female S. pleurospilus is the maternal effect on egg size and offspring growth that is induced by the quality of the female`s own filial environment, and which persists irrespective of the environmental quality during egg production. The adaptive significance and proximate mechanisms of this maternal effect will be studied in three laboratory experiments: (i) The performance of offspring hand-raised from small and large eggs against competitors and simulated predation attacks in poor and rich environments will be compared. (ii) I shall test whether the differential growth of young may be caused by a behavioural response of brood caring females towards perceived predation risk. (iii) I shall try to induce a maternal effect on egg size by exposing mothers to different levels of perceived predation risk during egg formation. In a combined field and experimental lab study I will investigate the physiological basis of early-environment effects on allocation strategies during development. Special emphasis will be given to the question whether there is a point of time in development when energy allocation trajectories become fixed for life. Finally, I shall investigate early-environment effects on female mate choice and male ornamentation. As early conditions affect phenotypic quality, poorly nourished males may exhibit less pronounced ornaments. At the same time poorly-raised females may reduce choosiness, which under certain conditions may even increase offspring fitness. Early-environment effects are of particular importance in variable environments. The stage in life when an environmental change occurs should strongly influence its effect on the developing phenotype. With a series of adaptive dynamics models I shall test the influence of timing and frequency of environmental changes on the simultaneous evolution of two reactions norms, age and size at maturation and the allocation of energy between growth and reproduction. - The effect of quality differences between juvenile and adult environments will be tested with two optimality models: how do spatial and temporal differences of the mean and variance of ecological variables in juvenile and adult environments determine whether mothers should use cues from their filial or ambient environments to determine offspring size? Predictions derived from these models will be tested with S. pleurospilus under natural conditions.

Without doubt the environment experienced by developing organisms greatly shapes their future growth and morphology, reproductive decisions and behaviour throughout life. The ability of a certain genotype to develop into different phenotypes when growing up under different environmental conditions is called "phenotypic plasticity". The range of phenotypes that can be expressed from organisms possessing a given set genes can be quite impressive; for example African mouthbrooding fish raised with limited food start to reproduce at a much smaller size, produce clutches faster and produce larger young than fish with the same genes but raised with plenty of food. Naturally, organisms can benefit from the ability to adjust flexibly to ambient environmental conditions. However, this ability does not come without costs. The ability to maintain the physiological and sensory machinery necessary to obtain information on the state of a changing environment and to perform the correct adjustment is expected to inflict energetic costs. In this project we used mathematical modelling to study the conditions under which it pays to stay plastic, and how the ability to adapt to a changing world should optimally change during life time. Performing experiments with African cichlid fish, we studied explicitly how animals adapt to environmental challenges by changing their growth, their learning ability and their behaviour, and whether plastic responses indeed confer benefits to animals. Our mathematical models investigated how much of its energy reserves an animal should use up either to reproduce or to maintain its body functions and hence ensure better survival. We found that unexpectedly animals should put all energy in reproduction when the environment is either very bad or very rich in food. When there is lots of food available it is not necessary to invest in self maintenance, whereas when food is scarce, survival chances are very low anyway, so that it is better to try to obtain some offspring before dying. When the conditions are intermediate, however, animals should spend rather more energy in own survival. Depending on how strongly the food availability fluctuates they should even skip reproduction altogether in environments with intermediate food availability. So far no theory was available to predict how the plasticity of animals should change throughout life. Our models predict that in many environments plasticity should be low at the beginning and end of life, whereas the responses to environmental change should be highest at intermediate ages. Very early in life the animals know very little about the quality of their environment and how variable it is, so that they first have to sample their environment. Close to the end of life changing one`s phenotypes would only be costly but cannot pay off anymore. Environmental responsiveness should therefore be highest at midlife. In our experiments, we first investigated how the time during adolescence when fish encounter a change in food ration affects their future life. Do they change important physiological traits when their ration is changed early in life, and are they still able to do so when food is changed rather late in their adolescence? We detected that even very basic physiological parameters, such as the metabolism and the efficiency to digest food still changes when food rations are switched rather late in the juvenile period. Even more interesting, varying food amounts strongly affect juvenile cognitive abilities. Juveniles receiving an experimental change in food ration were better able to solve a learning task than fish that were always kept on the same ration. Hence already a single change in resource availability early in life - be it from low to high food or form high to low food - permanently enhanced the learning ability of the fish, allowing them to better cope with changing environmental conditions. Second, we explored the abilities of mothers to prepare their young for the conditions they will encounter after birth by so called "non-genetic maternal effects", in our particular case by adjusting the egg size to the future environment of their young. We investigated whether the ability of mothers to lay eggs of different sizes in successive clutches can raise the survival prospects of their young, which should be the case if only specific egg size do well in certain environments. We found that in poor environments with little food, mothers lay large eggs because this gives their young a long-term size advantage, whereas individuals from small eggs had to spend more time foraging outside of shelter, which increased predation risk. In food-rich environments, however, juveniles behaved similarly, and hence egg size did not matter. In environments with high conspecific competition, we found that larger offspring only have a clear growth advantage over small ones when food was scarce. When food was plentiful small offspring grew even faster than their larger siblings. We were also able to show an egg-size dependent, molecular mechanisms plastic growth regulation. Finally we studied how the social environment, namely the presence or absence of parents, influences the development of social competence in highly social fish. Fish that had been raised with adult family members showed more often appropriate social behaviour in a wide range of different social challenges than fish that had been raised with siblings only, and they maintained this competence throughout life. This highlights the importance of the social group composition during raising, but also shows that the concept of social competence, the ability to show adequate social behaviour in different contexts through behavioural plasticity, can be applied to animals.