Life history decisions and indeterminate growth

Research project P 14327 Life history decisions and indeterminate growth Barbara TABORSKY 26.6.2000 I aim to study reproductive decisions of organisms which grow throughout their lives. Organisms with indeterminate growth, such as most fish and reptiles continue to grow after maturity. Fecundity increases with size, particularly in females, but resources used for growth during adulthood waste current reproductive potential in favour of potential future benefits. The continued change of body size during adulthood- requires that indeterminately growing animals decide about the allocation of energy between growth and reproduction during their entire lives. I shall investigate how variation in juvenile growth and in mortality affects the reaction norms of these life-long decision processes in female cichlid mouthbrooders. For this purpose, I shall integrate approaches from different fields of organismic biology, including life history theory, behavioural ecology, theoretical biology, and physiology. In particular, I shall study decisions about (i) the timing of maturation, (ii) the length of growth periods between clutches, and (iii) the amount of time and energy invested in reproductive events throughout life. Further, I shall investigate the functional significance of mouthbrooding for important life history traits. The project consists of four parts; (1) two long-term growth experiments to test predictions of shapes of reaction norms, one under social isolation of experimental fish and one in natural densities; (2) the development of a simulation model to predict how much females should invest in both, time and energy for reproduction, at each stage of their lifetime; this model will be tested with data from the field and from growth experiments; (3) an evaluation of costs and benefits of broodcare behaviour for both, females and offspring, with help of field data and a series of experiments; and (4) an investigation of the physiological mechanisms underlying the timing of maturity, which is one of the most central life history decisions. Three topics which have been widely ignored as yet in life history studies on indeterminately growing animals will be addressed explicitly in this project, namely the effects of juvenile growth on life history decisions during reproductive life, the possibility to make decisions on a continuous rather than on a seasonal time scale, and the effect of investment in broodcare on other life history decisions.

Current environmental conditions strongly influence decisions about key life-history variables such as the timing of reproduction, the size and number of offspring, or the speed of growth. I expected that environmental experience made during early life stages also shapes life history decisions of adults, which has been largely ignored in the zoological literature. I tested experimentally how past and present environmental conditions shape the decisions about timing of reproduction, growth and reproductive effort. As a model system I used African mouthbrooding cichlids, which were raised and kept singly under standardized conditions in a long-term experiment. Sixty fish were raised with high and the same number with low food availability. Females were the major target of this study of optimal life history decisions. After maturation, half of the females were switched to the opposite feeding regime, while the other half remained with the original treatment. As predicted, females in the high food treatment grew much faster, but postponed first reproduction and had longer intervals between broods than low-food females. Moreover, past and present ecological conditions influenced life history decisions interactively. Females kept under high-food conditions as adults but raised in `poor` environments compensated for their retarded growth slowly after receiving more food. However, they did so incompletely and only after some time lag, while they invested most of excess energy in reproduction. In a second approach I tested how environmental variation modulates reproductive schedules immediately, using the decision of mouthbrooders about the length of incubation as an example. I performed three lab experiments complemented by field observations in Lake Tanganyika. (1) I showed that females incubate young in their mouths 20% longer when predators are present. By this means females produced fitter young, but suffered from a reduced reproductive rate. (2) Some mouthbrooders feed young in their mouths. I showed experimentally that this exceptional behaviour is energetically costly. I hypothesized that for this reason, brooding females with access to food will reduce incubation time as young may develop faster. However, incubation time did not change when access to food was varied. Rather, fed young were larger, heavier and had a higher burst swimming speed at independence than unfed young. (3) In a few mouthbrooders, females transfer offspring to their mates half way during incubation. I tested whether and how such females can control their own brood care investment. Females tried to solicit the transfer of young already up to 4 days before the male finally accepted the young. Moreover, males accepted the offspring 2 days later when additional females (i.e. potential mating partners) were present. Apparently, in this sexual conflict about reproductive investment the male is the `winner`. With help of a theoretical model I studied the impact of environmental risk on reproductive schedules. In fish, mortality risk from predators rapidly decreases with increasing body size. I showed that when size-dependent mortality is high, two distinctly different morphs may evolve within the same population: small, early maturing, frequently reproducing individuals, and large ones that delay maturation and produce offspring at longer time intervals.