“The energetic cost of reproduction and its effect on optimal life-history strategies”
Asta Audzijonyte and Shane A. Richards (Oct 2018)
A model with energetic and size specific reproduction cost provides new explanation for the importance of large fish
Reproduction can be costly and will affect optimal maturation size and age
Why do some animals delay reproduction when this entails the risk of not leaving any progeny at all? Attempts to answer this question have motivated the development of life-history theory that aims to understand key determinants of maturation size and reproductive effort. Asta Audzijonyte and Shane Richards suggest that the energetic cost of reproduction, while often ignored or treated implicitly in life-history models, can be substantial and will influence optimal timing of reproduction events.
Audzijonyte and Richards present a life-history model that assumes that reproduction requires a minimum energy pool to cover the costs of reproductive behavior, such as mating, migration, or parental care. The researchers propose that for indeterminate growers, such as fish, this energetic cost of reproduction likely scales sublinearly with size, which implies that reproduction is relatively cheaper for larger individuals. Decreasing relative cost of reproduction with size means that delayed reproduction can be an optimal reproductive strategy and that relative reproductive output increases with size. Importantly, the energetic cost of reproduction sets limits on the smallest viable maturation size, which constrains a population’s ability to adapt to size-dependent and human-induced mortality, such as fishing. Audzijonyte and Richards suggest that the scaling of the reproduction cost with body size is a fundamental species’ parameter that allows for dynamically emergent maturation size and explains skipped reproduction events and high fitness of large individuals.
Trade-offs in energy allocation between growth, reproduction and survival are at the core of life-history theory. While age-specific mortality is considered to be the main determinant of the optimal allocation, some life-history strategies, such as delayed or skipped reproduction may be better understood when also accounting for reproduction costs. Here, we present a two-pool indeterminate grower model that includes a survival and energetic cost of reproduction. The energetic cost sets a minimum reserve required for reproduction, while survival cost reflects increased mortality from low post-reproductive body condition. Three life-history parameters determining age-dependent energy allocation to soma, reserve and reproduction are optimized, and we show that the optimal strategies can reproduce realistic emergent growth trajectories, maturation ages and reproductive outputs for fish. The model predicts maturation phase shifts along the gradient of condition related mortality and shows that increased harvesting will select for earlier maturation and higher energy allocation to reproduction. However, since the energetic reproduction cost sets limits on how early an individual can mature, increase in fitness at high harvesting can only be achieved by diverting most reserve into reproduction. The model presented here can improve predictions of life-history responses to environmental change and human impacts because key life-history traits such as maturation age and size, maximum body size, and size-specific fecundity emerge dynamically.