The relationship between offspring size and fitness: integrating theory and empiricism

Abstract

How parents divide the energy available for reproduction between size and number of offspring has a profound effect on parental reproductive success. Theory indicates that the relationship between offspring size and offspring fitness is of fundamental importance to the evolution of parental reproductive strategies: this relationship predicts the optimal division of resources between size and number of offspring, it describes the fitness consequences for parents that deviate from optimality, and its shape can predict the most viable type of investment strategy in a given environment (e.g., conservative vs. diversified bet-hedging). Many previous attempts to estimate this relationship and the corresponding value of optimal offspring size have been frustrated by a lack of integration between theory and empiricism. In the present study, we draw from C. Smith and S. Fretwell's classic model to explain how a sound estimate of the offspring size--fitness relationship can be derived with empirical data. We evaluate what measures of fitness can be used to model the offspring size--fitness curve and optimal size, as well as which statistical models should and should not be used to estimate offspring size--fitness relationships. To construct the fitness curve, we recommend that offspring fitness be measured as survival up to the age at which the instantaneous rate of offspring mortality becomes random with respect to initial investment. Parental fitness is then expressed in ecologically meaningful, theoretically defensible, and broadly comparable units: the number of offspring surviving to independence. Although logistic and asymptotic regression have been widely used to estimate offspring size-fitness relationships, the former provides relatively unreliable estimates of optimal size when offspring survival and sample sizes are low, and the latter is unreliable under all conditions. We recommend that the Weibull-1 model be used to estimate this curve because it provides modest improvements in prediction accuracy under experimentally relevant conditions.

Offspring size at weaning affects survival to recruitment and reproductive performance of primiparous gray seals

“…2011). In species producing few large offspring, such as marine mammals and seabirds, fewer parents are predicted to produce offspring near the physiological limit of viability and hence, the offspring fitness curve may approach the minimum viable offspring size relatively slowly (Rollinson and Hutchings 2013). Our results and those of Hall et al.…”

The positive correlation between maternal size and offspring size: fitting pieces of a life‐history puzzle

“…These explanations typically assume that maternal body size or morphology is optimized, then I becomes subject to limitations imposed by optimal maternal size. The term 'constraint' can be meaningful under the strictest definition of optimality (Orzack & Sober, 1994;Abrams, 2001;Rollinson & Hutchings, 2013b), but it adds confusion to the study of maternal-size correlations. The term 'constraint' can be meaningful under the strictest definition of optimality (Orzack & Sober, 1994;Abrams, 2001;Rollinson & Hutchings, 2013b), but it adds confusion to the study of maternal-size correlations.…”

Colonisation of toxic environments drives predictable life‐history evolution in livebearing fishes (Poeciliidae)

“…). Furthermore, life‐history theory predicts offspring size to become larger in low‐quality environments (theory: Rollinson & Hutchings ; empirical evidence for Atlantic salmon, Salmo salar : Rollinson & Hutchings ), as well as in habitats with increased resource competition (theory: Brockelman ; Jørgensen et al . ; empirical evidence for the guppy: Bashey ), as for example, induced by the increased population densities typically found in sulphide springs (J. Jourdan, D. Bierbach, M. Plath & R. Riesch, unpublished data).…”