The relationship between offspring size and fitness: integrating theory and empiricism
Date
2013-02
Authors
Rollinson, Njal
Hutchings, Jeffrey Alexander
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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.
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Citation
Rollinson, Njal, and Jeffrey A. Hutchings. 2013. "The relationship between offspring size and fitness: integrating theory and empiricism." Ecology 94(2): 315-324.