The correlated evolution of foraging mode and reproductive effort in lizards

Abstract

Life-history theory suggests that the optimal reproductive effort of an organism is affected by factors such as energy acquisition and predation risk. The observation that some organisms actively search for their prey and others ambush them creates the expectation of different energy needs and predation risk associated with each foraging behaviour, the so-called 'foraging-mode paradigm'. Although this paradigm has been around for decades, the empirical evidence consists of conflicting results derived from competing models based on different mechanisms. For instance, models within the foraging-mode paradigm suggest that widely foraging females have evolved low reproductive effort, because a heavy reproductive load decreases their ability to escape from predators. By contrast, a long-standing prediction of evolutionary theory indicates that organisms subject to high extrinsic mortality, should invest more in reproduction. Here, we present the first partial evidence that widely foraging species have evolved greater reproductive effort than have sit-and-wait species, which we attribute to a larger body size and greater mortality among mobile foragers. According to our findings, we propose a theoretical model that could explain the observed pattern in lizards, suggesting ways for evolutionary ecologists to test mechanistic hypotheses at the intraspecific level.

Keywords: clutch size; fecundity; maternal size; offspring size; phenotypic plasticity; predation.

Figure 1.

A conceptual model depicting putative relationships among foraging behaviour, energetics, predation risk and reproductive effort. The predicted relationships were derived from theoretical models of life-history evolution (see text for details).

Figure 2.

Random sample of stochastic character maps depicting the evolution of foraging mode in 485 species of lizards. Bars at the tips of the phylogeny represent log-transformed values of reproductive output for all lizards, but not the outgroup, Sphenodon punctatus. Pie charts on internal nodes represent posterior probability estimates resulting from 1000 simulations of the character histories. Major clades are enumerated as follows: (1) Gekkota, (2) Scincoidea, (3) Lacertoidea, (4) Anguimorpha, (5) Toxicofera and (6) Iguania. Lizard photos by Mark O’Shea. (Online version in colour.)

Figure 3.

Effects of maternal mass and foraging mode on the evolution of reproductive effort of lizards, as determined by phylogenetic generalized least-squares analysis. (a) Difference in reproductive effort among foraging modes assuming body mass as a proxy for energy. (b) Difference in reproductive effort among foraging modes assuming the scaled mass index as a proxy for energy. (Online version in colour.)

Figure 4.

A theoretical model relating the cumulative energy gain for reproduction, m, as a function of time spent foraging, t, and maternal size. (a) Larger females reach their maximum capacity at a higher value of m than small females. (b) Widely foraging females that are smaller but more efficient foragers may produce a greater reproductive output than larger sit-and-wait females. (c) Widely foraging females may also produce a greater reproductive output than sit-and-wait females if they are both larger and more efficient foragers. t_wf and t_sw in (b) represent the optimal foraging time of widely foraging females and sit-and-wait females, respectively. (Online version in colour.)