THE EVOLUTIONARY CONSEQUENCES OF ONE IN THE HAND VS. TWO IN THE BUSH

Many animals must forage. Foraging efficiency is the difference between benefit (energy consumed and converted) and cost (energy spent foraging) relative to time spent foraging. Increases in cost or time decrease efficiency. Both energetic and temporal cost can be divided into two components based on foraging phase: searching (the phase before recognition) and handling (pursuit/attack/consumption). The relative costs of searching (CS) vs. handling (CH) are a function of the forager’s sensory acuity and the density of local potential food. The less dense the food, the greater is the cost (energy and time) of searching. Some foragers encounter food-items sufficiently frequently that CH >> CS, and encounters are likely simultaneous, in which two or more food-items are detected. Other foragers encounter food-items so rarely that CS >> CH, and encounters are sequential, where one food-item at a time is detected. In this case, the forager does not choose between food-items, but chooses between the present food and continuing to search for better food.

Logically, if CH >> CS, then the forager can specialize because food-density is great enough that searching for better food is not costly. In contrast, a forager with CS >> CH must generalize and take available food because of the high risk associated with continued searching; the likelihood of finding food beneficial enough to compensate for the extra CS is low. This model has evolutionary consequences: species in habitats with high food densities are more prone to specialization, and if conditions persist over long time-scales, speciation. A comparison of predatory drilling gastropods provides one possible example: muricids often live in habitats (rocky nearshore, reefs) with very high densities of prey that lead to simultaneous encounters; many muricid populations have strong preferences for one prey species. In contrast, burrowing naticids often live in level-bottom settings in which prey are infrequent; naticids tend to be much broader in diet than muricids. There are > 1600 modern muricid species but < 300 naticid species; moreover, muricid speciation rates have been greater than those of naticids for most of their histories. Food-density may affect speciation rates. While other factors may contribute to differences in speciation, the hypothesis warrants further study.