Diurnal vertical migration: Adaptive significance and timing. Part 1. Selective advantage: A metabolic model1

Abstract

The diurnal vertical migrations of herbivores usually result in a major decrease in their exposure to food, a sacrifice widely considered warranted because visually orienting predators make daylight hours unsafe for grazers to feed near the surface. A new alternative to that hypothesis is proposed which emphasizes the photosynthetic rather than the visual significance of the day‐night cycle. Among the critical assumptions for this model are: that photosynthetic rate is great enough for algal biomass to increase appreciably from dawn to dusk; that the metabolic needs of grazers can be reduced by resting during nonfeeding hours in cooler waters at depth; and that the grazing rate of herbivores, following an extended interval of nonfeeding, is initially greater than steady state rate and then declines in a matter of a few hours. Calculations based on these assumptions demonstrate that nocturnal feeding by a grazer might be able to provide a greater net energy gain for growth and reproduction than continuous feeding. An unexpected prediction of this approach is that if maximizing net energetic gain is of greater selective importance than avoidance of visually orienting predators, grazers ought to migrate upward and begin feeding well before sunset, rather than after as the predation hypothesis presumes. The exact timing of the postulated feeding optimum depends on assumed values of several parameters, but most of the calculated advantage seems attainable by an onset of grazing 1–2 h before sunset. This prediction provides a simple first test of the general hypothesis.