SEXUAL DIMORPHISM IN SKATES (RAJIDAE) AND ITS POSSIBLE ROLE IN DIFFERENTIAL NICHE UTILIZATION.

Abstract

Adaptive radiation has been thought of generally as evolutionary divergence of species or higher taxonomic categories into differing zones or niches. Evidence is now accumulating to indicate that a lesser degree of adaptive radiation may also occur within animal populations, so that individuals may came to occupy different subniches or subzones (see Selander, 1966 and 1969, for a more complete discussion of this topic). When individuals within a population come to occupy differing subniches or adaptive subzones, the total niche of the species may be partitioned or even expanded in cases where the environmental situation will allow. Thus in many organisms sexual dimorphism may arise and result in a differential niche utilization by the sexes, which in turn may result in decreased intraspecific competition for food. The role of sexual dimorphism in birds as an adaptive agent in the reduction of intraspecific competition has been extensively reviewed and discussed by Selander (1966), and more recently additional data have been added by many authors. These are primarily cases in which morphological differences in the bill length between males and females indicates differences in foraging preferences between the sexes. Selander (1966) also suggested that behavioral polymorphism expressed as sexually dimorphic behavior could operate without concomitant modification of the feeding apparatus. This has been recently confirmed by Morse (1968) for wood warblers, Williamson (1971) for vireos, and Robins (1971) for a grassland sparrow. In these cases there were differences in foraging preferences between the sexes without expressed morphological differences in bill length. Birds have provided almost all of the examples of differential niche utilization by the sexes, primarily because foraging behavior is for the most part diurnal and easily observed. However, sexual differences which correlate with differences in foraging behavior have been recorded for several species of fresh-water fishes by Keast (1966), and for Anolis lizards by Schoener (1967, 1968). Sexual dimorphism in the feeding apparatus of skates has often been noted but almost no evidence concerning the significance of this phenomenon has been presented. Perhaps the most intriguing aspect of this phenomenon is that all skates (for which we have information) can be sorted roughly into three categories (Fig. 1): 1) essentially non-dimorphic, in which both sexes have pointed teeth adapted for grasping and tearing rather active prey; 2) dimorphic species in which teeth not brought into use are pointed, but the points are retained in the males while those in the females are quickly reduced to grinding buttons; and 3) dimorphic species in which the grinding character of the female dentition is genetically fixed and not just produced by wear. The purpose of this paper is to consider the possibility that the strongly dimorphic feeding apparatus in many skates represents another example of differential niche utilization by the sexes.