Assortative Mating for Size in Asellus Aquaticus (L.) (Isopoda)

Abstract

analysis of two hundred and one mating pairs of A. aquaticus collected in late February and early March of 1976 from a single population in Nottingham shire. Males and females were measured to the nearest millimeter, from the tip of the head to the tip of the abdomen, and the progeny of forty-Five selected pairs counted to the nearest five immediately after their release by the female. The distribution of male and female body lengths is shown in fig. 1. The mean length of the males is 11.3 mm and of the females 9.1 mm with variances of 0.68 for both groups. The difference of 2.2 mm between the means is highly significant (t = 2.67, /?<0.01, two-tailed test). The distribution of sizes about the means does not differ significantly in the two sexes (x2 = 5.95, df = 4, P= 0.20). The association of body lengths in mating pairs is shown in table I. A test for a linear trend in the table (Maxwell, 1964) shows a significant positive association between male and female body length in the pairs (^2 = 5Q? ?r = \y p = 0.025). In table I it can be seen that the males are tending to mate assortatively with females, more frequently choosing those about 2 mm shorter than themselves. Table I The association of male and female Asellus aquaticus (L.) in mating pairs accord ing to body length. Expected numbers in each cell given in brackets.