DEVELOPMENT OF BODY MASS AND SEXUAL SIZE DIMORPHISM IN DANISH RED FOXES (VULPES VULPES)

Abstract

In this study, we examine the development of body mass and sexual size dimorphism (SSD) in 178 juvenile wild Danish red foxes from 99 litters using piecewise analyses of regression lines for age versus weight. When fox cubs are younger than 100 days, only slight (SSD=1.7%) and no significant difference (t-test: t=1.2, p=0.24) was found in the mean weight of males (2.03± kg) and females (1.93± kg), and no significant difference was found in the slope of regression lines for males and females (F=0.97E-5, p = 0.99). In the growth period between 100 days of age and mating around 275 days of age, the regression line in males steepens more than that of females (difference in slopes, F=5.9, p<0.02) and the difference in mean weight of the sexes become highly significant (SSD=7.4%, difference in mean t=4.6, p=2.2E-5). After mating the growth curve levels off i.e. the slope of the regression lines for age versus weight is not significantly different from zero. Yearly variation was revealed in the growth rate of juvenile foxes (difference in slope for males; F=3.9, p<0.01 and females; F=8.6, p<0.001). Conclusion: SSD in red foxes mainly develop as a result of a faster grow rate in males between indepency and maturity. Ontogony of red foxes may genetically be disposed to prevent males outcompeting females in the early stages of life (<100 days), when cubs are still fed by adults and the increase in SSD before mating, may be an adaption to selective forces benefitting larger males. The growth rate of juvenile foxes of both sexes is influenced by environmental variation in different years.