Variability in body mass and sexual dimorphism in Danish red foxes (Vulpes vulpes) in relation to population density

TL;DR

In Danish red foxes from the same Jutland area, adult male body mass and sexual dimorphism varied with population density: males were heavier and dimorphism larger when density was low (1965–1977 and 2015/2016) than when density was high (2012–2014), while female mass did not differ significantly across periods. In winter 2015/2016, adult females had greater rump fat thickness than adult males, contrasting with 2012–2014 when no significant overall sex/age differences in rump fat were found.

Key findings

• Adult males were significantly heavier in low-density periods: mean 7.7 kg in 1965–1977 and 7.5 kg in 2015/2016 versus 6.8 kg in 2012–2014 (F = 6.3, p < 0.01; Hc = 10.9, p < 0.01). No significant differences were found in female weight across periods (adult females: 6.0, 6.1, and 6.2 kg).
• Sexual dimorphism in adult body weight was 7.6% in 1965–1977, 6.1% in 2015/2016, and 3.6% in 2012–2014; significant sexual dimorphism was found in all three periods for both adults and subadults.
• In winter 2015/2016, adult females had significantly greater rump fat thickness (mean 0.77 cm) than adult males (mean 0.58 cm) and subadult females; subadult females also had greater rump fat than subadult males (p < 0.02). In 2012–2014, no significant overall rump fat difference was found across sexes and age classes (F = 1.9, p = 0.14), though adult females were fatter than adult males in pairwise comparison.
• Adult and subadult males in 2015/2016 were significantly longer than those in 2012–2014: adult males 114.4 cm versus 109.5 cm, and subadult males 113.0 cm versus 109.1 cm (p < 0.001). No significant length difference was found for females.

Methods

• Three collections from agricultural Jutland, Denmark: 1965–1977 (collection A; 178 ear-tagged individuals from a mark-recapture study plus museum skeletons), 2012–2014 (collection B; 187 foxes >6 months old), and winter 2015/2016 (collection C; 187 shot foxes), totaling 552 foxes. Weights and lengths were recorded; age was determined by cementum annuli for B and C; rump fat thickness was measured for B and C.
• Population density was proxied by National Game Bag Records, specifically Jutland game bags: low density in 1965–1977 due to hunting pressure and in 2015/2016 following canine distemper, and high density in 2012–2014.
• Statistical analyses in PAST used one-way ANOVA and Kruskal-Wallis tests for period/region/age differences, t-tests and Mann-Whitney U-tests for pairwise comparisons, and sexual dimorphism was calculated as the percentage difference in mean length or mean weight, with cube root of mean body mass used for mass dimorphism.

Limitations

• Stated: length was compared only between collections B and C because collection A was measured by a different person; rump fat thickness was not available from the museum collection, so it was compared only between B and C. Age determination had uncertainty: cementum lines develop between August and October of the second year, so foxes without cementum lines could be 6–11 months old and were grouped as subadults. Game bag records as a density proxy may depend on hunting effort, hunting behaviour, and legislation, though the study compared recent fluctuations by age only.
• Stated: further studies are needed to fully understand the underlying mechanisms and subsequent effects of fluctuating sexual dimorphism.
• Evident: the design is observational and correlational, and the authors assumed genetic, geographical, and climatic factors were fixed over the study area and period without directly testing them; length and rump fat data were not available for all periods.