Reproductive strategies of the racoon dog and the red fox in Finland

TL;DR

Raccoon dogs and red foxes in southern Finland have nearly equal body size, similar diets, and stable populations, yet raccoon dog litters are far larger and relatively heavier, indicating that raccoon dog females invest more in reproduction. The author attributes this to differences in vole dependence, winter dormancy and fat storage, and juvenile predation mortality rather than to body weight or population status.

Key findings

• Mean litter size was 9.0 for the raccoon dog versus 5.1 for the red fox; mean relative litter weight was 18.3–24.0% of female body weight for the raccoon dog versus 10.4–12.5% for the red fox.
• Adult females of both species weighed 4–6 kg in spring/early summer (raccoon dog 5.2 kg, red fox 4.9 kg), so body size does not explain the different strategies; the raccoon dog showed no sexual dimorphism, while red fox males were slightly larger than females (male/female ratio 1.06).
• Vole cycles strongly affect red fox litter size but only weakly affect raccoon dog litter size (mean varies between 8 and 10); raccoon dog females also carried on average 850 g of extra fat (15.6% of body weight) in March, and winter dormancy buffered them against winter weather and food scarcity.
• Juvenile mortality during the first year was 87% for raccoon dogs in Finland versus 39–56% for red foxes in Sweden; because raccoon dog pup mortality is high and probably independent of litter size, natural selection favours large litters, whereas larger red fox litters would likely raise starvation-dependent pup mortality without increasing surviving young.

Methods

• Carcass material from hunters in southern Finland (hunting season August–April), 1986–1995; adults at least one year old were measured and weighed (to within 20 g and 0.5 cm) to determine body size, with age determined from long-bone ossification or canine tooth sectioning.
• Reproductive data came from 350 raccoon dog and 50 red fox uteri with placental scars or embryos/foetuses: placental scars were counted in autumn-killed animals and embryos (early pregnancy) or weighed foetuses (latter half of pregnancy) in spring-killed animals.
• A synthesis drawing on previously published work for part of the data (raccoon dog reproduction 1986–1990; raccoon dog body size 1986–1990) and on published literature for red fox diet (Lampio 1953) and population status.

Limitations

• The paper states no dedicated limitations section; it notes explicitly that when red fox data from Finland were lacking it referred to literature, and that the red fox social system in Finland and the effect of division of labour between the sexes on pup survival and litter size are "a subject for future research."
• Evident from the text: the red fox reproductive sample is much smaller than the raccoon dog sample (50 versus 350 uteri; e.g. n = 15 spring red fox litters at birth versus n = 77 for the raccoon dog), and several red fox comparisons (juvenile mortality, litter size range, life span) rely on Swedish or other non-Finnish studies.
• Evident from the text: red fox diet is taken from Lampio (1953), a study from a different period than the 1986–1995 carcass data, so diet comparisons between species rest on non-contemporaneous sources.