The largest litter size in raccoon dog Nyctereutes procyonoides recorded outside its native range

The raccoon dog (Nyctereutes procyonoides), a canid native to East Asia, was introduced to Europe in the 20th century, where it has become a successful invasive species with a wide distribution across the continent. It is known for its high reproductive potential, adaptability and tolerance to the presence of humans and poses a significant ecological and epidemiological threat. We report the first European case of an exceptionally large litter in a wild raccoon dog: a female found dead in Krakow, Poland, was carrying 19 fully formed embryos estimated at five weeks gestation. This litter size is the largest recorded outside the species’ native range since the 1960s and suggests excellent maternal condition, possibly favoured by the mild winter climate and abundant anthropogenic food resources in the region. Nevertheless, considering the high juvenile mortality of the species, the survival probability for many offspring was considerable. Our results underline the remarkable reproductive capacity of the raccoon dog in its introduced range and highlight the potential for population expansion under favourable environmental conditions. We recommend more intensive monitoring and research into raccoon dog population dynamics, reproductive ecology and impacts on native ecosystems in order to develop management strategies for this invasive predator.