by Kaarina Kauhala*
This article addresses possible reasons for the
spread of the raccoon dog
across much of the northern hemisphere, and attempts to account for
its current
distribution patterns in northern Europe.
The raccoon dog originated in the Far East and is a native of southeastern Siberia, China, North Vietnam, Korea and Japan (Figure 1). At least three different subspecies have been distinguished: Nyctereutes procyonoides procyonoides and N. p. ussuriensis in continental Asia and N. p. viverrinus in Japan.
The Japanese subspecies differs in three respects from the two mainland subspecies. Chromosome numbers are different (2n = 38 in Japan, 2n = 54 in the two mainland subspecies) (Mäkinen et al., 1986; Ward et al., 1987). The fur of the Japanese subspecies provides much poorer insulation than does that of ussuriensis and the energy intake of the Japanese raccoon dog in summer and early autumn (and consequently the seasonal change in weight) is lower than that of N. p. ussuriensis (examined on a fur farm in Finland) (Korhonen et al., 1991). Some differences in behaviour may also exist between the Japanese and continental Asian subspecies.
The large difference in chromosome numbers suggests that a process of speciation is under way and that the Japanese raccoon dog should probably be considered a separate species by now. The raccoon dog entered Japan from Korea or via Sakhalin 18,000-12,000 years ago, and it has adapted to the mild marine climate and a more or less constant supply of food throughout the year. In many of the areas inhabited by the raccoon dog on the Asian mainland the climate is much harsher and little food is available in winter.
The raccoon dog (subspecies ussuriensis) was introduced into the former Soviet Union, mainly the European part, in 1929-55 because it was considered a valuable fur animal (Lavrov, 1971). Altogether 9,100 animals were released, some of them not far from Finland (Figure 1). Many of the introductions were successful, and populations increased. It was not long before the raccoon dog began to colonize neighbouring countries, including Finland. The raccoon dog expanded its range at an average annual rate of 40 km, and by the mid-1970's its distribution included southern and central Finland, where the population density is now higher than that of the red fox (Helle & Kauhala, 1991). The species is very common in the Baltic states and many parts of eastern Europe, and is also found in Germany and Sweden. Some specimens have even been seen as far away as Norway, France, the Netherlands, Switzerland and Austria, and the species is still spreading westwards and southwards (see Helle & Kauhala, 1991).
The raccoon dog's colonization of Europe has been very rapid. In Finland, the growth rate of the population was highest in the south of the country, and the population reached its peak in the mid-1980's, according to game enquiries. Thereafter, the numbers first declined slightly in some areas, but have since stabilized and seem now to vary in a density-dependent manner (Kauhala, 1992). The raccoon dog would thus seem to have reached the carrying capacity of the environment in Finland. The annual catch has increased sharply in Finland during the past two decades, reflecting the rapid increase in population density. Only 818 raccoon dogs were killed during the 1970-71 hunting season but 75,000 were killed twenty years later. Although the annual tally does not always give a true picture of population density, an increase of this magnitude certainly reflects the rapid increase in numbers.
Thus, the raccoon dog can be considered a very successful species. It has many features in common with other mammals that have quickly invaded new areas: it is rather small, omnivorous, capable of living near human settlements and of utilizing man-made food resources, it has a very high rate of reproduction and is generally very adaptable. Moreover, the raccoon dog sleeps through the winter, which is a great advantage in areas where winters are harsh and food availability low. With short legs and small paws the raccoon dog is not able to move around in deep snow, so lethargy offers the best means of surviving the winter.
A comprehensive study of the ecology of the raccoon dog, directed by Dr. Eero Helle, began in 1986 in the Finnish Game and Fisheries Research Institute. This research included the collection and examination of thousands of raccoon dog carcasses, but radio-telemetry work was also carried out in the field. The following results are mainly based on this work.
One of the most important factors affecting the growth rate of any population is the speciesµ rate of reproduction. The reproductive potential of the raccoon dog is amazingly high in most areas studied: in southern Finland the mean litter size is 9 cubs (in some "good" years it may even reach 10) and the maximum litter size is 16 (Kauhala, 1992; Figure 2). Equally high figures have been reported from the Danube delta, Lithuania and the Russian Far East. The reproductive potential seems to be lower, however, in the eastern part of European Russia and in eastern Finland (North Karelia) (see Helle & Kauhala, in press).
The raccoon dog has one of the highest litter sizes to be found in the Family Canidae. Only the arctic fox (Alopex lagopus) and the African wild dog (Lycaon pictus) may have larger litters, and the arctic fox has very large litters only in years when small mammals are very abundant. The mean litter size of most canids is between four and six. Reproductive investment among canids can also be measured by comparing relative litter weights (litter weight/female weight). The relative litter weight of the raccoon dog is the highest of all canids, the mean weight of the litter being 24% of the weight of the female (Kauhala, 1992). The relative litter weight of most canids the size of the raccoon dog is around 11%.
Why is the raccoon dog able to invest so much in reproduction? One would expect the cost of reproduction to be too high for a female with nine or ten cubs year after year. However, there are several features of the raccoon dogµs ecology which may explain the high reproductive potential of the species (Kauhala, 1992).
Firstly, it is strictly monogamous (with the possible exception of the Japanese raccoon dog) and the male helps to rear the young. During the first week after parturition, both the male and female remain in the den with the cubs. Thereafter it is usually the female that forages for food because she needs a great deal of energy for lactation, while the male stays in the den with the cubs, guarding and keeping them warm. If the male did not participate in rearing the cubs, the female would not be able to obtain enough food to produce milk for such a large litter. The raccoon dog eats small food items and does not usually carry food into the den. On the contrary, the female prefers to eat as much as she can herself and feed the cubs entirely with her own milk during the first month.
Secondly, the raccoon dog is truly omnivorous, which means that some food is always available (Kauhala et al., 1993). It also utilizes man-made supplies of food, such as rubbish heaps, and so is rather insensitive to fluctuations in other food resources, e.g. voles, especially in areas near human settlements.
Thirdly, the raccoon dog sleeps during harsh winters. Winter is the critical season for many mammals in northern latitudes, because moving about in deep snow requires much energy and there is often a shortage of food. Adult raccoon dogs are always very plump in late autumn and are able to survive the winter in good condition, regardless of the weather and food supply. Thus, in March (when females come on heat) they still have ample extra fat, and can afford to invest heavily in reproduction.
Although the raccoon dog is very successful, it appears unable to colonize Lapland. The animalµs northern distribution limit lies between 65° N and the Arctic Circle, and so it is only rarely seen in Lapland. Distribution seems to be determined by climate, which is very similar near the northern limit in Finland and the northern limit in Russia: the mean annual temperature is just above 0° C, the snow cover about 80 cm, the duration of the snow cover 175 days and the length of the growing season 135 days.
The growth rate of the population and the present density are highest in southern Finland; both are explained rather well by the mean annual temperature. Thus, climate affects the distribution, regional abundance and growth rate of the raccoon dog population (Kauhala, 1992).
Climate also seems to have a marked effect on the productivity of the raccoon dog population, especially on the proportion of reproducing females (Figure 3). The time of the thaw (indicated by snow depth at the end of March) determines when the animal comes on heat and gives birth. Parturition, in turn, affects the length of the period during which the young can grow and accumulate fat reserves for the winter and, thus, their weight in late autumn. The weight of the females of the youngest age class largely determines how many of them will reproduce the following spring. This, in turn, determines the proportion of reproducing females in the population, because the youngest age class (individuals just under one year) constitutes more than 40% of the population in spring.
In Lapland, winter is so long and summer so short that cubs do not have not enough time to grow and accumulate fat reserves for the winter, and small cubs are not able to survive the long winter. Accordingly, the raccoon dog is not able to colonize Lapland permanently. In southern Finland, winter lethargy is an advantage, because there is time enough to accumulate fat reserves and the winter is not too long, but in northern areas total inactivity during the winter is a disadvantage.
Climate is also a determinant of population density. In southern Finland primary production is higher than farther north, and this naturally affects the amount of food available for animals. As a result, the home ranges are smaller and the population density higher in the south than in areas where the climate is more severe. For example, The mean home range size (calculated from population density) in the northwestern part of Russia (where climate is continental with long and very cold winters) is 10-20km². In the Novgorod area (where winters are shorter and warmer) the mean home range size is about 1.5km².
The numbers of all natural populations always fluctuate to some extent. The abundance of food, in particular of berries (mainly bilberries and lingonberries) in autumn, is largely responsible for the annual variation in the raccoon dog numbers in Finland (Kauhala, 1992). The abundance of voles may also have some effect, especially near the northern distribution limit, possibly because vole numbers fluctuate more sharply in northern than in southern Finland.
The abundance of berries probably affects the accumulation of fat reserves in autumn, because berries are one of the raccoon dogµs main sources of food. Juveniles in particular rely on them for building their fat reserves and surviving the autumn and winter. Juvenile mortality is indeed the most important mortality factor for the population (Figure 2), and seems to be directly dependent on the abundance of berries.
Climate can also affect the annual population density in the northern parts of the distribution area by causing heavy mortality, especially among juveniles, in severe winters.
Serious competition between the raccoon dog and other carnivores of similar size (red fox and badger) has not been observed. In fact, badger and red fox population densities increased slightly in southern Finland in the first half of the 1980's, even though raccoon dog numbers were rapidly increasing in the same area.
The diet of the red fox, badger and raccoon dog overlap to some extent since all three carnivores feed on items such as small mammals, invertebrates and plants. But the red fox specializes more on small mammals (voles), the raccoon dog on plant material (e.g. berries) and the badger on invertebrates (Ivanova, 1962; Kauhala et al., 1993). Thus, each species has a slightly different diet, which may help them to avoid competition.
Furthermore, at northern latitudes, competition for food is probably at its fiercest during the winter, when food is scarce. However, both badger and raccoon dog spend this season asleep, so there can be no food competition in winter.
The same three species may also compete for dens. The raccoon dog often uses old badger or fox dens, but both badger and fox tend to chase it away. Nonetheless, raccoon dogs are sometimes found in the same dens as badgers during the winter, possibly because the badger goes to the den earlier in autumn and wakes up later in spring, and so may not be aware that it is sharing the den with the raccoon dog.
In conclusion, the raccoon dog really does seem to have found a vacant ecological niche in its new range; there is enough food, no serious competition with other carnivores of similar size and predation on raccoon dogs is probably not very heavy.
Helle, E. & Kauhala, K. 1991. Distribution history and present status of the raccoon dog in Finland. Holarctic Ecology 14: 278-286.
Helle, E. & Kauhala, K. In press. Reproduction of the raccoon dog in Finland. Journal of Mammalogy.
Ivanova,G.I. 1962. Sravnitel'naja harakteristika pitanija lisicy, barsuka i enotovidnoj sobaki v Voronezskom zapovednike. Uch. Zap. Mosk. Gos. Pedagog. Inst. Lenina 186: 210-256.
Kauhala,K. 1992. Ecological characteristics of the raccoon dog in Finland. PhD thesis, University of Helsinki, Helsinki.
Kauhala, K., Kaunisto, M. & Helle, E. 1993. Diet of the raccoon dog, Nyctereutes procyonoides, in Finland. Z. Saugetierkunde 58: 129-136.
Korhonen,H., Mononen,J. & Harri,M. 1991. Evolutionary comparison of energy economy between Finnish and Japanese raccoon dogs. Comp. Biochem. Psysiol. 100A: 293-295.
Lavrov,N.P. 1971. Itogi introduktsii enotovidnoj sobaki (Npg) votdelµnye obblasti SSSR. Trudy kafedry biologii MGZPI 29.
Mäkinen, A., Kuokkanen, M.T. & Valtonen, M. 1986. A chromosome-banding study in the Finnish and Japanese raccoon dog. Hereditas 105: 97-105.
Ward, O.G., Wurster-Hill, D.H. & Ratty, F.J. 1987. Comparative cytogenetics of Chinese and Japanese raccoon dogs, Nyctereutes procyonoides. Cytogenet. Cell genet. 45: 177-186.
* Kaarina Kauhala completed her doctorate on raccoon dogs in 1992 at the University of Helsinki in Finland. Now she is organizing the II North-European symposium on the ecology of small and medium-sized carnivores for 1994. She still continues her research work on the raccoon dog and other small carnivores.
© 1994 International Union for the Conservation of Nature and Natural Resources
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