Being one of the most widespread taiga mammal species in the North-West of Russia, the forest lemming nevertheless remains one of the less studied representatives of the local theriofauna. This is explained, in addition to being near the southern borders of the distribution, by the extreme secrecy of its way of life and the associated difficulties of studying it.
M.Ya. Marvin (1959), one of the first teriofauna researchers writes about sporadic distribution and great rarity of the forest lemming in Karelia. However, in reality this species, although not numerous, is by no means rare and is found here regularly and everywhere. Its rarity in the samples of previous years is explained by the fact that the capture was then carried out with the help of break-back traps, and the forest lemming is known to be poorly caught in this way. Up to date its captures are recorded in all districts of Karelia, where trap-trench accounts were carried out, but collections from Kalevalsky, Kondopozhsky, Pitkyarantsky areas relating respectively to northern, average and southern Karelia are especially representative.
Nevertheless, in special literature devoted to mammals, information on this species is still extremely scarce and fragmentary. In this regard, the report, which aims to summarize, analyze and discuss materials collected over six decades (1958-2017) on the ecology of this species in Karelia, can, we hope, help at least partially fill this unfortunate gap.
The material for this work was the long-term collection of animals of the studied species, caught by two main methods: trap-lines (Hero break-back traps) and 30-meter trap trenches. 243600 trap-days were worked out by the first method, 24 forest lemmings were caught, 5400 trench-days were worked out and 192 individuals were caught.
Total number of captured and conducted through zoo-anatomical analysis animals of the studied species was 216. At that, the assessment of the number and territorial distribution of the animals, age determination, the analysis of the reproductive system, age and sex composition of the population, the study of shedding the coat and nutrition features were carried out according to the generally accepted method (reviews, see Novikov, 1953; Karaseva et al., 2008).
Abundance and biotopical distribution
The average number of forest lemmings in Karelia during the entire period of our research was 0.26 per 10 trench-days (1.9 % in small mammal catches) and 0.007 per 100 trap-days (0.09%). At the same time, the average long-term index of counting naturally decreases from North to South. For North Karelia, it is 0.8 per 10 trench-days (7.3 % in small mammal catches), in the middle part – 0.3 (1.8 %), and in South part – 0.2 (1.3%). In comparison with data from other, especially central parts of the area, these indicators look low.
As shown by censuring, in the taiga in the North-West of Russia forest lemmings mainly inhabit coniferous green-mossy forests with a powerful moss and shrub cover – pine and spruce forests. In this case, the presence of green mosses and the development of a moss cushion play the main part, but the closeness and composition of the stand, the drainage of the soil, illumination, the presence of undergrowth, etc.are not essential. The moss cover of the Karelian taiga is formed by the main types of green and bryophyte mosses: Tomenthypnumnitens, Aulacomnium sp., Brachythecium sp., Dicranum sp., Ptilidiumciliare, etc. Moss cushion often reaches a thickness of 10-15 cm and reliably covers not only the passages, but also the nests of lemmings, creating favorable homogeneous climatic conditions for the existence of animals and at the same time provides rich food supply for animals.
Typical habitats of this species are characterized by the presence of developed moss cover and good protective conditions provided by moss turf or permafrost cracks of the soil cover, dead wood or tussock bog, and more often by a combination of these factors. Bogginess and moisture do not always accompany its habitats, but in Western Siberia, forest lemmings often inhabit moss-sedge and tussock bogs and swampy margins of forest lakes (Yudin et al., 1976).
Lemmings find less favorable environmental conditions in deciduous and mixed forests, overgrown with shrub meadows, in deciduous light forests. Even during the years of maximal population growth, they do not live here permanently, but appear only during the period of settlement of young animals. Lemmings were not caught at all in lichen and rock pine forest, in clearings and on the moor edges.
Nevertheless, lemmings were regularly caught in landscapes heavily transformed by logging, especially in bilberry pine forests and even on the edges of swampy logging (Kurkhinen et al., 2006). Under these conditions, the preservation of a small but stable population of forest lemmings is possible even with a deep anthropogenic transformation of the landscape, provided that small arrays of coniferous forests with a powerful moss and shrub layer are preserved in felling areas. Intensive forest management ("Scandinavian technology"), accompanied by forest drainage, forest rejuvenation and the formation of coniferous monocultures, seems to be the main reason for the reduction of the range and number of forest lemmings in Scandinavia (H. Hentonen, oral report).
Activity. Nesting. Seasonal migration
The activity of forest lemmings is polyphasic (Gromov, Yerbaeva, 1995). Most often, they use various natural shelters; sometimes they make moves in the moss layer, but they do not dig real holes. At the same time, a characteristic feature of the ecology of the forest lemming is a relatively small mobility, the desire to limit their activities to a small area with optimal feeding and protective conditions. This leads to the lace character of the distribution of animals within the biotope and their rare trapping outside the settlement period. Lemmings compensate their low mobility by vigorous activity within a small territory. Here there are many paths and passages, gnawed in the moss cover, characteristic digging, droppings, etc.
Lemmings make their nests in simple burrows between the roots of trees, in moss bumps or among mossy stones. The material for the nest is dry grass and stalks of moss.
Table 1. Results of censuring of forest lemmings in the Kivach reserve during the peak years (1966-1967) (in the numerator — the number of animals per 10 trench-days, in the denominator – the share of the catch,%) (by: Ивантер, 1975)
|1966||1967||For the entire censuring period|
|Pinery with grass – green moss||4.0 / 14.3||0.8 / 50.0||—||2.3 / 10.3||4.3 / 27.3||1.9 / 12.2|
|Spruce grove with green moss||4.0 / 25.0||—||1.2 / 4.8||6.9 / 23.1||2.9 / 18.2||3.4 / 17.1|
|Ripe birch trees with spruce||—||—||—||1.0 / 4.3||1.4 / 11.1||1.1 / 7.2|
|Meadows among mixed forests near villages||—||—||—||1.8 / 11.3||1.4 / 8.0||0.9 / 8.2|
|On average for the entire reserve territory||2.0 / 11.7||0.2 / 7.7||0.3 / 1.7||2.5 / 11.9||2.3 / 14.3||1.4 / 10.3|
This rodent is characterized by seasonal migrations, sometimes taking on a mass character, especially in years of intensive breeding. A similar migration observed at the end of July 1960 in the valley of the middle course of the Kolyma River is vividly described by B. S. Yudin and co-authors (Yudin et al., 1976). According to their observations, during this period, young animals in the mass were evicted from the typical stations in those places, floodplain larch forests, to open habitats - horsetail-cereal meadows, grassy bogs. They swam the Kolyma feeders and often became prey for numerous pike. In the riverside of the Kolyma River corpses of drowned lemmings were found in the mass. The main movements of the animals occurred in the evening and at night, when the sun is very low above the horizon to the North of the Arctic circle. So, all forest lemmings that had swum across the Kolyma feeders were found after 22 o’clock. The width of the feeders in which they were observed reached 400 m. Among the animals, there were males and females weighing, as a rule, less than 20 g.
On the Kola Peninsula, mass migrations of this species also begin in mid-summer, but it lasts until October (Novikov, 1941). And among migrating individuals, young immature animals also predominate, although pregnant ones were also noted.
It can be assumed that the occurrence of such migrations is associated with the settlement of young lemmings in the years of their mass reproduction due to the resulting disproportion between a large number of consumers and limited stocks of green mosses, which serve as the main food base of this species throughout its vast range. According to Finnish zoologists (Kalela et al., 1963), in Northern Finland, forest lemming migrations occur in spring and autumn and are caused by seasonal changes in forage and habitat.
Forest lemmings specialize in feeding on green mosses. In places where they feed, we found some traces of their feeding on Pleurozium schreibery, Cyclocomium splendens, Polytrichum sp., Dicranium sp., and others. In this case, the frequency of eating a particular type of moss depends entirely on its abundance in the habitat. Direct estimates of the use of moss cover by lemmings obtained by V.N.Bolshakov and co-authors (Bolshakov et al., 1986) in the southern Urals have shown that the maximum share of moss-covered surface available and used by animals does not exceed 14% and is on average equal to 10% of the total projective moss cover. In the 32 lemming stomachs we studied, we also found mostly green mosses, only 6 stomachs contained the remains of herbaceous plants along with mosses, two – seeds and one – bilberries. Field observations showed that when lemmings ate green mosses intensively, they left characteristic and well-marked traces of feeding –bald spots in the moss litter in feeding areas. When kept in cages, they were most likely to eat the youngest parts of the moss, as well as in smaller quantities and less readily – the leaves and roots of sedge and leaves of marsh vetchling. Ground-based lichens, shrubs (cranberries, blueberries, ledum, butterbur, etc.) were completely ignored.
Specialized feeding of forest lemmings mainly with green mosses with a small share of other plant food was also observed in other parts of the species range: on the Kola Peninsula (Novikov, 1941), in Finland (Skaren, 1963), in the upper Lena valley (Raimers, Voronov, 1963), in southern Yakutia (Revin, 1968), in the far East (Yudin et al., 1976), in Barab (Glotov et al., 1978). Leafy and liverwort mosses, although go under the snow green, are not nutritious in their biochemical composition. In addition, they contain a large amount of poorly digestible cellulose and hemicellulose, as well as lignin and tannins, which completely deprives them of attractiveness for animals.
Low nutrition and poor digestibility of the main feeds of forest lemmings led to a significant development of its colon. The relative total length of the intestine of this species is also higher than that of Siberian lemmings and voles, whose feeds are more nutritious and easily digestible.
Reproduction and population structure
In our conditions, the beginning of reproduction of forest lemmings is timed to the second half of May. The male, caught 19.07.1967, had greatly enlarged sex glands (testis length 10.8, seminal vesicles 15.3 mm, weight of two testes 588 mg) in a state of active spermatogenesis: ripe spermatozoa were found in the tubules and appendage of the testis. At the same time, the adult female was caught 22.05.1971 which had not reproduced (the length of the uterine horns 18.4 mm, the weight of the uterus and ovaries 111 mg). In June, 3 adult females were caught, one of them was pregnant (5 embryos 11-12 mm long), and the other two fed the brood (24.06.1965, 29.06.2011). In July there is a height of the breeding season. All three adult females caught at this month were pregnant, and the wintering male was in a state of sexual activity (testis length 11 mm, seminal vesicles 15 mm, weight of two testes 604 mg). In August, breeding of adult lemmings continues, and in September it finishes. The length of the testes in males caught in September was 6.4-8, on average 6.9 mm, the seminal vesicles were 1.5-3 mm, on average 2 mm, the weight of two testes was 89-140 mg, on average 110 mg; there was no spermatogenesis. The last lactating female was caught 4.09.1970. This depletes all the information we have about the reproduction of the overwintered part of the population. Apparently, adult females produce at least 2-3 broods over the summer, and their participation in reproduction is 100%.
The first young animals appear in catches from mid-June (16.06.1967, 14.06.1998), but in large quantities - in July. At the end of July, some of the incremented lemmings of early broods can reach sexual maturity and participate in breeding. However, most of the young animals mature only in the spring of the following year, and the total number of reproducing young lemmings does not exceed 15% in females and 20% in males. A relatively small degree of participation of young females in breeding is also typical for lemmings in Finland. According to M. Ilmen and S. Lahti (Ilmen, Lahti, 1968), most of them overwinter immature at a weight of 18-20 g. In Yakutia, the total number of females that breed for the first time in their life and participate in breeding is 33.3% (Кrivosheev, 1971).
According to our data, the average brood size of a forest lemming in Karelia is 4.44 ± 0.26 with fluctuations from 3 to 6 (n = 18). This is close to the data for Eastern Finland (4.6 ± 0.42), but lower than in Yakutia (5.1 ± 0.24) and the Pechora-Ilych reserve (Krivosheev, 1971; Skaren, 1972; Bobretsov, 2004). The relatively low one-time fertility of lemmings (in comparison with bank voles and gray voles) is usually associated with good survival of young animals due to a stable food base and good protective conditions of the habitats (Кrivosheev, 1971). However, it is no less likely that this phenomenon is associated with a high frequency of brood births. For example, in Finland, the interval between broods in forest lemmings is usually 23–28 days: females mate 3-4 days after birth (Ilmen, Lahti, 1968).
Analysis of the age composition of the caught animals (table. 2) shows that until June inclusive, the population of forest lemmings is dominated by adults who have overwintered. Later, the share of them falls sharply, and in October, the catches consist exclusively of newly born lemmings. The sex ratio of this species, judging by the capture of young animals, deviates in favor of females. In Finland, this disparity is even greater: among embryos, newborns, and subsequent age groups, females make up 70-80 % (Kalela, Oksala, 1966). At the same time, it should be emphasized that the predominance of females in the forest lemming population is a very characteristic feature of this species, which, apart from us (Ivanter, 1975), was noted by many zoologists in different parts of the area (Novikov, 1941; Chernyavsky et al., 1978; Lyamkin et al., 1983; Bolshakov, Kubantsev, 1984; Gileva, Fedorov, 1984). It is explained by the presence of a unique system of genetic determination of sex, when along with normal females having two female (XX) chromosomes in the karyotype, a significant number of phenetically normal and fertile females with the male XY karyotype are present in the population (Fredga et al., 1977). Thus, according to the observations of E. A. Gileva and V. B. Fedorov (Gileva, Fedorov 1984), in the Northern Trans-Urals, the proportion of such females varied over the years from 33.3 to 56.5 %. In our collections, the percentage of females among youngsters varied by month in the range of 53-75 % and averaged about 60 % (see table. 2). As for adult (wintering) animals, among them the numerical deviation in favor of females (75 %) was noted only in July. In the remaining months, males reliably dominated (from 80 to 100 %). According to the authors who specifically discussed this aspect (Fedorov, 1992; Bobretsov, 2004), this can most likely be explained by the greater mobility of males during the peak of reproduction, which falls just in the middle of summer.
Table 2. Sex and age ratios in the forest lemming population in Karelia according to mass capture data
|Month||The number of increment specimens||The number of overwintering||Total number of animals|
A large proportion of youngsters in July catches (87.5 %) and their relative weight uniformity (12-17 g) indicate that the first spring-summer breeding of lemmings takes place in Karelia very amicably, and it is this, but not subsequent generations, that determines the high population size in the autumn.
As far as we can judge from the state of the scrapings and fur, the spring shedding of adult forest lemmings takes place in April – May. Young individuals shed for a long time, starting in July and until mid-October. In July, shedding animals accounted for 44 % of our collections, 53% in August, 60% in September, and 83% in October. In the second half of October, the shedding ends; all caught lemmings had fully formed winter fur.
Changes in the number
In some years, in the North-West of the taiga zone, there are sharp rises in the number of forest lemmings, which increases tenfold. In our region, and in particular in the Kondopoga and Pitkyaranta districts of Karelia, the mass appearance of this species was noted in 1966, 1970, 1987, 1992, 1998, 2003, 2008 and 2012-2013, when the catch rate per 10 trench-days increased from 0.0-0.09 to 0.3-1.3, i.e. more than 13 times. Accordingly, the share of this species in the catches of small mammals increased from 0.0–0.6 to 10-11%. Ten years earlier, in 1957, an even stronger outbreak of breeding and a mass "invasion" of forest lemmings were observed in North Karelia (in the Kalevala district). There was a real migration of animals. They came in an avalanche on the roads, ran into houses and barns, and died by the hundreds, unable to withstand the hardships of the way. For only 2 days of work, September 22 and September 24, 1957, on one of the streets of village Ukhta, employees of the field zoological unit collected 27 corpses of forest lemmings. In the same year, 1957, a sharp increase in the number of this species was observed in neighboring Finland. According to the U. Skaren (1963, 1972), in August 1957, the number of forest lemmings there was 6.2 specimens per 100 trap days (21.9% in Micromammalia catches). In 1963 a high concentration of lemmings was observed in Eastern Norway (Mysterud, 1968).
In the light of the above, the thesis of T. V. Koshkina (1958, 1970, 1971) that the number of secondary species of voles is always several times lower, and the amplitude of fluctuations in the number is ten times greater than that of the dominant species, deserves attention. At the same time, the small number of secondary species in comparison with the dominant ones is determined by their higher mortality, rather than a low rate of reproduction. Reproduction of the dominant species in the years of growth of their number is sharply reduced due to intraspecific competition and selection for the greatest aggressiveness in territorial competition and increased ability to master the space with a significant crowding of individuals. Individuals with these abilities are also characterized by low breeding potential (Ghitty, 1964).
Without sufficient material, we do not undertake to explain the reasons for the sharp increase in the number of forest lemmings. However, it is clear that the feed factor has nothing to do with it. Stocks of green mosses, which lemmings mainly feed on, do not change much over the years. It is not possible to link the lemming population level to weather conditions. At the same time, the synchronicity of the lemming population fluctuations with those of other rodent species (although this synchronicity is not complete) allows us to assume the general causes of their population dynamics. It is much easier to explain the sharp decline in the lemming population the year after the rise. Here, mortality and migration, regulated in accordance with the population density play the main role. It is known that an intensive increase in the number of rodents creates "population stress" that suppresses reproduction, increases the aggressiveness and mobility of animals and leads to an increase in mortality (Ghitty, 1960; Petrusevich, 1960; Bashenina, 1963; Christian, 1963, 1971; Koshkina, 1971; Shilov, 1972, etc.). It is possible that the forest lemming has similar intra-population mechanisms for population stabilization. As for the sharp fluctuations in its number, the duration of deep depressions, and the short duration of rises, all this indicates not the absence or imperfection of such mechanisms, but that they work without outrunning and are implemented mainly through migration and mortality. Changes in the intensity of reproduction associated with population density (in particular, the different degree of participation of young individuals in it) play a subordinate role in this case. The impact of dominant species is also significant (hence the duration of depressions).
Distinct autumn rises and spring declines in numbers typical of our other small mammals in the forest lemming occur with much smaller amplitude. The amplitude of long-term changes in the population is also low (only once, between 1969 and 1970, it exceeded the average for the entire period by 4 times, amounting to 5.6 times), which also demonstrate their absolutely random, ragged rhythm. For almost 50 years of censuring by trap trenches, from 1966 to 2014 (Priladozhsky station), only 5 fairly high rises were recorded there (1966, 1970, 1987, 1992, and 2003), repeated after 3, 17, 5, and 10 years. It is even worse with declines in numbers. They lasted from 3 to 17 years and alternated without any clear periodicity at intervals of 3-9 years.
Research shows that identified above ecological features peculiar to the forest lemming in the specific conditions of the southern periphery of the area, has considerable lability and a harmonious coherence both with the density and status of peripheral populations and external environmental factors. The variety of ecological adaptations, their transformation and change in time and space, adaptive dynamism and orientation to a specific ecological situation, which consists of the interaction of exogenous and endogenous factors, determine the relative stability and optimal state of the populations of the studied species in extreme frontier conditions. Instead of narrow special adaptations of individual rank in peripheral populations of such a widespread polytypical species as the forest lemming, wide structural-population-based adaptive complexes characterized by dynamism and high rate of compensatory adjustment are successfully implemented. Specific regulatory mechanisms of directed action that maintain the number at the level of an improved group organization play a certain role in the life and especially reproduction of forest lemming populations. However, in the conditions of the periphery of the area, the effect of these mechanisms is more limited and maintaining a dynamic balance of the population with the resources of its habitat is achieved with more significant participation of external factors.
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