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Ecological correlates of regional variation in life history of the moose Alces alces.


Regulation and limitation of populations through food shortage is a central paradigm in the population ecology of large herbivores (Caughley 1970, Sinclair 1977). Following Lack (1954), when resources are reduced seasonally, competition between the individuals for access to resources may increase, resulting in increased mortality (Sinclair 1977, Skogland 1985, 1990, Choquenot 1991) and decreased fecundity (McCullough 1979, Clutton-Brock et al. 1987, Fowler 1987). The fecundity of ungulates at high population densities may decrease through a resource-dependent reduction in body condition, often resulting in delayed maturity among young females (White 1983, Skogland 1990, Gaillard et al. 1992, Jorgenson et al. 1993) or reduction in the fecundity rates among adults (Franzmann and Schwartz 1985).

Regulation of population size (Sinclair 1989) through food limitation requires a rapid resource-dependent demographic response in the population (Maynard Smith 1974, Royama 1992). However, the establishment of such a feedback mechanism between resource level and population growth rate may be concealed by density-independent variation in demographic variables due to a stochastic environment. Both in tropical and temperate ungulates, environmental stochasticity may strongly influence population fluctuations (White 1983, Albon et al. 1987, Owen-Smith 1990). As in density-dependent cases, stochastic effects on fecundity often operate through body condition (Klein 1970, Saether 1985, Saether and Heim 1993, Langvatn et al., in press).

The present study compares patterns of covariation of life history characters in four different Norwegian moose populations living in very different range conditions (Hjeljord et al. 1994). We will examine whether differences in fecundity and mortality can be predicted from variation in resource availability. If food-limited regulation of moose populations is likely (cf. Messier 1994), we will expect, in the almost complete absence of large predators, a reduction in fecundity and an increase in mortality with reduced winter food availability.


The study was conducted in one southern (59.9 [degrees] N), one interior (60.7 [degrees] N), one alpine (61.3 [degrees] N), and one northern (69.8 [degrees] N) study area. Data were collected during the years 1985-1987 in the southern, 1985-1990 in the interior, 1984-1987 in the alpine, and 1984-1990 in the northern study area. The southern and interior study areas were located in coniferous forests, subject to relatively intense management for commercial forestry. In both areas the major food resources for moose were located near or on clear-cut areas. In the southern study area, the moose had a variety of both winter and summer food plants available. In the interior, Scots pine (Pinus sylvestris) and birch (Betula pubescens) were the dominant winter browse species. During summer, fireweed (Chamaenerion angustifolium) and birch leaves were a large proportion of the diet. For further description of these two areas, see Hjeljord et al. (1994).

The alpine area was located above the timberline, 850-1000 m above sea level. Birch was the only winter food for moose (Saether and Andersen 1990); winter range conditions were very poor due to a combination of deep snow and low biomass of browse, but during summer, many food plants were available (Saether et al. 1992). During the winter moose in the northern study area concentrated in broad river valleys where river flats were covered with dense stands of Salix species. In summer, many animals spread out and were found in rich birch forest with high production of preferred food plants. For a further description of those two study areas, see Saether and Andersen (1990), Andersen and Saether (1992), and Saether and Heim (1993).

In both the alpine and northern study areas, winter ranges were covered by snow most of the period from December to May. In the interior study area the snow cover was 35-50 cm during the winters 1985-1988, whereas in the last two study years the ground was free from snow most of the winter. In the southern study area the ground was covered with snow only during January-March with a maximum snow depth of 35 cm.


Data from radio-collared females

Data on the timing of calving, number of calves born, and juvenile survival were collected over several years from 11 adult radio-collared cows in the southern, 13 cows in the interior, 11 cows in the alpine, and 13 cows in the northern study areas. The capture procedures and the telemetry equipment are described in detail elsewhere (Saether and Andersen 1990, Andersen and Saether 1992).

Timing of calving was determined by locating and approaching the female on foot at regular intervals (usually 3-4 d) through the calving season from the middle of May to the end of June. Calving date was taken as the mid-date between the visits before and after calving. If a calf was found while still wet, calving was assumed to have occurred on the same day as the visit. All dates could be estimated within 3-4 d. Calf production was determined by the maximum number of calves recorded during the first 2 wk after calving. Because primiparous females calve later (Saether and Heim 1993) and have smaller litter size (Saether and Haagenrud 1983) than multiparous females (i.e., cows known to have calved previously), only data from multiparous cows were included in these analyses.

Juvenile survival rate was determined by recording the number of calves at regular intervals. Only females where the number of calves was known just prior to or in the first 2 d of the hunting period were included in the analyses of natural mortality. Losses of calves between the beginning of the hunting period and January were assumed to be caused by hunting. Two calves in the southern and one calf in the northern study area, killed during the winter by collisions with cars, were omitted from the analyses. Although we may underestimate natural mortality, relative differences in the natural mortality rate between the study areas should be comparable.

Age of first reproduction was recorded for 16 females in the northern and 10 females in the interior study area that were radio-collared as calves.

Data from females shot during hunting season

Data on age at maturity and timing of ovulation were collected from females shot during the hunting season, September-October, in the period 1972-1990. Females were considered to be calves or yearlings based on their tooth replacement patterns (Skuncke 1949, Markgren 1969). Age was estimated for older animals by counting annuli in the cementum and secondary dentine in the first incisor (Haagenrud 1978). Individuals were weighed just after they were shot. Carcass mass refers to mass of the skinned animal after removal of viscera, head, and lower legs (Langvatn 1977).

Ovaries were examined under a magnifying glass and the corpus luteum, corpus albicans, and corpus rubrum were counted. Presence of a corpus rubrum showed that [TABULAR DATA FOR TABLE 1 OMITTED] the cow had been pregnant the current year (Langvatn 1992). If a corpus luteum, but no corpus rubrum or corpus albicans, was present, the animal was identified as sexually mature the year of its death. When data were available from only one ovary, the individual was excluded from the analysis. Only females shot during October were included in the analyses.

Variation in age at maturity in relation to body mass was analyzed using a linear logistic model (McCullagh and Nelder 1989). If p is the probability of maturation, the model is

p = [exp([Alpha] + [[Sigma].sub.i] [[Beta].sub.i] [X.sub.i])]/{1 + [exp([Alpha] + [[Sigma].sub.i] [[Beta].sub.i] [X.sub.i])]},

where [X.sub.i] denotes the covariate i, [Alpha] the intercept, and [[Beta].sub.i] the slope parameter. The fit of the model was assessed by the use of a -2 log likelihood statistic and a goodness-of-fit statistic, computed by SPSS (Noriss 1990).

Posterior comparison of means was conducted by a Student-Newman-Keuls test (Sokal and Rohlf 1969) with a significance level [Alpha] = 0.05.


Body growth

In all age groups a significant difference was found between the study areas in body mass of the females shot during the hunting season (Table 1). Posterior comparisons of means showed that mean masses of the calves in the alpine area were significantly lower than the mean calf masses in the other two southern populations. Among the yearlings, mean masses in the alpine population were significantly lower and in the northern population were significantly higher than in each of the other three populations. As previously noted (Saether and Haagenrud 1985), relative body size differences among populations of adult females paralleled the youngest age-classes.

Mean winter masses of calves belonging to radio-collared females were suggestive of differences among study populations (Table 2, F = 2.39, 0.05 [less than] P [less than] 0.1, df = 3, 139). A posterior comparison of means showed that the calf mass in the alpine population was significantly different from the northern population. Compared to the mass of animals shot during the hunting season (Table 1), surprisingly large calves were found in the interior study area (Table 2). This was due to high rates of somatic growth during the snow-free winters in 1989 and 1990 [ILLUSTRATION FOR FIGURE 1 OMITTED], leading to large annual variation in calf masses. Mean masses of calves of radio-collared mothers from the interior study area differed significantly between the years ([ILLUSTRATION FOR FIGURE 1 OMITTED], F = 7.89, df = 4, 27, P [less than] 0.001); mean mass was 52 kg, or 40% higher in 1990 than in 1986.

Age at maturity

Logistic regressions of data collected from hunting seasons showed that the probability of ovulating as a yearling increased with body mass in the southern ([ILLUSTRATION FOR FIGURE 2 OMITTED], [[Chi].sup.2] = 5.12, P [less than] 0.05), interior ([ILLUSTRATION FOR FIGURE 2 OMITTED], [[Chi].sup.2] = 12.82, P [less than] 0.001), and alpine study areas ([ILLUSTRATION FOR FIGURE 2 OMITTED], [[Chi].sup.2] = 3.91, P [less than] 0.05), but not in the northern study area ([ILLUSTRATION FOR FIGURE 2 OMITTED], [[Chi].sup.2] = 0.63, P [greater than] 0.1) where only a small proportion of the yearlings ovulated. These relationships suggest a large increase in the probability of maturing as a yearling when the carcass mass becomes [greater than]140 kg. This suggests that in the southern part of Norway there may exist some threshold body mass for onset of breeding among yearling moose (cf. Saether and Haagenrud 1983).

Onset of reproduction of individually known females [TABULAR DATA FOR TABLE 2 OMITTED] that were collared as calves, was earlier in the interior than in the northern study area (Z = 1.97, P = 0.0492). In the interior study area, three females matured as yearlings; five females at 2 1/2 yr old and two at 3 1/2 yr old. In the northern populations, eight females matured at 2 1/2 yr old, whereas 8 females matured at older ages.

Timing of reproduction

The proportion of females older than 3 1/2 yr that were shot in the period 1-10 October that had corpora lutea differed significantly between the study areas (Table 2, [[Chi].sup.2] = 39.78, df = 3, P [less than] 0.001). Thus, the timing of ovulation was delayed with increasing latitude.

Differences in timing of ovulation were not reflected in a corresponding difference in the calving dates. To avoid bias in regional comparisons due to differences in age structure, primiparous females that usually calve later (Saether and Heim 1993) were excluded from the analyses. Mean calving dates differed by [greater than]14 d among populations across years (Table 2). The calving in the alpine study area occurred significantly later than in any of the other study areas (Kolgomorov-Smirnov two-sample test, P [less than] 0.005 in all comparisons). Furthermore, the calving occurred significantly later in the northern than in the two southermost populations (Table 2, P [less than] 0.05). The difference in mean calving date between the two southern populations was only [approximately equal to]2 d (P [greater than] 0.1). Thus, the calving dates differ regionally, dependent on both the gestation period and the timing of ovulation. The longest gestation period was found in the alpine study area with poor winter feeding conditions.

Fecundity rates

There was a significant difference among populations in number of calves at birth of multiparous females (F = 4.25, P [less than] 0.01, df = 3, 207). Posterior comparison of the means showed significantly lower fecundity rates (mean [+ or -] 1 SE, 0.93 [+ or -] 0.09 calves/yr) in the alpine study area than in each of the other three study areas. In contrast, in the most productive northern study area females averaged 1.35 [+ or -] 0.07 calves/yr. Also in the two southern study areas [greater than]1 calf was produced per female per year (1.24 [+ or -] 0.06 and 1.27 [+ or -] 0.09 calves [multiplied by] [female.sup.-1] [multiplied by] [yr-1] in the interior and southern study area, respectively).

No significant difference was found between the study areas in the twinning rate (number of calves produced per calving, F = 2.04, P [greater than] 0.1, df = 3, 123).

Mortality of calves

Mortality during the 1st yr was estimated by the loss of calves of radio-collared females. When calves shot during the hunting season were excluded, a significant difference in calf loss during the 1st yr of life was found between the study areas ([ILLUSTRATION FOR FIGURE 3 OMITTED], [[Chi].sup.2] = 11.51, df = 3, P [less than] 0.01). The highest losses occurred in the northern study area, where 20.6% of the calves born in this study area disappeared before the start of the hunting season. This figure is probably an underestimate because calves whose date of disappearance was not known were excluded from the analyses, even though some were likely to have died of causes other than hunting. No significant difference was found between the study areas in the winter losses of calves ([[Chi].sup.2] = 4.51, df = 3, P [greater than] 0.1).


Age at maturity and body growth

The rate of body growth was highest in the northern population (Table 1), where the quality and quantity of preferred summer food plants were highest (Saether et al. 1992, Saether and Heim 1993). This corresponds with previous studies of moose (Saether 1985, Saether and Helm 1993) as well as other ungulates (White 1983, Albon and Langvatn 1992), which have demonstrated a relationship between body mass and the range conditions during summer. However, the present study also demonstrates an effect of winter range conditions on body mass because significantly larger body masses were recorded in the interior study area after the snow-free winters in the last years of the study period. This increase in mass was related to a shift in winter diet from rather heavily browsed Scots pine to blueberry (Vaccinium myrtillus) (Saether et al. 1992). This food plant is highly preferred by moose, but in normal winters snow cover prevents access to this food source. Similarly, the lowest body masses were found in the alpine study area, where snow depth was highest and winter food supply was lowest. In this area, the rate of forage intake (Hjeljord et al. 1994) of the calves in March was 456 kJ [multiplied by] [kg.sup.-0.75] [multiplied by] [d.sup.-1], compared to 602, 678, and 828 kJ [multiplied by] [kg.sup.-0.75] [multiplied by] [d.sup.-1] in the interior, northern, and southern study areas, respectively. Thus, climatic variables influencing range conditions both during summer and winter affect body mass of moose.

The differences among populations in rate of body growth could not explain the regional variations recorded among the females in age at maturity. In the northern area with the largest yearlings (Table 1) only a small proportion of the yearlings ovulated [ILLUSTRATION FOR FIGURE 2 OMITTED]. Accordingly, none of the radio-collared females in this population gave birth to a calf at 2 yr old. In the three southern study areas, small yearling females do not ovulate but this probability increases rapidly when the carcass mass exceeds 140 kg, suggesting a body mass threshold for start of ovulation. However, this threshold for onset of maturation is less pronounced in moose than in red deer (Cervus elaphus) (Langvatn et al., in press) and wild reindeer (Rangifer tarandus) (Reimers 1983).

Two factors may explain why the age at maturity is delayed in the northern population: (1) In the north, the costs of early maturation may be larger than in the other areas, which should select for delayed maturation (Engen and Saether 1994). For instance, moose may reach the necessary body mass so late in the season that the chances for a successful reproduction are low (cf. Armitage 1981). If the length of the breeding season is limiting, we would expect a similar delayed maturation among the moose from the alpine areas where the period of snow-free ground is quite similar to northern Norway. This did not seem to be the case ([ILLUSTRATION FOR FIGURE 2 OMITTED]; see also Saether and Haagenrud 1985) so this hypothesis is unlikely to explain the delayed maturation in the north. (2) Several theoretical studies have shown that optimal age at maturity is strongly influenced by the probability of future survival (Stearns and Koella 1986, Charlesworth 1994, Engen and Saether 1994). In Norwegian moose, the main source of mortality is hunting. A very low proportion of adult females (often only 1-2%/yr) dies from other reasons (B.-E. Saether et al., unpublished data). However, the mortality rate during the 1st yr was higher among the offspring of radio-collared females in northern Norway than in the rest of Norway [ILLUSTRATION FOR FIGURE 3 OMITTED]. This was probably related to a higher risk of mortality due to difficult climatic and topographical conditions. In fact, on 3 of 16 occasions, first-time breeders lost their calves (Saether et al. 1992). Similarly, juvenile mortality was significantly higher in fawns of young mothers in three species of gazelle (Gazella sp.) (Alados and Escos 1991). We suggest that if calf mortality is higher among young than among older females, this may select for delayed maturation in these populations (Michod 1979, Engen and Saether 1994). Furthermore, it may be argued that predation on moose calves in northern Fennoscandia has been more intense in recent times than farther south, where the populations of large carnivores during the last two centuries have been more effectively reduced by man, and higher predation should select for delayed maturation. Skogland (1989) argued that high wolf predation on calves has selected for delayed maturation in caribou, compared to reindeer with low losses of calves. Furthermore, although maturation in general occurred 1 yr later in the north than in the south, within populations maturation depended on body mass (Saether and Heim 1993). This is in accordance with the predictions from the analyses made by Stearns and Koella (1986) that higher juvenile mortality, as was found in the northern population, should not change the shape of maturation trajectory in the size-age plane (see Engen and Saether 1994 for a qualitatively similar result). Thus, geographical variation in the juvenile survival rate may explain the regional difference that has evolved in the reaction norm of age at maturity [ILLUSTRATION FOR FIGURE 2 OMITTED]. A similar pattern has previously been described in reindeer (Skogland 1989, 1990).

The implications for population regulation

When moose face reduced resource availability during winter, several behavioral changes occur. As quality of food (expressed as digestibility) decreases, moose spend more time resting and decrease intake of browse (Saether and Andersen 1990). As a consequence, the rate of energy gain is reduced (Hjeljord et al. 1994). This low intake rate is associated with several life history changes. First, fecundity rate is decreased due to an increase in the interval between calvings (see Results: Fecundity rates). No change occurred in twinning rate. A decrease in the frequency of twinning was, however, recorded in Alaskan moose with a decline in habitat quality (Franzmann and Schwartz 1985). Thus, both the twinning rate and the proportion of cows with calf may decrease with poor range conditions. Second, in the alpine population, calvings occurred later in the season than in the other populations. Since timing of ovulation among adult females in this population did not deviate from expected based on latitude (Saether et al. 1992), the gestation period must have been prolonged. Some evidence suggests that the gestation length may increase with poor nutrition during the pregnancy (Verme 1965, Kiltie 1982, Bowyer 1991, Berger 1992). Third, delayed age at maturity was found in the alpine population, as expected from low yearling masses. A similar increase in age at maturity due to low body masses has also been described in other cervids (Skogland 1990, Gaillard et al. 1992, Jorgenson et al. 1993). Thus, poor winter conditions were associated with a reduction in the number of calves produced per adult female.

Stochastic variation in climate also affected the reproductive rate. In the interior study area, a large increase in winter live masses of calves occurred in the snow-free winters [ILLUSTRATION FOR FIGURE 1 OMITTED]. Since age at maturity is dependent on body mass [ILLUSTRATION FOR FIGURE 2 OMITTED], this led to an earlier age at maturity: 7.3% (n = 41) of yearling females shot during October in 1986-1988 had ovulated, compared to 22.6% (n = 93) among the females shot in the years 1989-1990 ([[Chi].sup.2] = 4.55, df = 1, P [less than] 0.05). Thus, the effects of winter food limitation on fecundity will also be influenced by stochastic climatic variables, affecting access to food resources. In moose, summer climate also affects body mass (Saether 1985, Solberg and Saether 1994), which may also influence the age at maturity of females (Saether and Heim 1993). Thus, the reproductive rate of the females seems to be greatly influenced by density-independent factors, operating both through summer and winter climate. Even though resource-dependent decreases in fecundity could be regulatory, their contribution to variation in the recruitment rate is likely to be less than the variation due to climate. Reproductive output is, however, still so high in the interior study area that the population growth rate will be [greater than]1.

Since the pioneering studies by Lack (1954, 1966), it has generally been assumed that population regulation (Sinclair 1989) operates through a positive relationship between mortality rate and resource limitation during the nonbreeding season. However, in the present study very poor winter feeding conditions in the alpine study area (Saether and Andersen 1990, Hjeljord et al. 1994) were not associated with an increase in the mortality rate among the calves. The highest mortality rates were found in the northern population [ILLUSTRATION FOR FIGURE 4 OMITTED] with good feeding conditions during both summer and winter. We do not know the reason for the high losses in this area. However, the peak in the calving period often coincides with the peak snow melting period in this mountainous area; small moose calves may easily be caught by flooded streams.

Our study has shown that one mechanism of resource-dependent reduction in reproductive rate is an increase in age at maturity. Small females mature later than large females (Saether and Haagenrud 1983, Saether and Heim 1993). A consequence of this is that a delay will occur between the occurrence of the ecological change (change in the winter feeding conditions) and the response to this change in the population. Such time delays are likely to generate complex population fluctuations (May 1981). Some evidence exists for very unstable population size on Isle Royale at very high densities (Messier 1991).

In an examination of the interactions between wolf (Canis lupus) and moose in 27 areas in North America, Messier (1994) suggested that multiple stable states are possible in this system. The high-density equilibrium is generated by a density-dependent decrease in the population growth rate through food competition. The present study suggests that such a decrease is likely first to occur at very high densities, close to the carrying capacity, K. This supports the suggestion by Fowler (1981) that the largest effects of density dependence in large mammals will occur at very high densities. Similarly, the delay in the demographic response to resource depletion may lead to an overshoot of K (May 1981). Thus, a stable high-density equilibrium between moose and their food resources is unlikely to occur. Hence, this study supports Messier's (1994) conclusion that predation is important for a regulation of moose population.


This study was generously funded by a grant from the Norwegian Directorate for Nature Management. A. Gravem was helpful in all stages of the project with collecting and organizing the data. We are grateful to E.O. Oen and M. Djupsjo for help with immobilization of the animals. O. Overrein, A. Slaen, E. Naess, and A. Gransjoen organized the collection of material from animals shot during the hunting season. We are grateful to M. Festa-Bianchet, J.-M. Gaillard, T.A. Hanley, T. Skogland, N.A. Slade; S. Stearns, and two anonymous reviewers for comments on previous drafts of the manuscript.


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Author:Saether, Bernt-Erik; Andersen, Reidar; Hjeljord, Olav; Heim, Morten
Date:Jul 1, 1996
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