Are moose only a large deer?: some life history considerations.

ABSTRACT: Body mass generally accounts for a large part of variation in life history traits of ungulates. However, phylogeny and ecological features such as habitat or diet have been shown to cause differences in life history patterns among species of similar size. To assess the factors that shape life history traits of moose (Alces alces), the largest deer (Cervidae) species, I fitted allometric relationships among ungulate species for a set of life history traits. I compared moose life history traits first with both traits expected from allometric equations and traits of similar-sized bovids. Both kinds of analyses led to the same results. While moose calves grow as expected from the size of their mothers, they start life at only about half the expected size. Moose populations have higher growth rates and shorter generation times as compared to similar-sized ungulates. Females reproduce earlier and have larger litters relative to their body size. The resulting faster than expected life cycle for moose cannot be accounted for by changes in survival patterns: moose closely fit the general pattern of ungulate population dynamics characterized by a low and variable juvenile survival as opposed to a high and constant survival of prime-age females. High reproductive output accounts for the fast life cycle of moose populations compared to other similar-sized ungulates. I propose that the high reproductive output has evolved in response to the unpredictable environmental conditions of early successional habitats preferred by moose. The evolutionary strategy of moose appears more similar to that of a very large roe deer (Capreolus capreolus) than that associated with larger deer in general.

ALCES VOL. 43:1-11 (2007)

Key words: allometry, birth mass, evolution, fitness, generation time, maternal care, reproductive output, survival patterns, ungulates

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Since the pioneering work by Stearns (1976), the study of variation in life history traits has become a popular task among evolutionary ecologists. The analyses of variation in life history traits can be performed at two different scales. First, the variation at the interspecific level is generally studied using species-specific data collected from the literature by using comparative analyses (sensu Harvey and Pagel 1991). Second, the existence of evolutionary trade-offs between fitness components or the assessment of life history variation generated by differences in phenotypic quality are usually performed at the intraspecific level by analyses of population- or individual-specific data (see Roff 1992, Steams 1992 for reviews). At the interspecific level, the variation in life history traits of vertebrates is mostly accounted for by three major structuring factors. Variation in body size generally accounts for more than half of the variation in most life history traits (Peters 1983, Calder 1984, Brown and West 2000 for reviews). In mammals, for instance, it is well-established that large mammals live longer, reproduce later, and produce fewer offspring per year than small ones (Stearns 1983, Gaillard et al. 1989). However, for a given size, taxa often show marked differences in life history traits. For example, it is well known that bats outlive similar-sized rodents. Thus, ecological correlates of life hi story traits also occur. Differences in diet and differences in habitat quality have been shown to generate differences in life history traits (Saether and Gordon 1994 for ungulates, Fisher et al. 2001 for marsupials).

Moose (Alces alces) are the largest members of the Cervidae family (from 200 to 825 kg, Novak 1993). Therefore, I expect that its large body size may have markedly shaped life history traits currently observed in moose populations. From comparative analyses of maternal care and demographic patterns reported in populations of moose and related ungulate species, I assessed whether moose life history can simply be accounted for by large size (i.e., moose are only large deer), or whether moose have specific life history traits independent of their size relative to other deer (i.e., moose are different than a large deer).

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