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Ims (1987a) hypothesized that the spatial distribution pattern of male voles may be influenced by the temporal distribution of females. When females enter estrus synchronously (i.e. are clumped in time) they represent a defendable resource and males should defend a territory containing a cluster of females. When females breed asynchronously, males should not be territorial. In this study we examined the role of female breeding synchrony on the spacing strategies of male meadow voles Microtus pennsylvanicus (Ord, 1815). Maternity was determined by the transfer of a unique combination of radionuclides from mother to offspring. The date of conception (to the nearest week) was determined by the weight at first capture of newly recruited voles. The number of litters conceived per week ranged from zero to four. The null hypothesis that the number of litters conceived per week was distributed randomly could not be rejected in 8 of 9 grid-years (4 years on 3 grids). This finding of breeding asynchrony in a vole species with nonterritorial males is consistent with Ims' hypothesis-.
We studied the influence of immigration on the dynamics ofMicrotus ochrogaster Wagner, 1842 andM. pennsylvanicus Ord, 1815 populations in alfalfa, bluegrass, and tallgrass prairie in east-central Illinois for 25 years. The numbers of immigrants in a site were positively correlated with overall population densities of the species in the vicinity of the study sites and within the study site. Population density of the other species was not correlated with immigration of either species. Immigrants did not differ significantly from residents with respect to sex-ratio and reproductive condition. Persistence of immigrantM. pennsylvanicus was lower than that of resident adults in all three habitats, while that of immigrantM. ochrogaster was lower than that of resident adults in alfalfa, where the species was most abundant. Neither the absolute number of immigrants nor the proportion of the population composed of immigrants indicated an effect of immigrants on among- and within-habitat differences in demography and population fluctuations of either species. Immigrants may have been an important factor in maintenance ofM. pennsylvanicus in alfalfa, a low-quality habitat, but the influence of immigration on the dynamics of populations inhabiting other habitats was insubstantial.
We studied factors affecting peak densities and amplitudes of fluctuation during 20 annual population fluctuations ofMicrotus pennsylvanicus Ord, 1815 in alfalfa and bluegrass habitats over a 25-year period. Survival was correlated with population density over the 25 years and was the most consistent variable associated with stoppage of population growth. Although not correlated with population density over the 25 years, a decline in the proportion of reproductively active adult females contributed to cessation of growth of population fluctuations that peaked in late autumn-winter, and to cessation of growth of eight of eleven population fluctuations that peaked during summer-early autumn. We conclude variation in survival to be the primary factor affecting peak densities and amplitudes of population fluctuation ofM. pennsylvanicus.
I tested the "reallocation" and "increased demand" hypotheses of seasonal al­location of energy using female meadow voles Microtus pennsylvanicus (Ord, 1815). I quantified the daily energy expenditure of adult females by the doubly labelled water method, both in the summer (reproductive) and winter (non-reproductive) seasons. Females were studied in field enclosures and were neither pregnant nor lactating, which made it possible to quantify the effect of season on non-reproductive costs. These costs were 20.6% higher in winter than in summer (3.40 vs 2,82 kj x g-1 x day"1). Laboratory data found in the literature showed that the cost of milk synthesis averages 3.53 kJ x g_1 x day"1 Assuming that these costs can be translated to natural conditions, daily energy expenditure of lactating females in the field is 225% that of non-reproductive ones. According to these results, females seem to follow a mixed strategy of both reallocation and increased demand of energy, although the observed pattern of allocation is much closer to an increased demand model. I discuss the assumption that lactation costs measured in captive voles can be transposed to field conditions, and propose an index of reallocation of energy that describes the strategy used by individuals. This index should permit the placement of each new species studied along the continuum of strategies that probably exists in mammals.
Social behavior of small mammals living under natural conditions often is inferred from live-trapping data, particularly from incidents in which two or more individuals are captured together in a trap. We examined whether multiple-capture data from a long-term study of prairie voles (Microtus ochrogaster) and meadow voles (Microtus pennsylvanicus) were consistent with well-known species differences in social behavior (whereas prairie voles are highly social and display monogamy, meadow voles are less social and promiscuous). When possible, we also examined multiple captures of two nontarget species, northern short-tailed shrews (Blarina brevicauda) and western harvest mice (Reithrodontomys megalotis). Percent of total captures that were multiple captures and percent of total adult captures that were male–female captures were highest for prairie voles and lowest for meadow voles; values for harvest mice and shrews were in between those of the vole species, but more similar to values for meadow voles. Repeat captures of the same male–female pair occurred most commonly in prairie voles, and multiple captures of this species typically involved individuals from the same social group. Multiple captures of adults and juveniles were more common in prairie voles than meadow voles, except for captures of at least one adult male and at least one juvenile, which did not differ between the two vole species. Multiple capture data for prairie voles and meadow voles were largely consistent with established species differences in social behavior, suggesting that such data can provide an accurate indication of social and mating systems of small mammals.
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