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14 October 2011 Local environmental factors affect reproductive investment in female edible dormice
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Abstract

Edible dormice (Glis glis) are exposed temporally and spatially to a tremendous variation in food resources. This variation strongly influences reproduction; in edible dormice reproduction is tightly linked to the availability of energy-rich seeds. Although most dormice reproduce in full mast years of beech or oak, entire populations skip reproduction in years without seed production; however, nearly 50% of all years are intermediate mast years, during which only part of the dormouse population reproduces. We investigated how the beech mast pattern, local habitat characteristics, and individual traits (body mass and age class) influence whether individual female edible dormice invest in reproduction in intermediate mast years. Our field study, conducted during 2006–2009 in the Vienna Woods, revealed that in intermediate mast years the probability of females reproducing increased with the age of trees but not with the proportion of beech trees within their home ranges. Mean litter size was larger in years with higher seed availability and also increased with the mean age of trees within the home range of the dormice. More adult than yearling females reproduced, but this effect was modulated by yearly and local variation in food availability. Whether a female edible dormouse reproduces in an intermediate mast year depends mainly on the local food availability and age of the individual.

In small mammals energy demands for reproduction are extremely high, and during peak lactation the level of energy turnover in females exceeds maintenance levels severalfold (e.g., Speakman et al. 2004). Food availability is one of the most important factors affecting reproduction (Hubbs and Boonstra 1997; Karels et al. 2000; Koskela et al. 1998, 2004). Food resources are typically not distributed uniformly, but vary on a spatial and temporal scale. Pulsed resources—occasional, short periods of resource superabundance followed by a decrease over time—represent a naturally occurring, extreme type of erratic food availability (Ostfeld and Keesing 2000; Yang et al. 2008). A prime example of pulsed resources are tree seeds (e.g., of beech [Fagus spp.] or oak [Quercus spp.]), which can be completely absent in some years but overabundant in mast years. Mast years of beech occur, on average, every 7 years and have been found to be fairly synchronized over large areas (Hilton and Packham 2003). Typical consumers of these seeds, such as a variety of rodents and wild boar (Sus scrofa), increase reproduction after a mast event, which often leads to a period of population growth (Bieber and Ruf 2005; Murúa and Briones 2005; Selås et al. 2002).

An interesting species in which to study responses to pulsed resources is the edible dormouse (Glis glis). This small mammal is a specialized seed predator that must cope with high annual variability in tree seed production. Edible dormice are born very late in the summer season, from July to August. Thus, the energetically expensive lactation period coincides with phases of high food availability. A supply of energy-rich food during autumn is essential for juveniles after weaning, because they need to gain sufficient body fat reserves within a very short time period to survive their 1st hibernation season (Bieber and Ruf 2004). Therefore, although adults can live on a variety of alternative food sources (including leaves, fruits, and insects), entire populations of edible dormice in deciduous forests of northern Europe typically reproduce in full mast years and skip reproduction in years with no seed production (Bieber 1998; Pilastro et al. 2003; Schlund et al. 2002). However, approximately 50% of all years are intermediate mast years (Hilton and Packham 2003). Variation in mean seed production across a particular area among years is primarily caused by the high variation among individual trees (Herrera 1998). Intermediate mast years therefore arise when only a part of the tree population produces seeds, rather than from all trees producing seeds at an intermediate level. In those intermediate mast years varying fractions of edible dormice reproduce (Kager and Fietz 2009; Ruf et al. 2006). Theoretically, the presence of a single, large, seeding beech tree in the home range of an edible dormouse should be sufficient to provide ample food resources to support the high energy requirements of reproduction.

We hypothesized that in years of intermediate tree seeding, investment in reproduction in edible dormice will be affected by local food availability, in particular by the proportion of beech trees in an individual's home range or by the age of trees, because older trees produce more seeds (Genet et al. 2009; Isagi et al. 1997; Yamauchi 1996). To test this hypothesis we collected data on the occurrence or absence of reproduction over a 4-year period of intermediate tree seeding in edible dormice inhabiting nest boxes in a deciduous forest in the Vienna Woods in Austria. We then used detailed forest inventory data to analyze if the proportion and average age of beech trees in an individual dormouse's home range influences whether a female invests in reproduction. Previous studies found no effect of body mass on reproductive investment in edible dormice, but these studies focused mainly on full mast and mast failure years (Bieber 1998; Fietz et al. 2005; Pilastro et al. 2003). Hence, we considered it possible that body mass affects reproduction in years of intermediate mast seeding. We also expected that food availability could affect different age classes differentially. Yearling edible dormice, which are not fully grown, might be less likely to reproduce under suboptimal local conditions and delay the onset of reproduction for another year. In other rodents low territory quality or high population density can lead to a delayed 1st reproduction (Tamiasciurus hudsonicus [Descamps et al. 2006] and Xerus inauris [Waterman 2002]). Consequently, we included both body mass and age class in our analysis.

Materials and Methods

Study species

Edible dormice are small (∼80–130 g—Bieber 1998; Lebl et al. 2010), nocturnal rodents living in mixed and deciduous forests in central and southern Europe (Vietinghoff-Riesch 1960). Dormice are hibernators and can spend up to 8 months in hibernation (Vietinghoff-Riesch 1960; Wyss 1932). Although dormice are arboreal during their summer active period, they hibernate in underground burrows (Vietinghoff-Riesch 1960). Edible dormice are principally solitary animals (Vietinghoff-Riesch 1960), but cases of communal breeding of females are recorded (Pilastro 1992), and males use huddling behavior (mainly in spring) as a mechanism for social thermoregulation (Fietz et al. 2010). Both yearlings and adults show extremely high site fidelity, suggesting that dormice disperse as juveniles (Bieber and Ruf 2009; Ruf et al. 2006; Vietinghoff-Riesch 1960). Reports of home-range size vary from ∼0.5 to 7 ha, with males having larger home ranges than females (Hönel 1991;