Differences in growth rates and pre-hibernation body mass gain between early and late-born juvenile garden dormice

Juvenile hibernators have to allocate energy to both growth and fattening, to survive winter, and to avoid possible disadvantages during their first reproductive season. Being born late in the active season may have important effects on growth and fattening. This study aimed at determining potential differences in rates and maximal level of growth, and in pre-hibernation body fat mass between early and late-born juvenile garden dormice (Eliomys quercinus), and kept in outdoor enclosures with ad libitum food and water. We first assessed mean pup mass in early and late-born litters (n = 31) from birth to their early weaning phase, at which time body composition was determined. Then, growth and body mass of early and late-born individuals (six males and six females, for each group) were measured weekly until hibernation onset (n = 24). We also assessed fat content in a group of juveniles during pre-hibernation fattening (n = 16) and after their first winter hibernation (n = 18). During the pre-weaning phase, young from early and late litters mainly grew structurally and gained mass at similar rates. After weaning, late-born juveniles grew and gained mass twice as fast as early born individuals. Body mass was positively associated with fat content during pre-hibernation fattening. Late-born females reached similar structural sizes, but had lower pre-hibernation fat reserves than early born females. Conversely, late-born males showed lower maximal size and pre-hibernation body fat content, compared with early born males. Thus, individuals born late in the season cannot fully compensate the lack of available time before the winter onset.