Developmental and social constraints on early reproduction in an asocial ground squirrel

Abstract

For short-lived species, selection for early reproduction should be strong, yet females often delay their first reproductive bout. Delay in age of first reproduction due to developmental constraints, such as food availability, or social constraints, such as the inhibitory presence of breeding adults, has been documented for social mammals, but effects on asocial species are less well known. We evaluated the influence of developmental and social factors on early reproduction in a short-lived, asocial species, the golden-mantled ground squirrel (Callospermophilus lateralis). We found that females who reproduced as yearlings had been weaned earlier in their natal summer and experienced early snow melt during their yearling spring, suggesting the importance of access to high-quality food at critical stages. Females were more likely to reproduce as yearlings when there were more adult males present during the breeding season, possibly because exposure to males accelerates reproductive maturity. Maternal presence had no effect on yearling reproduction, but yearlings with a littermate sister present were only 22% as likely to reproduce as females without a sister present, suggesting the effect of sibling competition well past weaning. Furthermore, the negative effect of a sister's presence, but not the presence of other females, suggests that relatedness affects reproductive competition in this asocial species.

Maternal survival costs in an asocial mammal

“…However, timing of emergence may have a more influential effect on offspring survival (Armitage et al, 1976 ; Rieger, 1996 ). Later emerging Uinta ground squirrel ( Spermophilus armatus ) offspring tend to emerge at a heavier weight (Rieger, 1996 ), but the rate of mass gain is significantly reduced for GMGS juveniles that emerge later in the summer (Wells & Van Vuren, 2018 ).…”

“…Furthermore, density‐dependent offspring sex bias, favoring the production of sons when density is high, is more pronounced when the number of kin within the local area (i.e., “locality”) is higher (Wells & Van Vuren, 2017 ). The presence of littermate sisters can also affect reproduction by delaying the age of first reproduction, but mother and other kin presence has no effect for GMGS (Wells & Van Vuren, 2018 ). Although kin presence can affect fitness (Viblanc et al, 2010 ) and reproductive output (Barra et al, 2021 ; Wells & Van Vuren, 2017 ) in ground squirrels, its explicit link to survival has been less explored.…”

Litter relocation behavior in two species of ground‐dwelling squirrels

“…Our study took place at the Rocky Mountain Biological Laboratory in the East River Valley, Gunnison County, Colorado (38°57′N, 106°59′W) at an altitude of 2,900 m. Long‐term studies of YBM (Armitage, ; Blumstein, ) and GMGS (Wells & Van Vuren, ) have been conducted at this location since 1962 and 1990, respectively. Common predators in this region include coyotes ( Canis latrans ), red foxes ( Vulpes vulpes ), long‐tailed weasels ( Mustela frenata ), and short‐tailed weasels ( Mustela erminea ) (Kneip, Van Vuren, Hostetler, & Oli, ; Van Vuren, ).…”

“…Female GMGS are solitary, although they often live in close proximity to conspecific neighbors (Wells & Van Vuren, ). For both species, females give birth in the natal burrow, where they nurse their pups for 25–30 days before the pups emerge and begin feeding on their own (Armitage, ; Wells & Van Vuren, ).…”

“…Each year, individuals of both species were trapped and marked, and pups were trapped upon emergence from their natal burrow. Intensive behavioral observations were conducted at four colonies for YBM (Armitage, ) and throughout a 13‐ha study area for GMGS (Wells & Van Vuren, ). All procedures followed guidelines of the American Society of Mammalogists (Sikes & Animal Care and Use Committee of the American Society of Mammalogists, ) and were approved by the Institutional Animal Care and Use Committees of the University of Kansas, University of California, Davis, and Rocky Mountain Biological Laboratory.…”