Juvenile survival curves in a solitary ground squirrel with a prolonged hibernation: effects of individual characteristics, environment, and maternal investment

Abstract

Juvenile survival is a key life-history influence on population dynamics and adaptive evolution. We analyzed the effects of individual characteristics, early environment, and maternal investment on juvenile survival in a large solitary hibernating rodent – yellow ground squirrel Spermophilus fulvus using Cox mixed-effects models. Only 48% of weaned pups survived to dispersal and 17% survived to hibernation. Early life expectancy was primarily determined by individual characteristics and to a lesser extent by the early environment. The strongest and positive predictor of juvenile survival was body mass which crucially affected mortality immediately after weaning. Males suffered higher mortality than females after the onset of dispersal; however, the overall difference between sexes was partly masked by high rates of mortality in the first days after emergence in both sexes. Later-emerged juveniles had lower life expectancy than the earliest pups. The overall effect of local juvenile density was positive. Prolonged lactation did not enhance juvenile survival: pups nursed longer survived shorter than the young nursed for a shorter period. Our findings support the hypothesis that females of S. fulvus cannot effectively regulate maternal expenditures to mitigate effects of unfavorable conditions on their offspring. The strategy to deal with seasonal time constraints on life history in female S. fulvus suggests an early termination of maternal care at the cost of juvenile quality and survival. This female reproductive strategy corresponds to a “fast-solitary” life of folivorous desert-dwelling S. fulvus and other solitary ground squirrels with prolonged hibernation.

Female hormonal profiles and vaginal cytology in a ground squirrel species with prolonged hibernation

“…Our ndings revealed fast transitions between phases of the female reproductive cycle and early maturation of juvenile females and therefore support our concept of fast life history strategy in yellow ground squirrels (Vasilieva and Tchabovsky 2018;Vasilieva et al 2022): we believe that the extremely long hibernation accelerates and shortens the key processes in S. fulvus life history. Short gestation and lactation, fast juvenile growth and development, low juvenile survival, early dispersal of juveniles, an early decrease of juvenile social activity, early maturation, and a short lifespan all constitute a consistent pattern in this species.…”

“…It turned out that all females in proestrus and estrus were trapped within the very rst days after their spring emergence; some females showed signs of pregnancy on the 1st-3rd day after the emergence. Subsequent observations of the litter emergence for the analyzed females uncovered a strong correlation between the date of female spring emergence and the date of litter emergence, as we reported previously (Vasilieva et al 2022). In addition, high variation of cell composition in the proestrus phase probably re ected fast cell proliferation and rapid transitions between proestrus and estrus.…”

“…The study species Yellow ground squirrels are big diurnal folivorous rodents that inhabit arid and semiarid zones of Central Asia, Iran, China, and Southern Russia (Thorington et al 2012; Shilova et al 2015). They live solitarily at low population density on open landscapes (Vasilieva et al 2022). Due to summer drought and heat, they enter estivation that is followed by hibernation; therefore, their active season is very short (~ 3-4 months; Vasilieva et al 2009).…”

“…It has been demonstrated in several ground squirrel species that early-bred females have higher reproductive success than late breeders do, and earlier-weaned juveniles survive better than later-weaned ones (Dobson et al 1999;Huber et al 2001). In yellow ground squirrels, we have previously documented positive effects of early reproduction on female reproductive success and on their offspring's body condition, growth, social activity, survival, and even their future reproduction (Vasilieva and Tchabovsky 2009Vasilieva et al 2022). Therefore, natural selection seems to favor short estrus and early conception for S. fulvus females.…”

“…S. fulvus is a large (up to 2 kg) ground squirrel with speci c life history traits: it is folivorous, inhabits harsh arid and semiarid zones, and hibernates up to 9 months per year (Vasilieva et al 2009;Shilova et al 2015). Fast juvenile growth and maturation, early dispersal, solitary life, and a short lifespan characterize "fast-solitary" life history strategy of yellow ground squirrels, which makes it exceptional among similar species (Vasilieva and Tchabovsky 2018;Vasilieva et al 2022). We hypothesized here that these traits of the annual cycle, especially the extremely short active season, determine the traits of the female reproductive cycle.…”