Mortality in relation to season, age, sex, and reproduction in Columbian ground squirrels (<i>Spermophilus columbianus</i>)

Abstract

Abstract:We investigated the timing of and age at mortality in Columbian ground squirrels (Spermophilus columbianus) in relation to sex and reproductive status. Life-history data were collected from 1994 to 1999 in the foothills of the Rocky Mountains, Alberta, Canada. We predicted increased mortality for males during mating and for females during lactation. Further, we expected reproductively successful females to have higher mortality than females that did not reproduce successfully. Finally we assumed that at some age reproductive success of females would decrease and mortality increase because of old age. For both sexes, survival over winter was high (ca. 90%) for adults. While there was a significant increase in mortality of adult males during the mating season, females did not have higher mortality during lactation than during the rest of the active season. Reproductive status influenced mortality in females: non-reproducing females had a higher chance of surviving than reproducing females. Females that weaned young were more likely to die after the weaning period than females that lost their litter during lactation. There was a positive correlation between maternal survival and survival of offspring to yearling age. Our results showed evidence of trade-offs between reproduction and survival of male and female Columbian ground squirrels.Résumé : Nous avons étudié la phénologie de la mortalité et déterminé l'âge au décès chez le Spermophile du Columbia (Spermophilus columbianus) en fonction du sexe et du statut reproducteur à partir de données démograph-iques recueillies de 1994 à 1999 sur les contreforts des Rocheuses en Alberta, Canada. Nous avons posé en hypothèse que la mortalité augmenterait chez les mâles au cours de la période des accouplements et chez les femelles durant l'allaitement et que les femelles qui réussiraient leur reproduction auraient une mortalité plus forte que les femelles qui la rateraient. Enfin, nous nous attendions à un déclin de la reproduction chez les femelles à partir d'un certain âge et à une augmentation de la mortalité due à la vieillesse. Chez les adultes mâles et femelles, la survie en hiver est élevée (ca. 90 %). Il se produit une augmentation significative de la mortalité des mâles pendant la saison des accouplements, mais les femelles n'ont pas un taux de mortalité plus élevé durant l'allaitement que pendant le reste de la saison d'activité. Le statut reproducteur influence la mortalité chez les femelles : en effet, les femelles non reproductrices ont de meilleures chances de survie que les femelles reproductrices. La probabilité de mourir est plus forte chez les femelles qui gardent leurs petits jusqu'au sevrage que chez celles qui perdent leur portée pendant l'allaitement. Il y a une corrélation positive entre la survie des femelles et la survie des jeunes au cours de la première année. Nos résultats indiquent qu'il s'établit des compromis entre la reproduction et la survie chez les mâles et les femelles du Spermophile du Columbia.[Traduit par la Rédaction] 470Neuha...

The costs of risky male behaviour: sex differences in seasonal survival in a small sexually monomorphic primate

“…Equal or even male-biased winter survival has been reported for other small mammalian species with annual male-biased mortality (e.g. Michener & Locklear 1990;Millesi et al 1999;Neuhaus & Pelletier 2001). Juvenile mouse lemurs of both sexes had substantially lower survival chances (age differential: Df females Z19%, Df males Z40%).…”

“…High breeding season mortality in males has been reported in many small mammals, but only a few studies have used a temporal scale fine enough to explore the potential proximate causes of this phenomenon in more detail (e.g. Boonstra et al 2001;Neuhaus & Pelletier 2001). We could not detect an effect of sex on survival outside the short mating period, which strongly supports the risky male behaviour hypothesis over the 'adverse effects of androgens' hypothesis, since roaming and physical male competition only occur during the short mating period (Eberle & Kappeler 2002, 2004, while testosterone levels are increased over a much longer period of time than the brief mating season (Perret & Aujard 2001).…”

“…Since females actually emerge from hibernation with a lower body mass than males (Schmid 1999) as Schü lke & Ostner (2007) already pointed out, resource scarcity (food and/or water) is unlikely to be the ultimate cause why females choose to sleep through a large part of the dry season. Indeed, a constant and high level of overwinter survival relative to breeding season survival has been reported for many species where both sexes undergo obligate hibernation (Michener & Locklear 1990;Millesi et al 1999;Neuhaus & Pelletier 2001;Schaub & Vaterlaus-Schlegel 2001;Meaney et al 2003;Sendor & Simon 2003;Smith & Nichols 2003).…”

Born with an advantage: early life and maternal effects on fitness in female ground squirrels

“…The colonial nature of the species allows for the collection of data from a large sample of individuals who share a very similar environment and allows for long-term monitoring over an individual’s lifespan. Columbian ground squirrels are a relatively long-lived rodent, with an average lifespan of 5 years in females, and some living more than ten years ( Neuhaus and Pelletier 2001 ). This species has relatively small litters (mean around three juveniles/year), and a short active season, spanning approximately 4 months of the year while hibernating for the other 8 months.…”

Inter-annual and inter-individual variations in survival exhibit strong seasonality in a hibernating rodent

“…Higher mortality rates and shorter lifespans in males than in females are commonly observed among mammal species (Neuhaus and Pelletier 2001;Clutton-Brock and Isvaran 2007;Kraus et al 2008). Various but interrelated hypotheses have been proposed to explain juvenile and adult male-biased mortality: 1) Lower immunocompetence of males, which increases their susceptibility to infectious disease and parasitism (Moore and Wilson 2002); 2) The costs of sexual size dimorphism in the larger sex, usually males in polygynous or promiscuous mammals (Promislow 1992;Moore and Wilson 2002); 3) The higher propensity of males to engage in potentially risky behaviours, such as dispersal, sexual competition and roaming (Greenwood 1980;Andersson 1994;Kraus et al 2008); 4) The potential effects of deleterious recessive alleles in the heterogametic sex (XY) (e.g.…”