Browsing by Author "Burger, Joseph Robert"

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    Burrow limitations and group living in the communally rearing rodent, Octodon degus
    (ALLIANCE COMMUNICATIONS GROUP DIVISION ALLEN PRESS, 2011) Ebensperger, Luis A.; Chesh, Adrian S.; Castro, Rodrigo A.; Ortiz Tolhuysen, Liliana; Quirici, Veronica; Burger, Joseph Robert; Sobrero, Raul; Hayes, Loren D.
    Group living is thought to evolve whenever individuals attain a net fitness advantage due to reduced predation risk or enhanced foraging efficiency, but also when individuals are forced to remain in groups, which often occurs during high-density conditions due to limitations of critical resources for independent breeding. The influence of ecological limitations on sociality has been studied little in species in which reproduction is more evenly shared among group members. Previous studies in the caviomorph rodent Octodon degus (a New World hystricognath) revealed no evidence that group living confers an advantage and suggest that burrow limitations influence formation of social groups. Our objective was to examine the relevance of ecological limitations on sociality in these rodents. Our 4-year study revealed no association between degu density and use of burrow systems. The frequency with which burrow systems were used by degus was not related to the quality of these structures; only in 1 of the 4 years did the frequency of burrow use decrease with decreasing abundance of food. Neither the number of females per group nor total group size (related measures of degu sociality) changed with yearly density of degus. Although the number of males within social groups was lower in 2008, this variation was not related clearly to varying density. The percentage of females in social groups that bred was close to 99% and did not change across years of varying density. Our results suggest that sociality in degus is not the consequence of burrow limitations during breeding. Whether habitat limitations contribute to variation in vertebrate social systems is discussed.
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    Sociality, glucocorticoids and direct fitness in the communally rearing rodent, Octodon degus
    (ACADEMIC PRESS INC ELSEVIER SCIENCE, 2011) Ebensperger, Luis A.; Ramirez Estrada, Juan; Leon, Cecilia; Castro, Rodrigo A.; Ortiz Tolhuysen, Liliana; Sobrero, Raul; Quirici, Veronica; Burger, Joseph Robert; Soto Gamboa, Mauricio; Hayes, Loren D.
    While ecological causes of sociality (or group living) have been identified, proximate mechanisms remain less clear. Recently, close connections between sociality, glucocorticoid hormones (cart) and fitness have been hypothesized. In particular, cort levels would reflect a balance between fitness benefits and costs of group living, and therefore baseline coil levels would vary with sociality in a way opposite to the covariation between sociality and fitness. However, since reproductive effort may become a major determinant of stress responses (i.e., the cart-adaptation hypothesis), cort levels might also be expected to vary with sociality in a way similar to the covariation between sociality and fitness. We tested these expectations during three years in a natural population of the communally rearing degu, Octodon degus. During each year we quantified group membership, measured fecal cortisol metabolites (a proxy of baseline cort levels under natural conditions), and estimated direct fitness. We recorded that direct fitness decreases with group size in these animals. Secondly, neither group size nor the number of females (two proxies of sociality) influenced mean (or coefficient of variation, CV) baseline cortisol levels of adult females. In contrast, cortisol increased with per capita number of offspring produced and offspring surviving to breeding age during two out of three years examined. Together, our results imply that variation in glucocorticoid hormones is more linked to reproductive challenge than to the costs of group living. Most generally, our study provided independent support to the cort-adaptation hypothesis, according to which reproductive effort is a major determinant, yet temporally variable, influence on cart-fitness covariation. (C) 2011 Elsevier Inc. All rights reserved.