Browsing by Author "Ortiz Tolhuysen, Liliana"

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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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    Ecological drivers of group living in two populations of the communally rearing rodent, Octodon degus
    (2012) Ebensperger, Luis A.; Sobrero, Raul; Quirici, Veronica; Castro, Rodrigo A.; Ortiz Tolhuysen, Liliana; Vargas, Francisco; Burger, Joseph Robert; Quispe, Rene; Villavicencio, Camila P.; Vasquez, Rodrigo A.; Hayes, Loren D.
    Intraspecific variation in sociality is thought to reflect a trade-off between current fitness benefits and costs that emerge from individuals' decision to join or leave groups. Since those benefits and costs may be influenced by ecological conditions, ecological variation remains a major, ultimate cause of intraspecific variation in sociality. Intraspecific comparisons of mammalian sociality across populations facing different environmental conditions have not provided a consistent relationship between ecological variation and group-living. Thus, we studied two populations of the communally rearing rodent Octodon degus to determine how co-variation between sociality and ecology supports alternative ecological causes of group living. In particular, we examined how variables linked to predation risk, thermal conditions, burrowing costs, and food avail-ability predicted temporal and population variation in sociality. Our study revealed population and temporal variation in total group size and group composition that covaried with population and yearly differences in ecology. In particular, predation risk and burrowing costs are supported as drivers of this social variation in degus. Thermal differences, food quantity and quality were not significant predictors of social group size. In contrast to between populations, social variation within populations was largely uncoupled from ecological differences.
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    Flight initiation distance is differentially sensitive to the costs of staying and leaving food patches in a small-mammal prey
    (CANADIAN SCIENCE PUBLISHING, 2009) Lagos, Patricio A.; Meier, Andrea; Ortiz Tolhuysen, Liliana; Castro, Rodrigo A.; Bozinovic, Francisco; Ebensperger, Luis A.
    Escape theory predicts that a prey should flee from an approaching predator at a point in which the cost of staying equals the cost of escape. We manipulated the cost of fleeing upon approaching human predators by providing the small mammal Octodon degus (Molina, 1782) with varying amounts of supplementary food likely to disappear while the animals are not in the food patch (e.g., hidden in their burrows). Simultaneously, we manipulated the risk of remaining in the patch by providing supplementary food at varying distances from the nearest burrow. Degus fled at a shorter distance to approaching predators when foraging in patches closer to the nearest burrow and supplied with relatively high abundance of food, but only when these rodents were foraging socially. Also, degus fled at a greater distance to approaching predators when foraging in patches far from the nearest burrow. Thus, functions linked to the loss of feeding opportunities and the risk of predation interact to influence flight initiation distance after a simulated attack. This study represented one of the few demonstrations of an interactive effect between cost and risks on antipredator behavior in a small, social prey mammal.
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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.
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    The influence of trap type on evaluating population structure of the semifossorial and social rodent Octodon degus
    (2009) Burger, Joseph R.; Chesh, Adrian S.; Castro, Rodrigo A.; Ortiz Tolhuysen, Liliana; Torre, Ignasi; Ebensperger, Luis A.; Hayes, Loren D.
    Trap type may influence captures of individuals in different age-sex categories in small mammal studies, resulting in biased population and demographic information. We deployed 4 live trap types at burrow systems of the rodent, Octodon degus Molina, 1782, in central Chile to determine trap efficacy in capturing individuals of 6 demographic categories. We captured 2672 individuals in 17 709 trap days (15.1% trapping success). Tomahawks were the most efficient trap capturing half of individuals during both years, followed by mesh Sherman traps, large Sherman traps, and medium Sherman traps in 2005. All trap types equally sampled sexes. Large and medium Sherman traps provided similar demographic structure, where half of the individuals captured were pups; Tomahawk traps sampled more adults than pups. Relative captures of pups were similar across different trap types, suggesting that pups are equally sampled by each of the deployed trap types. Relative captures of adults were lower in Sherman traps, suggesting that this age class avoided solid-walled traps. For Octodon degus, the sole use of Tomahawk traps may produce sufficient, unbiased demographic data. Only 4 trap mortalities occurred (0.15%). Researchers may minimize trap mortality without compromising sufficient demographic sampling by trapping during peak animal activity.