Browsing by Author "Hayes, Loren D."

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    Activational and organizational effects of testosterone on the number of mating partners and reproductive success in males of a social rodent
    (2024) Correa, Loreto A.; Aspillaga-Cid, Antonia; Riquelme, Juan; Ly-Prieto, Lvaro; Hayes, Loren D.; Ebensperger, Luis A.
    The timing of exposure to the steroid hormone, testosterone, produces activational and organizational effects in vertebrates. These activational and organizational effects are hypothesized to relate with the number of female mating partners and reproductive success in males. We tested this hypothesis by examining 151 wild degu ( Octodon degus) ) males across a 10-year study. We quantified the association between adult serum testosterone levels (i.e., an indirect index of adult activational effects) and anogenital distance (AGD) length (i.e., a direct index of fetal organizational effects), and their interaction on the number of female mating partners and reproductive success. We found no evidence of an association between adult male serum testosterone levels and the number of female mating partners, or between adult male serum testosterone levels and reproductive success. However, male AGD was positively associated with reproductive success, but not so with the number of female mating partners. Additionally, the positive association between male AGD and male reproductive success was mediated by the number of mates. Our findings do not support major roles of activational or organizational effects of testosterone on the number of female mating partners and its consequences on male reproductive success. Instead, our results suggest that compared with individual male attributes, the female social environment plays a more important role in driving male reproductive success.
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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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    Ecological predictors of range areas and use of burrow systems in the diurnal rodent, Octodon degus
    (2007) Hayes, Loren D.; Chesh, Adrian S.; Ebensperger, Luis A.
    Variation in animal space use patterns may be linked to numerous ecological factors affecting survival and reproduction. We examined the relationship between ecology and above- and below-ground components of space use by Octodon degus, a semi-fossorial rodent in Chile. We monitored the daytime minimum convex polygon and adaptive kernel range areas of 26 individuals and determined the number of burrow systems used by degus during night-time radiotelemetry and trapping of burrow systems on two study grids at Rinconada de Maipu, a semi-arid Matorral in central Chile. We quantified food biomass, soil hardness, distance to overhead vegetative cover, and density of burrow openings at putative nest burrows. Degus living on the grid with more shrub cover had larger range areas than degus living on the grid with less cover. The range areas of degus decreased with increasing distance from overhead vegetative cover. There was a weak (but statistically significant) negative relationship between the number of burrow systems used by degus and the distance to vegetative cover and density of burrow openings at burrow systems. Male and female degus had similar range areas. Our results suggest that overhead cover decreases the risk of predation to male and female degus. Degus probably balance the benefits of numerous burrow openings (reduced predation risk) with time and energy requirements of burrow construction and maintenance. Models of space use that consider the effect of multiple ecological variables should measure different dimensions of space use.
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    Effects of Radio-Collars are not Contingent on Socioecological Conditions in Degus
    (2021) Ebensperger, Luis A.; Quirici, Veronica; Bunster, Valentina; Leon, Cecilia; Ramirez-Estrada, Juan; Hayes, Loren D.
    Species-specific research on free-ranging mammals reveals a diversity of effects of radio-collars on behavior, body condition, and fitness. Although these studies indicate rather limited direct effects, radio-collars may cause effects influenced by socio-ecological conditions. Using a 7-year study on a natural population of group-living degus (Octodon degus), we tested the hypothesis that ecological (food availability, burrow density) and social (group size, group male-to-female ratio) conditions modulate effects of radio-collars on body condition (e.g., body mass, ecto- and endoparasite loads, fecal cortisol metabolites) and direct fitness (litter size, adult survival). We determined the effect of radio-collar use on degus by contrasting the presence or absence of radio-collars, quantifying the effects of the number of days carrying a radio-collar, and the relative mass of radio-collars worn by degus in central Chile between 2009 and 2015. Radio-collar use was not associated with direct effects on litter size, adult survival, or with body mass and fecal cortisol metabolites but was linked to low ecto- and endoparasite loads. These seemingly positive effects may reflect decreased mobility, or a research bias for radio-collaring larger, healthier individuals. There was no evidence that ecological and social conditions modulated radio-collar effects on degu body condition and direct fitness. These findings are consistent with evidence from other mammal studies that reported no appreciable detrimental direct or indirect effects of radio-collars. (c) 2021 The Wildlife Society.
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    Fecal cortisol levels predict breeding but not survival of females in the short-lived rodent, Octodon degus
    (2013) Ebensperger, Luis A.; Tapia, Diego; Ramirez-Estrada, Juan; Leon, Cecilia; Soto-Gamboa, Mauricio; Hayes, Loren D.
    The cort-adaptation hypothesis indicates that an association between glucocorticoid (cort) levels and fitness may vary with the extent to which reproduction or breeding effort is a major determinant of cort levels. Support for a context dependent association between cort and fitness comes mostly from relatively long-lived, bird species. We tested the hypothesis that there are gender and context (life-history) specific cort-fitness relationships in degus, a short-lived and generally semelparous social rodent. In particular, we used demographical records on a natural population to estimate adult survival through seasons and years and linked that to records of baseline cort (based on fecal cortisol metabolites). We found no evidence for a direct relationship between baseline cort and adult survival across seasons, and this lack of association was recorded irrespective of sex and life history stage. Yet, cort levels during early lactation predicted the probability that females produce a second litter during the same breeding season, supporting a connection between baseline cort levels and breeding effort. Overall, the differential effects of cort on survival and breeding supported that the extent of cort-fitness relationships depends on the fitness component examined. (C) 2013 Published by Elsevier Inc.
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    Fitness consequences of group living in the degu Octodon degus, a plural breeder rodent with communal care
    (2009) Hayes, Loren D.; Chesh, Adrian S.; Castro, Rodrigo A.; Tolhuysen, Liliana Ortiz; Burger, Joseph Robert; Bhattacharjee, Joydeep; Ebensperger, Luis A.
    The fitness consequences of plural breeding vary considerably among social vertebrates. We tested three hypotheses for the direct reproductive fitness consequences of group living in the degu Octodon degus, a social rodent endemic to central Chile. To test the 'benefits of communal care' hypothesis, we determined the relationship between the number of females per group, per capita direct fitness and offspring survival. To test the 'food abundance and quality' hypothesis, we determined the relationship between the biomass of preferred foods at burrow systems, group size, per capita direct fitness and offspring survival. To test the 'predation risk' hypothesis, we determined the relationship between group size, the density of burrow entrances to which social groups had access, per capita direct fitness, and survival of adults and offspring. Group size of core females (i.e. those with 50% or more nightly overlap) was negatively correlated with per capita direct fitness, but not with the number of females per group or total group size. Group living did not enhance the survival of offspring. Greater biomass of food (at 3 m and 9 m) and burrow density were not linked to larger groups and offspring survival. Our results did not support predictions of the 'benefits of communal care', 'food abundance and quality' or 'predation risk' hypothesis. Pending microsatellite analyses, we hypothesize that survival benefits linked to foraging group size and not reproductive fitness benefits may explain the evolution of sociality in degus. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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    Fitness consequences of group living in the degu Octodon degus, a plural breeder rodent with communal care
    (2009) Hayes, Loren D.; Castro Bustamante, Rodrigo Alfredo; Ebensperger Pesce, Luis Alberto
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    Fitness consequences of variation in social group size are not population-specific but are associated with access to food in the communally breeding rodent, Octodon degus
    (2024) Hayes, Loren D.; Strom, Madeline K.; Leon, Cecilia; Ramirez-Estrada, Juan; Grillo, Sara; Gao, Cuilan L.; Vasquez, Rodrigo A.; Ebensperger, Luis A.
    Studies that concurrently investigate the functional benefits of group living in multiple populations of the same species are rare. Over a 3-year period (2014-2016), we examined two ecologically contrasting populations to test multiple hypotheses for the adaptive significance of group living in the communally breeding rodent Octodon degus. We quantified the size of social units (number of adults, number of adult females), edible vegetation at burrow systems, and per capita offspring weaned (PCOW) in each population. Contrary to expectations, we did not observe population-specific associations between group size and edible vegetation or PCOW nor universal benefits of group living. In one population, PCOW increased in mid-sized groups with more edible vegetation. However, this trend was not consistent across years. Notably, we observed a complete reproductive failure in one population during the first year of study, one that was characterized by low rainfall and no detectable edible vegetation. This result is important because reproductive failure occurred regardless of group size, suggesting that communal living may not buffer degus against the harshest of environmental conditions. Examining how social organization shapes individual reproductive success under extreme variation in food availability is an important step towards understanding how populations will respond to a changing climate.
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    Habitat type influences endocrine stress response in the degu (Octodon degus)
    (2013) Bauer, Carolyn M.; Skaff, Nicholas K.; Bernard, Andrew B.; Trevino, Jessica M.; Ho, Jacqueline M.; Romero, L. Michael; Ebensperger, Luis A.; Hayes, Loren D.
    While many studies have examined whether the stress response differs between habitats, few studies have examined this within a single population. This study tested whether habitat differences, both within-populations and between-populations, relate to differences in the endocrine stress response in wild, free-living degus (Octodon degus). Baseline cortisol (CORT), stress-induced CORT, and negative feedback efficacy were measured in male and female degus from two sites and three habitats within one site during the mating/early gestation period. Higher quality cover and lower ectoparasite loads were associated with lower baseline CORT concentrations. In contrast, higher stress-induced CORT but stronger negative feedback efficacy were associated with areas containing higher quality forage. Stress-induced CORT and body mass were positively correlated in female but not male degus across all habitats. Female degus had significantly higher stress-induced CORT levels compared to males. Baseline CORT was not correlated with temperature at time of capture and only weakly correlated with rainfall. Results suggest that degus in habitats with good cover quality, low ectoparasite loads, and increased food availability have decreased endocrine stress responses. (C) 2013 Elsevier Inc. All rights reserved.
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    Immunocompetence of breeding females is sensitive to cortisol levels but not to communal rearing in the degu (Octodon degus)
    (2015) Ebensperger, Luis A.; Leon, Cecilia; Ramirez-Estrada, Juan; Abades, Sebastian; Hayes, Loren D.; Nova, Esteban; Salazar, Fabian; Bhattacharjee, Joydeep; Ines Becker, Maria
    One hypothesis largely examined in social insects is that cooperation in the context of breeding benefits individuals through decreasing the burden of immunocompetence and provide passive immunity through social contact. Similarly, communal rearing in social mammals may benefit adult female members of social groups by reducing the cost of immunocompetence, and through the transfer of immunological compounds during allonursing. Yet, these benefits may come at a cost to breeders in terms of a need to increase investment in individual immunocompetence. We examined how these potential immunocompetence costs and benefits relate to reproductive success and survival in a natural population of the communally rearing rodent, Octodon degus. We related immunocompetence (based on ratios of white blood cell counts, total and specific immunoglobulins of G isotype titers) and fecal glucocorticoid metabolite (FGC) levels of adults immunized with hemocyanin from the mollusk Concholepas concholepas to measures of sociality (group size) and communal rearing (number of breeding females). Offspring immunocompetence was quantified based on circulating levels of the same immune parameters. Neither female nor offspring immunocompetence was influenced by communal rearing or sociality. These findings did not support that communal rearing and sociality enhance the ability of females to respond to immunological challenges during lactation, or contribute to enhance offspring condition (based on immunocompetence) or early survival (i.e., to 3 months of age). Instead, levels of humoral and cellular components of immunocompetence were associated with variation in glucorcorticoid levels of females. We hypothesize that this covariation is driven by physiological (life-history) adjustments needed to sustain breeding. (C) 2014 Elsevier Inc. All rights reserved.
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    Individual behavioural variation does not affect social organization or reproductive success in a cooperative small mammal
    (2024) van der Marel, Annemarie; Johnson, Nicholas E.; Grillo, Sara; Riquelme, Juan; Vasquez, Rodrigo A.; Gillam, Erin H.; Ebensperger, Luis A.; Hayes, Loren D.
    Recent evidence indicates that individual behavioural variation in animals, defined as consistent individual differences in behaviour across contexts and time, influence ecological and evolutionary processes, and a growing number of studies demonstrate that individual behavioural variation can play a large role in shaping grouping dynamics among social animals. We studied the common degu, Octodon degus, a social rodent, to evaluate whether individual behavioural variation underlies social organization and the reproductive success of individuals within groups. We examined social groups in a population in central-north Chile during one breeding season, tested 67 adults in an open field test (i.e., the propensity to explore an unfamiliar environment) and 62 adults in determined assortment based on individual behavioural differences across 19 social groups, and performed genetic analyses to assess reproductive success. We found that the response to the poke test was repeatable, while none of the behaviours from an open field test were. The repeatable behaviour during the poke test was not associated to components of social organization (group composition), or to reproductive success. These findings imply that individual behavioural varia- tion did not affect grouping patterns or direct fitness in this degu population.
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    Instability Rules Social Groups in the Communal Breeder Rodent Octodon degus
    (2009) Ebensperger, Luis A.; Chesh, Adrian S.; Castro, Rodrigo A.; Tolhuysen, Liliana Ortiz; Quirici, Veronica; Burger, Joseph Robert; Hayes, Loren D.
    It has been hypothesized that animal groups in socially cohesive species are inherently unstable, ultimately the result of constraints to independent breeding, and proximately the product of adult fidelity and offspring philopatry. Other processes, including emigration of individuals that join pre-existing groups would be less important. We examined the persistence and variation in the composition of members of social groups in Octodon degus, a communal breeding rodent in which limitations to independent breeding are less obvious. This analysis was conducted during subsequent years, as well as during different seasons within years. Similar to social species in which constraints to independent breeding influence sociality, groups in degus were unstable in that they were short lived and ruled by an extensive turnover of group members across years. A relatively high turnover of group members was also recorded within years. Variation in the composition of groups was caused mostly by disappearance (presumably mortality) and immigration of adult members. Adult fidelity and offspring philopatry and dispersal played secondary roles in affecting the composition of social groups between and within years. Future studies should reexamine the importance of habitat limitations and its proximate determinant, natal philopatry, in driving the stability of social groups.
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    Long-term field studies on rodents
    (2017) Hayes, Loren D.; Ebensperger Pesce, Luis Alberto; Kelt, Douglas A.; Pillay, Neville; Viblanc, Vincent A.; Schradin, Carsten; Meserve, Peter L.
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    Male group members are costly to plurally breeding Octodon degus females
    (2019) Hayes, Loren D.; Correa Kaempfe, Loreto Alejandra; Abades T., Sebastián R.; Gao, Cuilan L.; Ebensperger Pesce, Luis Alberto
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    Octodon degus kin and social structure
    (2016) Davis, Garrett T.; Vasquez, Rodrigo A.; Poulin, Elie; Oda, Esteban; Bazan Leon, Enrique A.; Ebensperger Pesce, Luis Alberto; Hayes, Loren D.
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    One for all and all for one: phenotype assortment and reproductive success in masculinized females
    (2021) Correa, Loreto A.; Leon, Cecilia; Ramirez-Estrada, Juan; Ly-Prieto, Alvaro; Abades, Sebastian; Hayes, Loren D.; Soto-Gamboa, Mauricio; Ebensperger, Luis A.
    Homophily by morphological and behavioral traits has been described in several species of vertebrates, but its functional consequences remain poorly studied. Homophily by plurally breeding females may improve direct fitness by enhancing reproductive success. Female mammals may exhibit phenotypical masculinization due to exposure to androgens during early development, a condition that is associated with maternal performance during subsequent breeding. Our goal was to assess whether female composition (in terms of masculinization) of plurally breeding groups influences female fitness in a natural population of degus (Octodon degus). We assessed if plurally breeding female degus assort themselves by anogenital distance (AGD), an accurate measure of masculinization level. We also quantified if homophily by AGD phenotype affects female reproductive success and the reproductive output of the group. Plurally breeding groups typically included similarly masculinized (i.e., long AGD) females or similarly feminized (short AGD) females, indicating a strong degree of homophily. Females weaned more offspring in plurally breeding groups with more masculinized females. Additionally, standardized variance in the number of offspring weaned decreased in plurally breeding groups with mostly masculinized females, indicating greater reproductive equality in these groups. We conclude that female degus organize into homophilic social groups of similar AGD, and that social groups of masculinized females exhibit a higher reproductive success.
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    Postnatal development of the degu (Octodon degus) endocrine stress response is affected by maternal care
    (2016) Bauer, Carolyn M.; Ebensperger Pesce, Luis Alberto; León, Cecilia; Ramírez Estrada, Juan Carlos; Hayes, Loren D.; Romero. L. Michael
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    Reproductive correlates of social network variation in plurally breeding degus (Octodon degus)
    (ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, 2013) Wey, Tina W.; Burger, Joseph R.; Ebensperger, Luis A.; Hayes, Loren D.
    Studying the causes and reproductive consequences of social variation can provide insight into the evolutionary basis of sociality. Individuals are expected to behave adaptively to maximize reproductive success, but reproductive outcomes can also depend on group structure. Degus (Octodon degus) are plurally breeding rodents, in which females allonurse indiscriminately. However, communal rearing does not appear to enhance female reproductive success, and larger group sizes are correlated with decreasing per capita pup production. To further investigate mechanisms underlying these patterns, we asked how differences in sex, season and average group reproductive success are related to degu association networks. We hypothesized that if reproductive differences mirror social relationships, then females (core group members) should show stronger and more stable associations than males, and female association strength should be strongest during lactation. We also hypothesized that, at the group level, social cohesion would increase reproductive output, while social conflict would decrease it. Females did have higher association strength and more preferred partners than males, but only during lactation, when overall female associations increased. Females also had more stable preferred social partnerships between seasons. A measure of social cohesion (average association strength) was not related to per capita pup production of female group members, but potential social conflict (heterogeneity of association strengths) was negatively related to per capita pup production of female group members. Our results highlight temporal and multilevel patterns of social structure that may reflect reproductive costs and benefits to females. (C) 2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.