Ecological drivers of group living in two populations of the communally rearing rodent, Octodon degus

Abstract

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 availability 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.

Effects of Habitat and Social Complexity on Brain Size, Brain Asymmetry and Dentate Gyrus Morphology in Two Octodontid Rodents

“…degus live in relatively large groups ranging from 1 to 12 adult individuals , and females of this species exhibit communal care of offspring [Ebensperger et al, 2004]. These characteristics have been documented in at least four populations of this species [Ebensperger et al, 2004[Ebensperger et al, , 2012Jesseau, 2004;Sobrero et al, unpubl. data], implying a high frequency of social interactions and probably a need for cognitive skills underlying these interactions.…”

“…The original research reported herein was performed under guidelines established by the Pontificia Universidad Católica de Chile Bioethical Committee (CBB-042/2011) and adhered to Chilean laws [permits 1-154.2010 (7989), 1-109.2011 (6749), 1-90.2011 (4731) and 1-95-2012 (4486) [Ebensperger et al, 2004[Ebensperger et al, , 2012, we placed traps near burrow openings and inside patches with high shrub cover and baited them with rolled oats, fruity cereals and sunflower seeds. During each capture, we recorded sex, body mass (to 0.1 g) and reproductive status (whether a female had a perforated vagina, was pregnant or lactating) of all degus, and each animal was marked with an ear tag (Monel 1005-1; National Band and Tag Co., Newport, Ky., USA).…”

“…Adult-aged degus were fitted with a radio collar weighing 7-9 g (RI-2D; Holohil Systems Limited, Carp, Ont., Canada; SOM-2190A, and BR radio collars; AVM Instrument Co., Colfax, Calif., USA). At the end of our study, all radio-collared animals were recaptured and radio collars were removed [Ebensperger et al, 2004[Ebensperger et al, , 2012.…”

“…O. degus uses relatively open savannas or open scrub environments in central Chile, but more closed scrub patches and ravines in northern populations [Quispe et al, 2009;Ebensperger et al, 2012]. In these environments, O. degus excavate and use burrow systems connected aboveground by runways or trails used during foraging [Fulk, 1976;Lagos et al, 1995;Vásquez et al, 2002].…”

The evolution of sociality in rodents: Is the traditional socioecological approach universal?

“…However, this is not an ecological factor. Many hypotheses that explain the evolution of sociality are based on the acceptance of a balance or a tradeoff between the positive and negative effects of the group living (Alexander, 1974;Bertram, 1978;Madison et al, 1984;Pulliam and Caraco, 1984;Ebensperger et al, 2011). Advantages include a decrease in the risk of predation, in particular due to specific alarm calls, e.g., in ground dwelling sciurids (Barash, 1973;Armitage, 1981;Hoogland, 1981); a reduction in energy spent searching for food, foraging, burrowing, and the maintenance of complex burrows (Jarvis, 1981;Jarvis et al, 1994;Ebensperger and Bozinovic, 2000); and the successful survival of the offspring due to collective thermoregulation, which acquire greater body weight before dispersal, the con tinuity of experience of adult individuals, and living through an adverse season within a group (Barash, 1974;Armitage, 1981Armitage, , 1999Armitage, , 2007Arnold, 1988Arnold, , 1990aArnold, , 1990bArnold, , 1993Hayes, 2000).…”