Brief Periods of Positive Peer Interactions Mitigate the Effects of Total Social Isolation in Young Octodon degus Valentina Colonnello 1,2 Paolo Iacobucci 1,2 Mary Patricia Anderson 1 Jaak Panksepp 1 1 Department of VCAPP College of Veterinary Medicine Washington State University Pullman, Washington 99164-6351 E-mail: jpanksepp@vetmed.wsu.edu 2 Department of Psychology University of Rome ‘‘Sapienza’’, Rome 00185, Italy ABSTRACT: We investigated whether positive daily peer-interactions counteract the effects of isolation in Octodon degus. Twenty-five-day-old degus were either isolated (ISO), socially housed (SOCIAL), or isolated and allowed 1-hr daily peer interaction (PARTIAL-ISO). The animals were observed over 4 weeks. Just prior to isolation and after 2 weeks of individual housing, the subjects were assessed for response to pleasant stimuli via a sucrose preference test and to fearful situations in open field and startle tests. Two weeks after the previous tests, the subjects were retested as above and observed in novelty and sociability tests. Only the ISO group showed significant alterations in sensitivity to reward and increased risk-taking behavior in fearful situations. The ISO group consumed more sucrose, spent less time freezing in the startle test and exhibited increased exploration in open field and novelty tests compared to PARTIAL-ISO and SOCIAL groups. In the sociability test, the SOCIAL group vocalized more than the other two groups during encounters with an unfamiliar degus. Our findings suggest that (i) chronic isolation induces alteration of hedonic, emotional and social profiles, with a maturational delay in fear-related responses; (ii) friendly interaction attenuates most behavioral changes induced by total social isolation. However, the positive effects of daily social interactions did not fully counteract deficits in social vocalizations. Our study represents one of the few available studies focused not only on the consequences of negative life events in this species, but also the protective role of relatively short periods of positive social activity on subsequent emotional development. ß 2010 Wiley Periodicals, Inc. Dev Psychobiol 53: 280–290, 2011. Keywords: emotional development; positive affect; impulse control; fear; vocal- izations INTRODUCTION According to developmental theories and clinical and preclinical studies, early peer relationships play a crucial role in shaping emotional and cognitive development (Bukowski, Brendgen, & Vitaro, 2007; Rubin, Bukowski, & Parker, 2006). In both humans and animals, the link between impaired emotional development and negative events during early development is well established (Deater-Deckard, 2001; Hol, Van den Berg, Van Ree, & Spruijt, 1999; Tanaka, Osako, & Yuri, 2010). Both social isolation and exclusion increase the likelihood of vulner- ability for internalizing (Gazelle & Ladd, 2003; Larson, Raffaelli, Richards, Ham, & Jewell, 1990) and external- izing disorders (Deater-Deckard, 2001; Fone & Porkees, 2008; Lapiz et al., 2003; Lukkes, Watt, Lowry, & Forster, 2009). Also, the positive effects of peer social interaction have been extensively demonstrated across mammalian species (Asher, Renshaw, & Hymel, 1982; Ladd, 2005; Ozbay, Fitterling, Charney, & Southwick, 2008; Parker, Rubin, Erath, Wojslawowicz, & Buskirk, 2006; Rubin, et al., 2006), though in primates peer-rearing is not as beneficial as maternal rearing (Bastian, Sponberg, Sponberg, Suomi, & Higley, 2003; Spinelli et al., 2009). Friendly peer relationships during the juvenile period play Developmental Psychobiology Received 15 April 2010; Accepted 18 October 2010 Correspondence to: J. Panksepp Contract grant sponsor: Hope for Depression Research Foundation Published online 22 December 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/dev.20520 ß 2010 Wiley Periodicals, Inc.