Psychoneuroendocrinology 25 (2000) 259 – 276 Influence of a natural stressor (predator odor) on locomotor activity in the meadow vole (Microtus pennsylanicus ): modulation by sex, reproductive condition and gonadal hormones Tara Perrot-Sinal *, Klaus-Peter Ossenkopp, Martin Kavaliers Neuroscience Programme, Uniersity of Western Ontario, London, Ont., Canada, N6A 5C2 Received 12 March 1999; accepted 21 July 1999 Abstract Sex differences in a variety of non-reproductive behaviors have been indicated to occur in seasonally breeding polygynous/promiscuous rodents such as the meadow vole, Microtus pennsylanicus. The present study was designed to assess the effects of reproductive and hormonal status on the locomotor responses of meadow voles following brief exposure to the odors of a natural predator, the Red fox (Vulpes ulpes ). Adult male and female meadow voles, which are seasonal photoperiodically-induced breeders, were housed in either mixed sex pairs under a long, reproductively stimulatory photoperiod (simulating breeding: long light cycle, paired; LLC +P) or in same-sex pairs under a short, reproductively inhibitory photoperiod (simulated non-breeding: short light cycle, non-paired; SLC-NP). On 2 consecu- tive days following 1 day of baseline activity monitoring, voles were exposed individually for 3 min to fox odor and a novel pungent control odor (extract of almond). The levels of various measures of activity that were displayed by the voles were assessed by an automated Digiscan activity monitoring system. LLC + P (simulated breeding) voles displayed higher basal levels of activity relative to SLC +NP (simulated non-breeding) voles, with males displaying greater activity than females. LLC +P (simulated breeding) males displayed a significant reduction in activity levels following exposure to fox odor relative to control odor. The reductions in activity following fox odor exposure were related to plasma testosterone levels such that a larger behavioral response (i.e. greater reduction) was associated with www.elsevier.com/locate/psyneuen * Corresponding author. Present address: Department of Physiology, School of Medicine, Bressler Bldg., University of Maryland, 655 W. Baltimore St., Baltimore, MD 21201, USA. Tel.: +1-410-706- 2654; fax: +1-410-706-8341. E-mail address: tperr001@umaryland.edu (T. Perrot-Sinal) 0306-4530/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII:S0306-4530(99)00054-2