Behav Ecol Sociobiol (1984) 15: 9-17 Behavioral Ecology and Sociobiology 9 Springer-Verlag 1984 Dynamics of social nesting in overwintering meadow voles ( Microtus pennsylvanicus) : possible consequences for population cycling Dale M. Madison, Randall W. FitzGerald, and William J. McShea Department of Biological Sciences, State University of New York at Binghamton, Binghamton, New York 13901, USA Received May 13, 1983 / Accepted November 12, 1983 Summary. A small population of meadow voles (Microtus pennsylvanicus) in a field enclosure was studied from August to February in Apalachin, New York, USA. Radiotelemetry provided direct measures of intraspecific spacing and social nesting through the fall-winter transition. Data on weather and predation were collected concurrently. A total of 32 voles were radiotracked during 6 tracking sessions, with an average of 17.3 voles (11 to 25 range) tracked per session (Figs. 1, 2a). Discrete social nesting "constellations" first occurred dur- ing October, primarily as a result of the formation of extended maternal families (Figs. 1, 2e). Re- cruitment of adult males and offspring into these early nesting groups was male biased. The average number of voles in these groupings varied from 3.2 (Jan; 3-4 range) to 7.0 (Oct; 4-10 range); but the average number of voles that slept together at any given time, the nesting "cluster," remained steady at 2.4 (2-5 range) (Fig. 2e). During late De- cember and early January under the protection of snow, many voles shifted their home areas and nesting affiliations with the result that non-lineage nesting constellations formed (Fig. 1, 3). The ther- moregulatory benefits of huddling and the threat of predation appear to be important governors of movement, group formation and dispersion. The existence of an optimum group size produces and "Allee Effect" that may contribute importantly to population lows and multi-annual cycles. Introduction Microtine rodents regularly undergo annual fluctu- ations in numbers, and are often credited with ex- hibiting multi-annual cycles of abundance with a period of 2 to 4 years (Finerty 1980). Why these populations increase does not evoke much contro- versy, but why they level off and decline, some- times precipitously, has stimulated considerable debate (Krebs and Myers 1974; Krebs 1978; Chris- tian 1978). Microtine populations typically peak during fall and decline through late winter, but weather conditions, 1Lechnical limitations and com- peting academic schedules have limited the number of direct observational studies of population de- clines at this time of year. Perhaps the most noticeable change in the be- havior of microtine rodents with the coming of winter, besides the general curtailment of repro- ductive activity (Christian 1980), is the appearance of social nesting (Madison 1984a). The thermoreg- ulatory benefit and possible moisture conservation resulting from this behavior has been demon- strated in several microtine species (e.g., Geb- czynski 1969, 1975; Gorecki 1968; Hansson and Grodzinski 1970; Trojan and Wojciechowska 1968; Wiegert 196JL). The occurrence of group nesting or "huddling" in the field is clearly demon- strated in Microtus pennsylvanicus (Webster and Brooks 1981a, b) and M. xanthognathus (Wolff and Lidicker 1981), and is strongly suggested for many other microtines (Madison 1984a; West 1977; West and Dublin 1984). The existence of nesting groups during winter raises interesting questions concerning differential survival, population regulation, and multi-annual cycling in microtine rodents. Is the unit of disper- sion upon which selective agents act during the winter the individual or the group? Is there selec- tion for diffuse or clustered family group members ? Is there strong selection for social toler-