Basking aggregations in the adder (Vipera berus): attraction to conspecific cues or to scarce suitable microhabitats?

Snakes live a predominantly solitary and secretive life, and are most often considered to be “non-social”. We here provide quantitative information on the occurrence of basking aggregations, i.e., gatherings of two or more adders having clear physical contact with each other, in the adder (Vipera berus). Overall, approximately 1 out of 10 observed adders was seen in groups that consisted predominantly of two individuals and rarely of 3 up to 7 adders. The incidence of aggregations was higher in the adults than in the younger age classes, and in the adult males than in the adult females. Males and especially females were found much more than expected in single-sex clusters, whereas male–female groups were seen much less frequently. Aggregations of adult males were seen almost exclusively during early spring, and groups with only adult females were mainly noticed during mid-summer. For both sexes, aggregated adders were seen most often during periods when the observation frequency of solitary adders was also highest. Hence, the seasonal variation of adder groupings is not induced by varying levels of the tendency to actively associate with conspecifics. There were no indications that adders actively aggregate in large groups and individual snakes did not exhibit a stronger tendency toward gregariousness than others. Overall, our analyses indicate that the observed aggregations are mainly non-social, but rather reflect the tendency of individual adders to simultaneously select scarce thermally suitable microhabitats.