Movements, Mating, and Dispersal of Red-Sided Gartersnakes (Thamnophis sirtalis parietalis) from a Communal Den in Manitoba

We studied the spring emergence of red-sided gartersnakes (Thamnophis sirtalis parietalis) from a large communal den in Manitoba to clarify the context in which courtship and mating occur. In particular, is the mating system in such a massive aggregation (> 20,000 snakes) substantially different from that at smaller dens? Radio-tracked female snakes stayed near the den for a few days postemergence, apparently waiting until they recovered locomotor ability after the long hibernation period, before setting out for their summer ranges. Females dispersed from the den in all directions, rather than following distinct migration corridors. Male snakes moved frequently and spent much of their time far from the den in surrounding woodland. Most males remained near the den for only a small proportion of the entire mating season, apparently because of high rates of energy expenditure during mate-searching and courtship. Courting groups contained from 1–62 males and were largest close to the den. Most courting and mating occurred in groups of less than five males, often more than 20 m from the den. These small group sizes resulted from the females' dispersal prior to mating. Larger groups did not induce female cooperation (mating) more quickly than small groups. Despite the spectacular aggregations of courting snakes within the den, most reproductive activity in this population (as in other gartersnake populations) occurs in small groups, widely dispersed over a broad area.

Fig. 1.

Movement patterns of four radio-tracked male gartersnakes (Thamnophis sirtalis parietalis) in central Manitoba during the spring mating season in 1998. Data for four snakes are shown, as examples of the patterns observed. Some males remained relatively close to the den throughout the monitoring period (numbers 47 and 50, upper diagram), whereas the activity of others was centered away from the den (numbers 28 and 51, lower diagram). The den itself is represented by the black area near the center of each diagram; the circle around it represents a fence (to exclude tourists), and the hatched area represents thick groves of aspen trees. The unhatched area between the aspen groves represents open grassland. Dots show successive daily locations of each telemetered snake, connected by straight lines

Fig. 2.

The proportion of telemetry locations at which radio-tracked gartersnakes were close enough to the den to be within reception range of the telemetry signal, as a function of the time since their release at the beginning of the study. Reception range was typically 20–50 m for males, 50–100 m for females

Fig. 3.

Movement patterns and directions of dispersal for radio-tracked female gartersnakes (Thamnophis sirtalis parietalis) in central Manitoba during the spring mating season in 1998. Most females stayed for a few days near the den then moved away rapidly in a consistent direction. The figure only shows the first 50 m of their dispersal movement, but in some cases, we followed the animals for > 1 km. See caption to Figure 1 for explanation of symbols and shading

Fig. 4.

The distribution and attributes of courting groups of gartersnakes at and near a den in central Manitoba. The upper figure (A) shows that most groups were found very close to the den, but some were a considerable distance away. The middle figure (B) shows that the numbers of males per group declined rapidly with increasing distance from the den. The lower figure (C) shows that group sizes varied considerably, but most groups consisted of < 5 males

Fig. 5.

Rates of loss in body mass of male gartersnakes during the mating season. Each snake is represented by only one data point. The line shows least-squares regression of best fit; see text for statistical results

Accepted: 14 Jul 2000