Chastity belts in gartersnakes: the functional significance of mating plugs

Male red-sided gartersnakes (Thamnophis sirtalis parietalis) deposit a thic k gelatinous plug that occludes the female cloaca after copulation. Previou s workers have interpreted the plug as a sexually-selected adaptation to (1 ) physically prevent re-mating by the female, and/or (2) provide pheromonal cues to discourage courtship by rival males or to decrease recepticity by females. Our data support the former hypothesis, but not the latter. Plugs serve as effective physical barriers to additional copulation for <72 h, bu t this is long enough for most females to become unreceptive, and/or disper se from the mating aggregation. Experimental removal of plugs immediately a fter copulation results in some re-mating by females, but plug removal seve ral hours later does not rekindle sexual receptivity. Contrary to previous work, our experiments show that fluids associated with copulation (rather t han the plug per se) are responsible for the rapid decline of male interest in mated females. Thus, the plugs primary Function is to physically preven t matings rather than as a source of pheromonal cues to manipulate the beha vioour of females or rival males. Plug mass is determined not only by a mal e's body size, but by his prior mating history (plug mass decreases with re peated mating) and by the size of his partner (males allocate larger plugs to larger females). Gartersnakes are unusual not only in their production o f mating plugs, but also in their brief duration of copulation compared to other snakes. Mating plugs may have evolved in gartersnakes to reduce matin g times, because of the extremely high 'opportunity cost' of prolonged mati ng to a male gartersnake in a mating aggregation. (C) 2000 The Linnean Soci ety of London.