Males Biting Female Decoys. —

Forty-five mating attempts were observed in our experiments, and biting was involved in 4 of them (9%). In observations 1 and 2, the male interrupted his mating attempt and moved toward the head of the female decoy and bit the back of its neck (Fig. 1A). In both cases, the male also appeared to scratch the female decoy’s neck. In observation 3, the male interrupted his mating attempt, moved behind the female decoy, and bit the rear edge of its carapace. In observation 4, the male interrupted his mating attempt, moved behind the female decoy, but bit the area between its hind leg and tail. In this last case, the exact location of the bite was hidden by the rear edge of the female decoy’s carapace. The motion of the male, however, strongly suggested that he attempted to bite the female decoy twice.

Males Biting Live Females. —

We captured 3 instances (observations 5, 6, and 7) in which a male bit a free-ranging female. In observation 5, 2 males interacted with the same female. One male appeared to perform a display in front of the female, while the other male appeared to attempt to mate with the female. The latter male then moved behind the female and bit the rear edge of her carapace in a similar location and fashion as the male in observation 3 (Fig. 1B). The female reacted by slightly moving forward, and the male briefly held onto her carapace. In observation 6, a male seemingly attempting to mate with a female moved behind the female and bit her right foot. The female reacted by pulling her foot in and the male held on as her foot was pulled in (Fig. 1C). The female then moved out of the camera field of view, and the male continued to follow her. The male behavior in this case was similar to that of males in observations 3 and 4 involving the decoys. In observation 7, biting was not directly observed, but inferred from the male’s actions and the female’s reactions. In this case, both the male and the female were partially out of the camera’s field of view. The male was facing the female’s tail, but his head was hidden by her carapace. The male appeared to be pushing himself against the tail area, and, as he did so, the female moved forward. The male appeared to be dragged as the female moved forward, suggesting that he was holding on with his mouth. The female then apparently attempted to kick the male away with her foot. The observations of males biting real females confirm that this behavior is not unique to the interactions between males and irresponsive female decoys.

Biting may serve 2 nonmutually exclusive functions: tactile signaling and sexual coercion. Sexual coercion usually takes 1 of 3 forms: forced copulation, harassment, or punishment (Clutton-Brock and Parker 1995). Forced copulation appears to be physically impossible in northern map turtles because of the diminutive size of males compared to females (Bulté and Blouin-Demers 2009). Harassment involves repeated mating attempts with the same female. Punishment involves physically harming females rejecting mating attempts. The goal of harassment and punishment is to raise the cost of mating resistance above the cost of mating itself (Clutton-Brock and Parker 1995). Harassment and punishment can lead to convenience polyandry in which females mate with multiple males, not because it increases their fitness, but because not doing so reduces it (Rowe et al. 1994). Polyandry is the norm in our study population, but the fitness benefits to females appear minor (Banger et al. 2013), suggesting that female choice alone may not be the only cause for polyandry. It is thus possible that polyandry in northern map turtles is, at least in part, an outcome of coercion and that biting is a form of punishment. Males were observed to bite decoys only as part of mating attempts, which may thus show how certain males react to unreceptive females. The coercion hypothesis is also consistent with male-avoidance behaviors recently documented in females at our study site (Bulté et al. 2024).

Male biting behavior may also help explain the differences in the relationship between bite force and head width in northern map turtles. Male map turtles have smaller heads than females relative to their body size and thus produce lower absolute bite forces than females, but the allometric coefficient between head width and bite force is significantly higher in males than in females (Bulté et al. 2008b). When accounting for head width, bite force is 1.7 times higher in males than in females (Bulté et al. 2008b). The diet of males is mostly composed of soft caddisfly larvae and small molluscs (Bulté et al. 2008a), and, at least in our study population, the hardest prey ingested by males are well under their maximum bite force capacity (Bulté et al. 2008b). Diet therefore does not explain the relatively stronger bite force in males. Males with relatively stronger jaws may be better at coercing females into mating than males with weaker jaws, hence creating selection for stronger bite force. Intrasexual selection on bite force was documented in male collared lizards (Crotaphytus collaris) that bite each other to defend territories (Lappin and Husak 2005). Bite force may also be under positive selection in species with coercive mating tactics. The presence of sexual weapons associated with biting (tomiodonts) in male painted turtles indicates that selection on bite performance can be important (Hawkshaw et al. 2019; Moldowan et al. 2020a, 2020b).

Coercion may be used by males to gain rapid mating opportunities when females are briefly aggregated around an essential resource, as is the case for green sea turtles (Chelonia mydas) at nesting beaches (Lee and Hays 2004), for red-sided garter snakes (Thamnophis sirtalis parietalis) at overwintering sites (Shine et al. 2005), and for European common frogs (Rana temporaria) at breeding ponds (Dittrich and Rödel 2023). In our study population of northern map turtles, mating occurs under similar circumstances. Hundreds of turtles share the same hibernacula, and mating occurs in late fall and early spring when they are aggregated (Bulté et al. 2018, 2021). Communal overwintering appears to be caused by the rarity of suitable overwintering sites (Feng et al. 2019; Bulté et al. 2024b), so females may be constrained to share space with numerous reproductively active males.

We showed that biting is part of the reproductive behavior of male northern map turtles. Although the function of biting is not yet entirely clear, our previous work on this population suggests a coercive function. Interestingly, G. geographica is the most basal Graptemys species (Thomson et al. 2018), so biting may be an ancestral behavior and may thus be part of the mating systems of other species in the genus. This potentially coercive mating strategy was unexpected given the direction of sexual size dimorphism in map turtles.