Dermal bite wounds as indicators of reproductive seasonality and behaviour in the Atlantic stingray, Dasyatis sabina

Elasmobranch fishes exhibit a series of complex courtship and mating behaviours in which males inflict significant bite wounds to the body of female mates. However, the variety and frequency of mating wounds are not known across a full reproductive season for any species. We examined the distribution and abundance of dermal wounds in adult Atlantic stingrays, Dasyatis sabina, which have a protracted and defined mating season to determine (1) if dermal wounds can be used as indicators of mating activity, (2) whether different categories of bite wounds can be associated with specific mating behaviours, and (3) whether the skin thickness in females is sexually dimorphic. Adults of both sexes showed fresh wounds during the full duration of the mating season (October-June) and there was no relationship between ray size and wound density. Females had more total wounds than males in every month with a maximum average of 20.2 wounds per female in April. Mating wounds were categorized into five distinct forms: single track, double track, bite, margin abrasion and excision. Wounds were randomly distributed over the body of males but concentrated on the posterior half of the disc in females. Each wound type occurred in approximately equal proportions among sexes with the exception of the precopulatory and copulatory-induced margin abrasions which accounted for 13.7% of the total wounds in females but only 3.1% in males. We suggest that the pronounced and concurrent appearance of single track, double track and bite wounds among males results from random premating courtship attacks by males because females cannot be visually discriminated. However, the concentration of wounds on the posterior disc of females is consistent with the possible presence of olfactory cues (e.g. pheromones) that are released at the cloaca. The pectoral fin dermis of females was 50% thicker than that of males, which eliminated the excision of margins during male grasping and functions to reduce female injury. The temporal occurrence of wounds from October through June and peak in April is consistent with previous reproductive studies that show fresh sperm in the reproductive tract of females over the protracted mating period and also ovulation in late March or early April. The importance of social reproductive biting is discussed in relation to the reproductive induction hypothesis proposed to possibly explain the protracted mating of this species. Monitoring of dermal wounds provides a useful non-invasive technique to determine reproductive activity and a means for inference of social relationships in elasmobranch populations.