Addled by antlers: Synchronous disruption to female caribou antler phenology

Abstract Caribou are the only deer species in which females also grow antlers; however, female antler phenology differs from males. While males grow antlers over summer and hold them through autumn rut, female caribou grow antlers in summer and carry the polished antlers through winter until parturition. The pathways associated with antler casting in females are thought to be related to hormonal changes during parturition. Yet not every female caribou is pregnant every year, and nonpregnant individuals must use an alternative cue to trigger antler casting. While populations are typically consistent in antler phenology there are observations of individual caribou on an alternate phenological timeline, casting antlers 1 month prior to parturition, and growing velvety antlers a few inches long during the calving time. Indeed, this is generally observed in barren females and only in small proportions of the population. Here we report on observations of a population‐wide change in antler phenology on Fogo Island, Newfoundland. We observed several individuals growing velvety antlers during calving season, earlier than in previous years, including two individuals who were confirmed to have been pregnant. Additionally, we observed no female caribou with polished antlers from the previous autumn, when our field observations generally show ~20% of the adult female caribou in this population have antlers at this time. We discuss possible disruptions to this phenology, commenting specifically on environmental cues that differed in comparison to years with the more commonly observed phenology. We found evidence suggesting that years with early antler casting had earlier spring warming, as well as earlier reduction in snow cover when compared to previous years. Our observations suggest that changing climate patterns may impact antler growth in female caribou, causing growth to coincide with the increased nutritional and energetic demands associated with gestation and lactation. Specifically, it is possible that earlier warming patterns may change which cue triggers antler growth in pregnant individuals from a hormonal cue during pregnancy to an environmental cue more commonly used by barren individuals. We highlight potential research questions surrounding the resilience of antler phenology, and the potential fitness consequences of disruptions to antler growth.