Nest emergence phenology of the Northern Map turtle (Graptemys geographica) near its northern range limit
Northern Map Turtle , turtle , emergence , hatchling
Turtles are arguably the most threatened order of vertebrates, yet we still know surprising little about the ecology and annual cycle of most species across their respective ranges. This is also true of the eight freshwater turtle species that occur in Ontario, Canada, all of which are considered at risk. My Masters thesis focused on one of these species, the Northern Map Turtle (Graptemys geographica), for which we have limited data on nest phenology from only two locales in the USA. Hatchling emergence from the nest is not a straightforward product of time since oviposition, varies among species, and is not well understood. Map turtle hatchlings have been shown to be capable of overwintering in the nest, but do not always do so. I investigated the emergence phenology of an urban population of Northern Map Turtles near the northern limit of the species range to better understand the ecology of the species as a whole, and these local populations specifically. I sought to answer two broad questions: 1. What are the rates of survival and timing of emergence of hatchlings at this location relative to other sites and species? And 2. What proximate and ultimate factors might contribute to emergence timing? I found, protected, and installed temperature data loggers in 61 Northern Map turtle nests in 2018 and 2019 in Kingston, Ontario, Canada and nearby rural locations near the Queen’s University Biological Station. I compared emergence timing to nest temperature, air temperature and rainfall. The overall survival rate was 70% and 65% for overwintering clutches. 25% of clutches emerged in the fall and 75% of clutches remained in the nest over winter. I found that winter nest temperature below a threshold of -8°C negatively impacted the survival of overwintering clutches. Most notably, I found that higher amounts of rainfall and a smaller nest-air temperature difference increase the odds of emergence. I found some evidence of a latitudinal trend in proportion of fall versus spring emergence in temperate North American turtles implying adaptation or plasticity in response to local environmental conditions. These findings help fill the gaps in our knowledge of this species phenology and will help us to be better able to appreciate and conserve them.