Sexual size dimorphism (SSD) is a common phenomenon in animals. In many species females are substantially larger than males. Because body size plays a central role in modulating the body temperature (T b) of ectotherms, intersexual differences in body size may lead to important intersexual differences in thermoregulation. In addition, because SSD is realized by differences in growth rate and because growth rate is strongly temperature dependent in ectotherms, a conflict between male reproductive behaviour and thermoregulation may affect the expression of SSD. In this study, we investigated the thermal implications of SSD in a reptile exhibiting spectacular female-biased SSD: the northern map turtle (Graptemys geographica). Over three seasons, we collected >150,000 measurements of T b in free-ranging adult and juvenile northern map turtles using surgically implanted miniature temperature loggers. Northern map turtles exhibited seasonal patterns of thermoregulation typical of reptiles in northern latitudes, but we found that large adult females experienced a lower daily maximum T b and a narrower daily range of T b than adult males and small juvenile females. In addition, despite more time spent basking, large adult females were not able to thermoregulate as accurately as small turtles. Our findings strongly suggest that body size limits the ability to thermoregulate accurately in large females. By comparing thermoregulatory patterns between adult males and juvenile females of similar body size, we found no evidence that male reproductive behaviours are an impediment to thermoregulation. We also quantified the thermal significance of basking behaviour. We found, contrary to previous findings, that aerial basking allows northern map turtles to raise their T b substantially above water temperature, indicating that basking behaviour likely plays an important role in thermoregulation.
Study species and study site
The northern map turtle is a widely distributed freshwater turtle in eastern and central North America. It is characterized by an extreme SSD with females being more than twice the length of males and no overlap in size at maturity (Bulté and Blouin-Demers 2009). We conducted this study between 2004 and 2006 in Lake Opinicon, a small mesotrophic lake at the Queen’s University Biological Station, 100 km south of Ottawa, Ontario, Canada.
Biologging and radio-telemetry
In late April and early May, we captured map turtles while snorkelling at a communal hibernation site. We surgically implanted temperature loggers (Thermochron iButton DS 2422 and DS1921; Dallas Semiconductor, Sunnyvale, Calif.) in the abdominal cavity of 14 juvenile females (mean carapace length 140 mm; SE = 1.67; range = 130–151 mm), 19 adult males (mean carapace length = 136 mm; SE = 1.33; range = 127–148), and 18 adult females (mean carapace length = 238 mm; SE = 3; range = 217–260). Details of the anaesthetic and surgical procedures for the logger implantation can be found in Edwards and Blouin-Demers (2007). The loggers could hold 2,048 (DS 1921) or 8,192 (DS 2422) temperature readings. Some adult females were implanted with two loggers (model DS 1921). Using these loggers, we obtained T b readings every 110 min (single DS 1921), 55 min (two DS 1921), or 25 min (single DS 2422) between May and September. Each turtle implanted with a logger was also equipped with a radio-transmitter (SI-2FT and SB-2FT; Holohil Systems, Carp, ON, Canada) bolted to the outside edge of the carapace. We located each individual every 2–3 days. When tracking turtles, we applied special care not to disrupt normal basking behaviour. We recaptured the turtles to remove the transmitters and the loggers the fall or spring following implantation.