Summary
Sexual dimorphism in body size and in trophic morphology are common in animals and are often concordant with patterns of habitat use and diet. Proximate factors leading to intersexual differences in habitat use, however, are challenging to unravel because these differences may stem from sexual dimorphism or may be caused by intersexual competition. Intersexual differences in diet and habitat use are common in size dimorphic reptiles. In this study, we investigated factors contributing to intersexual differences in diet and habitat use in a population of northern map turtles (Graptemys geographica (Le Sueur, 1817)) from Ontario, Canada. Using radiotelemetry, we showed that in a lake map turtles do not exhibit intersexual differences in habitat use, in contrast to river populations. Patterns of habitat use were also inconsistent with prey distribution. The lack of intersexual habitat use differences in our lake population, despite marked differences in prey distribution, also indicated that intersexual habitat use differences documented in river populations are a consequence of sexual dimorphism in swimming capacity. Using stable isotope analysis and fecal analysis, we found a large dietary overlap between males and females, indicating no intersexual competition for food. Patterns of prey selection in females, however, were concordant with the reproductive role hypothesis.
Methodology
Study site
We studied northern map turtles between May 2004 and September 2007 in Lake Opinicon (44°34′N, 76°19′W) at the Queen’s University Biological Station, approximately 100 km south of Ottawa, Ontario, Canada. Lake Opinicon is a small (788 ha) and shallow (mean depth 4.9 m) mesotrophic lake that is part of the Rideau Canal waterway linking the cities of Ottawa and Kingston. We captured northern map turtles with basking traps and by hand while snorkelling.
Radiotelemetry and habitat use
We tracked 53 northern map turtles with radiotelemetry. Turtles equipped with radio transmitters were selected to fit in one of the following categories: adult females (plastron length = 201–234 mm, n = 17), juvenile females overlapping in size with males (plastron length = 114–135 mm, n = 18; hereafter small females), and adult males (plastron length = 111–125 mm, n = 18). We attached the radio transmitters (model SI-2FT and SB-2T; Holohil Systems, Carp, Ontario) to the rear marginal scutes of the carapace with stainless steel bolts and nuts. The edges of the transmitters were smoothed with nontoxic aquarium silicone to prevent snagging on aquatic vegetation. Individual turtles were followed for one to three active seasons. We located each individual every 2–3 days from late April to early September and once a week from mid-September to mid-October. Each individual location was plotted in the field on a detailed map of the lake and UTM coordinates (NAD1983) were later obtained from the electronic version of the same map with the software ArcGIS version 9.0 (Environmental Systems Research Institute, Inc. 2000).
We used water depth and distance to shore as our metrics of habitat use. Depth was obtained from a bathymetric chart of the lake. We are aware that males and females can differ on other habitat variables, or that ontogenetic changes can be apparent on habitat variables measured at a finer scale. Depth and distance to shore, however, are regularly associated with extreme SSD in aquatic reptiles (Pluto and Bellis 1986; Shine 1986; Bodie and Semlitsch 2000; Lindeman 2003; Carrière 2007). In addition, most biotic (e.g., macrophyte cover, prey distribution) and abiotic (e.g., temperature, dissolved oxygen) variables in lakes are dictated by either depth or distance to shore. Thus, our two habitat variables should integrate most habitat variables likely to vary between sexes or ontogenetically. We broke down the depth of the lake into three classes (0–2, 2–4, and >4 m) and calculated the proportion of observations in each depth class for each group (males, small females, and large females). Because the proportions of each depth class used by an individual are compositional data (i.e., they always sum to one), they are not independent from each other and thus must be transformed (Aebischer et al. 1993). The linear independence of each xi component (i.e., the depth classes) can be achieved with the following transformation: yi = ln(xi/xj), where xj is one of the components and yi is the transformed variable (Aitchison 1986). This transformation requires the exclusion of one component from the analysis (i.e., xj). We used the class >4 m as xj because this class comprised <2% of the turtle observations. We excluded from the depth analysis all the observations for which the turtles were basking out of the water (692 of 2963 observations).