Summary
- Sexually dimorphic traits often reflect factors limiting the reproductive success of animals. Thus, most sexually dimorphic traits can be directly linked to the reproductive role of each sex. Sexual dimorphism in trophic structures (e.g. beak, jaws, teeth), however, often lacks a direct link to reproduction.
- Trophic structures can be linked indirectly to reproductive allocation via energy acquisition. The reproductive role hypothesis (also known as the dimorphic niche hypothesis) posits such an indirect link, but has received heretofore little direct empirical support. We tested this hypothesis in a molluscivorous turtle exhibiting marked female‐biased trophic morphology dimorphism.
- Bite force analysis showed that females have stronger jaws than males and dietary analysis revealed that females ingest snails closer to their maximum biting capacity than males. Body condition of both sexes and reproductive output of females increased with relative head width, indicating that fitness is tightly linked to head size and bite force.
- Our study provides strong evidence that reproductive role contributes to sexual dimorphism in trophic morphology. Our findings should apply to any animal in which energy intake is limited by trophic morphology.
Methodology
Study species and study site
We studied northern map turtles between May 2004 and June 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. Turtles were captured with basking traps and by snorkelling. All captured turtles were brought to the laboratory where we measured maximum plastron length (PL) with a forestry calliper (± 0·5 mm) and head width (HW) with a digital calliper (± 0·01 mm). We marked turtles individually by drilling small holes in the marginal scutes.
Bite force analysis and prey hardness
Bite force was measured in 52 turtles with an isometric Kistler force transducer (type 9023, Kistler Inc., Wintherthur, Switzerland) connected to a Kistler charge amplifier (type 5058a, Kistler Inc.). We induced turtles to bite forcefully on the free ends of the bite force device (following Herrel & O’Reilly 2006). We measured bite force five times for each turtle, with a short rest (30–40 s) between successive bites. If the turtle did not bite effectively, it was allowed to rest for 30 min before retesting. The highest bite force obtained from each session was taken as the maximal bite force for that individual. The distance between the biting plates was adjusted according to the size of the animal to standardize the gape angle. Care was taken to ensure that each turtle bit the plates in the same orientation.
We determined the maximum hardness of ingested prey by reconstructing the size and hardness of consumed snails (Viviparus georgianus Lea) from the size of the opercula recovered in the faeces of map turtles. V. georgianus is the most important prey item of male and female map turtles in Lake Opinicon and is also the hardest (G. Bulté, unpublished data). We collected faeces by keeping turtles individually overnight in plastic bins filled with lake water. Water containing faeces was filtered and the solid phase was preserved in ethanol until examination under a dissecting scope. For each sample, we measured the largest operculum.
To reconstruct snail hardness, we first determined the relationship between the length of the operculum and the shell length (SL) of the snail based on 90 snails collected in Lake Opinicon. Operculum length (OL) was a strong predictor of SL (R 2 = 0·95, F (1,88), P < 0·0001: SL = –0·878 + 1·906 × OL). We then used the reconstructed SL to predict hardness of the snails using the equation specific to V. georgianus provided by Osenberg and Mittlebach (1989) assuming no important geographical variation in snail hardness.
Each faeces sample represents the prey ingested over a short period of time (a few days). Consequently, a given sample may not contain a snail operculum representing the maximum potential prey size for the individual from which the sample was obtained, and any relationships drawn from all the samples will underestimate the maximum capacity of the turtles. To circumvent this problem and to identify the maximum realized capacity for an individual of a given HW or PL, we used cyclical regressions to partition the data (Thomson et al . 1996). This approach involves a series of linear regressions (in our case, prey hardness regressed on HW or PL) in which the data are successively divided according to the sign of the residuals. The first cycle thus includes all the data, the second cycle includes only the data falling above the line of best fit of the first cycle (i.e. with positive residuals) and the third cycle includes only data falling above the line of best fit of the second cycle.