Testing ‘bigger is better’ and maternal effects hypotheses in hatchlings of the sexually dimorphic spiny softshell turtle (Apalone spinifera)

The ‘bigger is better’ hypothesis (BIBH) predicts that fitness increases with body size. Eastern spiny softshell turtles (Apalone spinifera) exhibit sexual size dimorphism (SSD) at maturity, and the much larger female body size is attributed to selection favouring greater reproductive output. Hatchlings exhibit sexually dimorphic markings, but hatchling SSD has not been investigated. The BIBH predicts that hatchlings will not exhibit SSD because reproductive benefits to larger, mature females drive size dimorphism at maturity, but selection on hatchlings relates only to survival. We tested this prediction with A. spinifera hatchlings (N = 2630) incubated in controlled conditions to avoid confounding environmental effects on phenotype. Hatchling mass was similar between sexes, suggesting similar nutritional allocation to male and female offspring. Hatchling size (carapace length) was also similar between sexes. Hatchling mass and body size varied among clutches and were positively related to maternal size, suggesting morphological constraints on egg size. Surprisingly, clutch size was related only weakly to maternal size. Taken together, our results suggest that hatchling size in A. spinifera is consistent with the BIBH and that offspring quality increases with maternal age, but that maternal effects drive a large proportion of the variation in clutch size and offspring quality.