Unusual sexual dimorphism and small adult size for olive ridley sea turtles are linked to volumetric geometric constraints

Abstract

Determination of sexual size dimorphism (SSD) in marine turtles is challenging because females are observed on nesting beaches, while males remain at sea and are rarely encountered. Data obtained from a sea turtle bycatch survey and a nesting monitoring program conducted in the same area in the Republic of the Congo give access to the body size of both male and female olive ridleys (Lepidochelys olivacea). Curved carapace length (CCL) in male and female olive ridleys reveals SSD, with males being larger than females. This large male SSD contrasts with the female-biased or absence of SSD commonly observed in sea turtles. The ultimate and proximate mechanisms potentially at play are discussed here and tested where possible. One of the main drivers of smaller female SSD could be egg production. We show that egg production in olive ridleys in the Congo is optimal for intermediate female CCL when corrected to nesting date. When females are longer, egg production decreases, probably because of the constrained three-dimensional volume. This counterintuitive relationship between CCL and egg production could be the driving force behind the unusual evolution of larger male SSD in olive ridley turtles, and ultimately, the small size of this species as compared to other marine turtles.