Exploring the origin of egg protein in an oviparous water snake (Natrix maura)

Reptiles are typical capital breeders that fuel reproduction by the use of lipids stored in fat bodies. However, little is known about the origin (exogenous or endogenous) of egg protein. We have examined the origin of egg protein by means of the analysis of protein content of liver and carcass (skeletal muscle) of the oviparous snake Natrix maura throughout an annual cycle. We have also measured monthly variation of the digestive-content mass and the ovarian mass. Results showed that protein in liver peaked during vitellogenesis according to the role of the liver in the synthesis of vitellogenin. Partial correlations showed the path of protein from the prey (digestive-content) to the liver, and finally to the ovaries, as well as an inverse relation between carcass protein and ovarian mass. Carcass muscle is the only body part that may act as a potential reserve for endogenous protein, although we did not find significant variation in carcass protein during female reproduction. As females with large follicles did not stop foraging activity, we assumed that egg protein was derived from the diet as partial correlations indicated. Our results suggest that N. maura is a capital breeder for lipids and tend to be income breeder for protein. This conclusion contrasts with that observed in capital breeders for which egg protein was derived from muscle. We discuss the idea that flexibility in the origin of egg protein could affect the body condition in post-reproductive females.