Are Black Swamp Snakes (Seminatrix pygaea) income or capital breeders? Experimental evidence using stable isotopes to unravel the timing of reproductive allocation

In life history studies, a dichotomy is often drawn between “capital” and “income” reproductive allocation strategies, while recognizing that such alternatives may represent extremes of a continuum. For snakes, many species become anorexic during pregnancy, thereby limiting the potential for income breeding; thus, most snake studies suggest capital breeding as the predominate strategy. Recently, we documented that Black Swamp Snakes (Seminatrix pygaea) feed readily during pregnancy and we hypothesized that they used income breeding to fuel reproduction following prolonged drought-induced aestivation. Still, direct evidence of nutrient transfer from recently-ingested prey to offspring is largely nonexistent in snakes. Here, we manipulated the concentration of a naturally occurring stable isotope (¹⁵N) in prey items (Lumbricus terrestris) to test the hypothesis that S. pygaea transfer energy consumed during vitellogenesis and/or pregnancy to their offspring. The δ¹⁵N of our labeled prey (300.9 – 481.7 ‰) was significantly elevated above natural levels for S. pygaea (5.5 – 9.0 ‰) and their natural prey (4.0 – 6.7 ‰), providing an ideal tool for documenting income breeding. We offered pregnant S. pygaea prey items every 7-12 days from capture until parturition but altered the introduction of labeled prey among three treatment groups: control (unlabeled prey), early (labeled prey early in gestation), and late (labeled prey late in gestation). Our experiment provides direct evidence that S. pygaea can transfer ¹⁵N to offspring a minimum of 77 days prior to parturition, but probably not as late as 23 days prior to giving birth. Further, one female consumed labeled prey throughout gestation and displayed elevated ¹⁵N in maternal tissues, but failed to transfer incoming ¹⁵N to her offspring. Our experiment unequivocally demonstrates that S. pygaea can transfer income energy to offspring, but suggests that individual variation may exist in reproductive allocation strategy.