A comparison of reproductive and energetic states in a Marine Apex predator (The Tiger shark, Galeocerdo cuvier)

To fuel the high energetic demands of reproduction, vertebrates employ different tactics of resource use. Large sharks exhibit long gestation periods and have relatively few well-developed young, which likely incurs high energetic costs. However, information on the relationship between the reproductive and energetic states for most shark species is lacking. In the present study, we used a noninvasive approach to assess relationships among reproductive stage, plasmatriglyceride levels, bodycondition, and circulating reproductive hormones in free-ranging female tiger sharks (Galeocerdo cuvier). A total of 57 sharks were sampled (19 immature, 15 mature/nongravid, and 23 gravid). Circulating plasma triglycerides did not significantly differ among female tiger sharks of different reproductive stages, but body condition values were significantly higher for mature/nongravid sharks (1:29±0.23) compared with gravid (1:15±0.08) and immature (1:13±0.07) sharks. For gravid and mature/nongravid sharks, no significant correlations existed among values of reproductive hormones, plasma triglycerides, and body condition. However, for immature sharks, estradiol values were negatively correlated with body condition values. Additionally, progesterone levels were positively correlated with testosterone levels in immature females. Our results suggest that this large generalist predator may not necessarily be easily characterized as a pure capital breeder, as has been previously hypothesized for ectotherms, but may rely on a mix of energy stores and opportunistic feeding to support reproduction. We present several hypotheses to explain these patterns and discuss our results in terms of energetic reproductive strategies.