Reproductive Fitness in Male Tuatara (Sphenodon punctatus) and its Implications

Abstract

<p><b>Sexual selection and variation in mating success in the mate-limited sex provide the underpinning for the evolution of many unique traits. Understanding the evolutionary costs of and relationships among these traits can inform our knowledge of sexual selection across the tree of life and provide species-specific improvements in conservation practices. Despite facing an ongoing extinction crisis, the evolution of sexual traits and variation in reproductive fitness is understudied in reptiles. </b></p> <p>Tuatara (Sphenodon punctatus) are culturally significant reptiles endemic to Aotearoa New Zealand. The sole survivors of the once widespread reptile order Rhynchocephalia, they are of extreme evolutionary, and thus conservation, importance. While many conservation successes have been achieved since tuatara were at their lowest numbers following severe human-associated range restrictions, we still know very little about reproductive fitness in males of this species. Thus, I undertook a large study investigating male sperm quality, dietary intake, immune function, morphological characteristics, and the relationship among these factors in a population of tuatara living on Takapourewa (Stephens Island). </p> <p>I provide the first description of tuatara, and thus Rhynchocephalian, mature sperm, including: summary statistics of sperm head length, midpiece length, tail length, percent viability, and curvilinear velocity. I found that tuatara sperm are long and fast but have poor viability relative to other reptile species. I did not find any significant trade-off between percent viability and curvilinear velocity in male tuatara, but there was weak support for the influence of tail length and midpiece length on sperm swim speed. Tail length was positively associated with faster sperm swim speeds, while midpiece length was negatively associated with faster sperm swim speeds. I also carried out preliminary work exploring the efficacy of different buffers and cryopreservatives at maintaining sperm cell membrane integrity and swim speed after freezing and thawing. I found that PBS was an effective buffer and that Lake’s cryoprotectant is not suitable for use in this species. </p> <p>Next, I used morphological measurements and stable isotope analyses (Δ13C and Δ15N) to establish that tuatara with larger gapes have higher levels of marine-associated (seabird) carbon in their diets. Additionally, tuatara with larger gapes tended to have better overall body condition, be longer (larger snout-vent length), and have larger caudal fat stores. Tuatara are sexually dimorphic, with males being larger and larger males dominating mating opportunities. These results indicate that male tuatara most likely to mate consume a disproportionately larger amount of seabird material on Takapourewa. </p> <p>Using historical blood smears collected from tuatara living on both Takapourewa and a resource-restricted island (North Brother Island), I established reference intervals for tuatara leukocyte profiles for the first time and explored the factors influencing tuatara immune function. I found that tuatara leukocytes display circannual rhythm and that the source population and date of sample collection were the most consistent predictors of leukocyte profile makeup in this species. Interestingly, I found that individuals from a resource-restricted, inbred population with poor body condition and showing evidence of chronic stress (determined via increased heterophil: lymphocyte ratio) were still able to mount responses to immune challenge, as measured by nitric oxide production. </p> <p>Finally, I looked at the trade-off between the many factors explored in this thesis: morphological characteristics known or thought to be under selection from female mate choice (body condition, gape size, and dorsal and nuchal spine number/area), dietary intake (Δ13C and Δ15N), immune status (heterophil:lymphocyte ratio, total number of leukocytes, and ectoparasite load), and sperm quality (viability and curvilinear velocity). I found evidence of an immunocompetence handicap, with both measures of sperm quality being negatively associated with body condition and the ratio of heterophils:lymphocytes. These relationships are likely the result of testosterone-induced immunosuppression and an increase in metabolic rate. I also found evidence of signal dishonesty in relation to tuatara sperm swim speed. Faster sperm were associated with smaller spine area, a trait known to be negatively associated with female mate choice. Lastly, I found evidence of the effects of diet on tuatara sperm swim speeds. Tuatara with higher (less negative) Δ13C values had slower swimming sperm, possibly a consequence of oxidation damage associated with marine-derived dietary items. These results combine to provide support for a trade-off between immune and endocrine system investment, mediated by diet, in this unique reptile. This work expands our understanding of sexual selection and the costs of reproductive fitness to a new order and provide important context for future conservation practices in this species.</p>