Slow Isotope Turnover Rates and Low Discrimination Values in the American Alligator: Implications for Interpretation of Ectotherm Stable Isotope Data

Abstract

Stable isotope analysis has become a standard ecological tool for elucidating feeding relationships of organisms and determining food web structure and connectivity. There remain important questions concerning rates at which stable isotope values are incorporated into tissues (turnover rates) and the change in isotope value between a tissue and a food source (discrimination values). These gaps in our understanding necessitate experimental studies to adequately interpret field data. Tissue turnover rates and discrimination values vary among species and have been investigated in a broad array of taxa. However, little attention has been paid to ectothermic top predators in this regard. We quantified the turnover rates and discrimination values for three tissues (scutes, red blood cells, and plasma) in American alligators (Alligator mississippiensis). Plasma turned over faster than scutes or red blood cells, but turnover rates of all three tissues were very slow in comparison to those in endothermic species. Alligator d 15N discrimination values were surprisingly low in comparison to those of other top predators and varied between experimental and control alligators. The variability of d 15N discrimination values highlights the difficulties in using d 15 N to assign absolute and possibly even relative trophic levels in field studies. Our results suggest that interpreting stable isotope data based on parameter estimates from other species can be problematic and that large ectothermic tetrapod tissues may be characterized by unique stable isotope dynamics relative to species occupying lower trophic levels and endothermic tetrapods.

Coastal leatherback turtles reveal conservation hotspot

“…Skin samples were collected in preference to other superficial tissues (e.g. blood), because the isotopic turnover of skin in large reptiles is on the scale of months to years6061. Thus, it should reflect foraging conditions at the previous feeding areas, and not vary significantly over the nesting season.…”

Urinary iodine and stable isotope analysis to examine habitat influences on thyroid hormones among coastal dwelling American alligators

“…Isolated keratin samples were dried for 48 hours at 60 °C and ground to fine powder. Alligator mississippiensis scute tissue has a slow rate of turnover (mean turnover = 590 days for δ 13 C and 414 days for δ 15 N) (Rosenblatt and Heithaus, 2013) thus stable isotope signatures of scute keratin represent incorporation of long-term dietary patterns (> 1 year).…”

“…For isotope mixing model analyses we used a Bayesian mixing model within the ‘ siar ’ package (version 4.2, of R; hereafter SIAR) (Parnell and Jackson, 2013) to estimate the proportion of prey resources contributing to the diet of individual A. mississippiensis . We used discrimination factors determined for A mississippiensis keratinized scute tissue by Rosenblatt and Heithaus (2013): $$\mathrm{normal\Delta }{}^{13}\mathrm{C}_{\text{tissue-diet}}=0.61‰\pm 0.1212‰false(\text{mean}\pm \text{SD}false)$$ $$\mathrm{normal\Delta }{}^{15}\mathrm{N}_{\text{tissue-diet}}=1.22‰\pm 0.08‰false(\text{mean}\pm \text{SD}false)$$…”

Isotope incorporation in broad-snouted caimans (crocodilians)

“…Indeed, reptilian metabolism and regulatory physiology are distinct, and parameter estimates obtained from validation studies in other taxonomic groups may lead to inappropriate conclusions about these ectotherms. Only eight studies on isotope incorporation in reptiles have been published [tortoise (Murray and Wolf, 2012), freshwater turtles (Seminoff et al, 2007), sea turtles (Seminoff et al, 2006; Reich et al, 2008; Seminoff et al, 2009), snakes (Fisk et al, 2009), lizards (Warne et al, 2010), and alligators (Rosenblatt and Heithaus, 2013)]. Nonetheless, the results of these few studies have been used to interpret field data from these species as well as guide future research and new study questions [e.g.…”