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Incubation Temperature Effects on Hatchling Performance

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Incubation Temperature Effects on Hatchling Performance INCUBATION TEMPERATURE EFFECTS ON HATCHLING PERFORMANCE IN THE LOGGERHEAD SEA TURTLE (CARETTA CARETTA) A thesis submitted in partial fulfillment of the requirements for the degree MASTER OF SCIENCE in MARINE BIOLOGY by LEAH FISHER OCTOBER 2012 at THE GRADUATE SCHOOL OF THE COLLEGE OF CHARLESTON Approved by: Dr. David Owens, Thesis Advisor Dr. Allan Strand Dr. Eric McElroy Dr. Melissa Hughes Dr. Amy T. McCandless, Dean of the Graduate School ABSTRACT INCUBATION TEMPERATURE EFFECTS ON HATCHLING PERFORMANCE IN THE LOGGERHEAD SEA TURTLE (CARETTA CARETTA) A thesis submitted in partial fulfillment of the requirements for the degree MASTER OF SCIENCE in MARINE BIOLOGY by LEAH FISHER OCTOBER 2012 at THE GRADUATE SCHOOL OF THE COLLEGE OF CHARLESTON Incubation temperature has significant developmental effects on oviparous animals, including determining sex for several species. For the Northwest Atlantic loggerhead sea turtle (Caretta caretta), apparent population-wide female-biased hatchling sex ratios contrast with observations of juvenile populations, where sex ratios have remained constant at about 2 to 1 female-biased over the past 30 years. It has been suggested that some unknown factor is affecting loggerhead survival resulting in an unexplained differential loss of ~60% of female hatchlings per year. One theory to explain this hatchling mortality is tested in this project, that incubation temperature affects traits that influence survival. Furthermore, there may be differential survival between male and female hatchlings. I conducted laboratory experiments to test for an effect of incubation temperature on performance of loggerhead hatchlings. I tested 68 hatchlings produced from eggs incubated at 8 different constant temperatures ranging from ~27ºC to ~32.5ºC. Following their emergence from the eggs, I tested righting response, crawling speed, and conducted a 24- hour long hatchling swim test. Data indicate an effect of incubation temperature on survivorship, righting response time, crawling speed, change in crawl speed, and overall swim activity, with hatchlings incubated at 27ºC showing decreased locomotor abilities. No hatchlings survived when incubated at 32ºC and above. Differences in survivorship of hatchlings incubated at high temperatures are important in light of projected higher sand temperatures due to climate change, and could indicate increased mortality from incubation temperature effects. TABLE OF CONTENTS Introduction 1 - 10 Materials and Methods 10 - 17 Egg Collection and Incubation Protocol Hatching Protocol Experimental Protocol Righting Response and Crawl Test Swim Test Video Analysis Statistical Analysis Results 17 - 27 Hatch Success and Survivorship Incubation Temperature Effects on Performance Variables Composite Rank Clutch Effect on Performance Variables Discussion 27 - 38 Hatch Success and Survivorship Performance + Incubation Temperature Implications Beyond Controlled Incubation Facing Climate Change Conclusions 39 Future Directions 40 - 42 Acknowledgements 43 Literature Cited 44 - 52 INTRODUCTION Temperature plays a critical role in animal development. In particular, incubation temperature has significant developmental effects on oviparous animals, including determining sex for several species (Deeming and Ferguson 1991). Temperature- dependent sex determination (TSD) is found in seven orders of fish (Conover 2004), and many reptiles: all crocodilians, tuataras, many turtles, and some lizards (reviewed in Valenzuela and Lance 2004). TSD is a type of environmental sex determination (ESD), and contrasts with the more common genetic sex determination (GSD). For reptiles with TSD, climate change impacts, particularly rising temperatures around the world, dictate a need for an improved understanding of how even subtle temperature changes in the nest environment will influence the survivorship of both individuals and species in the next few decades. All sea turtles exhibit TSD, with temperatures greater than ~29°C producing females for six of the seven species, and temperatures below producing males (review in Raynaud and Pieau 1985, Mrosovsky 1988). It has been predicted that females born in warm temperatures would have an increased fitness compared to males born at that same temperature, and the reverse for cool temperatures. Within this prediction is another hypothesis, stating that at the extreme incubation temperatures where only one sex is produced, hatchlings could reach optimal fitness for that sex. On the other hand, close to pivotal temperature may be optimal for both sexes. The fitness of sea turtle hatchlings in terms of lifetime fecundity is nearly impossible to test directly due to their slow growth and late maturity (Hamann et al. 2007). Thus, phenotypic measures have been used to estimate hatchling fitness based on other effects of incubation temperature. 1 Incubation Temperature Effects Incubation temperature influences several factors in sea turtle egg incubation besides hatchling sex, including development time, incubation duration, size, mass, and amount of yolk content converted to hatchling tissue (Foley 1998, Reece et al. 2002, Booth et al. 2004). Embryonic development is faster at warmer temperatures, resulting in shorter incubation periods (Davenport 1997). Incubation temperature can also affect physical traits of hatchlings, such as size and locomotor performance for turtles. It is generally assumed that an increase in locomotor activity, for example crawling faster or swimming longer, is advantageous because it may allow hatchlings to better avoid predators (Janzen 1993). In turn, this increased performance could affect lifetime fitness. For example, a study on the snapping turtle (Chelydra serpentina) found that even after six months that included a period of hibernation, incubation temperature maintained an effect on growth rates and water temperature preferences for male and female turtles (O’Steen 1998). Another study on the Ouachita map turtle (Graptemys ouachitensis) showed an effect of incubation temperature on performance both immediately after hatching and after one year (Freedberg et al. 2004). For freshwater turtles, it has been reported that hatchlings show decreased performance ability at the upper and lower extremes of incubation temperature (Micheli- Campbell et al. 2011, Booth et al. 2004, Freedberg et al. 2004). For loggerhead (Caretta caretta) and green sea turtles (Chelonia mydas), several studies (Booth and Astill 2001, Booth et al. 2004, Booth 2006, Hamann et al. 2007) have suggested that hatchlings produced at warmer temperatures may be smaller, swim faster, and possibly even grow faster (Glen et al. 2003); however, these studies did not test the upper limit of incubation 2 temperature, and the fitness consequences remain untested or unconfirmed (Hawkes et al. 2009). More specifically for green sea turtles, hatchlings incubated at 26°C were seen to have lower flipper stroke rate frequency and force output than hatchlings incubated at 28°C and 30°C (Burgess et al. 2006, Booth et al. 2004). This suggests that male hatchlings, coming from cooler temperatures, have a decreased locomotor ability compared to female hatchlings. In contrast for the loggerhead, one study reported that at cooler incubation temperatures, larger loggerhead eggs produced larger hatchlings, which swam faster but accumulated more lactate and exhibited slower growth rates (Foley 1998). No examination of sex was done in this study. A different study on loggerheads suggests that female hatchlings from warmer temperatures may be larger, possibly helping them evade predators (Reece et al. 2002). However, this study used different methods than similar studies, such as Burgess et al. (2006) and Booth et al. (2004), by physically moving eggs during incubation to manipulate incubation temperatures. Along with Foley (1998), these are the only two published studies on loggerhead hatchlings focusing on possible fitness consequences from physical differences based on incubation temperature. In an unpublished thesis experiment on loggerheads, a crawling test was used as a correlate to fitness, and it was reported that longer incubation duration, produced by cooler incubation temperatures, resulted in faster hatchlings (Nicar and Weber 2002). This would suggest that male hatchlings perform better, contrasting with Reece et al. (2002) and green turtle hatchling performance. These sparse and contradictory results indicate that a study designed to test physical and behavioral differences of loggerhead hatchlings incubated at a wide range of temperatures is needed. 3 Incubation temperature also determines if sea turtle hatchlings survive incubation because successful incubation only occurs within a small range of temperatures (Miller 1985). At temperatures above the upper range of optimal temperatures, approximately 33°C and higher for sea turtles, an increase in morphological abnormalities and decrease in hatching success are seen (Miller 1985, Miller et al. 2003). Even within the optimal range of incubation temperatures, Reid et al. (2009) found that hatchlings are more likely to have supernumerary, or irregularly patterned carapace scutes, with increased incubation temperature. In Mon Repos, Australia, from 2005-2007, average incubation temperatures towards the hotter extreme were correlated with a decrease in hatching success for loggerhead hatchlings (Chu et al. 2008). Chu et al. (2008) state that a prolonged exposure to hotter incubation temperatures results in an overall weakening of hatchlings.
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