Dr Daniel A. Warner Post-Doctoral Fellow Department of Ecology, Evolution and Organismal Biology Iowa State University Ames, Iowa 50011, USA e-mail: dwarner@iastate.edu http://www.public.iastate.edu/~fjanzen/

Past Research Education Publications Abstracts and Presentations Grants and Awards

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In general, I am interested in how environmental and maternal factors influence variation in phenotypic traits, and their implications on the evolution of life-history traits.  The majority of my research investigates how the embryonic environment and maternal factors modify various fitness-related traits in oviparous reptiles (specifically lizards), and, in turn, how these traits influence individual survival and reproduction (i.e., fitness).  The current and past research projects that I have been involved with are briefly outlined below.

2003-2007: Adaptive significance of temperature-dependent sex determination

My current research focuses on the ecology and evolution of temperature-dependent sex determination (TSD) in reptiles.  Species with TSD typically do not have sex chromosomes, and offspring sex is determined by the temperatures embryos experience during development.  This type of sex-determining mechanism is found in all crocodilians, most turtles, and several lizard species.  The numerous independent occurrences of TSD within and among the major reptile groups raise several interesting questions about the evolution and adaptive significance of TSD.

The major component of my PhD research focuses on testing the differential fitness model proposed by Charnov and Bull (1977. Nature 266:828-830).  This model provides an explanation for why environmental sex determination would be favoured over genetic sex determination.  In terms of the adaptive significance of TSD, this model proposes that incubation temperatures differentially influence the fitness of male and female offspring, such that male-producing temperatures are best-suited for males and female-producing temperatures are best-suited for females.

One problem with testing the differential fitness model is that it is logistically difficult (if not impossible) to evaluate the long-terms effects of developmental temperatures on lifetime reproductive success (i.e., fitness), especially on extremely long-lived, late-maturing TSD reptiles, such as turtles and crocodilians.  However, with the recent discovery of TSD in many short-lived agamid lizards in Australia, evaluating the long-term effects of incubation history on reproductive success now appears to be feasible.

I am using the jacky dragon (Amphibolurus muricatus) to test the differential fitness model.  Jacky dragons are common agamid lizards found in coastal heathland habitat in southeast Australia (see photo taken at Royal National Park, below).  Jacky dragons have an interesting pattern of TSD, whereby low and high incubation temperatures produce females, and intermediate temperatures produce both sexes.  These short-lived lizards can reach sexually maturity at an age of 9 months depending on how early they hatched in the previous season.  Their nesting season occurs from October to February and they can produce up to four clutches of eggs within a single season.

During the first year (2003-2004) of my PhD research, I initiated a long-term experimental test of four hypotheses for the adaptive significance of TSD; each hypothesis falls within the framework of the differential fitness model (see Warner and Shine. 2005. Evolution 59:2209-2221).  The overall objective of this project is to evaluate the long-term effects of incubation temperature on male and female fitness.  This project involves raising offspring to sexual maturity in semi-natural field enclosures to evaluate the sex-specific effects of incubation history on survival and actual reproductive success.  Thus, I intend to continue this project for at least two more reproductive seasons in order to evaluate incubation and phenotypic influences on life-time reproductive success.

Research during my second season (2004-2005) focused on maternal influences on offspring sex ratios in the jacky dragon.  I evaluated two types of maternal effects that likely play a role in sex determination; the first type is maternal nest-site selection behaviour, and the second is maternal allocation of endogenous steroid hormones into the yolks of eggs (in collaboration with Dr. Matthew Lovern, Oklahoma State University).  I also conducted experiments to determine if the quality of the maternal diet and the operational sex ratio of the adult population influences sex allocation when eggs are incubated under a temperature that produces a 50:50 sex ratio.

During my third season (2005-2006), my research will focus on the nesting behaviour of females under natural field conditions.  The objective of this study is to monitor field nest temperatures and determine the influence of natural incubation temperatures on offspring sex ratios under natural conditions.  In addition, because environmental temperatures change over the course of the nesting season (October to February), I will evaluate how (or if) offspring sex ratios shift during the hatching season.  This research will also determine the environmental characteristics that females consider when selecting a nest site.

Female jacky dragon with radio transmitter digging nest

Average clutch of eggs in nest before being buried

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1998-2003:  Environmental and maternal influences on phenotypes and survival of hatchlings lizards

 

For my MS degree (advised by Dr. Robin Andrews, Virginia Tech), I examined the relative contributions of the incubation moisture environment, maternal yolk investment, and clutch to variation in offspring phenotypes and survival of the eastern fence lizard (Sceloporus undulatus).  This research took place in southwest Virginia in Jefferson National Forest.
 

After completing my MS degree (January 2001), I extended this research in collaboration with Travis Robbins (University of South Florida).  We investigated the effects of flutctuations in the moisture environment during incubation on phenotypes and survival of hatchling Sceloporus undulatus from a population in west-central Florida.

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2001-2003:  Population biology and fisheries management of marine arthropods

After finishing my MS degree, I moved to Florida and lined up a job at the Florida Marine Research Institute with the Crustacean Fisheries Research Group (supervised by Dr. Theresa Bert) in St. Petersburg.  I was involved with various research projects on the population biology and fishery management of stone crabs (Menippe adina & M. mercenaria), blue crabs (Callinectes sapidus), shrimp (penaeid species), and horseshoe crabs (Limulus polyphemus).

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1996-1998:  Population biology of the six-lined racerunner in northwestern Illinois

As an undergraduate student I worked in the laboratory of Dr. Fredric Janzen (Iowa State University) for about 3 years.  During this time I undertook an independent research project to study the demography and population biology of six-lined racerunners (Cnemidophorus sexlineatus) in northwest Illinois.  I was also involved with various other projects that focused on the demographics of western hognosed snakes (Heterodon nasicus), and nesting patterns and TSD in painted turtles (Chrysemys picta) and snapping turtles (Chelydra serpentina).  During my time in the Janzen lab, I spent a summer (1997) working with John Tucker on his demographic studies of red-eared slider turtles (Trachemys scripta elegans) in west-central Illinois.

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Iowa State University, Ames, Iowa, USA.
Bachelor of Science in Animal Ecology.  May 1998.
Advisor:  Dr. Fredric Janzen

Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA.
Master of Science in Biology.  January 2001.
Advisor:  Dr. Robin Andrews

The University of Sydney, Sydney, New South Wales, Australia.
PhD candidate in Biology,  July 2003 - May 2007.
Advisor:  Prof. Richard Shine

Iowa State University, Ames, Iowa, USA.
Post-doctoral Fellowship, Department of Ecology, Evolution and Organismal Biology, July 2007

Hatchling Varanus rosenbergi from Royal National Park

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Last updated: June 2006

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