Evolutionary physiology of side-blotched lizards: correlational selection on performance
A key issue in evolutionary physiology has been the difficulty in linking physiological traits directly with evolutionary forces (natural selection, sexual selection, etc.). The research I am currently conducting with Barry Sinervo at UC Santa Cruz uses complementary approaches (lab and field-based studies, under natural and experimental conditions) to allow direct measurements of the interactions between genotype, phenotype, physiological performance, and Darwinian fitness.
Male side-blotched lizards (Uta stansburiana) in Los Banos, California exhibit one of three morphs, identified by throat color. Each morph uses a different mating strategy (territory defense, mate defense, or "sneaking"), and morphs coexist in what has been termed a "rock-paper-scissors" game. The morphs differ in social status, territorial behavior, and other traits, and are primarily genetically determined.
I am measuring physiological traits (e.g. stamina, field metabolic rate) associated with the different morphs, as well as natural selection on the different traits via measurements of field survivorship and fitness. The unique nature of this system allows me to directly measure traits such as reproductive success and survival, as well as physiological traits, across multiple generations. Ultimately, I will determine correlations (linkages) among traits and evaluate how selection is acting on this suite of characters.
There have been few studies of the biology of neonate or juvenile reptiles, and little is known of reptilian nutritional physiology. As a postdoctoral fellow with Dr. Kenneth Nagy at UCLA, I conducted basic physiological research on a threatened species, the desert tortoise (Gopherus agassizii), with implications for conservation and management of declining populations. In the field, I conducted studies of juvenile tortoise foraging, in collaboration with Dr. David Morafka (California State University, Dominguez Hills) and Dr. Olav Oftedal (Smithsonian Institute). In the laboratory, I investigated the nutritional quality of four natural food plants to determine which nutrients limited growth of wild tortoises and to evaluate the potential nutritional impact of invasive plant species on tortoise growth and survivorship. I determined that phosphorus and nitrogen were potentially limiting nutrients, and future work in this area may include determinations of the ecological importance of phosphorus nutrition for tortoises.
Desert tortoise populations face many threats, including habitat loss, disease, and subsidized predation. Head-start programs, in which tortoise eggs and juveniles are maintained in semi-natural pens for some period of time to protect them from predation, have been proposed as one mechanism for assisting population recovery. However, the effects of long-term residency in such pens on juvenile tortoise behavior have not been fully assessed. In collaboration with Dr. David Morafka, I tracked dispersal behavior by neonate and juvenile (8-9 year old) tortoises after release from long-term nursery pens at the Army's National Training Center at Fort Irwin.
Tortoises were released about 60 m away from the pen in early September. Neonates dispersed randomly and quickly selected hibernation burrows apparently at random, but juveniles showed a strong tendency to (a) attempt to return to their home pen, (b) take longer to select hibernation burrows, and (c) select burrows with a preferred orientation (southeast). In a followup study, juveniles released 500 m from their natal pen did not attempt to return to the pen.
There was no mortality in the initial study, but in the followup study, 7 of 16 juveniles were killed by a single raven over a period of about 6 weeks, showing the impact a single predator can have on tortoise populations. There was a clear size threshold for predation (only tortoises under 125 g were taken), suggesting that maintaining tortoises in nurseries until they reach that threshold and then releasing them at least 500 m from the nursery may be an effective means of boosting tortoise populations.
Osmoregulatory physiology of desert and intertidal lizards
For my dissertation research (with Dr. Vaughan Shoemaker at UC Riverside), I studied ion secretion by the nasal salt glands of lizards, which supplement renal ion excretion. The salt glands of lizards, unlike those of marine birds, sea turtles, or sea snakes, are unique in their ability to vary the composition of the secreted fluid, secreting potassium chloride as well as sodium chloride depending on dietary ion content.
I examined the extrinsic factors (salts and other loads) and intrinsic factors (hormones and neurotransmitters) controlling rate and composition of secretion by the desert iguana (Dipsosaurus dorsalis), an herbivorous desert lizard. Desert iguanas normally secrete potassium chloride to help eliminate the high amounts of potassium found in the desert plants they eat.
Other vertebrate salt glands secrete in response to any osmotic challenge (e.g. NaCl, sucrose). Desert iguana salt glands respond specifically to increases in plasma potassium or chloride. They do not respond to sodium alone or other osmotic challenges (sucrose, histidine acetate). Effects of potassium and chloride appear to be additive.
How does this relate to the ecology of desert iguanas? Is it coincidence that the two ions that initiate secretion are also the two most ecologically relevant ions for this species? Future studies will address these issues.
To further examine the ecological importance of salt glands for lizards, I studied a newly-described intertidal lizard (Uta tumidarostra) in Baja California. This species feeds on salty intertidal crustaceans, and I determined that its hypertrophied salt gland must be responsible for at least one third to one half of the lizard's daily salt excretion. My investigations of these two species provoked questions about the evolution of salt glands in lizards and other reptiles, which I plan to pursue in the future.
Future plans: Evolution of salt glands in lizards
Most evolutionary physiology studies have focused on topics such as locomotor performance, energetics, or thermoregulation, but vertebrate osmoregulation is relatively unstudied. Salt glands are far less complex than other osmoregulatory organs such as kidneys, which also excrete nitrogenous waste and other waste products. Salt glands excrete only simple ions and water; however, the ions excreted vary with diet and species. This makes them ideal models for the study of the evolution of ion-excreting systems. Additionally, because salt glands allow salt excretion with minimal water loss, those species that have them may be able to feed on otherwise unavailable high-salt foods. Thus, the salt gland may play a role in determining distributions of species in harsh environments. My research on lizard salt glands will investigate the evolution of salt glands in different families of lizards and explore in detail variation within one family, allowing me to examine salt gland evolution at multiple levels.
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