Research

Research Foci of the DeNardo Lab

Research in our lab examines physiological processes from an integrative perspective. Studies examine how physiological processes shape the ecology of a species, influence indiviudal behavior, and provide driving forces for evolution. We are also interested in the counter relationship of how the ecology and behavior of an animal influence physiological processes. Most of our research projects incorporate both laboratory experiments that allow for close assessment of subjects under controlled conditions and intensive field studies which allow for more complex ecologically relevant assessments. While challenging, simultaneously undertaking both laboratory and field studies provides a powerful means to thoroughly address our research questions. Field research includes both observational studies and manipulative experiments (e.g., hormone manipulation, food or water supplementation). Our work addresses broad physiological questions and typically uses squamate reptiles (lizards and snakes) as models for these studies. Each project incorporates at least one but usually more of the following broad areas:

Physiological capabilities

A key focus of our lab is exploring the physiological potential of our study species. By understanding the capabilities of an organism, we are able to appreciate the physiological constraints that may limit an animal's distribution, habitat use, and reproductive activity. This is especially important given current climate change predictions. Climate change at the rapid rates that are predicted will limit the value of evolutionary adaptation of many species. Instead, physiological tolerances and plasticity will be critical indicators of a species ability to cope with these anticipated changes. Some of the physiological measures we frequently assess include metabolic rate, evaporative water loss, hydration state, body condition, growth, and digestion.

Physiological regulation

In addition to recognizing the physiological potential of an organism, we are also interested in how these processes are regulated. Therefore, we sometimes examine hormone profiles of our study organisms and conduct manipulative experiments that alter circulating hormone levels.

Physiological trade-offs

Our lab is not only interested in how animals physiologically cope with environmental challenge, but also how adjustments to meet one challenge affect other physiological processes. Of particular interest is the relationship among water balance, thermoregulation, and energy balance. For example, evaporative water loss aids thermoregulation, but comes at a potential cost to water balance. We are interested in how the current state of the animal influences the behavioral and physiological responses to environmental challenge.

Environmental use

We are very interested in how organisms use their environment to meet physiological needs and how they avoid physiologically challenging situations. Similar to our interests in the physiology of homeostatic trade-offs, we are interested in how homeostatic trade-offs affect activity patterns. For example, underground burrows provide critical refuge from the hot arid surface conditions of the desert, but most animals must surface in order to forage and maintain energy balance. The timing of activity bouts and the physiological implications of such bouts are areas of interest to our lab.

Proximate mechanisms for evolution

Some studies in the lab examine proximal mechanisms that may explain traits that are often considered "adaptive". While we cannot determine what selective pressures led to the evoultion of particular traits, we conduct studies that assess proximate mechansims and their potential as driving forces for evolution. For example, we have examined proximate mechanisms for sexual size dimorphism in rattlesnakes and the potential for the regulation of the developmental environment to serve as a driving force for the evoution of endothermy.

Recent and current projects

Some of the recent and current projects being conducted in the lab are briefly outlined here. More details for each of these projects can be found in the research focus statement of each lab member (by clicking on their photo within the Team DeNardo directory). The lead particant of each project are provided in parentheses.

What are the implications and motivations of python egg-brooding behavior? (Zach Stahlschmidt) All female pythons exhibit a brooding behavior that involves tightly coiling around a clutch of eggs throughout the incubation period (more than 80 days in some species). Because python egg brooding is dynamic yet limited in complexity, it is an

exceptional model for examining how specific parental behaviors are balanced to meet the developing offspring's myriad of needs, as well as how abiotic and biotic factors influence investment into various parental behaviors. Thus, python egg brooding can provide insight into the adaptive significance of more complicated examples of female-only nest attendance, the most taxonomically diverse mode of parental care. This study specifically addresses: (1) How do egg-brooding behaviors impact the developmental environment and offspring phenotype? (2) Do abiotic and biotic proximate cues influence python egg-brooding behavior? (3) Do python eggshell permeability dynamics mediate an embryonic respiration-hydration tradeoff?

Does facultative endothermy during brooding improve the developmental environment of python offspring? (Jake Brashears) This study has two primary objectives. First, it evaluates the endothermic capability of various python species. Second, the study utilizes controllable snake models (i.e., "psedoserpents") to determine

whether stepwise increases in endothermic capability provide progressively enhanced control of the developmental environment and whether such enviornmental control enhances offspring quality. This study provides an empiracal test of Farmer's reproductive model for the evolution of endothermy.

What are the proximate mechanisms regulating python brooding behavior? (Jake Brashears) This project examines the environmental and hormonal signals that produce and maintain python brooding behavior. Specifically, this project aims to determine the cues (chemical, tactile, etc.) that the female python may receive from the clutch and which steroid hormones are involved in priming her to correctly receive and interpret these cues.

How do ontogenetic differences in physiology affect environmental sensitivity and behavior in Gila monsters? (Karla Moeller) Due to differences in size, physiological tolerances, and physiological buffering capabilities, juveniles and adults may vary in their

sensitivity to environmental challenges and thus may require different survival strategies. As recruitment of juveniles into the adult population is critical for species survival, the physiological and behavioral influences that affect or mitigate juvenile sensitivity must be considered for a complete assessment of a species’ vulnerability to stochastic environmental conditions. This project predominantly uses laboratory experiments to compare physiological limitations and buffering capabilities of neonate and adult Gila monsters and to link physiological differences to varied behavioral responses that contribute to survival under xeric conditions.

Does state-dependent foraging theory apply to low energy organisms like the Gila monsters? (Christian Wright) To survive in challenging environments, organisms must integrate behavioral and physiological responses to address critical needs, and foraging effort represents one such example of the integration of these two responses. Although predation risk and food availability strongly influence foraging decisions, so too can physiological condition (i.e., State-Dependent Foraging, SDF). Although SDF is well studied, particularly in species with high energy intake and expenditure, studies of the applicability of SDF theory to low-energy, infrequently feeding systems are limited.

This project uses a combination of lab-based and manipulative field studies to examining the interaction between physiological state and foraging behavior in the Gila monster, Heloderma suspectum, an actively foraging lizard that has relatively low energy demands and feeds infrequently.

Are rattlesnake groups random associations of individuals attracted to a common resource (aggregations) or true societies of individuals that can recognize each other, have social bonds, and may exhibit cooperation? (Melissa Amarello) Social network analysis is being used to describe the structure of an Arizona black rattlesnake (Crotalus cerberus) population and determine whether there is active companionship and avoidance behavior. Rattlesnakes likely evolved from a non-social ancestor, providing an ideal opportunity to examine ecological correlates of sociality that evolved independently of previously studied.

Does ecotourism negatively impact marine iguanas? (Dale DeNardo & Susannah French) Large scale disturbances such as those associated with urbanizatiion, fragmentation, and global climate change are receiving considerable attention in regards to their impact on wild populations of animals. However, little attention is being paid to smaller scale disturbances. In collaboration with Susannah French at Utah St. University, we are evalauting the effect of ecotourism on the immune function of the Galapagos marine

iguana (Amblyrhynchus cristatus). Thus far, we, along with others, have demonstrated that exposure to ecotourism alters immune function. We are continuing our work to detail these effects and to determine whether the effects are driven, at least partially, by altered foraging effort.

What dictates skin water resistance in snakes? (Dale DeNardo & Oli Lourdais) Snake skin is relatively water resistant, but there is considerable variation among species. To date, most comparative studies have looked at only a few snake species at a time.

Our goals is to conduct a thorough assessment of transcutaneous evaporative water loss across snakes. This will provide insight into the the degree to which phylogeny, size, and habitat influence water flux.