Standing Variation and the Capacity for Change: Are Endocrine Phenotypes More Variable Than Other Traits?

Circulating steroid hormone levels exhibit high variation both within and between individuals, leading some to hypothesize that these phenotypes are more variable than other morphological, physiological, and behavioral traits. This should have profound implications for the evolution of steroid signaling systems, but few studies have examined how endocrine variation compares to that of other traits or differs among populations. Here we provide such an analysis by first exploring how variation in three measures of corticosterone (CORT)-baseline, stress-induced, and post-dexamethasone injection-compares to variation in key traits characterizing morphology (wing length, mass), physiology (reactive oxygen metabolite concentration [d-ROMs] and antioxidant capacity), and behavior (provisioning rate) in two populations of tree swallow (Tachycineta bicolor).

Captivity influences immune responses, stress endocrinology, and organ size in house sparrows (Passer domesticus).

Studies using wild animals in laboratory-based research require bringing wild-captured organisms into a novel setting, which can have long-lasting impacts on physiology and behavior. In several species, captivity stimulates stress hormone production and can alter immune function. Despite this, there is little consensus on how captivity influences stress hormone regulation, or if captivity-induced changes in stress hormone production and regulation mediate changes in immune function.

Exercise training reveals trade-offs between endurance performance and immune function, but does not influence growth, in juvenile lizards.

Acquired energetic resources allocated to a particular trait cannot then be re-allocated to a different trait. This often results in a trade-off between survival and reproduction for the adults of many species, but such a trade-off may be manifested differently in juveniles not yet capable of reproduction. Whereas adults may allocate resources to current and/or future reproduction, juveniles can only allocate to future reproduction.

Effects of Water Loss on New Mexico Spadefoot Toad (Spea multiplicata) Development, Spleen Cellularity, and Corticosterone Levels.

Amphibian metamorphosis is complex and larval morphology and physiology are completely restructured during this time. Amphibians that live in unpredictable environments are often exposed to stressors that can directly and indirectly alter physiological systems during development, with subsequent consequences (carryover effects) later in life. In this study, we investigated the effects of water level reduction on development rate, spleen size and cellularity, and examined the role of corticosterone levels in premetamorphic, metamorphic, and postmetamorphic New Mexico spadefoot toads (Spea multiplicata).

Baseline hormone levels are linked to reproductive success but not parental care behaviors.

Consistent behavioral differences among individuals, or personalities, have been hypothesized to arise as a result of consistent individual differences in hormone levels. Individual variation in baseline hormone levels or hormonal similarity within a breeding pair may be related to reproductive success, as suggested by the corticosterone-fitness hypothesis and the hormonal similarity hypothesis, respectively. In a population of Eastern bluebirds (Sialia sialis) with repeatable behavioral expression and coordination of behavior within pairs, we tested if baseline androgen and corticosterone levels are related to behavioral expression, if coordination in behavior within pairs is facilitated by hormonal coordination, and if baseline hormone levels are related to fledging success at the individual or pair level.

Heightened aggression and winning contests increase corticosterone but decrease testosterone in male Australian water dragons.

Water dragons (Intellegama [Physignathus] lesueurii) are large (to >1m) agamid lizards from eastern Australia. Males are fiercely combative; holding a territory requires incessant displays and aggression against other males. If a dominant male is absent, injured or fatigued, another male soon takes over his territory.

Experimentally decoupling reproductive investment from energy storage to test the functional basis of a life-history trade-off.

The ubiquitous life-history trade-off between reproduction and survival has long been hypothesized to reflect underlying energy-allocation trade-offs between reproductive investment and processes related to self-maintenance. Although recent work has questioned whether energy-allocation models provide sufficient explanations for the survival cost of reproduction, direct tests of this hypothesis are rare, especially in wild populations. This hypothesis was tested in a wild population of brown anole lizards (Anolis sagrei) using a two-step experiment.

Variation in steroid hormone levels among Caribbean Anolis lizards: endocrine system convergence?

Variation in aggression among species can be due to a number of proximate and ultimate factors, leading to patterns of divergent and convergent evolution of behavior among even closely related species. Caribbean Anolis lizards are well known for their convergence in microhabitat use and morphology, but they also display marked convergence in social behavior and patterns of aggression. We studied 18 Anolis species across six ecomorphs on four different Caribbean islands to test four main hypotheses.

The maternal environment affects offspring viability via an indirect effect of yolk investment on offspring size.

Environmental conditions that reproductive females experience can influence patterns of offspring provisioning and fitness. In particular, prey availability can influence maternal reproduction and, in turn, affect the viability of their offspring. Although such maternal effects are widespread, the mechanisms by which these effects operate are poorly understood.

Socially regulated reproductive development: analysis of GnRH-1 and kisspeptin neuronal systems in cooperatively breeding naked mole-rats (Heterocephalus glaber).

In naked mole-rat (NMR) colonies, breeding is monopolized by the queen and her consorts. Subordinates experience gonadal development if separated from the queen. To elucidate the neuroendocrine factors underlying reproductive suppression/development in NMRs, we quantified plasma gonadal steroids and GnRH-1- and kisspeptin-immunoreactive (ir) neurons in subordinate adults and in those allowed to develop into breeders, with or without subsequent gonadectomy.

Structural coloration signals condition, parental investment, and circulating hormone levels in Eastern bluebirds (Sialia sialis).

Many of the brilliant plumage coloration displays of birds function as signals to conspecifics. One species in which the function of plumage ornaments has been assessed is the Eastern bluebird (Sialia sialis). Studies of a population breeding in Alabama (USA) have established that plumage ornaments signal quality, parental investment, and competitive ability in both sexes.

Acute and persistent effects of pre- and posthatching thermal environments on growth and metabolism in the red-eared slider turtle, Trachemys scripta elegans.

Many ectotherms possess the capacity to survive a wide range of thermal conditions. Long-term exposure to temperature can induce acclimational and/or organizational effects, and the developmental stage at which temperature exposure occurs may affect the type, degree, and persistence of these effects. We incubated red-eared slider turtle embryos at three different constant temperatures (T(inc); 26.

Elevated plasma corticosterone increases metabolic rate in a terrestrial salamander.

Plasma glucocorticoid hormones (GCs) increase intermediary metabolism, which may be reflected in whole-animal metabolic rate. Studies in fish, birds, and reptiles have shown that GCs may alter whole-animal energy expenditure, but results are conflicting and often involve GC levels that are not physiologically relevant. A previous study in red-legged salamanders found that male courtship pheromone increased plasma corticosterone (CORT; the primary GC in amphibians) concentrations in males, which could elevate metabolic processes to sustain courtship behaviors.

Maintenance of the spinal nucleus of the bulbocavernosus neuromuscular system is not influenced by physiological levels of glucocorticoids.

The spinal nucleus of the bulbocavernosus (SNB) neuromuscular system mediates sexual reflexes, and is highly sexually dimorphic in rats. While maintenance of this system in adulthood is mainly dependent on androgens, there is also evidence to suggest that glucocorticoids may have a catabolic effect. We conducted a series of studies to fully examine the influence of basal glucocorticoids on the size of the SNB motoneurons and the associated bulbocavernosus (BC) and levator ani (LA) muscles.

Transdermal delivery of corticosterone in terrestrial amphibians.

Stressors elicit allostatic responses that allow animals to cope with changing and challenging environments and also cause release of glucocorticoid hormones (GCs). Compared to other vertebrate classes, relatively little is known about amphibian behavioral and physiological responses to GCs. To understand the effects of elevated plasma GCs in amphibians, exogenous application of GCs is necessary, but traditional methods to elevate GCs require handling and/or anesthesia which themselves are stressors.

The effects of diet on plasma and yolk steroids in lizards (Anolis carolinensis).

Steroids present in egg yolk have been shown to vary as a result of numerous social and environmental influences and to produce both positive and negative phenotypic outcomes in offspring. In the present study, we examined how quality of the diet affects plasma and yolk steroids in the green anole (Anolis carolinensis), a lizard species with genotypic sex determination. We documented the effects of body condition on plasma testosterone (T) and corticosterone (CORT)-steroids with frequently opposing effects-in breeding females and on the T and CORT content of their eggs.

Effects of sex, age, and season on plasma steroids in free-ranging Texas horned lizards (Phrynosoma cornutum).

The Texas horned lizard (Phrynosoma cornutum) is protected in several states due to its apparently declining numbers; information on its physiology is therefore of interest from both comparative endocrine and applied perspectives. We collected blood samples from free-ranging P. cornutum in Oklahoma from April to September 2005, spanning their complete active period.

Maternal nutrition affects reproductive output and sex allocation in a lizard with environmental sex determination.

Life-history traits such as offspring size, number and sex ratio are affected by maternal feeding rates in many kinds of animals, but the consequences of variation in maternal diet quality (rather than quantity) are poorly understood. We manipulated dietary quality of reproducing female lizards (Amphibolurus muricatus; Agamidae), a species with temperature-dependent sex determination, to examine strategies of reproductive allocation. Females maintained on a poor-quality diet produced fewer clutches but massively (twofold) larger eggs with lower concentrations of yolk testosterone than did conspecific females given a high-quality diet.

Faster lizards sire more offspring: sexual selection on whole-animal performance.

Sexual selection operates by acting on variation in mating success. However, since selection acts on whole-organism manifestations (i.e.

Effects of testosterone on the development of neuromuscular systems and their target tissues involved in courtship and copulation in green anoles (Anolis carolinensis).

Male green anole lizards court females using a red throat fan (dewlap) and copulate by intromitting one of two penises (hemipenes). These structures begin sexually monomorphic, but by adulthood males have larger dewlaps, only males have hemipenes, and many of the neuromuscular components of both systems show male-biased dimorphisms. We hypothesized that testosterone (T), which increases in juvenile males but not females about a month after hatching, facilitates masculinization.

The green anole (Anolis carolinensis): a reptilian model for laboratory studies of reproductive morphology and behavior.

The green anol (Anolis carolinensis) is an excellent reptilian model for studying reproductive behavior and the neural and muscular morphology that supports it. This lizard has been the subject of behavioral and ecological study for more than 100 yr, and a rich literature exists on its natural history. Both courtship and copulatory behaviors reveal sex and seasonal differences, which allow for the study of mechanisms regulation naturally occurring variation in performance at multiple levels within a single animal model.

Sex steroids in green anoles (Anolis carolinensis): uncoupled maternal plasma and yolking follicle concentrations, potential embryonic steroidogenesis, and evolutionary implications.

The sex steroids testosterone (T) and estradiol-17beta (E2) play important roles in vertebrate reproduction and development. However, little is known about the relationship between plasma steroid levels (which can influence reproductive function) and yolk steroid levels (which can influence embryonic development) in oviparous species. Therefore, we examined the extent to which T and E2 are coupled in plasma and yolking follicles in adult females and explored the dynamics of yolk and embryo steroid content during egg incubation in green anoles (Anolis carolinensis).

Form emergence and fixation of head bobbing displays in the green anole lizard (Anolis carolinensis): a reptilian model of signal ontogeny.

Signal ontogeny was examined in green anoles (Anolis carolinensis). From 1,246 head bob displays given by 114 juveniles, it was found that juveniles possessed all 3 display types (A, B, and C) described for adults and that C displays were present at hatching, but A and B displays appeared to emerge gradually from a common precursor. Durations of the head bobs and pauses that make up juvenile displays tended to be more variable (i.

Yolk testosterone varies with sex in eggs of the lizard, Anolis carolinensis.

In the green anole (Anolis carolinensis), a lizard with genotypic sex determination, yolk testosterone (T) concentration is greater in male-producing than female-producing eggs at oviposition, but the source and potential effects were not clear from previous studies. If yolk T levels are also sex-specific before eggs are laid, a period during which embryonic steroidogenesis is unlikely, it would strongly suggest that the difference in yolk T is maternally derived. We collected yolk samples from eggs shelling within the oviducts of anesthetized females, and then allowed these females to lay the eggs naturally.