Species of lizard
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Urosaurus ornatus, commonly known as the ornate tree lizard, is a species of lizard in the family Phrynosomatidae. The species is native to the southwestern United States and northwestern Mexico. The species, which was formerly called simply the "tree lizard", has been used to study physiological changes during the fight-or-flight response as related to stress and aggressive competition. Its life history and costs of reproduction have been documented in field populations in New Mexico and Arizona. This species has been fairly well studied because of its interesting variation in throat color in males (within a population) that can correlate with different reproductive strategies,
Appearance.
The ornate tree lizard may grow to a snout-to-vent length (SVL) of up to . As adults, all males have paired turquoise patches of skin on the abdomen; females lack this abdominal coloration. Male ornate tree lizards are found in a variety of colors. While not all populations contain more than one or two colors, 9 color types have been documented within "U. ornatus". A population documented in Verde River, Arizona, has two types of coloration patterns among male tree lizards that account for 45% of all males. The first is characterized by a blue spot in the center of a larger orange patch on the throat fan ("dewlap"). The second has a solid orange throat fan ("dewlap"). The orange-blue males are more aggressive and defend territories that can include up to four females. The orange males have longer, leaner body types and are much less aggressive. Orange males can be nomadic during dry years, and during rainy years tend to occupy small territories.
Hierarchies.
The differences in throat color can impact many different behaviors and create hierarchies within these lizard populations. It is thought that the blue patch on their throats acts as a badge that signals fighting ability and social status across age or sex classes. In most encounters between males with an orange background and blue spot (OB) and males with just the orange background (O), the OB males are almost always dominant. In the case of an interaction between two OB males, the male with the larger blue spot tends to be more dominant. There is a direct correlation between spot presence/size to social status and dominance within the lizard population. Color variation in the throat can also affect reproduction tactics between different males. Males with a throat color that has a solid central greenish-blue patch within an orange background (OB) are territorial and more aggressive than males that lack the greenish-blue patch (O), therefore suggesting that OB males will fight more for their mate. These two types of males, therefore, are thought to express alternative reproductive strategies due to their differences in territoriality. The blue patch can be described as a characteristic that heavily contributes to sexual selection since females tend to mate with males that have larger blue spots.
Multiple male genders.
Some, such as Stanford professor and biologist Joan Roughgarden, have suggested multiple male genders in this species. Among differently colored male tree lizards, there are different hormonal profiles. On the day a male tree lizard hatches, researchers think that high blood levels of progesterone and then later, as a juvenile, higher testosterone levels will cause him to develop into an orange-blue type; low progesterone and later lower testosterone levels, as a juvenile, may lead the male to develop into an orange type. During dry weather conditions, orange-type males' corticosterone levels increase, which causes testosterone to decrease, leading them to be more likely to leave their territory and become nomadic. Orange-blue males do not have this hormonal response to the weather, and remain in their territories regardless of climatic conditions.
Speed and stamina.
The differences in throat color can also be related to speed and stamina in male lizards. OB males or dominant males are significantly faster than O males and have higher lengths of stamina than O males. The link between social hierarchy and locomotor performance can be traced to the suite of displays that identify a dominant individual being the presence/size of the blue spot on their throats. These activities may be affected by different intensities and duration and it is possible for an O male to have better speed and stamina than an O male, it is just uncommon. The OB male having territorial and defensive traits makes them spend more time fighting or displaying and asserting their dominance over the O lizards and exerting that amount of physical strength allows them to develop the higher rates of speed and longer durations of stamina. When the O male competes against the OB male and loses, the O male does not get the opportunity to utilize their speed and stamina and therefore causes them to have decreased rates of speed and shorter periods of stamina.
Significance of throat color.
The throat color of this lizard represents important information about their level of development. Juvenile males will firstly have an orange throat color. Monomorphic males will maintain this orange throat coloration for life, however, polymorphic males will either retain their orange throat color or they will develop a yellow throat color as they mature. The ontogenic color change of the throats for polymorphic males have important social functions. Females and young males with an orange throat coloration inhibits aggression from male adults. Dominant males that develop different throat colors late in their development will typically have blue and blue-green throats. This suggests that these colors are visual indicators of social dominance. Researchers have hypothesized that the different throat colors of these lizards have important social functions. This is because the throat coloration of male ornate tree lizards affects their social relationships. Additionally, since throat coloration is a genetically determined trait, it is possible that the different throat colors seen in polymorphic males during developmental stages are responses to changing social roles, enabled by phenotypic plasticity, as the lizards mature.
Additionally, throat coloration is also associated with size. 82% of hatchlings develop an orange throat color within 15 days of hatching and the lizards that had a delayed onset of throat coloration are typically smaller for their age. Therefore, the size of the ornate tree lizard is a better indicator of their throat color than their age.
Population structure.
"Urosaurus ornatus" has a complicated social hierarchy causing O males and OB males to fight for resources. The social systems that allow for the stability of the lizard population cause a mixed evolutionary stable strategy for the continuation of their genes. In a mixed evolutionarily stable strategy, both morphs pursue different life history strategies that allow them to achieve equal fitness. O and OB males behave differently in different situations, but most times having the OB male be dominant over the O male. While OB males are more dominant socially, O males are usually larger and grow faster than OB males, both in captivity and in the wild. O males will reach a larger size before breeding time, but once both O and OB males reach adulthood they are the same size.
Effects of habitat.
Lizards are ectotherms and because of their temperature dependent lifestyle they may not follow predictions of density‐dependent habitat selection models because temperature strongly influences their habitat selection and population structure. Since competition for resources can cause a decrease in fitness, increased population size should cause a decrease in body size and growth rates across the population. In these lizards there is population density-dependent growth for females of this species but not for males. The temperature quality of the environment that these lizards live in also determines body size and composition. Lizards that are raised and produced in crowded spaces and environments with poor resources and lower temperatures grow smaller than lizards in higher quality environments with better suited temperatures and good resources.
It has been suggested that the abundance of rainfall that the ornate tree lizard experiences induce a multitude of changes in this lizard's behaviors. During a four-year study in the grapevine hills of Big Bend National Park in Texas, researchers observed the behavior of this lizard from 1974 to 1978. During 1974 and 1976, precipitation was abundant, however, during 1975 and 1977, there was little precipitation. During the dry years, there is a decrease in available prey and the ornate tree lizard's individual foraging success, growth rates, body masses, and prehibernation lipid levels were significantly lower. Population density was also reduced. These results have led researchers to propose that dry years are stressors which induces changes in diet, behaviors, and health of individuals. Since drought induces food scarcity, there is increased intra-specific competition for resources. Studies have demonstrated that "Sceloporus merriami" and the "U. ornatus" lizards also compete interspecifically for resources. Limited resources lowers population density of both lizard species. There is seasonal variation in the intensity of competition between these species which is highly correlated to food availability and the amount of rainfall. Where rainfall varies, the intensity of interspecific competition also varies. Therefore, the state of the environment directly affects competition and food availability for the ornate tree lizard.
Geographic range.
"Urosaurus ornatus" is one of the most widespread and abundant of North American lizards. It inhabits a wide range of states throughout the United States, including California, Nevada, Utah, Colorado, Wyoming, Arizona, New Mexico, and Texas. It is arboreal, semi arboreal, or saxicolous throughout its wide living range and habitats. It can also be found in areas in Mexico including states such as Sonora, Sinaloa, Chihuahua, and Coahuila.
There are large temperature ranges throughout the southwestern United States and these lizards adapt to the different climates that they live in. Their mean body temperatures also change depending on where they are located. If the male lizard was in a montane habitat or a desert habitat their body temperature would be 35 degrees Celsius, but if the male lizard was in a desert riparian habitat its body temperature would be around 38 degrees Celsius. Their changes in body temperatures reflect the climates they live in. These temperature differences also occur between male and female lizards. On average the body temperature of a female is lower than the males body temperature. There is a possibility that this difference in body temperature could be related to the presence or absence of sexual differences between males and females and could also affect the health of the clutch and the size of the clutch.
Size.
The size of each lizard is closely related to their age and the seasonal state of their environment. During winter months (January through March), 75% of adults and 25% of juveniles are active and 13% of juveniles are active in the spring season. Additionally, at the beginning of April, 87% of the ornate tree lizards are in a reproductive state. This is because the beginning of April marks the beginning of their breeding season. By the month of May, however, all the lizards have reproductive potential.
Growth.
The lizards will reach their minimum adult size within a year. Male ornate tree lizards will grow approximately 0.16 to 0.29 mm per month. During the spring and summer seasons, however, the male growth rate increases to an average of 2.1 mm per month. 
Female ornate tree lizards are typically smaller than males when they reach adulthood. When the females grow to 45 mm, they are considered 'mature'. For males, there is a small range in their size. Researchers have proposed that there is a stabilization in growth rate as they approach their maximum size.
Locomotion.
These lizards tend to be active during every month except December. Both male and female ornate tree lizards emerge only on the warmest days in November and January, however, they engage in extended period of activity in February. When the lizards are active, they tend to occupy open locations in small or large boulders.
Anti-predator behavior.
Male "Usosaurus ornatus" lizards express chemical and behavioral responses to predators. Chemical responses of the males when encountering a predator can include changes in their levels of testosterone and corticosterone while behavioral responses shown tend to be hiding and flight initiation distance (FID, the distance between the animal and the predator before escape is attempted). Levels of expression in both chemical and behavioral responses can differ depending on color variation of the throat. Chemical levels in both O and OB males can change significantly when predators are spotted. When encountered with a predator, the lizard's corticosterone levels in both O and OB males