Gila Monsters
Field Sites

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About the Gila Monster:

treadmill movie

The Gila monster and desert survival
Temperate deserts can be seasonally inhospitable environments, which impose significant challenges to resident species because of extreme temperatures and low precipitation and productivity. The climate at our field site in the Arizona Upland subdivision of the Sonoran Desert is representative of the highly seasonal climate characteristic of the region. For example,

climate diagrams illustrate three distinct seasons during the Gila monster active season (March – mid-November) at our field site:(1) a cool wet season following emergence from overwinter refugia. Rainfall occurs unpredictably and maximum shaded air temperature (Ta) are near the species’ preferred body temperature (Tp, 29°C), while nighttime low Ta is significantly below Tp. In Gila monsters, diurnal surface activity occurs during the initial month of this season, but activity becomes crepuscular as the season progresses and the temperatures rise. Basking is typically observed only during this season in our study population.  

(2) a hot dry season during which daytime high Ta averaging 38°C (9°C above Tp) is exacerbated by little or no rainfall (the summer drought at our field site is typically >80 d). Nighttime lows approach Tp and overall productivity of the desert is also low during this time. Many animals, including the Gila monster, reduce surface activity, become nocturnal, or both to avoid the most challenging conditions.   

(3) a hot wet season defined by relief of seasonal drought by 6-8 weeks of summer monsoon rains which produce 60% of the annual rainfall at our field site (5 yr mean total annual rainfall = 12 cm). Maximum and minimum Tas are similar to the hot dry season, yet overall desert productivity and animal surface activity rapidly and dramatically increase on many trophic levels demonstrating the importance of monsoon rainfall.

Gila monsters are large-bodied (adult body mass 350-600 g and snout-vent length 300-360 mm), long-lived (~20 yrs), venomous lizard primarily inhabiting the Sonoran Desert of Arizona and Mexico. Gila monsters are exceptionally well-suited to life in the Sonoran Desert despite its limited and highly seasonal resource availability. These lizards capitalize on pulsatile resources by binging on vertebrate nest contents and water when available. For example, a 600g free-ranging female Gila monster
consumed four neonate desert cottontails (Sylvilagus audobonii) which weighed a total of 90 g or 15% of her body mass. Whenever possible, Gila monsters will maintain considerable fat stores abdominally and caudally. By measuring tail volume
via water displacement, we can generate a simple body condition index to assess energetic status of free-ranging lizards. Gila monsters binge similarly on water and may imbibe 15-20% of their body mass. Drinking bouts can last several minutes and are characterized by stereotyped drinking postures. Gila monsters will then store water as dilute urine in the urinary bladder. Using portable ultrasonography, we can determine the water content of the urinary bladder and use this information to better understand the animal’s physical condition, the climatic correlates that have contributed to the animals’ conditions, and whether bladder and hydration states affect other physiological processes or behaviors (e.g., evaporative water loss, thermal preference, surface activity, etc.).

In Gila monsters, the combination of a low metabolic rate (Beck, 1990) and longevity means that energy and hydric budgets can be balanced over months or years. A more pressing day-to-day concern is likely thermoregulation. The relatively low Tp (29°C) of this species has direct implications on these lizards’ thermal ecology and behavior. Gila monsters meet thermal requirements in several ways:

(1) vary the duration of surface activity seasonally such that Gila monsters are inactive from mid-November to early-March, active about 17% of the time in the cool wet season, 10% during the hot dry season, and peaking at 30% in response to the monsoon rains during the hot wet season. Each year at the onset of monsoon rains, >80% of the Gila monsters we study are surface active on the first 1-2 nights of rainfall. This rapid and widespread response occurs before the monsoon-induced dramatic increase in prey availability and suggests the importance of water to this species.  

(2) vary the timing of surface activity seasonally such that Gila monsters are diurnal or crepuscular during the cool wet season and then become nocturnal during the hot dry and hot wet seasons. These activity patterns allow Gila monsters to reduce exposure to temperature extremes.

(3) shift macroenvironment occupied seasonally such that some Gila monsters migrate from their active season home ranges in the bajada (flats surrounding hills and buttes) to rocky hillsides and buttes near the end of the active season. Other Gila monsters simply overwinter in the bajada in their active season home range. Gial monsters emerge from their overwintering sites in early March, and, by mid-April, the hills lizards migrate back to their active season home range.

(4) shift microenvironment occupied seasonally such that Gila monsters use rock burrows
most often from the end of the active season through March, dirt burrows
during the last month of the cool wet season and first month of the hot dry season, and packrat middens
during the rest of the active season. Interestingly, thermal profiles of refugia suggest that this seasonal shift is motivated by shifts in thermal quality of the refugia across the active season.     

Though Gila monsters become predominantly nocturnal during the hottest months of the year (May-August), they may be thermally challenged by air temperatures that typically exceed 40°C at sunset and can remain above 35°C for several hours into the night. Perhaps to cope with these high temperatures, Gila monsters have a relatively high evaporative water loss (EWL) rate for their size compared to other lizards. In fact, studies in our lab have demonstrated that Gila monsters use cloacal evaporative cooling to suppress body temperature when it becomes critically elevated (> 37.5°C). In addition, Gila monsters also reduce EWL and maintain favorable body temperatures by foraging infrequently and selecting thermally and hydrically favorable refugia when inactive.

The Gila monster urinary bladder:
We recently demonstrated that like many chelonians and amphibians, Gila monsters use their urinary bladder as a water reservoir – defined here as an anatomical location where water can be sequestered from and later returned to general circulation. Gila monsters will store water in the urinary bladder and absorb it over several weeks to prevent dehydration. A full urinary bladder can contribute an additional 15-20% total body water to the lizard. This is an atypical attribute in lizards and suggests the importance of water to this species. However, a full urinary bladder can significantly increase body mass as well. Thus, in addition to investigating osmoregulatory implications of this physiological reservoir, we also examine ways in which bladder water and an animal’s hydration state directly influence other fundamental physiological processes including thermoregulation, water loss, and endurance. We study these relationships in the laboratory and in free-ranging Gila monsters in the field using a variety of techniques to ensure that lab experiments are ecologically relevant and to clarify how physiological condition influences ecophysiology and behavior under natural conditions.          

Contribution to osmoregulation – During the beginning and end of the active season, free-ranging Gila monsters store dilute urine in the urinary bladder. Recent lab studies have demonstrated that water can be absorbed into circulation from the urinary bladder and when deprived of food and water under controlled lab conditions, Gila monsters beginning with a full urinary bladder dehydrate nearly 2.5 times slower than those with an empty urinary bladder (78.5 d vs. 33.3 d). This effect is ecologically significant as well considering that these lizards typically experience seasonal drought lasting about 80 d in desert habitats such as our field site. This is currently a primary focus of our field research – please read about Jon Davis’ research for details.       

Effect on evaporative water loss (EWL) - EWL rates of Gila monsters are affected by hydration state, but it is unknown whether reservoir condition (full or empty) also influences EWL. In conditions where environmental temperature exceeds body temperature, cooling can only occur via EWL. Thus, the bladder reservoir may indirectly support thermoregulation, energy balance, and reproduction by enabling Gila monsters to cool via EWL, thereby reducing heat load and extending activity bouts without jeopardizing osmoregulation. Ongoing studies in our lab seek to clarify the influence of bladder condition (full or empty) on EWL.

Effect on thermal preference – Hydration state has been correlated with Tp in several reptiles; with animals typically selecting cooler temperatures when dehydrated compared to when hydrated. EWL decreases with decreasing body temperature, so reducing Tp is often considered a response to reduce EWL. Thus, we are examining Tp of hydrated and dehydrated Gila monsters in a thermal gradient in the lab. Additionally, we are studying the relationship between hydration state (plasma osmolality and bladder condition) and body temperature at multiple field sites to determine whether hydration state, bladder condition, or water availability influence the thermal biology of free-ranging Gila monsters. For these field studies, we use a combination of radiotelemetry and biotelemetry to monitor nearly forty Gila monsters.              

Effect on endurance – Interestingly, free-ranging Gila monsters do not regularly store water in the urinary bladder early in the summer monsoon season despite its relatively high availability and the risk of missing the opportunity to refill the ‘canteen’. Perhaps this occurs in part because storing a large volume of water in the coelomic cavity may have negative consequences on Gila monsters. For example, similar coelomic cavity space is needed to accommodate the increased mass and volume associated with reproduction or consumption of a large meal, and both are known to have deleterious effects on locomotor performance and predator avoidance in many species. In fact, Gila monsters with full urinary bladders have significantly lower endurance when exercised on a treadmill compared to when their urinary bladders are empty. Because Gila monsters are widely-foraging nest predators, the potentially significant energetic consequences of reduced endurance warrants further study. Similarly, effects of dilute urine storage on activity energetics (e.g., cost of transport, maximum aerobic speed) remain to be studied.

The Gila monster and water availability:
Storage of water and behavioral responses to rainfall and drought suggest that water availability significantly influences Gila monster biology. However, water may influence these lizards directly (through individual-level effects on physiology and thus behavior) or indirectly (through community-level effects on resource availability). To discriminate between these hypotheses, we have undertaken a multiple field site comparative study in which the physiology and behaviors of Gila monster populations inhabiting undeveloped natural areas are being compared to those of urban populations living in developed, water-supplemented, semi-natural habitat. We experimentally eliminate seasonal dehydration in half of our natural population by administering 50ml of water via an intragastric tube monthly during the active season. Urban Gila monsters we study inhabit golf courses and vacation resorts in which housing density is low, variable sized natural habitat patches have been conserved, and most importantly, irrigation provides constant water availability to the area throughout the year, resulting in high productivity on many trophic levels. Thus, we are able to compare three groups of Gila monsters: (1) Control – Gila monsters experience natural seasonal drought without supplemental water, (2) Water - Gila monsters remain hydrated throughout the year, but experience the  natural seasonality in water availability, and (3) Urban – Gila monsters remain hydrated throughout the year and experience similar climate, but augmented water, and thus prey, availability.  

Urban life - Urban life, for Gila monsters, presents an interesting paradox because augmented water and food resources may benefit growth, reproduction, immune function, but are likely accompanied by drawbacks for Gila monsters. For example, urbanization often requires conversion and fragmentation of continuous natural habitat which may or may not disrupt movement patterns of some animals. Population density of Gila monsters may increase, resulting in greater competition for resources (e.g., mates, refugia, energy). Predator abundance and diversity (natural predators plus dogs and cats) may also increase as will vehicle and pedestrian traffic, all of which can lead to disturbance or death of these lizards. Furthermore, the use of treated reclamation water as irrigation water may have unknown individual- and community-level consequences. Urban life is clearly complex and deserves attention from researchers because urbanization is increasing, especially in the southwestern US, and the effects of urbanization on animal physiology (and the subsequent influence physiology has on behavior and ecology), have received relatively little notice.

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