Activity patterns and thermoregulation

Activity patterns influenced by seasonal, daily, or other rhythms probably have been the targets of more herpetolog-ical research than any other aspect of behavior except diet. This is partly a consequence of the development of miniaturized radiotelemetric equipment and partly the result of the simple fact that understanding any animal requires a sense of the actions it performs and when it performs them. Numerous investigators are interested in energy budgets and related phenomena, and activity patterns tell us about most of the energy costs sustained by animals. Feeding success tells us about energy gains. The construction of activity budgets is an important step in the calculation of energy budgets. Many her-petologists consider the study of activity patterns a necessary starting point, once the basic questions of taxonomic allocation are at least tentatively answered. It could be hypothesized that we could predict activity patterns and energy expenditures if we knew the resource needs of a species and the distribution of these resources in the habitat. Although this reasoning is sound, rarely do we know about all the resource needs of a species, and we know even less about their distribution, especially in the perception of the animals. Knowledge of resource needs is an important goal, but instead of using the information to predict activity patterns, we frequently use data on the patterns to infer resource needs and distributions.

The longest migration by any reptile is the oviposition movement of some female green seaturtles (Chelonia mydas) from their feeding grounds in the kelp forests off the coast of Brazil to the beaches of Ascension Island in the South Atlantic, about midway between Brazil and the west coast of Africa (approximately 1,600 mi [1,900 km] from Brazil). Other populations of green seaturtles and all other seaturtles make substantial annual movements to nesting beaches, although none of these migrations is as long as the swims of the subpopulation of green seaturtles using Ascension Island. The sensory basis of this remarkable migration involves geomagnetic sensitivity, chemical cues, and probably celestial cues. Even more remarkable is the movement of hatchlings from Ascension Island to the Sargasso Sea and later to the Brazil kelp beds, the turtles again using the same variety of sensory systems. Somewhat less spectacular migrations are known in other reptiles, especially snakes in temperate zones, where hibernation sites and feeding sites are separated by distances on the order of 1-7 mi (1.6-11.2 km). The snakes make an annual vernal migration to the feeding grounds and an autumnal migration back to the hibernacula. Sensory bases for these movements have not been studied sufficiently, but it seems clear that chemical cues are involved, and there is reason to suspect a role for geomagnetic, celestial (especially solar), and visible landmark cues.

Homing has been observed in a variety of reptiles that have returned to sites from which they were experimentally displaced. Alligators, several species of snakes and turtles, and a few species of lizards have been studied, but we know relatively little about the sensory basis of this behavior. Geomagnetic cues were probably used by the alligators, chemical cues were certainly involved in several of the turtle studies, and solar cues were well documented in several of the snake studies. In most other cases, however, our knowledge has not advanced beyond the fact that homing or related behavior (e.g., y-axis orientation) occurred. Some studies have shown no evidence of return to a site occupied before displacement. This lack of evidence gives rise to interesting speculation that the motivation to return depends on the quality of the initial habitat and the quality of the habitat in which the test animals were released. An important implication is that the animals assess their habitats and use this information to decide whether to move toward "home" or to adopt a new home for themselves. If such behaviors occur, the cognitive mechanisms involved in habitat assessment will prove to be even more interesting than the sensory mechanisms mediating orientation and movement toward home. The literature on reptile homing behavior contains many anecdotes that have substantially influenced our collective thinking. For example, rattlesnakes have been removed from a human property (usually a backyard, field, or a common area in a community) and released a distance away (usually on the order of a mile or two [1.6-3.2 km]) only to return to its initial location within a few days. Another interesting case involved a black ratsnake (Elaphe obsoleta) removed from a farm several times but always returning within a few days. In cases of this sort, the snakes were not marked at the time of original capture, so it is not absolutely certain that the returning snake and the removed snake were one and the same. On the basis of size, coloration, and unique features such as scars, we can be reasonably confident, but the following case, described by the late Charles M. Bogert, urges caution.

Bogert was collecting reptiles in New Mexico and caught a striped whipsnake (Masticophis taeniatus) in a particular bush, which Bogert marked with a golf tee. The next day, while passing the same bush, Bogert caught a second striped whipsnake in it. Later on the second day, Bogert visited the bush again, and, sure enough, a third specimen of the species presented itself. There is no question that the snakes were three different individuals because Bogert caught and preserved each one. He was interested in what might have been special about the bush, but he was able to identify nothing. Perhaps this was pure chance, always a possibility with anecdotes. On the other hand, maybe the bush was in a prey-rich area or in a migration path or in a spot containing an ideal refuge from the sun or from predators. If any of these conditions existed, it would not be surprising that removal of an initial occupant might be followed by the arrival of another. If the first occupant had not been captured and kept out of the habitat, we might easily misinterpret the second and third snakes as being the initial one. Although anecdotal reports of homing should not be discounted, they should be regarded cautiously.

Closely related to homing is territoriality, the defense of a resource, usually in a particular place but sometimes mobile, as in the case of potential mates that might move a considerable distance but are nevertheless defended in each place they occupy. Numerous lizards exhibit unambiguous territoriality, defending feeding areas from conspecifics and sometimes from heterospecifics that compete for the same foods. Clever experiments have shown that adding food to territories results in shrinkage of the area defended, whereas depleting food from territories results in expansion of the area defended. Results of such studies leave no doubt that the territorial behavior of the resident lizard is sensitive to the quantity and quality of the food contained within the territory. The results of the studies also leave no doubt that the lizards assessed their food supplies and responded accordingly. Food, however, is not the only resource that lizards defend. Ovipo-sition sites are important in some species, and refuge is important in others.

Crocodilians are known to behave in a territorial manner, especially mature females who defend their nests. Large males also appear to be territorial, particularly during the breeding season. Turtles and snakes present a far more ambiguous situation, except for the few species in which females attend their eggs or young. Even these cases are not clear examples of territoriality, because there is no evidence that the females defend their eggs or oviposition sites against conspecifics. It is possible that mothers defend their eggs or neonates only against heterospecific oviphages or predators, in which case the term territoriality may not be appropriate. Territoriality implies intraspecific interaction. At present we cannot state with certainty that any species of turtle or snake exhibits true territorial behavior. Anecdotal evidence exists for both groups, and behavioral phenomena among captive specimens also are suggestive. The possibility of territoriality among these reptiles should be considered, but caution should be exercised in interpretation of anecdotal evidence.

Another approach to the study of activity patterns of reptiles is to record the number of individuals seen or captured (usually in pitfall traps) during each month of the year. In temperate North America, such projects reveal several annual patterns: a single peak during the warmest months with sharply reduced activity before or after; a single peak during the warmest months but fairly broad activity period such that substantial numbers of individuals are active before and after the warmest months; and two peaks, one in spring and one in autumn. Some species, such as garter snakes, appear to remain active all year if the temperature remains high enough. Other species appear to be endogenously programmed to become inactive during winter months, even if the temperature is artificially elevated. Having maintained many rattlesnakes in captivity, we have found that in species such as the prairie rattlesnake and the western diamondback, some individuals remain active all year if the temperature is kept at 79°F (26°C) or higher. Other individuals "shut down" for several months under the same conditions, refusing food for one to three months each year.

Closely associated with the circannual studies of reptile activity are parallel studies of reproduction. Periods of high activity or capture success can correspond with periods of intense feeding behavior, but they can also correspond with periods of courtship and copulation, particularly because of the likelihood of capturing males active in searching for re-productively motivated females. Pregnant female prairie rattlesnakes, for example, are relatively inactive, perhaps even congregating in birthing rookeries, whereas nonpregnant females have made a vernal migration to hunting grounds. The females that migrate are the ones likely to develop ripe ova and to become sexually motivated. Consequently, they are the ones mature males pursue, making themselves vulnerable to traps or to capture by hand. Although a number of reproductive strategies are observed across the various families of reptiles, most of these strategies involve seasonally increased activity by one sex or the other. It is safe to conclude that cir-cannual activity patterns are partly correlated with ovarian and testicular cycles or with courtship and copulatory seasons in taxa that exhibit a dissociation between reproductive behavior and genital physiology (i.e., some species breed at times of the year different from the times gametes are produced; in such cases, activity is influenced by the breeding season rather than the gamete-producing season).

In addition to circannual studies of reptile activity, there have been many studies of circadian rhythms, that is, changes in activity during the 24-hour period of a typical day in the animal's active season. Some species are strictly diurnal (active only during daylight), others are strictly nocturnal (active only at night), and other species are crepuscular (active at dawn and dusk). Juvenile kingsnakes (Lampropeltis getula flori-dana) exhibit crepuscular and nocturnal patterns, whereas adults are diurnal. The shift toward diurnal behavior occurs when the snakes are approximately 35 in (90 cm) snout-vent length and capable of defending themselves against a variety of predators, particularly birds, that are active during daylight. Nocturnal reptiles exhibit relative cessation of activity during periods of full moon, when ambient illumination at night favors detection by predators.

In some species, daily activity patterns shift with average daily temperature. Among plains garter snakes (Thamnophis radix) and western diamondback rattlesnakes, individuals are diurnal at the relatively low temperatures of early spring and late autumn, but during the hottest days of summer, these same individuals become nocturnal. During intermediate parts of the year, such as late spring and early summer, the snakes are crepuscular, because it is too hot during the day and too cold at night. For species with very broad geographical ranges, it is possible that individuals in northern latitudes are diurnal whereas individuals to the south are crepuscular or nocturnal, depending on thermal conditions.

Temperature is a primary modulator of many physiological processes influencing digestion, reproduction, and locomotion in reptiles. Herpetologists have focused on this factor more than any other. Each species has a lethal minimum and maximum temperature, below or above which life ceases immediately. Within this range, which might be 17.6-104°F (-8°C to +40°C), there is a second range from the critical thermal minimum (CTMin ) to the critical thermal maximum (CTMax), which might be from 35.6°F to 89.6°F (2°C to 32°C). Survival below the CTMin or above the CTMax is possible for brief periods, longer below CTMin than above CTMax, but death occurs relatively rapidly as the temperature converges with the respective lethal extremes. Yet another range is nested with the boundaries of CTMin to CT Max. This range runs from the voluntary minimum to the voluntary maximum, within which the reptile prefers to remain when choice (i.e., behavioral thermoregulation) is possible. This range defines the animal's normal or preferred activity range, and its midpoint corresponds to the animal's voluntary or preferential mean temperature. The precise temperatures corresponding to these boundaries shift during the course of the year, a phenomenon called acclimation. For example, in early spring, a reptile might prefer to keep its body temperature between 64°F and 82°F (18°C and 28°C), but later in the summer, the voluntary minimum and maximum might shift to 68°F (20°C) and 86°F (30°C), respectively. As this acclimation effect occurs, the lethal minimum, CTMin, lethal maximum, and CTMax all shift upward by 3.6-5.4°F (2-3°C).

One of the dominating necessities of any reptile is to keep its body temperature within the preferred range of its species. Because all extant reptiles are ectotherms, temperature regulation involves a large suite of behaviors, some obvious and some remarkably subtle. Reptiles also exhibit many anatomical and physiological adaptations. Among the more obvious thermoregulatory behaviors are locomotory activities by which the animals shuttle between sunlight and shade as necessary to keep their body temperatures within the optimal range. Basking is another behavior by which body surfaces are exposed to sunlight to increase core body temperature. Reptiles of some species can thermoregulate by altering the amount of surface area exposed to sunlight. They do so with inconspicuous alterations of posture, sometimes associated with color change. Large snakes are known to retain fecal material and to use it for storing heat. If a snake needs to reduce its heat load, one option is to eliminate the feces. Coiling the body and aggregating with other snakes can profoundly reduce the rate of heat loss, such that snakes in winter dens can have higher body temperature than would be indicated by the ambient climate. Because of the many behavioral and other devices they have for regulating body temperature, reptiles can maintain their temperatures within a surprisingly narrow range. The typical standard error of the mean of repeated measurements of an individual reptile's preferred body temperature is less than 1.8°F (1°C). These measurements were made under close to ideal conditions; nevertheless, such studies show how precise reptiles can be when heat sources and refuge are readily available and when no obstructions are placed in the way of locomotion. Although reptiles lack the endothermic mechanisms of birds and mammals and only a few species of reptiles engage in shivering thermogenesis, when solar or other heat sources are available during the active season, many reptiles can maintain high body temperatures with surprisingly little variability.

The temperature at which reptiles maintain their bodies during the active season can be influenced not only by acclimation but also by disease. Infection with pathogenic bacteria can cause the reptile to prefer a higher than usual temperature. This so-called behavioral fever kills the bacteria within a few days. This interesting phenomenon has been found to occur among most ectothermic vertebrates and is straightforwardly analogous to the endogenous fever response of endothermic vertebrates to pathogenic bacteria. Another thermal phenomenon is emotional fever. Lizards handled briefly by humans regulate their body temperature between one and two degrees higher than normal. This elevation may be related to a flight or escape response or to a metabolic response to the immunosuppressive effects of stress. The presence of food in the stomach also induces reptiles to elevate their body temperature and digest the food more rapidly and more thoroughly than would otherwise be the case. Pregnant females prefer higher temperatures, which facilitate gestation. As we discover these fascinating events, our appreciation of the precision of reptile behavior increases, as does our ability to care for these animals in captivity.

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