By the end of April some kinds of deciduous trees have not yet begun to leaf out, and in most other kinds the leaves are still in an early stage of development. Absence of a leaf canopy during April permits the skinks to utilize the spring sunshine to maintain their body temperatures at almost the same high level that they maintain in the same situations in hot summer weather.
Table 2. Temperatures (in Degrees Centigrade) of Skinks Found Under Flat Rocks Exposed to Sunshine, Contrasted With Air Temperatures; Spring of 1953.
| Date | Age and sex | Skink temperature | Air temperature |
| March 23 | Ad. ♀ | 20.8 | 12.4 |
| March 23 | young | 24.7 | 12.4 |
| March 25 | Ad. ♂ | 22.8 | 12.5 |
| March 25 | young | 21.0 | 12.5 |
| March 25 | young | 25.7 | 14.5 |
| March 25 | young | 22.5 | 14.5 |
| March 27 | Ad. ♂ | 26.6 | 16.5 |
| March 27 | young | 22.0 | 16.5 |
| March 27 | Ad. ♀ | 22.5 | 16.5 |
| March 27 | Ad. ♀ | 20.5 | 16.2 |
| March 27 | Ad. ♀ | 26.5 | 19.3 |
| March 27 | Ad. ♀ | 30.7 | 19.3 |
| April 4 | young | 22.0 | 18.1 |
| April 5 | Ad. ♀ | 26.0 | 13.0 |
| April 6 | Ad. ♂ | 31.5 | 13.5 |
| April 6 | Ad. ♂ | 23.7 | 16.0 |
| April 6 | Ad. ♀ | 22.2 | 16.0 |
| April 6 | Ad. ♂ | 20.0 | 16.0 |
| April 6 | Ad. ♀ | 20.0 | 16.0 |
| April 6 | Ad. ♀ | 26.5 | 20.3 |
| April 20 | Ad. ♀ | 29.7 | 17.2 |
| April 20 | Ad. ♀ | 25.8 | 17.2 |
Recent studies by Cowles and Bogert (1944:288-289) and Bogert (1949:198) have brought out the fact that terrestrial poikilotherms, and especially lizards, maintain fairly high and constant body temperatures through behavioral thermoregulation, during their periods of activity. For genera and species of lizards, there are optimum body temperatures, which the individual tends to maintain, fluctuating within a range of only a few degrees while it is active. Forms that are not closely related may differ notably in their optimum temperatures, although within any one genus the range is slight. For example in the iguanid genus, Sceloporus, Bogert found that different species from such distant regions as Arizona and Florida agreed in having body temperatures approximating 35° or 36°C., while different members of the teiid genus Cnemidophorus in the same two regions were found to approximate 41°C. in mean temperatures. In commenting on the distribution of North American lizards as affected by opportunity for behavioral thermoregulation by direct insolation, Bogert (op. cit.:205) wrote: “Such secretive lizards as skinks (principally Eumeces in North America) with low body temperature preferences approximating 30°C. are dominant in Florida and the Gulf Coast, in contrast to the Teiidae and Iguanidae (several genera in the United States), which are far more abundant in the arid regions of the Southwest.” Bogert and Cowles (1947:19) record that in a large individual of Eumeces inexpectatus taken near the Archbold Biological Station in Florida, the body temperature was 33.2°C.
In the 1952 season, a small thermometer of the type described by Bogert (op. cit.:197) was frequently carried on collecting trips, and cloacal temperatures were recorded for the lizards collected. For those found in traps the opportunity for behavioral thermoregulation was limited, and temperatures usually approximated those of the air. The circumstances of capture, and the air temperatures were recorded for most of the skinks taken. For those found under rocks or in other shelter, the temperature usually approximated that of the immediate surroundings, and averaged much lower than for those taken in the open, but some found in such shelters had temperatures many degrees higher than their surroundings, and were fully active, having evidently just taken to cover to escape notice as the collector approached. As soon as a lizard was secured it was held in a leather glove or heavy cloth to prevent conduction of heat from the collector’s hand, and a reading was taken within a few seconds. Most of the skinks found in the open could not be caught immediately but were secured only after minutes of maneuvering on the part of both collector and lizard. In most instances this maneuvering probably entailed some loss of heat by the lizard, as it interrupted its thermoregulatory behavior to run to a place of concealment, usually in shadow on a tree trunk, or in or beneath ground litter. Excluding all those not found active in the open, the mean temperature, in a sample of 41, was 31.5°C. ± .60. This figure is thought to be slightly too low because of heat loss by many of the skinks in the time required to capture them.
In order to test the range of tolerance and verify the preferred optimum temperature of the five-lined skink, an experimental terrarium was set up providing extremes of temperature at each end. A false floor of 1⁄8 inch wire screen was provided, with a seven-inch strip of galvanized sheet metal beneath it at each end. Beneath the screen and sheet metal at one end the space was filled with chopped ice, and “dry ice.” Observations were made on hot, clear summer days, with the terrarium arranged so that the half of it containing ice, was in shadow, and the other half was in sunshine. The strip of metal, warmed by direct sunlight, became uncomfortably hot to the touch while at the other end the sheet metal and overlying screen were cooled by the ice. A narrow zone across the middle of the terrarium had screen but no underlying sheet metal and was the only part within which the lizard could maintain normal temperature, one end being uncomfortably hot and the other end too cool. A large dead skink left on the metal strip in direct sunlight for five minutes had a cloacal temperature of 45.3°C., and after five minutes on the screen at the cool end, its temperature had dropped to 25.5°C. On several occasions a number of skinks were put in the terrarium and their temperatures taken at brief intervals. Temperatures ranged from 21.6°C. to 37.7°C. but were mostly within a much narrower range, from 28° to 36°C. One skink that seemed to be sick was sluggish in behavior, not responding to the extremes of temperatures as readily as the other individuals and his temperature fluctuated widely and irregularly. Eliminating this individual, 66 temperature readings taken, from five other skinks, gave a mean of 32.6°C. ± .235. While nearly all the temperature readings were within a range of ten degrees, two of the readings were outstandingly low and perhaps should be discarded. If this is done, a mean of 33.8°C. ± .19 is obtained for the remaining 64. There is distinct bimodality in this series however, with a mean of 34.2° for the 49 higher readings, and a mean of 28.8°C. for the 15 lower temperatures. A similar bimodality is evident in the readings obtained from skinks caught in the open under natural conditions. It seems that the lower readings result from lags in the skinks’ response when body temperature drops slightly below the optimum. The skink is quick to make adjustment whenever its temperature appreciably exceeds this optimum level, and is in extreme discomfort at only a few degrees higher temperature. At slightly lower temperatures, however, the skink experiences no discomfort, and only slightly decreased efficiency in its various functions, and its thermoregulatory behavior in making readjustment toward the optimum is likely to be leisurely and interrupted unless its temperature drops below 28°C.
Catching the skinks in the experimental terrarium at frequent intervals to take their temperatures involved some disturbance to them, interrupting their thermoregulatory behavior. The experimenter’s first attempt to grasp a skink sometimes failed, and it then dashed about the terrarium for several seconds, probably altering its temperature somewhat. Nevertheless most of the lizards’ movements were motivated by thermoregulation. This was especially evident when they were left undisturbed, and is illustrated by the following notes on behavior of an adult female and half-grown young of fasciatus and a young E. obsoletus on the afternoon of July 21, 1952.
Having once emerged from its hiding place a skink becomes more or less independent of the temperature of the air and substrate, as it is capable of thermoregulation through insolation. However, after a period of cooling and inactivity in dormancy, or merely resting for the night in temporary shelter, the skink is dependent on warmth from the air or substrate or both to become sufficiently activated so that it can emerge and take advantage of direct sunlight. About 10:00 a. m. on April 13, 1951, when the air temperature was a little less than 10°C., a large adult male rustling among dry leaves attracted my attention. Obviously recently emerged from hibernation, he was caked with dried mud and his eyelids were nearly sealed shut. He had been sunning, however, and was active enough to elude my attempts to catch him, as he scurried into a deep crevice under the ledge. On the morning of March 24, 1951, while the temperature was still between 10° and 15°C., a subadult skink, the first one of the season, was seen sunning itself at the entrance of a deep crevice under the ledge. This skink was still not fully active, and its movements were stiff, yet it was alert and wary, and it quickly retreated back into the crevice. During the first week of May, 1952, skinks were active in abundance and numbers were caught daily in funnel traps and pitfalls. On May 9, however, the maximum air temperature was 16.5°C. with cloudy sky and occasional showers. Under these conditions skinks stayed under cover; none was seen in the open nor caught in a trap, and several found under rocks were slow and sluggish. On May 10 a terrarium with several adults was placed in dilute sunshine beside a window in an unheated room. After a period of basking the skinks were stimulated to activity, but were unable to attain normally high temperatures, and as a result their movements were like slow motion caricatures of the normal behavior. Males approached each other with menacing demeanor, with heads turned, snouts depressed, and forequarters standing high. Frequently one would edge up to another and bite hard at its flanks. The several males were sexually aroused by the presence of the two females, but were capable of only the preliminary phases of courtship, in delayed and protracted form. The temperature of one was 18.2°C. when the sun had nearly set and activity was tapering off, at an air temperature of 16.2°C. At 16°C. skinks in a terrarium with no access to sunshine for the most part showed no interest in food and kept out of sight under cover. When exposed their activity was directed almost entirely toward burrowing into the substrate or searching for objects beneath which to hide. One adult female was partly exposed by scraping away loose soil into which she had burrowed. A mealworm was then dropped just in front of her head. She tested it several times with her tongue and then ate it without emerging, her movements being much less brisk than they normally are in feeding. Probably this approximates the threshhold temperature for feeding behavior. At 19.5°C. the several skinks in this terrarium were moving about in the open although they were not exposed to sunshine, and they accepted food avidly when it was offered, but were much slower than at optimum temperatures. On May 16, 1951, when a pair of skinks were put together in a terrarium in the laboratory at 21°C., copulation ensued but it was of longer duration than in other observed instances, seemingly because of the relatively low temperature.