V I I . DESERT FISHES
447
acute environmental conditions are now recognized, in part at least, as
regulatory in function, activities which place the animal under the most
optimal conditions available within a given environmental spectrum. This
represents a selection of the most favorable habitat available along an environmental gradient in which the biotic elements assume a somewhat
dominant position. Refinement of such behavior varies considerably, as
does the detail and quality of its observation.
Behavioral selection of favorable conditions permits a population to
continue its normal activities. Alterations of normal behavior, i.e., changes
in activity patterns and changes in the population dispersion in the habitat,
apparently enhance survival of those forms possessing a sufficiently labile
behavioral repertoire. Behavioral thermorégulation in fishes, for example,
has only recently been described, and Norris (1963) reviewed available
evidence for the phenomenon. The responses seem relatively widespread
in fishes as a group and are extremely well developed and important in
desert species. Lowe and Heath (1969), for instance, showed that desert
pupfish at Quitobaquito Spring, Arizona, commonly (and preferentially)
occupied water at 40°-41°C, which is within 2° or 3 °C of the level for
thermal death. Barlow (1958a) also noted C. macularius in shore pools
of the Salton Sea moving to occupy waters just cooler than 36°C and
avoiding waters above or below that level, if possible. In temperature
gradients of streams fed by hot springs near Tecopa, California, C. n.
calidae occurred most abundantly at 36°C (Miller, 1948). Maximum activity of C. n. nevadensis in the marsh at Saratoga Springs, Death Valley,
California, occurred between 30° and 34°C, and was severely restricted
above 38°C.
More recently, observations of another population of Cyprinodon occupying outflows of newly constructed artesian wells a few miles north of
Tecopa indicate a somewhat higher voluntary tolerance, causing the population to follow the seasonally changing 42°C isotherm as it moves up or
down the stream channel (Soltz and Brown, 1970). Male Crenichthys
baileyi from Crystal and Mormon springs, Nevada, oriented evenly between a minimum available temperature of 32°C and 36°C, in Mimbres
Hot Spring, New Mexico, and could be chased into 38°C water (Hubbs
and Hettler, 1964). Gambusia marshi was most abundant at 39.5°-40.5°C
in Ojo Caliente, Coahuila (Section III,A,5).
Thermal conditions need not be extreme for behavioral responses to occur. LaBounty (1968) presented data on activity of C. n. nevadensis from
Zzyzx Pond, California, on four different occasions during the year. Temperature ranges were 10.9°-21.8°C on February 27 and 26.3°-32.2°C on
August 31; thus, lethal temperatures were never approached. The fish consistently selected the warmest temperatures in the pond and were most
447
acute environmental conditions are now recognized, in part at least, as
regulatory in function, activities which place the animal under the most
optimal conditions available within a given environmental spectrum. This
represents a selection of the most favorable habitat available along an environmental gradient in which the biotic elements assume a somewhat
dominant position. Refinement of such behavior varies considerably, as
does the detail and quality of its observation.
Behavioral selection of favorable conditions permits a population to
continue its normal activities. Alterations of normal behavior, i.e., changes
in activity patterns and changes in the population dispersion in the habitat,
apparently enhance survival of those forms possessing a sufficiently labile
behavioral repertoire. Behavioral thermorégulation in fishes, for example,
has only recently been described, and Norris (1963) reviewed available
evidence for the phenomenon. The responses seem relatively widespread
in fishes as a group and are extremely well developed and important in
desert species. Lowe and Heath (1969), for instance, showed that desert
pupfish at Quitobaquito Spring, Arizona, commonly (and preferentially)
occupied water at 40°-41°C, which is within 2° or 3 °C of the level for
thermal death. Barlow (1958a) also noted C. macularius in shore pools
of the Salton Sea moving to occupy waters just cooler than 36°C and
avoiding waters above or below that level, if possible. In temperature
gradients of streams fed by hot springs near Tecopa, California, C. n.
calidae occurred most abundantly at 36°C (Miller, 1948). Maximum activity of C. n. nevadensis in the marsh at Saratoga Springs, Death Valley,
California, occurred between 30° and 34°C, and was severely restricted
above 38°C.
More recently, observations of another population of Cyprinodon occupying outflows of newly constructed artesian wells a few miles north of
Tecopa indicate a somewhat higher voluntary tolerance, causing the population to follow the seasonally changing 42°C isotherm as it moves up or
down the stream channel (Soltz and Brown, 1970). Male Crenichthys
baileyi from Crystal and Mormon springs, Nevada, oriented evenly between a minimum available temperature of 32°C and 36°C, in Mimbres
Hot Spring, New Mexico, and could be chased into 38°C water (Hubbs
and Hettler, 1964). Gambusia marshi was most abundant at 39.5°-40.5°C
in Ojo Caliente, Coahuila (Section III,A,5).
Thermal conditions need not be extreme for behavioral responses to occur. LaBounty (1968) presented data on activity of C. n. nevadensis from
Zzyzx Pond, California, on four different occasions during the year. Temperature ranges were 10.9°-21.8°C on February 27 and 26.3°-32.2°C on
August 31; thus, lethal temperatures were never approached. The fish consistently selected the warmest temperatures in the pond and were most