Ant-eating frogs are active over a temperature range of at least 16 deg.
C. to 37.6 deg. C. They tolerate high temperatures that would be lethal
to many other kinds of amphibians, but are more sensitive to low
temperatures than any of the other local species, and as a result their
seasonal schedule resembles that of the larger lizards and snakes more
than those of other local amphibians. The latter become active earlier
in the spring.
Earliest recorded dates when the frogs were found active in the course
of the present study from 1950 to 1955 were in April every year; the
20th, 25th, 24th, 2nd, 25th, and 21st. Latest dates when the frogs were
found in the six years of the study were: October 22, 1949; October 13,
1950; October 7, 1951; August 24, 1952; August 18, 1953; and October 27,
1954 (excluding two late stragglers caught in a pitfall on December 5).
Severe drought caused unseasonably early retirement in 1952 and 1953.
Body temperatures of the frogs were taken with a small mercury
thermometer of the type described by Bogert (1949: 197); the bulb was
used to force open the mouth and was thrust down the gullet into the
stomach. To prevent conduction of heat from the hand, the frog was held
down through several layers of cloth, at the spot where it was
discovered, until the temperature reading could be made. This required
approximately five seconds.
[Illustration: FIG. 1. Temperatures of ant-eating frogs grouped in
one-degree intervals; upper figure is of frogs found active in the open,
and lower is of those found under shelter. The frogs are active over a
temperature range of more than 20 degrees, and show no clear cut
preference within this range.]
Most of the 79 frogs of which temperatures were measured, were found
under shelter, chiefly beneath flat rocks. The rocks most utilized were
in open situations, exposed to sunshine. Most of the frogs were in
contact with the warmed undersurfaces of such rocks. Forty-three of the
frogs, approximately 54.5 percent, were in the eight-degree range
between 24 deg. and 31 deg. C. Probably the preferred temperatures lie
within this range. The highest body temperature recorded, 37.6 deg. C.,
was in a frog which "froze" and remained motionless in the sunshine for
half a minute after the rock sheltering it was overturned. Probably its
temperature was several degrees lower while it was sheltered by the
rock. Other unusually high temperatures were recorded in newly
metamorphosed frogs found hiding in piles of decaying vegetation near
the edge of the pond, on hot afternoons of late August. Temperatures
ranged from 17.0 deg. to 30.7 deg. in frogs that were found actually
moving about. Several with relatively low temperatures, 22 deg. to 17
deg., were juveniles travelling in rain or mist on cool days. These
frogs, having relatively low temperature, were sluggish in their
movements, as compared with individuals at the upper end of the
temperature range.
[Illustration: FIG. 2. Body temperatures and nearby air temperatures for
frogs found under natural conditions. Dots represent frogs found under
shelter; circles represent those found in the open.]
After the first frost each year the frogs usually could not be found,
either in the open or in their usual hiding places beneath rocks. They
probably had retired to deep subterranean hibernation sites. The only
exception was in 1954, when two immature frogs were found together in a
pitfall on the morning of December 5 after a rain of .55 inches ending
many weeks of drought. Air temperature had been little above 10 deg. C.
that night, but had often been below freezing in the preceding five
weeks.
Reactions of these same two individuals to low temperatures were tested
in the laboratory. At a body temperature of 11 deg. C. they were
extremely sluggish. They were capable of slow, waddling movements, but
were reluctant to move and tended to crouch motionless. Even when they
were prodded, they usually did not move away, but merely flinched
slightly. At 6 deg. C. they were even more sluggish, and seemed
incapable of locomotion, as they could not be induced to hop or walk by
prodding with a fine wire. When placed upside down on a flat surface,
they could turn over, but did so slowly, sometimes only after a minute
or more had elapsed. Respiratory throat movements numbered 46 and 60 per
minute.