University of Kansas Publications

Museum of Natural History

Volume 8, No. 4, pp. 275-306, 9 figs. in text

February 10, 1956

A Field Study
of the Kansas Ant-Eating Frog,
Gastrophryne olivacea

BY

HENRY S. FITCH

University of Kansas
Lawrence
1956

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. Wilson

Volume 8, No. 4, pp. 275-306, 9 figs. in text
Published February 10, 1956

University of Kansas
Lawrence, Kansas

PRINTED BY
FERD VOILAND. JR., STATE PRINTER
TOPEKA, KANSAS
1956

25-7819

A FIELD STUDY
OF THE KANSAS ANT-EATING FROG,
GASTROPHRYNE OLIVACEA

By

Henry S. Fitch

INTRODUCTION

Nearly all ant-eating frogs seen on the Reservation have been caught and examined and individually marked. By November 1, 1954, 1215 individuals had been recorded with a total of 1472 captures. In the summer of 1950, Richard Freiburg studied this frog on the Reservation and his findings (1951) led to a better understanding of its natural history. The numbers of frogs studied by him however, were relatively small and the field work was limited to the one summer. The data now at hand, representing six consecutive years, 1949 through 1954, serve to supplement those obtained by Freiburg, corroborating and extending his conclusions in most instances, and also indicating that certain of his tentative conclusions need to be revised.

While the present report was in preparation, Anderson (1954) published an excellent account of the ecology of the eastern species G. carolinensis in southern Louisiana. Anderson's findings concerning this closely related species in a much different environment have been especially valuable as a basis for comparison. The two species are basically similar in their habits and ecology but many minor differences are indicated. Some of these differences result from the differing environments where Anderson's study and my own were made and others certainly result from innate genetic differences between the species.

The frog with which this report is concerned is the Microhyla carolinensis olivacea of the check list (Schmidt, 1953: 77) and recent authors. De Carvalho (1954: 12) resurrected the generic name, Gastrophryne, for the American species formerly included in Microhyla, and presented seemingly valid morphological evidence for this plausible generic separation.

G. olivacea is obviously closely related to G. carolinensis; the differences are not greater than those to be expected between well marked subspecies. Nevertheless, in eastern Oklahoma and eastern Texas, where the ranges meet, the two kinds have been found to maintain their distinctness, differing in coloration, behavior, calls, and time of breeding. Hecht and Matalas (1946: 2) found seeming intergrades from the area of overlapping in eastern Texas, but some specimens from this same area were typical of each form. Their study was limited to preserved material, in which some characters probably were obscured. More field work throughout the zone of contact is needed. The evidence of intergradation obtained so far seems to be somewhat equivocal.

Besides G. olivacea and typical G. carolinensis there are several named forms in the genus, including some of doubtful status. The name mazatlanensis has been applied to a southwestern population, which seems to be a well marked subspecies of olivacea, but as yet mazatlanensis has been collected at few localities and the evidence of intergradation is meager. The names areolata and texensis have been applied to populations in Texas. Hecht and Matalas (1946: 3) consider areolata to be a synonym of olivacea, applied to a population showing intergradation with carolinensis, but Wright and Wright (1949: 568) consider areolata to be a distinct subspecies. G. texensis generally has been considered to be a synonym of olivacea. Other species of the genus include the tropical G. usta, G. elegans and G. pictiventris.

Of the vernacular names hitherto applied to G. olivacea none seems appropriate; I propose to call the species the Kansas ant-eating frog because of its range extending over most of the state, and because of its specialized food habits. The type locality, originally stated to be "Kansas and Nebraska" (Hallowell, 1856: 252) has been restricted to Fort Riley, Kansas (Smith and Taylor, 1950: 358). Members of the genus have most often been referred to as toads rather than frogs because of their more toadlike appearance and habits. However, this family belongs to the firmisternial or froglike division of the Salientia and the terms "frog" and "toad," originally applied to Rana and Bufo respectively, have been extended to include assemblages of related genera or families. Members of the genus and family usually have been called "narrow-mouthed" toads from the old generic name Engystoma, a synonym of Gastrophryne. G. olivacea usually has been referred to as the Texas narrow-mouthed toad, or western narrow-mouthed toad. The latter name is inappropriate because the geographic range is between that of a more western representative (mazatlanensis) and a more eastern one (carolinensis). The names texensis, areolata and carolinensis have all been applied to populations in Texas, and it is questionable whether typical olivacea even extends into Texas.

HABITAT

In the northeastern part of Kansas at least, rocky slopes in open woods seem to provide optimum habitat conditions. This type of habitat has been described by several earlier workers in this same area, Dice (1923: 46), Smith (1934: 503) and Freiburg (1951: 375). Smith (1950: 113) stated that in Kansas this frog is found in wooded areas, and that rocks are the usual cover, but he mentioned that outside of Kansas it is often found in mesquite flats that are devoid of rocks. Freiburg's field work was done almost entirely on the Reservation and was concentrated in "Skink Woods" and vicinity, where much of my own field work, both before and afterward, was concentrated. On the Reservation and in nearby counties of Kansas, the habitat preferences of the ant-eating frog and the five-lined skink largely coincide. In an account of the five-lined skink on the Reservation, I have described several study areas in some detail (Fitch, 1954: 37-41). It was on these same study areas (Quarry, Skink Woods, Rat Woods) that most of the frogs were obtained.

Although G. olivacea thrives in an open-woodland habitat in this part of its range, it seems to be essentially a grassland species, and it occurs throughout approximately the southern half of the Great Plains region. Bragg (1943: 76) emphasized that in Oklahoma it is widely distributed over the state, occupying a variety of habitats, with little ecological restriction. Bragg noted, however, that the species is rarely, if ever, found on extensive river flood plains. On various occasions I have heard Gastrophryne choruses in a slough two miles south of the Reservation. This slough is in the Kaw River flood plain and is two miles from the bluffs where the habitat of rocky wooded slopes begins that has been considered typical of the species in northeastern Kansas. It seems that the frogs using this slough are not drawn from the populations living on the bluffs as Mud Creek, a Kaw River tributary, intervenes. The creek channel at times of heavy rainfall, carries a torrent of swirling water which might present a barrier to migrating frogs as they are not strong swimmers. The frogs could easily find suitable breeding places much nearer to the bluffs. Those using the slough are almost certainly permanent inhabitants of the river flood plain. The area in the neighborhood of the slough, where the frogs probably live, include fields of alfalfa and other cultivated crops, weedy fallow fields, and the marshy margins of the slough. In these situations burrows of rodents, notably those of the pocket gopher (Geomys bursarius), would provide subterranean shelter for the frogs, which are not efficient diggers.

The frogs may live in many situations such as this where they have been overlooked. In the absence of flat rocks providing hiding places at the soil surface, the frogs would rarely be found by a collector. The volume and carrying quality of the voice are much less than in other common anurans. Large breeding choruses might be overlooked unless the observer happened to come within a few yards of them. Most of the recorded habitats and localities of occurrence may be those where the frog happens to be most in evidence to human observers, rather than those that are limiting to it or even typical of it.

On September 20, 1954, after heavy rains, juveniles dispersing from breeding ponds were in a wide variety of situations, including most of the habitat types represented on the Reservation. Along a small dry gully in an eroded field formerly cultivated, and reverted to tall grass prairie (big bluestem, little bluestem, switch grass, Indian grass), the frogs were numerous. Many of them were flushed by my footsteps from cracks in the soil along the gully banks. In reaching this area the frogs had moved up a wooded slope from the pond, crossed the limestone outcrop area at the hilltop edge, and wandered away from the woods and rocks, out into the prairie habitat. In this prairie habitat there were no rocks providing hiding places at the soil surface, but burrows of the vole (Microtus ochrogaster) and other small rodents provided an abundance of subterranean shelter. In the summer of 1955 the frogs were seen frequently in this same area, especially when the soil was wet from recent rain. When the surface of the soil was dry, none could be found and presumably all stayed in deep cracks and burrows.

Anderson (1954: 17) indicated that G. carolinensis in Louisiana likewise occurs in diverse habitats, being sufficiently adaptable to satisfy its basic requirements in various ways.

BEHAVIOR

Less frequently, undisturbed individuals have been seen wandering on the soil surface. Such wandering occurs chiefly at night. Diurnal wandering may occur in relatively cool weather when night temperatures are too low for the frogs to be active. Wandering above ground is limited to times when the soil and vegetation are wet, mainly during heavy rains and immediately afterward.

Pitfalls made from gallon cans buried in the ground with tops open and flush with the soil surface were installed in 1949 in several places along hilltop rock outcrops where the frogs were abundant. The number of frogs caught from day to day under varying weather-conditions provided evidence as to the factors controlling surface activity. After nights of unusually heavy rainfall, a dozen frogs, or even several dozen, might be found in each of the more productive pitfalls. A few more might be caught on the following night, and occasional stragglers as long as the soil remained damp with heavy dew. Activity is greatest on hot summer nights. Below 20° C. there is little surface activity but individuals that had body temperatures as low as 16° C. have been found moving about.

Frogs uncovered in their hiding places beneath flat rocks often remained motionless depending on concealment for protection, but if further disturbed, they made off with the running and hopping gait already described. Although they were not swift, they were elusive because of their sudden changes of direction and the ease with which they found shelter. When actually grasped, a frog would struggle only momentarily, then would become limp with its legs extended. The viscous dermal secretions copiously produced by a frog being handled made the animal so slippery that after a few seconds it might slide from the captor's grasp, and always was quick to escape when such an opportunity was presented.

TEMPERATURE RELATIONSHIPS

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.

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° and 31° C. Probably the preferred temperatures lie within this range. The highest body temperature recorded, 37.6° 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° to 30.7° in frogs that were found actually moving about. Several with relatively low temperatures, 22° to 17°, 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.

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° 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° 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° 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.

BREEDING

In 1954 the frogs were recorded first on April 25, but these were under massive boulders, and were still semi-torpid. Frogs were found fully active, in numbers, under small flat rocks on May 7. They were found frequently thereafter. On the afternoon of May 13, the third consecutive day with temperature slightly above 21° C., low croaking of a frog was heard among rocks at an old abandoned quarry. Throughout the remainder of May, calling was heard frequently at the quarry on warm, sunny afternoons. Often several were calling within an area of a few square yards, answering each other and maintaining a regular sequence. In the last week of May rains were frequent, and the precipitation totalled 2.09 inches. On June 1 and 2 also, there were heavy rains totalling 2.26 inches. On the evening of June 2 many frogs were calling at a pond ½ mile south of the Reservation, and one was heard at the pond on the Reservation. By the evening of June 4, dozens were calling in shallow water along the edge of this pond in dense Polygonum and other weeds. There was sporadic calling even in daylight and there was a great chorus each evening for the next few days, but its volume rapidly diminished.

In mid-June a system of drift fences and funnel traps was installed 200 yards west of the pond in the dry bottom of an old diversion ditch leading from the pond. The ditch constituted the boundary between bottomland pasture and a wooded slope, and therefore was a natural travelway. The object of the installation was to intercept and catch small animals travelling along the ditch bottom. The drift fence was W-shaped, with a funnel trap at the apex of each cone so that the animals travelling in either direction would be caught. The numbers of frogs caught from time to time during the summer provided information as to their responses to weather in migrating to the pond.

Table 1. Numbers of Frogs Caught Within Two Days After Rain in Funnel
Traps in 1954, from Mid-June, to the Time of First Frost.

From the positions of the traps and drift fences, it was obvious that all of the frogs that were caught were travelling toward the pond. Capture of an equal number moving away from the pond a few days afterward might have been expected but none at all was caught while making a return trip. Therefore it seems that the frogs returned by a different route to their home ranges after breeding. Of necessity they make the return trip under conditions drier than those that prevail on the pondward trip, which is usually made in a downpour. Probably the return travel is slower, more leisurely, and with more tendency to keep to sheltered situations.

The call is a bleat, resembling that of a sheep, but higher, of lesser volume, and is not unlike the loud rattling buzz of an angry bee. The call is usually of three to four seconds duration, with an interval several times as long. Calling males were floating, almost upright, in the water within a few yards of shore, where there was dense vegetation. The throat pouch when fully expanded is several times as large as the entire head. When a person approached to within a few yards of frogs they usually stopped calling, submerged, and swam to a place of concealment.

Having heard the call of typical G. carolinensis in Louisiana, I have the impression that it is a little shorter, more sheeplike, and less insectlike than that of G. olivacea. The call of Gastrophryne is of such peculiar quality that it is difficult to describe. Different observers have described it in different terms. Stebbins (1951: 391) has described the call in greatest detail, and also has quoted from the descriptions of it previously published. These descriptions include the following: "high, shrill buzz"; "buzz, harsh and metallic"; "like an electric buzzer"; "like bees at close range but more like sheep at a distance"; "bleating baa"; "shrill, long-drawn quaw quaw"; "whistled whēē followed by a bleat."

Stebbins observed breeding choruses (mazatlanensis) at Peña Blanca Springs, Arizona, and stated that sometimes three or four called more or less together, but that they seldom started simultaneously. Occasionally many voices would be heard in unison followed by an interval of silence, but this performance was erratic. At the pond on the Reservation I noted this same tendency many times. After a lull the chorus would begin with a few sporadic croaks, then four or five or even more frogs would be calling simultaneously from an area of a few square yards. Anderson (op. cit.: 34) found that in small groups of calling G. carolinensis there was a distinct tendency to maintain a definite pattern in the sequence of the calls. One "dominant" individual would initiate a series of calls, and others each in turn would take up the chorus.

Pairing takes place soon after the breeding aggregations are formed. On the night of June 4, 1954, a clasping pair was captured and kept in the laboratory in a large jar of water. This pair did not separate, and spawning occurred between noon and 1:30 P. M. on June 5. When the newly laid eggs were discovered at 1:30 P. M. most of them were in a surface film. Some were attached to submerged leaves and a few rested on the bottom. The pair was still joined, but the male was actually clasping only part of the time, and as the frogs moved about in the water, it became evident that they were adhering to each other by the areas of skin contact, which were glued together by their dermal secretion. They were unable to separate immediately, even when they struggled to do so. They were observed for approximately 15 minutes before separation occurred, and during this time they were moving about actively. As they separated, the area of adhesion was discernible on the back of the female. It was U-shaped, following the ridges of the ilia and the sacrum.

On August 2, 1954, after a rain of 3.22 inches, the previously mentioned funnel trap in the ditch had caught 31 ant-eating frogs. Water had collected to a depth of several inches in the depression where the trap was situated. A dozen of the trapped frogs were clasping pairs. These frogs struggled vigorously as they were removed from the traps, handled and marked. As a result most of the clasping males were separated from the females. In handling those of each pair I noticed that they were glued together by dermal secretions, as were those of the pair observed on June 5. The areas of adhesion were of similar shape and location in the different pairs, and included the U-shaped ridge of the female's back and the male's belly, and the inner surfaces of the male's forelegs with the corresponding surfaces of the female's sides where the male clasped.

This adhesion of the members of a pair during mating may be a normal occurrence. The copious secretion of the dermal glands is of especially glutinous quality in Gastrophryne. The adhesion of members of a pair may have survival value. These small frogs are especially shy, and in the breeding ponds they respond to any disturbance with vigorous attempts to escape and hide. Under such circumstances the adhesion may prevent separation. Also, it may serve to prevent displacement of a clasping male by a rival. Anderson (op. cit.) who observed many details of the mating behavior of G. carolinensis, both in the laboratory and under natural conditions, mentioned no such adhesion between members of a pair.

Anderson (op. cit.: 31) discussed the possibility that reproductive isolation might arise in sympatric populations, such as those of G. carolinensis in southern Louisiana, through inherent differences in time of spawning. However, in G. olivacea at least, such isolation would be prevented by individual males returning to breed at different times in the same season. Furthermore, individual differences in choice of breeding time probably result from environmental factors rather than genetic factors in most instances. In G. olivacea in Kansas, time of breeding is controlled by the distribution of heavy rainfall creating favorable conditions. Onset of the breeding season may be hastened or delayed, or an entire year may be missed because of summer drought. If favorable heavy rains are well distributed throughout the summer, frogs of age classes that are not yet sexually mature in the early part of the breeding season, may comprise the bulk of the breeding population in late summer.

DEVELOPMENT OF EGGS AND LARVAE

GROWTH

In 1954, an opportunity to investigate the early growth was afforded by unusually favorable circumstances. The population of frogs that emerged from hibernation in the late spring of 1954 included few, if any, that were below adult size; drought had prevented successful breeding in 1952 and 1953. Heavy rains in the first week of June, 1954, and again in the first week of August, resulted in the production of two successive crops of young so widely spaced that they were easily distinguishable. Some young may have been hatched after other minor rains, but certainly these were relatively few. Young from the eggs laid in the first week of August were metamorphosing during the last week of August. Growth in the frogs of this group can be shown by the average size and the size range of the successive samples collected.

Table 2. Growth in Frogs Metamorphosed in the Last Week of August, 1954.

The frogs hatched in June were present in relatively small numbers compared with those hatched in August, and were not observed metamorphosing. In late August a sample of 33 judged to belong to the June brood averaged 26.2 (22-28) mm. long. A sample of 39 from the first week of October averaged 28.1 (24.5-32) mm. Frogs of this group thus were approaching small adult size late in their first growing season. Such individuals possibly breed in the summer following their first hibernation, when they are a year old or a little more. Because recaptured frogs were not sacrificed to determine the state of their gonads, the minimum time required to attain sexual maturity was not definitely determined. The available evidence indicates that sexual maturity is most often attained late in the second year of life, at an age of approximately two years. The darkened and distensible throat pouch of the adult male probably is the best available indicator of sexual maturity.

Frogs that metamorphose in late summer have little time to grow before hibernating, and still are small when they emerge in spring. The smallest one found was 19 mm. long (May 19, 1951), and in each year except 1954 many such young were found that were less than 25 mm. in length in May or early June. None of the frogs marked at or near metamorphosing size has been recaptured, but the trend of early growth is well shown by Table 2 and Fig. 3. However, many juveniles that were captured and marked within a few weeks of metamorphosis were recaptured as adults. The selected individuals in Table 3 are considered typical of growth from "half-grown" to small adult size. Growth in many other individuals is shown in Figs. 6 and 7.

Table 3. Growth in Frogs Marked as Young and Recaptured as Small
Adults.