Field Study of Kansas Ant-Eating Frog Part 1

Field Study of Kansas Ant-Eating Frog.

by Henry S. Fitch.

INTRODUCTION

The ant-eating frog is one of the smallest species of vertebrates on the University of Kansas Natural History Reservation, but individually it is one of the most numerous. The species is important in the over-all ecology; its bioma.s.s often exceeds that of larger species of vertebrates. Because of secretive and subterranean habits, however, its abundance and effects on community a.s.sociates are largely obscured.

The Reservation, where my field study was made, is the most northeastern section in Douglas County, Kansas, and is approximately 5 miles north and 2 miles east of the University campus at Lawrence. The locality represents one of the northernmost occurrences of the species, genus, and family. The family Microhylidae is a large one, and most of its representatives are specialized for a subterranean existence and a diet of termites or ants. The many subfamilies of microhylids all have distributions centering in the regions bordering the Indian Ocean, from South Africa and Madagascar to the East Indies, New Guinea, and Australia (Parker, 1934). Only one subfamily, the Microhylinae, is represented in the New World, where it has some 17 genera (de Carvalho, 1954) nearly all of which are tropical. _G. olivacea_, extending north into extreme southern Nebraska (Loomis, 1945: 211), ranges farther north than any other American species. In the Old World only _Kaloula borealis_ has a comparable northward distribution. Occurring in the vicinity of Peiping (Pope, 1931: 587), it reaches approximately the same lat.i.tude as does _Gastrophryne_ in Nebraska. The great majority of microhylid genera and species are confined to the tropics.

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. .h.i.therto 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 a.s.semblages 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 gra.s.sland 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 gra.s.s prairie (big bluestem, little bluestem, switch gra.s.s, Indian gra.s.s), 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

Ordinarily the ant-eating frog stays beneath the soil surface, in cracks or holes or beneath rocks. Probably it obtains its food in such situations, and rarely wanders on the surface. The occasional individuals found moving about above ground are in most instances flushed from their shelters by the vibrations of the observer's footsteps. On numerous occasions I have noticed individuals, startled by nearby footfalls, dart from cracks or under rocks and scuttle away in search of other shelter. Such behavior suggests that digging predators may be important natural enemies. The gait is a combination of running and short hops that are usually only an inch or two in length. The flat pointed head seems to be in contact with the ground or very near to it as the animal moves about rapidly and erratically. The frog has a proclivity for squeezing into holes and cracks, or beneath objects on the ground. The burst of activity by one that is startled lasts for only a few seconds. Then the frog stops abruptly, usually concealed wholly or in part by some object. Having stopped it tends to rely on concealment for protection and may allow close approach before it flushes again.

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 RELATIONs.h.i.+PS

Ant-eating frogs are active over a temperature range of at least 16 C.

to 37.6 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.

[Ill.u.s.tration: 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 suns.h.i.+ne. 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 suns.h.i.+ne 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.

[Ill.u.s.tration: 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 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

Many observers have noted that breeding activity is initiated by heavy rains in summer. In my experience precipitation of at least two inches within a few days is necessary to bring forth large breeding choruses.

With smaller amounts of precipitation only stragglers or small aggregations are present at the breeding ponds. Tanner (1950: 48) stated that in three years of observation, near Lawrence, Kansas, the first storms to bring large numbers of males to the breeding ponds occurred on June 20, 1947, June 18, 1948, and May 1, 1949.

In 1954 the frogs were recorded first on April 25, but these were under ma.s.sive 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 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 const.i.tuted 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.

Date Precipitation No. of in inches caught frogs July 1 2.02 8 July 10 .11 none July 16 1.26 none July 20-21 .94 3 July 24 .38 2 July 28 .29 none August 1-2 3.22 31 August 6-7-8 2.43 none August 12 .28 none August 16 .29 none August 19-22 .70 none August 27-28 1.05 none September 9 .50 none September 29-30 .38 none October 4 .74 none October 12-14 3.51 none

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 sh.o.r.e, 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 wh[=e][=e] followed by a bleat."

Stebbins observed breeding choruses (_mazatlanensis_) at Pena 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 sp.a.w.ning 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.