Field Study of Kansas Ant-Eating Frog Part 2

On many occasions individuals were found beneath the same rock on two consecutive days, or occasionally on several successive days. Rarely, such continued occupancy of a niche lasted several weeks. In 1949, a frog was found under the same rock on June 4, 6, 26, 27, and July 1, 3 and 11. This was an immature female, presumably metamorphosed late in the summer of 1948. During the five weeks period covered by the records, it grew from 27 mm. to 34 mm. In 1952, another individual was found under its home rock on June 23 and 30, July 2 and 3, and August 14 and 20. In 1952 a juvenile was found under a rock on May 30, June 4, and June 17. These three individuals were exceptional in their continued occupancy of the same niches. Among the hundreds of others recorded, none was found more than twice in any one place.

Despite the fact that field work was concentrated on small areas which were worked intensively, only eight per cent of the frogs recorded were ever recaptured, and most of those were recaptured only once. Only 13 individuals yielded series of records, well s.p.a.ced, in two or more different years. These few individuals recaptured frequently may not be typical of the entire population. The low incidence of recaptures indicates that relatively few of the frogs present on an area at any one time have been taken. Because of their secretive and subterranean habits most of the frogs are missed by a collector who searches by turning rocks, or trapping with pitfalls. Therefore, even though a marked frog may survive and remain within a radius of a few hundred feet of one point for months or even years, the chances of recapture are poor.

One female was caught first as a juvenile on June 8, 1950. On April 24, 1951, when first recaptured, she had grown to small adult size, and was only 18 feet from the original location. On July 30, 1951, however, she was recaptured 750 feet away. At a fourth capture on May 21, 1952, she had s.h.i.+fted 70 feet farther in the same direction. At the final capture on June 24, 1952, she was approximately 140 feet from both the third and fourth locations. The sequence of these records suggests that the frog had already settled in a home range at the time of her first capture in 1950, and that approximately a year later she s.h.i.+fted to a second home range, which was occupied for the following year, at least.

[Ill.u.s.tration: FIG. 8. Distances between captures in frogs marked, and recaptured after substantial intervals including one or more hibernations. Distances are grouped in 25-foot intervals. For longer distances the trend is toward progressively fewer records, indicating that typical home ranges are small.]

In several instances, after recaptures as far as 400 feet from the original location, frogs were again captured near an original location, suggesting that for some individuals, at least, home ranges may be as much as 400 feet in diameter.

Figure 8 shows that for movements of up to 400 feet, numbers of individuals gradually decrease with greater distance. For distances of more than 400 feet there are comparatively few records. Of the 59 individuals recaptured after one or more hibernations, only nine had moved more than 400 feet from the original location. Twenty-five were recaptured at distances of 75 feet or less. The mean distance for movement for all individuals recaptured was 72 feet. A typical home range, therefore, seems to average no more than 75 feet in radius. Of the 59 individuals recaptured after one or more hibernations, 47 were adults and probably many of these had made round-trip migrations to the breeding pond. This was not actually demonstrated for any one individual, but several were captured in each of three or four different years near the same location.

[Ill.u.s.tration: FIG. 9. Distances between captures and elapsed time in months in marked frogs recaptured. Few records are for distances more than 400 feet. There is but little tendency to longer movements in those caught after relatively long intervals.]

The trend of movements differed in the s.e.xes. Males are more vagile. Of 21 adult males recaptured, none was less than 40 feet from its original location, whereas six of the 26 adult females were less than 40 feet away from the original point of capture. Of seven frogs that had wandered 700 feet or more, five were males.

FOOD HABITS

According to Smith (1934: 503) stomachs of many specimens, from widely scattered localities in Kansas, contained only large numbers of small ants. Tanner (1950: 47) described the situation of a frog found on the Reservation buried in loose soil beneath a flat rock, beside an ant burrow, where, presumably, the frog could snap up the pa.s.sing ants without s.h.i.+fting its position. Anderson (_op. cit._: 21) examined alimentary tracts of 203 specimens of _carolinensis_ from Louisiana, representing a year round sample for several different habitats. He found a variety of small animals including ants, termites, beetles, springtails, bugs, ear-wigs, lepidopterans, spiders, mites, centipedes, and snails. Most of these prey animals were represented by few individuals, and ants were much more numerous than any of the other groups. Anderson concluded that ants, termites, and small beetles were the princ.i.p.al foods. He noted that some of the beetles were of groups commonly found in ant colonies. Tanner reported that in a large number of the frogs which he collected in Douglas, Riley, Pottawatomie, and Geary counties, Kansas, the digestive tracts and feces contained only ants. Wood (1948: 226) reported an individual of _G. carolinensis_ in Tennessee found under a flat rock in the center of an ant nest.

Freiburg (_op. cit._: 383) reported on the stomach contents of 52 ant-eating frogs collected near the Reservation. Ants const.i.tuted nearly all these stomach contents, though remains of a few small beetles were found. The ants eaten were of two kinds, _Lasius interjectus_ and _Crematogaster_ sp. The latter was by far the more numerous.

Although I made no further study of stomach contents, the myrmecophagous habits of _Gastrophryne_ have come to my attention frequently in the course of routine field work. Individuals kept in confinement for a day or more almost invariably voided feces which consisted mainly or entirely of ant remains, chiefly the heads, as these are most resistant to digestion.

Often upon examining frogs I have found ants (_Crematogaster_ sp.) or their severed heads, attached with mandibles embedded in the skin. To have been attacked by ants, the frogs must have been in or beside the ants' burrow systems. Frequently the frogs that were uncovered beneath rocks were adjacent to cl.u.s.ters of ants or to their nests or travelways, in a position strategically located to feed upon them, as described by Tanner. Often the feces of the frogs were found in pitfalls or under flat rocks. Although these feces were not a.n.a.lyzed, they seemed to consist mainly or entirely of ant remains.

The species of _Crematogaster_, which is the chief food of _Gastrophryne_ in this region, is largely subterranean in habits, and is extremely abundant. Any flat rock in damp soil is likely to harbor a colony beneath it. Colonies are situated also in damp soil away from rocks, beneath almost any kind of debris, and in hollow weed stalks and decaying wood. Live-traps for small mammals, having nest boxes attached, almost always were occupied by colonies of _Crematogaster_, if they were left in the field in warm, humid weather. Occasionally the ants attacked and killed small mammals caught in such traps. Among the thousands of kinds of insects occurring on the Reservation, this ant is one of the most numerous in individuals, one of the most important on the basis of bioma.s.s and provides an abundant food source for those predators that are ant eaters. Food supply probably is not a limiting factor to populations of _Gastrophryne_ on the area.

PREDATION

Young copperheads are known to feed upon ant-eating frogs occasionally (Anderson, 1942: 216; Freiburg, 1951: 378). Other kinds of snakes supposedly eat them also. The common water snake (_Natrix sipedon_) and garter snake (_Thamnophis sirtalis_) probably take heavy toll of the adults at the time they are concentrated at the breeding pools. Larger salientians may be among the more important enemies of the breeding adults, the tadpoles, and the newly metamorphosed young. Bullfrogs (_Rana catesbeiana_) and leopard frogs (_Rana pipiens_) are normally abundant at the pond on the Reservation. These large voracious frogs lining the banks are quick to lunge at any moving object, and must take heavy toll of the much smaller ant-eating frogs that have to pa.s.s through their ranks to reach the water. The newly metamorphosed young often are forced to remain at a pond's edge for many days, or even for weeks, by drought and they must be subject to especially heavy predation by ranid frogs. Even the smallest newly metamorphosed bullfrogs and leopard frogs would be large enough to catch and eat them.

As a result of persistent drought conditions in 1952 and 1953, bullfrogs were completely eliminated from the pond by early 1954. Re-invasion by a few individuals occurred in the course of the summer; these probably made long overland trips from ponds or streams that had persisted through the drought. Leopard frogs reached the pond in somewhat larger numbers, but their population in 1954 was only a small percentage of that present in most other years. Notable success in the ant-eating frog's reproduction in 1954 may have been due largely to the scarcity of these large ranids at the breeding ponds.

Freiburg (_loc. cit._) noted that many of the ant-eating frogs he examined were scarred, and some had digits or limbs amputated. He did not speculate concerning the origin of these injuries. However, it seems likely that many or all of them were inflicted by the short-tailed shrew (_Blarina brevicauda_). Five-lined skinks living on the same area were likewise found to be scarred by bites which I identified (Fitch, 1954: 133) as bites of the short-tailed shrew. This shrew is common on the Reservation, especially in woodland. Many have been trapped in the pitfalls. On several occasions when a short-tailed shrew was caught in the same pitfall with ant-eating frogs, it was found to have killed and eaten them. Like the frogs, the shrews were most often caught in pitfalls just after heavy rains. Once in 1954 a shrew was found at the quarry in a pitfall that had been one of those most productive of frogs.

The bottom of the pitfall was strewn with the discarded remains (mostly feet and skins) of perhaps a dozen ant-eating frogs. All had been eaten during one night and the following morning, as the trap had been checked on the preceding day. On other occasions shrews caught in pitfalls with several frogs had killed and eaten some and left others unharmed.

SUMMARY

In northeastern Kansas the ant-eating frog, _Gastrophryne olivacea_, is one of the more common species of amphibians. This area is near the northern limits of the species, genus, and family. The species prefers a dry, rocky upland habitat often in open woods or at woodland edge where other kinds of salientians do not ordinarily occur. It is, however, tolerant of a wide variety of habitat conditions, and may occur in river flood plains or cultivated land. In these situations where surface rocks are absent, cracks and rodent burrows presumably furnish the subterranean shelter that it requires.

This frog is secretive and spends most of the time in subterranean shelter, obtaining its food there rather than in the open. Only on warm rainy nights is it inclined to venture into the open. Then, it moves about rapidly and with a scuttling gait, a combination of running and short hops. However, it may be flushed in daylight from a hiding place by the vibrations from footsteps of a person or an animal, or it may move about in the daytime when temperatures at night are too low for activity. Though not swift of foot, the frogs are elusive because of their tendency to keep under cover, their slippery dermal secretion, and the ease with which they find and enter holes, or crevices to escape.

Breeding occurs at any time from late May through August and is controlled by the distribution of rainfall. Heavy precipitation, especially rains of two inches or more, stimulates the frogs to migrate in large numbers to breeding ponds. Even though there are several well s.p.a.ced periods of unusually heavy rainfall in the course of a summer, each one initiates a new cycle of migration, mating and sp.a.w.ning. Heavy rainfall is a necessity, not only to ensure a water supply in temporary pools where the frogs breed, but to create the moist conditions they require for an overland migration. An individual male may migrate to a pond and breed at least twice in the same season. Whether or not the females do likewise is unknown. Amplexus and sp.a.w.ning occur mainly within a day or two after the frogs reach the ponds. The males call chiefly at night, but there may be daytime choruses when breeding activity is at its peak. Many males concentrate within a few square yards in the choruses and float upright usually beside or beneath a stem or leaf, or other shelter, rendering them extremely inconspicuous. The call is a bleat of three seconds duration, or a little more. In amplexus the members of a pair sometimes become glued together by their viscous dermal secretions. The eggs hatch in approximately 48 hours. The tadpoles metamorphose in as few as 24 days. Newly metamorphosed frogs are 15 to 16 mm. in length, or, rarely as small as 14.5 mm. They are thus much larger than newly metamorphosed _G. carolinensis_, which have been described as 10-12 mm. or even as small as 8.5 mm. The newly metamorphosed frogs disperse from the breeding ponds as soon as there is a heavy rain. The young grow a little more than one mm. in length per week. Those metamorphosed in early summer may attain minimum adult size before hibernation which begins in October. It seems that s.e.xual maturity is most often attained in the second season, at an age of one to two years.

_Gastrophryne_ belongs to a family that is primarily tropical in distribution, and frogs of this genus have much higher temperature thresholds than most other amphibians of northeastern Kansas, with a correspondingly short season of activity. For more than half the year, mid-October to early May the frogs are normally in hibernation. Body temperatures of active frogs ranged from 17.0 C. to 37.6 C., but more than two-thirds were within the relatively narrow range, 24.0 to 31.

Near the date of the first autumn frost the frogs disappear from the soil surface and from their usual shelters near the surface, presumably having retired into hibernation in deep holes and crevices.

The natural enemies include young of the copperhead. The bullfrog and leopard frog probably take heavy toll of both the adults and the newly metamorphosed young at the breeding ponds. Reproductive success of the ant-eating frogs was much greater in 1954 when these ranids were unusually scarce. The short-tailed shrew is an important enemy. On occasion it took heavy toll of frogs trapped in pitfalls, and many of the larger adults were scarred or mutilated from bites, probably of the shrew.

Each of several frogs was found consistently under the same rock for periods of weeks. The hundreds of other frogs that were marked were rarely found twice in any one spot. Usually an individual recaptured after weeks or months was still near the original site. In many instances the distance involved was only a few yards, but there is some evidence that home ranges may be as long as 400 feet in greatest diameter. Of those caught in two or more different years only 15 per cent were shown to have moved more than 400 feet. These few exceptionally long movements, up to 2000 feet, involve s.h.i.+fts in home range or migrations motivated by reproductive urge.

LITERATURE CITED

ANDERSON, P.

1942. Amphibians and reptiles of Jackson County, Missouri. Bull.

Chicago Acad. Sci., 6: 203-220.

ANDERSON, P. K.

1954. Studies in the ecology of the narrow-mouthed toad, Microhyla carolinensis carolinensis. Tulane Studies in Zool., 2: 15-46.

BLAIR, A. P.

1950. Note on Oklahoma microhylid frogs. Copeia, 1950: 152.

BOGERT, C. M.

1949. Thermoregulation in reptiles, a factor in evolution.

Evolution, 3: 195-211.

BRAGG, A. N.

1943. Observations on the ecology and natural history of Anura, XV.

The hylids and microhylids in Oklahoma. Great Basin Nat., 4: 62-80.

de CARVALHO, A. L.

1954. A preliminary synopsis of the genera of American microhylid frogs. Occas. Papers Mus. Zool. Univ. Michigan, no. 555: 19 pp., 1 pl.

DICE, L. R.

1923. Notes on the communities of vertebrates of Riley County, Kansas, with especial reference to the amphibians, reptiles and mammals. Ecology, 4: 40-53.

FITCH, H. S.

1954. Life history and ecology of the five-lined skink, Eumeces fasciatus. Univ. Kansas Publ. Mus. Nat. Hist., 8: 1-156.

FREIBURG, R. E.

1951. An ecological study of the narrow-mouthed toad (Microhyla) in northeastern Kansas. Trans. Kansas Acad. Sci., 54: 374-386.

HECHT, M. K., and MATALAS, B. L.

1946. A review of the Middle American toads of the genus Microhyla.

American Mus. Novitates, no. 1315: 1-21.

LOOMIS, R. B.

1945. Microhyla olivacea (Hallowell) in Nebraska. Herpetologica, 2: 211-212.

MITTLEMAN, M. B.

1950. Miscellaneous notes on some amphibians and reptiles from the southeastern United States. Herpetologica, 6: 20-24.

PARKER, H. W.