Differences in size between species and geographic variation in size in Gastrophryne have been given little attention by herpetologists, but if understood, would help to clarify relationships. Hecht
and Matalas stated in their revision (1946: 5) that size is of no importance as a taxonomic character, as typical carolinensis, olivacea, and mazatlanensis all averaged approximately the same—26 to 28 mm.—females slightly larger than males. However, they arbitrarily classed as adults all individuals 22.5 mm. in length or larger, having found individuals this small that showed the darkened and distensible throat pouches characteristic of adult males. From the trend of my own measurements of G. olivacea in northeastern Kansas, I conclude that either many immature individuals were included in their samples, or that the populations sampled included some with individuals that were remarkably small as adults.
The population which I studied may be considered typical of G. olivacea. They averaged large, including individuals up to 42 mm. in length, well above the maximum sizes for any reported in the literature. At metamorphosis these olivacea are of approximately 50 percent greater length than G. carolinensis as reported by Wright and Wright (1949: 573) and Anderson (1954: 41). Yet Blair (1950: 152) observed that in eastern Oklahoma, where the
ranges of olivacea and carolinensis overlap, the latter is larger. On the basis of field and laboratory observations he tentatively concluded that one of the main barriers to interbreeding was the reluctance of the males of carolinensis to clasp the smaller females of olivacea.
That size differs in different populations, and is still poorly understood, is illustrated by the following discrepant figures from various authors.
Table 4. Size Range of Adults in Various Populations of Gastrophryne.
| Species or subspecies | Geographic population sampled | Authority | Size range of adults in mm. |
|---|---|---|---|
| olivacea | Douglas Co., Kansas | present study | 31 to 42 |
| olivacea | entire range | Wright and Wright (1949) | 19 to 38 |
| carolinensis | entire range | Wright and Wright (1949) | 20 to 36 |
| carolinensis | southern Louisiana | Anderson (1954) | 22 to 35 |
| areolata | southeastern Texas | Wright and Wright (1949) | 23 to 29 |
| mazatlanensis | Arizona and New Mexico | Wright and Wright (1949) | 22 to 30 |
| mazatlanensis | Santa Cruz Co., Arizona | Stebbins (1951) | 25.2 to 31.5 |