Size and Proportions.—Frogs of the genus Osteocephalus are moderate to large hylids. The largest species is taurinus, attaining a snout-vent length of 103.1 mm; the smallest is pearsoni, which attains a length of 54.7 mm. Considerable intraspecific geographic variation occurs in adult size, especially in taurinus. Females of all species attain a noticeably larger size than males, but no significant differences are apparent in proportions ([Table 2]).
Table 2.—Comparison of Size and Proportions in the Species of Osteocephalus.
(Means are given in parentheses below observed ranges)
| Species | N | Snout-vent Length | Tibia Length/ S-V L | Foot Length/ S-V L | Head Length/ S-V L | Head Width/ S-V L | Tympanum/ Eye | |
| O. buckleyi | ♂ | 30 | 37.9-48.1 | 0.478-0.580 | 0.375-0.444 | 0.319-0.357 | 0.329-0.368 | 0.608-0.820 |
| (43.3) | (0.520) | (0.408) | (0.343) | (0.351) | (0.711) | |||
| ♀ | 31 | 48.6-75.1 | 0.476-0.599 | 0.363-0.469 | 0.310-0.358 | 0.318-0.367 | 0.574-0.905 | |
| (61.7) | (0.553) | (0.428) | (0.333) | (0.348) | (0.734) | |||
| O. leprieurii | ♂ | 21 | 41.2-48.4 | 0.514-0.571 | 0.383-0.430 | 0.308-0.357 | 0.326-0.368 | 0.652-0.884 |
| (44.7) | (0.538) | (0.408) | (0.335) | (0.348) | (0.777) | |||
| ♀ | 21 | 46.6-61.5 | 0.516-0.592 | 0.382-0.453 | 0.314-0.343 | 0.328-0.363 | 0.698-0.909 | |
| (57.1) | (0.539) | (0.404) | (0.329) | (0.349) | (0.785) | |||
| O. pearsoni | ♂ | 2 | 45.3-46.2 | 0.481-0.504 | 0.404-0.437 | 0.322-0.335 | 0.327-0.342 | 0.660-0.673 |
| (45.8) | (0.493) | (0.421) | (0.329) | (0.335) | (0.666) | |||
| ♀ | 1 | 54.7 | 0.521 | 0.405 | 0.318 | 0.346 | 0.862 | |
| O. taurinus | ♂ | 59 | 40.3-84.6 | 0.512-0.576 | 0.387-0.445 | 0.296-0.345 | 0.301-0.355 | 0.638-0.896 |
| (66.3) | (0.541) | (0.416) | (0.318) | (0.324) | (0.752) | |||
| ♀ | 45 | 45.1-103.1 | 0.520-0.577 | 0.391-0.448 | 0.306-0.334 | 0.308-0.347 | 0.640-0.817 | |
| (75.8) | (0.542) | (0.420) | (0.321) | (0.327) | (0.758) | |||
| O. verrucigerus | ♂ | 11 | 50.4-54.3 | 0.494-0.552 | 0.409-0.442 | 0.322-0.346 | 0.328-0.344 | 0.623-0.804 |
| (53.0) | (0.519) | (0.427) | (0.333) | (0.337) | (0.730) | |||
| ♀ | 3 | 63.1-65.8 | 0.532-0.561 | 0.435-0.463 | 0.345-0.347 | 0.348-0.379 | 0.692-0.808 | |
| (64.5) | (0.545) | (0.448) | (0.346) | (0.358) | (0.731) | |||
Coloration.—All Osteocephalus are predominantly brown frogs usually with some darker dorsal markings (Figs. 1 and 2). Osteocephalus verrucigerus has a nearly uniform dark brown dorsum and no distinct transverse bars on the limbs, whereas all of the other species have distinct bars on the limbs. The dorsal markings on the body consist of irregular blotches in buckleyi, pearsoni, and taurinus but are narrow transverse marks in leprieurii. A narrow middorsal cream or yellow stripe is present in some individuals of buckleyi and taurinus but absent in all individuals of the other species. The flanks are uniform pale tan in leprieurii and uniform reddish brown in verrucigerus; in the other species the flanks are cream to brown with dark brown or black spots (also dark diagonal marks in some buckleyi). A creamy white anal stripe is present in some specimens of leprieurii but absent in all individuals of other species.
The postocular region, encompassing the tympanum, is dark brown in most specimens. In adults of pearsoni and taurinus the upper lips are dark brown. A pale cream or tan suborbital spot is present in pearsoni and in some taurinus; in some specimens of taurinus the suborbital spot is expanded posteriorly forming a labial stripe on the posterior part of the lip. The labial markings of verrucigerus are similar to the latter pattern, except that in females a distinct, light labial stripe extends the length of the lip. Osteocephalus leprieurii has a distinct, broad, pale labial stripe. The lips are barred cream and dark brown in buckleyi.
The venter is uniform creamy white or pale tan in leprieurii, uniform white in some buckleyi (most males), and uniform tan in some taurinus. The other species and some individuals of taurinus and buckleyi (most females) have dark ventral markings. These markings are most distinctive in verrucigerus, in which the venter is white with bold black mottling and spots ([Fig. 3c]). Those individuals of taurinus having ventral markings usually have indistinct, diffuse brown spots on the throat and chest ([Fig. 3b]). Osteocephalus pearsoni is characterized by a fine brown reticulation on the venter and on the anterior and posterior surfaces of the thighs in adults ([Fig. 3a]). Individuals of buckleyi that have ventral markings vary between the patterns illustrated for pearsoni and taurinus ([Figs. 3b and c]).
Ontogenetic change in coloration is slight or non-existent in buckleyi, pearsoni, and taurinus, except that juveniles lack ventral markings. A dark blotch on the back and distinct transverse bars on the limbs are evident in juveniles of verrucigerus; these markings are obscured in the adults. Juveniles of leprieurii are olive-brown with yellow dorsolateral stripes; the transverse dark marks, characteristic of the adults, appear before the stripes are lost.