Fig. 22: Altitudinal distributions of Syrrhophus and Tomodactylus. Widths of the columns are proportional to the numbers of species at a given altitude; narrowest width equals one species.
Syrrhophus is closely allied to another Mexican leptodactylid genus, Tomodactylus, which was revised by Dixon (1957), who along with numerous other authors noted the close relationship between the two genera. There is an almost complete lack of sympatry between the two genera; in very few places in México do they coexist ([Fig. 21]). Tomodactylus has its greatest diversity in the Cordillera Volcánica and Sierra Madre del Sur, whereas Syrrhophus reaches its greatest diversity in the Sierra Madre Oriental and eastern foothills. The species of both genera are about the same size and presumably have similar requirements insofar as food, breeding sites, and habitat selection.
Four cases of intergeneric sympatry are known for the two genera: 1) the Chilpancingo region of Guerrero, 2) the lowlands of Colima and the mountains just inland in Jalisco, 3) the lowlands of central Nayarit, and 4) the Sierra Madre Occidental on the Durango-Sinaloan border. The apparent sympatry in the Chilpancingo region involves four species: S. pipilans, T. albolabris, T. dilatus, and T. nitidus. Of the four, T. dilatus appears to be completely allopatric in that it occurs at higher altitudes (above 2000 meters), whereas the other three occur below 1800 meters in the region (Davis and Dixon, 1965). In the Colima-Jalisco region, Tomodactylus tends to occur higher (Dixon and Webb, 1966) than some of the Syrrhophus, but one subspecies of Tomodactylus nitidus is a lowland frog, occurring sympatrically with the lowland Syrrhophus modestus. A similar situation is observed in Nayarit; the lowland Tomodactylus occurs sympatrically with the small Syrrhophus pallidus. In both cases the Syrrhophus is smaller than the Tomodactylus.
Frogs of the genus Syrrhophus tend to occur at lower elevations than do their close relatives of the genus Tomodactylus ([Fig. 22]). This generalization is complicated by the occurrence in the Sierra Madre Oriental in relatively high altitude Syrrhophus (up to 2000 m.) and the occurrence in Michoacán of low altitude Tomodactylus (to sea level). There are no Tomodactylus in the Sierra Madre Oriental, whereas the genus Syrrhophus is represented in the lowlands of western México (modestus group). Syrrhophus and Tomodactylus exhibit essentially parapatric distributions. The two genera as now composed can be characterized as low to moderate elevation frogs (Syrrhophus) and moderate to intermediate elevation frogs (Tomodactylus).
Literature Cited
Baird, S. F.
1859. Reptiles of the Boundary. United States and Mexican Boundary Survey, pp. 1-35, pls. 1-41.
Barbour, T.
1923. The reappearance of Batrachyla longipes. Proc. New England Zool. Club, 8:81-83.
Barbour, T., and A. Loveridge