North America is often divided between the United States, with areas south being referred to as Middle America or Central America. However, the landmasses are connected and migration of species is very possible. We continue to look at comparisons and differences among the families of frogs found in the northern New World.

Hylidae

Hylidae, or the Treefrogs, are fairly well represented in North America, but reach a much higher diversity in Central America. Treefrogs spend much of their life in trees and may descend to ponds for breeding, although this is observed more readily in North America. The utilization of ponds may be attributed to the presence of both permanent and vernal pools during the spring and early summer when North American Treefrogs are breeding. In Middle America, Treefrogs utilize many breeding locations including streams, puddles, permanent and temporary pools, and phytotomata or tree holes. Despite other frogs sharing the trees with them (Centrolenidae), the differences in breeding strategies (Centrolenids lay their eggs on leaves where they eventually hatch and fall into slow moving streams below) have likely released them from much competition, allowing Hylids to also radiate and thrive in Middle America.

Bufonidae

Bufonidae, the True Toads, are slightly more speciose in Middle America. Some of these frogs are brightly colored, advertising against predation because of toxins. All toads contain toxins in their parotid glands, but many in North America are brown and drab in color, choosing cryptic protection first. However, high levels of toxins in the skin and bright coloration may allow the toads to move more freely into other habitats and fulfill more niches. This may have allowed for a diversification of toads in Middle America that is greater than in North America.

Leptodactylidae

Leptodactylidae is an incredibly diverse family and includes the largest genus in the world, Eleutherodactylus (recently split into Eleutherodactylus and Craugaster but will be considered here as one taxon group). Poorly represented in North America (barely entering the southern reaches), this family exploded in number in Middle America after moving north from South America. The earliest fossil frog found in Middle American belongs to the Eleutherodactylus group and this long history has allowed for much diversification. All members of the Eleutherodactylus group have direct development reproductive strategies; eggs hatch directly into small froglets with the tadpole stage completely skipped. This has released the frogs from a dependency on standing water (moisture is still required to prevent desiccation of the eggs and frogs) and allowed them to move into many habitats. The Eleutherodactylus group can frequently be found a meter or more off the ground in the forest while toads (Bufonidae), which are able to travel further from water sources due to a thicker, more protective skin, are found more commonly on the ground. This may have resulted from interspecific competition where one species released to moving up or down to avoid competition with the other.

Ranidae

Finally, Ranidae shows a reversal in diversity compared to other families in common between the two regions, with a greater diversity in North America. Ranids lay their eggs in relatively permanent ponds and tadpoles, at times taking two or three seasons, will mature into froglets in this environment. In Middle America, such ponds are often not available as dry seasons do not facilitate the year-round existence of sufficient water sources. Rana warszewitschii, which is found in lower Middle America, breeds in mountain streams, laying eggs in pools along the side of streams and tadpoles can often be found among the leaf litter in such pools. However, as mentioned above, these ponds are often occupied by the tadpoles of Centrolenidae, which may out-compete the Ranids for resources.

Through the past two blogs we have looked at the families of frogs that reside in North America and Middle America, which technically is part of North America. We have seen that most families of frogs are represented in both areas, but numbers and lifestyles vary, likely being linked to climate and available habitat.

Photo credit: Julie M. Ray