About Engystomops petersi Jiménez de la Espada, 1872
Engystomops petersi (described by Jiménez de la Espada in 1872) are relatively small frogs. Males measure 21–31 mm (0.83–1.22 in) in snout–vent length, while females measure 25–39 mm (0.98–1.54 in). Their dorsal coloration is variable. The dorsal skin is warty, bearing small tubercles with scattered larger tubercles. This species lacks maxillary and premaxillary teeth. It also has T-shaped terminal phalanges and two generally visible parotoid salivary glands. The hands and feet have bulbous fingers, and the first finger is longer than the second finger. Overall body build ranges from slender to stocky. This species can be distinguished from other similar frogs by its tympanum (eardrum) trait: some females have concealed tympani, while males have prominent tympani. The larval morphology of E. petersi has been thoroughly studied and compared to other members of the genus Engystomops. E. petersi larvae are generally the largest among all Engystomops larvae. Larval structure consists of the body, snout, oral disc, vent tube, tail, dorsal fin, ventral fin, and other organs. Total larval length is usually around 22.7 mm, with body length making up slightly less than half of the total length. It has a broad, wide, rounded snout, with nostrils that have slightly discolored outer rims. Its oral disc is filled with small papillae arranged in rows, with papillae varying in tip length and shape. The jaw is dark-colored and marked with serrations that follow an arc-shaped pattern. E. petersi tadpoles have a vent tube attached to the body's ventral fin. Their tails make up approximately 60% of total length, and have a narrowly rounded tip. A distinguishing feature of E. petersi larvae compared to other Engystomops species is the presence of prominent elliptical paravertebral glands. Only the preserved larval coloration of this species has been reported. In preservative, larval coloration ranges from mainly dark to light brown, with a visible spottty mesh pattern and speckles across certain body regions. The tail is light brown with uniform speckles, and the fins are translucent with threadlike markings. Engystomops petersi is a common species that inhabits primary forests, secondary forests, and forest edges. It is a terrestrial, nocturnal frog that is often found in leaf litter and prefers humid environments. Though terrestrial, it breeds near water bodies, specifically along the edges of lakes, ponds, and pools. Males produce mating calls while floating on their backs in the water. The genus Engystomops is a frog lineage found in the Andean foothills of Ecuador, Peru, and Bolivia, extending into the Amazon basin of Brazil. Speciation events within the genus are well studied, and are typically defined through research on mitochondrial DNA, mating calls, and cytogenetic variation among regional frogs. Engystomops petersi forms the northwestern clade of the genus Engystomops, and is recognized as a distinct species found in northern Peru and Ecuador. Puyo, Ecuador is a central location for E. petersi, and this population has been used for comparative karyotypic analysis with other regional frog species. Its karyotype is unique compared to other Engystomops species in the area, and it is especially distinct from the southwestern clade E. freibergi, which is found in southern Peru and Brazil. The breeding period of E. petersi coincides with the rainy season. The male call consists of a prefix and a "whine" component, with an additional third "squawk" component present only in some populations. Eggs are laid in foam nests.
