Spikethumb frog

Male spikethumb frogs of some species, such as P. guatemalensis, are often found calling from the crevices between rocks of streams. They utilize the same location along a stream for extended periods of time, where they can call for females, mate with females, and guard their fertilized eggs. The vocalizations of males have been recorded,[3] however further research is required to determine whether vocalizations are used for displaying aggression towards nearby males.[7] Vocalization is not universal amongst males in the genus however, as P. Hartwegi lack vocal sacs and vocal slits.[8]

Research suggests that males engage in male-male combat to maintain control of their mating sites, located within the crevices between rocks, as mating sites are likely a limiting resource.[7] The prepollex may be used during this combat, leaving scars on the head and forelimbs of males. These scars are not observed on females.[5]

Spikethumb frogs reproduce in streams using axillary amplexus, a process that can take hours or days.[5][8] Male spikethumb frogs display two secondary sexual characteristics that are used during reproduction. These include swollen upper lips and long premaxillary and maxillary teeth that protrude down beyond the lip.[9] The upper lip of females contains ordinary mucous glands, and these differ from the glands in the male's upper lip, which largely contains specialized mucous glands, also called breeding glands.[8] Ordinary glands are often found equally distributed throughout the body, which differ from specialized glands that are specifically located, often to perform a specific function.[10]

In Plectrohyla, the specialized mucous glands in the lips of males produce sodefrin precursor-like factors (SPF), which function as allohormones.[8] Allohormones are substances that are directly transmitted from one individual to another, circumventing the special senses, and can have various effects on copulation, including increased female receptivity, inhibition of remating, and altered sperm storage in females.[11] Male spikethumb frogs deliver allohormones via tramautic mating, in which the male presses its swollen upper lip and protruding premaxillary and maxillary teeth into the back of the female, leaving scratches on the females back.[5] These scratches allow for direct transmission of the SPF allohormone into the bloodstream of the female, preventing loss to the environment.[8] These differ from pheromones, which are released into the environment and detected via the special senses.[12] Upon transmission into the bloodstream, SPF allohormones function by shortening courtship duration, however the exact effect of SPF allohormones on the physiological and behavioral changes to the female have not been determined.[8] Shortened courtship duration reduces risks of predation during copulation, increasing fitness.[8]

This copulatory mechanism resembles the condition of the plethodontid salamander, in which males use their elongated teeth to scratch the backs of females to deliver allohormones directly from the mental glands on their chin.[5] These allohormones increase female receptivity, which results in increased probability that the female will accept the male's sperm.[11]