Despite their peculiar mating method characterizing Amphibians, frogs have only 4740 species worldwide with a presence in every continent except Antarctica. However, their unique mating process does not permit male penetration to female genitalia.
This has led the male species to adopt a reproductive behavior known as amplexus. It involves the male frog clinging to its female counterpart to fertilize her eggs externally.
The males will have to grasp the female before the specific egg laying moment, which may lead the couples to pair for long periods amounting to hours, days, and in some rare cases months, depending on the species. Here is some more information on why frogs stack upon each other.
The mystery of Amplexus
While the most common method involves the male, using its forelimbs to grab its counterpart around the torso, known as inguinal amplexus. The other known amplexus method is axillary amplexus, where the male grasp just behind the females’ forelimbs.
Despite the copulating embrace, there are challenges encountered by the pair. The female finds it hectic having to navigate while carrying an equivalent weight to its own, more so while trying to feed. The males have also to wade off the unpaired males attempting to dislodge them from their intimacy.
This can be hard, especially in the case of the Columbian Spotted Frogs, which engage in a ferocious scramble to assume the grasping position. This has led to morphological adaptation that eases the amplexus process.
Morphological changes
The amphibian skin, including frogs, is very slippery, making it hard for the male species to hang on, especially when battling to stay on top. To adapt to this, male frogs have flexible forearm structures that aid in gripping females, like enlarged thumbs or swollen forearms.
This anatomy can be identified on pickerel frogs, leopard frogs, and American toads. We can also use these structures to determine the sex of the frog. Further studies identified that successful frogs had enlarged nuptial pads and thicker forelimbs.
In Ascaphus truei, grows a tail that aids in copulating, becoming one of the rare frog species that achieves internal fertilization. The Coqui female in Puerto Rico locks the male legs with her legs to aid internal fertilization after prolonged inactivity. The males are exempted from grasping the female.
Reproduction Process
The mating process is initiated with the male frogs making loud advertisement calls to attract the females. To please the females, the dominant male is identified by making mating calls that last longer and are more frequent or nest-building ability.
The females then approach the males for breeding, and the males will have to release their sperm by grasping at the female’s backs. Some situations are dire where males form a breeding ball surrounding the female, which may drown the female.
The males have to release the sperm as the female deposits her eggs to fertilize the eggs. The humorous part of this process is the male’s impaired vision, which may lead them to grasp on decomposing females, plastic cups, and other inanimate objects, different species leading to hybrid and, in other cases, grasp on other males.
In this instance, the bottom frog then lets a release call that identifies it as a male, and the grasping male has to release and continue his search for a female. The copulating position facilitates the success of reproduction. The embrace position is also unique with specific species to discourage crossbreeding.
