Red eyed green frog (Agalycnis callidryas) mating is an event visible to few visitors of the Neotropical forests. These frogs live high up in the forest canopy and only come down to swamps to breed in large numbers. The economy of nature limits reproductive effort of females. Egg production is costly compared to sperm therefore not all females can be ready to mate when the mating time comes. There’s always less females in a given mating aggregation and males put up a big display and even fight to secure a mate. The economy of nature restricts female reproduction. However, this can change in other species whose sex roles have reversed such as the seahorses and pipefishes where males raise babies in their brood pouches.
In frog life cycle tadpole stage is the most vulnerable. There are many predators in and out of the water. Red eyed green frogs evolved a different survival strategy from other frog species: They lay their eggs on vegetation hanging over the water. If a predator attacks eggs, tadpoles can escape by hatching prematurely and throw themselves into the water below. This early hatching ability is an example of phenotypic plasticity. The video clips below show two examples of induced hatching as an anti-predatory behavior.
The following two videos have been recorded by Karen Warkentin of Boston University showing how red eyed green frog tadpoles hatch prematurely when they sense the threat of two different predators by vibrations on the leaf.
Predation by wasp:
Predation by snake:
So here are the existential paths a female frog could follow shaped by natural selection: Larger males will sire larger tadpoles who can metamorphosize quickly during the tadpole stage and escape aquatic predators by crawling into the land. On the other hand, it is rather difficult to carry a large male on her back. Amplexus can last for many days. Climbing onto vegetation to lay eggs without loosing balance can be rather difficult while carrying a large male. Females choose an optimal sized male within these constraints.
External fertilization has these kinds of problems. Male clings onto female until she’s ready to oviposit. Males want to maximize their fatherhood by being the only male to fertilize the eggs. Variation of the same theme is seen in many species using external fertilization mode of reproduction including horseshoe crabs and fireworms.
Of course there’s a reason why red eyed green frogs lay their eggs outside the water. Lakes and ponds are full of nasty aquatic predators. When tadpoles escape a predator through induced hatching into the water body they may face even more deadly predators like the notoriously voracious larvae of dragonflies:
Dragonfly newt larvae predation:
Dragonfly nymph hunting fish: