It's a rainy, mid-monsoon night on the Andaman Islands in the Bay of Bengal, and Charles Darwin’s frogs (Minervarya charlesdarwini) are up to something strange. They're having sex in a completely unique way; upside-down in tree holes.
Toes wedged into the bark, the honey-brown female points her body head-down and, with the male still attached, lays her eggs on the surface walls of a tree hole or a buttress root mini-pond.
Once the eggs hatch, the tadpoles drop into the safety of the watery pool below. As each in-tree pond rarely holds more than a soup-bowl’s worth of liquid, a female lays only a few large eggs above each one.
But why mate upside down? To answer this, a joint Indian-US team spent 55 nights in the deep jungles of this remote volcanic archipelago.
After arduous observation, the scientists discovered that these strange sexual practices are not, as they first thought, due to the constraints of mating in very cramped conditions. Instead, it seems to be much more about the mating pair avoiding interference from other males, during those crucial moments when eggs are being laid, fertilised and stuck to the tree bark.
This is important because M. charlesdarwini males complete very vigorously for females.
Defending his pond, resident males have a special rival-repelling call. If that doesn’t work, they actively bite any potential interlopers.
Females, attracted to the resident male’s mate-summoning call, also face a difficult time as, before the upside-down position is adopted, other males attempt to push themselves between a mating pair.
''Frogs have the greatest array of reproductive strategies of all limbed vertebrates” says Harvard University’s Radcliffe Fellow, S. D. Biju, who led the study.
“But, of all 7,708 species of frogs known to science, M. charlesdarwini is the only one known to breed in this way, both using tree holes and an upside-down egg-laying posture”.
As to why the frogs choose tree holes at all, the scientists point to the fact that these form safe havens, free from both fish and other potential predators, and from the risk of strong currents washing away the vulnerable, weak-swimming, tadpoles.
Words: Adrian Barnett
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