Glassfrog eggs, laid on leaves hanging over streams in tropical rainforests, are tasty snacks for snakes, insects, and other predators until they hatch and drop into the streams to begin life as tadpoles. Biologists had thought the eggs of most glassfrog species were on their own during this vulnerable stage, without any help from mom or dad.
But walking along a stream in Panama late one night, Jesse Delia (GRS’18) spotted a glassfrog mother sitting on her clutch of eggs.
Delia, a PhD student in the lab of Karen Warkentin, a professor of biology, went on to observe nighttime parental behavior among 40 species of glassfrogs. Spending a total of 18 months walking up and down South and Central American streams every night, Delia found that in many species, glassfrog mothers brood their eggs during the night the eggs are fertilized, and that this care improves the survival of the eggs. In almost a third of species, glassfrog fathers stay on guard for much longer periods.
The research, which Delia completed with Laura Bravo-Valencia, a graduate student at the Universidad de los Andes in Bogotá, Colombia, was published in the Journal of Evolutionary Biology in March 2017.
Jesse Delia (GRS'18) during fieldwork. Photo by Laura Bravo-Valencia
The discoveries were based on “a tour de force of extreme fieldwork,” says Warkentin. Delia and Bravo-Valencia pursued their project over six rainy seasons at 22 sites along streams in Colombia, Ecuador, Mexico, Panama, and Peru. The project also included field monitoring of parental behavior throughout the duration of embryonic development for 13 species.
Some of these streams were in warm lowland forests and others up in the Andes mountains. “In Colombia, we would take buses…into the mountains and try to find somebody who would put us up, somewhere close to a forest, a couple hours’ hike into the stream,” says Delia. “Streams in the Andes are really steep, with impassable waterfalls every so often, and in many sites, they are cascades of freezing cold water.”
Delia and Bravo-Valencia also performed experiments on two species of frogs in Panama. Monitoring the eggs daily until they hatched, which could take almost 20 days, the researchers found the eggs given maternal care survived significantly better than those that did not receive care.
The key to this survival was that the frogs were soaking up water from damp spots on leaves and delivering it to the eggs. The jelly surrounding the eggs would swell up with water and grow about four times thicker. Offering these swollen egg packages to katydids, crickets that prey on frog eggs, the biologists saw the predators become frustrated. “Each embryo is surrounded by this protective layer of jelly, so when the katydid bites, it’s getting mouthfuls of jelly, and it generally gives up,” says Warkentin.
Bravo-Valencia and Delia also found the glassfrog mothers were dedicated guardians in the first few crucial hours, refusing to abandon their posts, even when gently pinched or poked.
“These are relatively well-studied, charismatic frogs, yet we were fundamentally wrong about the reproductive behavior of most glassfrog species,” Warkentin says. “In glassfrogs, maternal care helps embryos survive, but they seem to do the bare minimum. It seems that fathers not only took over the job, when mothers were already doing it, but they also greatly elaborated the amount of care. Even after eggs have started hatching, fathers keep caring.”
“In many animals, mothers are on duty when offspring need intensive care, whereas fathers care when it’s easy or help out when more is needed. This common pattern has influenced how we think about parental sex-roles,” Delia says. “Glassfrogs do the complete opposite: moms do the minimum—at least time-wise—while fathers go to extremes.”