Frog Fathers Provide Transport, Piggyback Style
Bijal P. Trivedi
National Geographic Today
|August 7, 2002|
Absentee dads are common in the animal kingdommost sow their seed
and leave without further ado. But in remote tropical rain forests of
Papua New Guineawhere biodiversity literally drips from the
branches and new species abounda researcher has discovered some
frog fathers that not only guard the eggs but also provide transport,
piggyback style, for up to 24 froglets after they emerge from the shell.
The two species who practice piggyback paternal careLiophryne schlaginhaufeni and Sphenophryne cornutabelong to a family of frogs called microhylids.
"The microhylid frogs of New Guinea are the only known large group of terrestrial vertebrates in which male care predominates," wrote David Bickford, an evolutionary biologist at the University of Miami in Coral Gables, Florida, in a report published in the August 8th issue of the journal Nature.
Microhylid frogs of Papua New Guinea, of which there are at least 150 species, all undergo "direct development." That is, they bypass the tadpole stage and develop from larvae to miniature versions of the adult inside the egg. Direct development is a key adaptation that allows the frog to reproduce in regions without bodies of water nearby.
After the mother lays the eggs in the leaf litter on the forest floor, she is long gone. The father dutifully fertilizes the eggs. Then, like a mother hen, he shelters the clutch, warding off predators and hydrating the eggs for about a month until they're ready to hatch.
As the tiny froglets emerge from their shells, usually a brood of up to 25, each climbs onto the father's back. He waits patiently until every last froglet is aboard.
Then the father departs with his brood on his back, like an overloaded school bus, for an odyssey that lasts up to nine days.
Traveling only after dark, he hops as far as 50 feet or so per night. Along the way, one by one, the froglets leap off to begin their own independent lives.
"This is really fascinating because, though males are known to guard eggs, they have never been known to transport the froglets," said Janalee P. Caldwell, specialist in amphibian biology at the University of Oklahoma. "What makes this particularly interesting is that the fathers are transporting froglets that are fully developed and completely independent at this point."
"One explanation for the father's behavior may be that he wants to be sure that the progeny are his, and guards the clutch to prevent other males from fertilizing the eggs," Bickford hypothesized. The advantage of this system is that froglets are dispersed over a broad area, reducing the chance of future inbreeding and competition for food, said Bickford.
"It is an interesting approach to ensure survival," said Darrel Frost, curator in the department of herpetology at the American Museum of Natural History in New York.
Tales of these "frog transport events" have been reported from Papua New Guinea, but neither the sex nor the species were documented.
Bickford discovered this unusual form of parental care via daily surveys of the rain forest floor during a four-year stay in the Chimbu Province of Papua New Guinea, where he trained the local Pawaia people as research assistants. Here he combed plots of damp leaf litter on his hands and knees searching for frogs. When he discovered a vigilant frog father hugging his clutch, he marked the area and returned at night when the frogs are most active. But things became most interesting when the eggs hatched and the father began to "transport."
A couple of hours before sunset, Bickford or a colleague would return and mount a "24/7 vigil." Remaining about ten feet (three meters) behind, the researcher would follow the father by candlelight or with a dim red flashlight, noting how far he traveled each night and when each froglet leapt to independence.
"It was really tough, frustrating work," said Bickford. "We lost a lot of frogs with our do-not-disturb attitude."
Male Parental Care Is Rare
The world of frogs harbors a wide spectrum of parental care.
There are some species of poisonous frogs in South America where the males transport tadpoles, says Caldwell. "The males crouch down in the leaf litter next to the hatching eggs and the tadpoles wriggle up onto the father's back and he transports them to water."
Mothers of the Jamaican cave frog speciesEleutherodactylus cundallicarry their froglets from the cave into the rain forest. It is the only known example of female froglet transport.
Male parental care is rare. There are isolated examples in nature: the American burying beetle watches its larvae, and male fish do the majority of egg guarding and cleaning. Male seahorses take parental care to an extreme, assuming all responsibility for the pregnancy and actually carrying the eggs around in a chemically controlled pouch until they hatch.
Bickford anticipates that weighing the costs and benefits of such varied parenting styles will reveal exactly how such systems evolved.
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