BERKELEY — UC Berkeley herpetologist
Jim McGuire was slogging through the rain forests of Indonesia’s
Sulawesi Island one night this past summer when he grabbed what he
thought was a male frog and found himself juggling not only a frog but
also dozens of slippery, newborn tadpoles. The
newly described fanged frog Limnonectes larvaepartus (male, left, and
female) on the Indonesian island of Sulawesi. Jim McGuire photos.
He had found what he was looking for: direct proof that the female of
a new species of frog does what no other frog does. It gives birth to
live tadpoles instead of laying eggs.
A member of the Asian group of fanged frogs, the new species was
discovered a few decades ago by Indonesian researcher Djoko Iskandar,
McGuire’s colleague, and was thought to give direct birth to tadpoles,
though the frog’s mating and an actual birth had never been observed
before.
“Almost all frogs in the world – more than 6,000 species – have
external fertilization, where the male grips the female in amplexus and
releases sperm as the eggs are released by the female,” McGuire said.
“But there are lots of weird modifications to this standard mode of
mating. This new frog is one of only 10 or 12 species that has evolved
internal fertilization, and of those, it is the only one that gives
birth to tadpoles as opposed to froglets or laying fertilized eggs.” Newborn tadpole (top and bottom views) of the newly described fanged frog.
Iskander, McGuire and Ben Evans of McMaster University in Ontario, Canada, named the species Limnonectes larvaepartus and fully describe it in this week’s issue of the journal PLOS ONE. External vs. internal fertilization
Frogs have evolved an amazing variety of reproductive methods, says
McGuire, an associate professor of integrative biology and curator of
herpetology at UC Berkeley’s Museum of Vertebrate Zoology. Most male
frogs fertilize eggs after the female lays them. About a dozen species,
including California’s tailed frogs, have evolved ways to fertilize eggs
inside the female’s body. However, the mechanisms of internal
fertilization are poorly understood in all but California’s two species
of tailed frogs, the latter of which have evolved a penis-like organ
(the “tail”) that facilitates sperm transfer. Whereas the tailed frogs
deposit their fertilized eggs under rocks in streams, the other frogs
previously known to have internal fertilization give birth to froglets –
miniature replicas of the adults.
Although internal fertilization is extremely rare among frogs, there
are many other bizarre reproductive variations. Some frogs carry eggs in
pouches on their back, brood tadpoles in their vocal sac or mouth, or
transport tadpoles in pits on their back. The two known species of
female gastric brooding frogs, both of which are now extinct, were
famous for swallowing their fertilized eggs, brooding them in their
stomach, and giving birth out of their mouths to froglets. Two genera in
Africa engage in internal fertilization and give birth to froglets
without going through a free-living tadpole stage. A
male of the species L. larvaepartus sits next to a pool containing
tadpoles (yellow circle), and may be guarding them, a typical male
behavior in some frog species.
Fanged frogs – so-called because of two fang-like projections from
the lower jaw that are used in fighting – may have evolved into as many
as 25 species on Sulawesi, though L. larvaepartus is only the
fourth to be formally described. They range in size from 2-3 grams – the
weight of a couple of paper clips – to 900 grams, or two pounds. L. larvaepartus is in the 5-6 gram range, McGuire said.
The new species seems to prefer to give birth to tadpoles in small
pools or seeps located away from streams, possibly to avoid the heftier
fanged frogs hanging out around the stream. There is some evidence the
males may also guard the tadpoles. Sulawesi a biodiversity hotspot
McGuire first encountered the newly described frog in 1998, the year he
began studying the amazing diversity of reptiles and amphibians on
Sulawesi, an Indonesian island east of Borneo and south of the
Philippines. The island is a geographical hodgepodge, having formed from
the merger of several islands about 8-10 million years ago.
“Sulawesi is an incredible place from the standpoint of species
diversity endemic to the island as well as in situ diversification,” he
said, noting that most places on the island are home to at least five
species of fanged frogs living side by side.
Although many vertebrate species have diversified on the island after
arriving by overwater “sweepstakes” dispersal, most – such as the
flying lizards and black-crested macaque monkeys – have speciated in
such a way that their geographic ranges are non-overlapping, with their
ranges meeting like pieces in a jigsaw puzzle. The fanged frogs are
special, McGuire says, because they appear to represent a virtually
unexplored adaptive radiation with many species occurring at the same
sites but adapted to occupy distinct ecological niches. While
searching for frogs at Gunung (Mount) Balease in the central core of
Sulawesi, McGuire came across a green pit viper. Sean Reilly photo.
“We are really interested in understanding how much of Sulawesi’s in
situ diversification was initiated on the paleo-islands, or if much or
even all of the diversification was postmerger,” he said.
Much of McGuire’s work to date has been with the simpler non-adaptive
radiations of the flying lizards and macaques. Fanged frogs present an
even more exciting challenge, he says, because their diversification
likely was influenced not only by the dynamic tectonics of Sulawesi, but
also by adaptive radiation via ecological diversification.
McGuire and his colleagues and students have collected reptiles and
amphibians throughout the island – flying lizards are his particular
love – and taken genetic samples to reconstruct the evolution of species
over time and perhaps shed light on how and when the islands came
together.
He also is working with Iskandar to prepare a monograph on the
identification, distribution and biology of the fanged frogs on the
island.
McGuire’s research is supported by the National Science Foundation (DEB 0328700, DEB 0640967).
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