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e. bicollor
e. curtipes
edalorhina-perezi
e-crenunguis
e-choloronotus
e-croceoinguinis
bufo-cognatus
eleuth-lepto
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Before looking at frogs in detail, I wish to introduce you to the other
two groups of amphibians. Salamanders have long tails and four relatively
short legs (except for one North American family which has only front
legs). The two pairs of limbs are well separated (markedly so in elongate
salamanders). They have large protruding eyes but lack ears. Most people,
unfamiliar with them, mistake them for some small lizard. Like other amphibians,
salamanders remain in wet places and several groups have taken up permanent
existence in the water, even to the point of retaining their gills as
adults and never developing a tongue. Their need for a wet environment
largely explains the fact that most salamanders are either nocturnal (when
humidy rises as the temperature falls) or burrow in the soil. Salamanders
are very well-represented in North America and to a much lesser extent
in Eurasia. Tropical America, especially Central America, is also richly
endowed with salamanders. For all its richness in amphibians, Colombia
has a very modest salamander fauna (certainly fewer than 30 species).
Salamanders are mostly small vertebrates, ranging from 50 mm total length
to the Giant Salamanders of China and Japan which can exceed a meter in
length.The other group of amphibians is the Gymnophiona or caecilians
(neither the technical name nor the common name is very commonly heard).
These are tropical creatures. In the Americas they are distributed from
Mexico to Uruguay. They also occur in subsaharan Africa, India, southeast
Asia, the Philippines and Indonesia, and on the Seychelles Islands in
the middle of the Indian Ocean. Caecialians are peculiar for several reasons.
They are greatly elongated animals lacking any evidence of once having
had limbs and look very much like large (to very large) earthworms or
eels. Only three families of caecilians have tails and those tails are
very small (short) structures. The eyes are small and usually not visible
externally (in some they are beneath the bones of the head). Most caecilians
burrow, coming to the surface infrequently (at night and during very heavy
rains) but an American family is nearly completely aquatic, found in lakes
and rivers over much of South America. Perhaps recalling the fish ancestry
of amphibians (and tetrapods), many caecilians have bony scales but these
are conceaned inside pockets in the folds on their bodies. The smallest
are less than 100 mm in total length and the largest exceed 1.5 meters
in length.
Frogs don't resemble caecilians or salamanders. Frogs all have four limbs
but the hindlimbs are markedly larger than the forelimbs. Nealy all have
protruding eyes and most have obvious external ears. Something which is
very obvious is that they appear to not have tails and to have a very
short trunk (the space between the shoulders and the hips). The large
hindlimbs would seem to be an adaptation for jumping and although many
frogs are good jumpers, others can hardly be encouraged to do so. There
are some structural features peculiar to frogs. The two bones of the lower
frontleg are fused as are the two bones of the second joint of the hindleg.
The hindleg has an "extra" joint between the shank (second segment) and
the foot. The vertebral column is short (10 vertebrae or fewer) and extends
only about half the length of the body. The second half of the body is
supported by long, forward-extending bones of the hips and a long bone
(coccyx) lying between these. This peculiar series of structures once
again would seem to be some sort of modification for jumping.
Frogs are found on all continents except Antarctica. The richest continent
is South America with more than 35 % of all frog species. Frogs range
in size from extremely diminutive frogs in southeastern Brasil and Cuba
in which the females only reach 10 mm head-body length (males are smaller
still) to the Goliath frog from the Camaroons which reaches a head-body
length 300 mm (in total length, snout to tip of its toes, to 1 meter).
Europeans had two general names for these animals (frogs and toads) and
those names (or ideas) spread with the colonial interests of Europeans.
To the Europeans, frogs were species closely associated with water, having
thin, smooth skins and webbed feet whereas toads were very often markedly
terrestrial (found in fields far from ponds and streams), with thick,
warty skins and less daintly webbed feet. In the middle of the last century,
an American anatomist found an anatomical correlate of these ecological
differences. The two halves of the shoulder girdle overlapped on the midline
in toads (family Bufonidae) but were fused at the midline in frogs (family
Ranidae). This anatomical differences was examined in frogs of other families
and seemed to sort frogs into two sorts, those sharing the shoulder girdle
morphology of bufonids and those sharing the girdle morphology of ranids.
The other features, long-used in Europe, presented recurring mosaics on
nearly every continent. Even within the "true" toads (Bufonidae), there
are slender, smooth-skinned animals who are superficially very frog-like
and within the "true" frogs there are toad-like members living on various
continents.
Most people do not encounter very many frogs during their lifetimes because
these animals like moist places and are frequently nocturnal. In the temperate
regions, many breed early in the spring, especially during rains, weather
conditions that most people avoid. In the warmer tropics, frogs are seldom
seen except at night and many respond very positively to the onset of
the rainy season. Tropical travelers may hear the croaks and clucks and
trills of many frogs but are reluctant to venture forth at night into
vegetation-choked water to seek these denizens out, perhaps because their
minds are fired by tales of pirannas, snakes, crocodiles, tigers, and
other creatures. Ecotours are now available in the Amazon Basin but that
environment is not optimal if one wishes to see a great many frogs. The
treasure chests are hidden in the cloud forests, especially of the northern
Andes, visited by few aside from biologists and guerrillas.
Whereas temperate region frogs are mundane when it comes to reproduction
(nearly all charge breeding ponds, call furiously, then the female deposits
her pigmented eggs in the water with the male discharging a cloud of sperm
to fertilize them, the fertilized eggs are then abandoned by the parents
and hatch a vegetarian tadpole which looks like a fish but grows to a
particular size and undergoes a dramatic transformation [metamorphosis]
in structure and biology, becoming a carnivore as it metamorphoses from
a tadpole to a small froglet, and takes up its "terrestrial" existence
in the same habitat used by the adults when they are not breeding), tropical
frogs offer a wide variety of reproductive behaviors. Many tropical frogs
have a reproductive behavior scarcely distinct from their temperate relatives
but others seem very peculiar, even incredulous. An Australian colleague
once wrote a paper describing the reproductive biology of a now-extinct
Australian frog (Rheobatrachus silus) wherein the female swallows the
fertilized egg, the embryo grows in her stomach, eating her stomach lining,
and after transforming emerges (is birthed) by the female vomiting up
the froglet. He sent the paper to the prestigious journal Nature where
it was rejected, presumably because the editor thought an Australian was
making a joke. Tyler persevered and his report was published in Science.
Because I study a very species-rich genus (Eleutherodactylus, with more
than 600 species distributed from the southwestern United States to northern
Argentina) having a peculiar (to temperate-zone biologists) reproductive
biology, I will describe it first as a tropical departure. It is probably
the second most-common reproductive mode among frogs in general being
represented abundantly in several families in Africa, Australia, Indonesia,
and Madagascar. Frogs of the genus Eleutherodactylus are mostly small
animals (few exceed 40 mm in head body length) and are mostly nocturnal,
climbing onto vegetation within easy reach. They have large (relative
to the size of the adult) eggs and most fertilized these externally (two
are known to exhibit internal fertilization) in a terrestrial site (within
a bromeliad, under a rock or log, or in the leaf litter that carpets the
forest floor). These are moist places, a prerequisite for successful reproduction
and are also the same places in which juveniles and adults live. The large
fertilized egg is deposited with the others in a clutch, far from standing
water. In the West Indies, the male guards the clutch until hatching but
on the mainland (for the few species granting observations), the female
stays with the egg mass a few days and then abandons it (or she hurries
away at the first opportunity). The embryo remains suspended inside the
jelly capsule of the egg and goes through its development, obtaining all
its nourishment from the yolk provided by its mother. After 30 or so days,
the embryo uses an egg tooth to slit the jelly capsule and emerges as
a miniature of the adult. While an embryo, it had a tail, used as a respiratory
organ, but the tail (a vertebrate feature) was resorbed prior to hatching.
At no time was there a tadpole. This reproductive mode is called direct
development (no tadpole). Direct development is seen in a good many kinds
of frogs and many employ exactly the same sequence as do Eleutherodactylus.
Others seem to be very different, but are doing the same thing. In the
family Hylidae ("treefrogs"), most species of the subfamily Hemiphractinae
either glue the eggs to the female's back or places them inside a skin
pouch on her back (a backpack). She is able to move about in the environment,
regulating for her (and her clutch) the moisture regieme and the temperature
regieme (something that Eleutherodactylus and many other genera cannot
do). Certain species of the family Pipidae (aquatic frogs) use their backs
in a different way. When a female is ready to reproduce, the skin of her
back thickens and develops a rich vascularization. Fertilized eggs fall
upon the skin surface and are enveloped by this vascularized skin into
small pits. Once development is complete, the froglet crawls out of its
pit and takes up an existence within the same aquatic environment used
by its parents. Not all hemiphractines or American pipids demonstrate
direct development to the same degree. All have large eggs and use pouches
or highly vascularized skins. Several hemiphractines (Gastrotheca) discharge
tadpoles from the pouches into ponds for a "normal" cycle as a tadpole
(some are well along in their development and others less so - in the
genus Flectonotus, the tadpole is so close to transforming that they do
not feed as tadpoles, rather they simply wander around a bit and then
transform). In some Pipa (family Pipidae), what emerges from the pits
are not froglets but rather small tadpoles who then make their way as
vegetarians until metamorphosis.
Another common variant of the "normal" reproductive biology is that seen
in glass frogs (family Centrolenidae) and several treefrogs (family Hylidae).
These animals deposit their relatively large unpigmented or pigmented
eggs on leaves (rarely rocks) overhanging some body of water. The early
development of the embryo is therefore carried out in air but within the
moist confines of the jelly coat of the egg. Upon hatching, the tadpole
drops into the water below. In the case of leaf frogs (family Hylidae,
subfamily Phyllomedusinae) thisd is always into a pond whereas in the
case of glass frogs, this is always into a stream (slow streams in the
lowlands, fast streams in the Andes or lesser mountain ranges).
Poisondart frogs are often very pretty, decorating calenders and coffee
mugs with their reds, blues, and yellows. These are also mostly diurnal
frogs, hopping about in the leaf litter or on low vegetation in plain
sight of birds and other potential predators. Few animals attempt to eat
these frogs because they possess cocktails of alkaloids in their skins
which either render them unpalatable or toxic. These frogs have large
eggs, suggesting direct development, and the initial part of their reproductive
biology is simple direct development except that they then hatch into
tadpoles. Often, the male guards the developing clutch, perhaps against
other males. At any rate, after the tadpoles hatch, writhing about in
the moist decomposing jelly mass, the male positions himself within the
mass and allows the tadpoles to crawl upon his back (in some cases only
one or two tadpoles, in other cases all of them). The male then carries
the tadpoles about during his periginations along the forest floor for
part of a day or for several days. During this time the tadpoles are not
feeding. The male transports the tadpoles to a body of water where they
will complete their development. The body of water might be a slow stream,
a puddle, a water-filled tree hole, or the axilla of a bromeliad, even
many meters up some tree. In most cases, the tadpole is then on its own
(frogs are not very sentimental) but in a few cases parental care continues.
Tree holes and bromeliads are often rather sterile biologically and food
hard to come by. The female may "remember" or it may be the male who "remembers"
but the female alone, or the female led by the male in one spectacular
case from Amazonia (the report of this case was accepted by the journal
Nature), revisits the tadpole to deposit unfertilized eggs which serve
as the tadpole's food.
I see these as the three most obvious tropical departures from the temperate
"norm". The first, Direct Development, occurs in the neotropical families
Brachycephalidae, Bufonidae, Hylidae, Leptodactylidae, Microhylidae (perhaps),
and Pipidae. The second, Aereal Early Development, is known for only two
neotropical families (Centrolenidae [all] and Hylidae [all phyllomedusines
and some Hylinae]), and the third, Early Direct Development with Subsequent
Tadpoles (and parental transport), is exclusive to the Dendrobatidae although
a case could be made to claim that the behaviors of certain Pipa, Flectonotus,
and some Gastrotheca represent the same mode but carried out in a different
way or in a different environment. The traditional (in the minds of people
not acquainted with the tropics) or "normal" development is seen in many
neotropical frogs - in northwestern South America there are many "toads"
(Bufonidae, Hylidae, Leptodactylidae, and Pseudidae) and "frogs" (Microhylidae
and Ranidae) with pedestrian reproductive biologies, little different
from some "typical" frog or toad from Europe or the United States.
There are other, less common, modifications of reproductive biology that
seem peculiar to someone seldom-traveled in the world I occupy. A relatively
successful (= speciose) group of leptodactylid "toads" in the American
tropics uses a foam nest. The foam is produced by mucous secretions which
are used to capture air bubbles as the frogs beat their hindlegs. At the
same time (proving that these frogs could walk and whistle at the same
time if need be), the female is ejecting eggs and the male sperm. The
fertilized eggs are swept up into and mixed with the bubbles of the foam
nest. One finds these nests floating on the water surface or concealed
within a burrow excavated by the male on dry land, depending upon the
species or genus concerned. The foam regulates development so that early
embryology occurs until just before hatching and then stops for most of
these frogs. With a subsequent rainfall, the foam nest is destroyed and
the nearly hatched tadpole falls into its aquatic environment but with
a considerable headstart on those frogs arriving at the newly formed body
of water to call and attract mates. In the Americas, this reproductive
mode is the property of the subfamily Leptodactylinae (not all employ
it) but other frogs, from other continents, employ something similar (Myobatrachidae,
subfamily Limnodynastinae, in Australia, and Rhacophoridae in Africa).
I view all of these as exhibiting a "fossil" adaptation (an exapation)
to the uncertainty of available moisture, so as to reproduce successfully.
Limnodynastines use their forelibs to beat the mucous to form a foamnest
and Chiromantis (Rhacophoridfae) construct their nests in trees above
ponds. Three obvious convergences upon the same solution on three continents,
isn't evolution a wonderful idea, much better than the idea of some vaporous
hominoid (phrase thanks to Ernest Haeckel) playing dice with the universe?
One might subdivide reproductive modes into a bewildering array (as did
W. E. Duellman and L. Trueb in their monumental Biology of Amphibians)
0f more than 30 modes they recognized, I think that they erred in their
use of a simple combination of Where (in water, on land, on [or in] the
body of a parent) and Degree of Parental Care to divide up frogs. Many
frogs provide parental care, nearly independent of reproductive mode.
Female Leptodactylus bolivianus and L. colombiensis "guard" their tadpoles,
and, in the case of L. bolivianus, go so far as to excavate "escape" routes
for the tadpoles as the pond begins to dry-up. I find it difficult to
invent a separate reproductive mode merely because the female attends
the group of tadpoles after they hatch. There is a similar care by Hemisus
(an African toad-like frog in its own family) but without the intervention
of water. More than 20 years ago, I observed the very curious behavior
of a centrolenid frog, Centrolene petrophilum, wherein a male, dislocated
from his clutches, returned to those cutches and pressed his snout against
them, in spite of our presence and lights. During their reproductive season,
males of this species have spines (= glands?) on theior snoutsd and these
are pressed against the clutch, afixed to a rock face above the stream,
as soon as the male returns. This appeared to me then, and appears to
me now, as some sort of parental care (and/or adult-embryo communication).
Nonetheless, this observation of the natural history of one species seems
inappropirate as the basis for designating some additional reproductive
mode. There is probably a novel sense organ involved but I have no way
to homologize this with the behavior of a Centrolene geckoideum, which
always, in my experience, leaves a hole in the middle of its egg mass,
perching atop the egg mass (sometimes, but not always).
No one is going to confuse a frog with any other sort of animal. Frogs
(and toads, if you will) share too many peculiarities). It seems to me
that it is much more difficult to convince someone of the Amphibia (technically,
the Lisamphibia [Caecilans + Frogs + Salamanders]) as a natural group
(my apologies, for the cogniscenti, to David Cannatella, Linda Trueb,
and the late Ernest Williams). The rub here is "what is a primitive frog"?
There are two positions. One holds that such creatures as Ascaphus truei
and the Leiopelma (of New Zeeland, of all places) are sufficient creatures
to serve as analogues of frog ancestors (because they share some very
dubious features with salamanders). At the moment, the fossil record seems
(emphasis mine) to support such a position. The heterodox position, advanced
originally by the late Joseph Tihen, is that leptodactylids (my favorite
sort of frogs) include the most primitive of this tribe. Ascaphids occur
in the northwestern part of the United States and leiopelmatids are confined
to New Zeeland. There are early fossils of these sort(s) of frogs in Argentina.
Hence, the traditional view seeks an origin of frogs in one of these three
places (one of which is today the richest place of frogs). Tihenīs (and
my) position places the origin of frogs (with no fossil evidence) on the
richest continent, South America, to which leptodactylids are nearly endemic.
However, to paraphrase Gertrude Stein (and my colleague Robert F. Inger),
a FROG is a FROG is a FROG. Frogs are not a minor element of the biology
of the earth - more frogs (species) are now recognized than there are
of mammals. Our provinical attachment to the Mammalia (essentially, a
radiation of bats and mice) has occluded our ability to reason for a century
or more.
I admit to being biased - I like frogs and have never met one I didn't
like. Frogs can be appreciated as biological curiosities but I like to
think of them as biological jewels - probably because I've devoted 35
years of my life to thinking of them. I use frogs to study arcane subjects,
something for which I'll apologize to no one, such as biogeography and
speciation (subjects which fill no stomachs nor cure any disease). Like
it or not, but the world has more humans than are required - hence, the
search for cures to cancer or AIDS do not represent an urgent necessity,
whereas research devoted to the study of ānimal or plant extinction is
[biologically] important. Perhaps I've become a misanthrope, such is for
the reader [or viewer, in this new century] to decide.
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