Our two visits to Chinaja and Toocog afforded the opportunity to gather
data on the ecology of the rainforests of southern El Peten and to study
the relationships between the environment and members of the
herpetofauna. Tropical rainforests present the optimum conditions
for life, and it is in this environment that life reaches its greatest
diversity. Here, too, biological inter-relationships are most complex.
This complexity is illustrated by the presence of many species of some
genera, all of which are found together in the same geographic region.
In the rainforests of southern El Peten there are six species of
_Anolis_, five of _Hyla_, four of _Bothrops_, and three of
_Coniophanes_. Obviously, the diversity of ecological niches in the
rainforest is sufficient to support a variety of related species. Of the
examples mentioned above, fairly adequate ecological data were obtained
for most of the species of _Anolis_, which will be used to show the
ecological diversity and vertical stratification of sympatric species in
the rainforests.
Of the six species of _Anolis_, all except _A. sericeus_ are typically
found in humid forests. _Anolis sericeus sericeus_ is poorly represented
in the collections from southern El Peten, where it may be in
competition with _Anolis limifrons rodriguezi_ that resembles _Anolis s.
sericeus_ in size, coloration, and habits. Therefore, _Anolis sericeus
sericeus_ is excluded from the following discussion. The common
terrestrial species is _Anolis humilis uniformis_; sometimes this small
species perches or suns on the bases of small trees or buttresses of
some large trees. When disturbed it takes to the ground and seeks cover
in the leaf litter or beneath logs or palm fronds. _Anolis lemurinus
bourgeaei_ is about twice the size of _Anolis humilis uniformis_ and is
usually observed on buttresses of large trees or on the lower two meters
of tree trunks. Individuals were seen foraging on the ground along with
_Anolis humilis uniformis_. At no time were _Anolis lemurinus bourgeaei_
observed to ascend the trunks of large trees; they always took refuge
near the bases of trees. _Anolis limifrons rodriguezi_ is found on the
stems and branches of bushes. It is a small species that sometimes is
observed on the ground but was never seen ascending large trees. _Anolis
capito_ is about the same size as _Anolis lemurinus bourgeaei_ and lives
on the trunks of large trees. In the tops of the trees lives a large
green species, _Anolis biporcatus_.
Similar segregation habitatwise can be demonstrated for other members of
the herpetofauna. The avoidance of interspecific competition in feeding
is well illustrated by three species of snakes that probably are the
primary ophidian predators on frogs. _Drymobius margaritiferus
margaritiferus_ is diurnal and terrestrial; it feeds on frogs at the
edges of breeding ponds by day. Also during the day _Leptophis mexicanus
mexicanus_ feeds on frogs in bushes and trees. At night the activities
of both of these species is replaced by those of _Leptodeira
septentrionalis polysticta_, which not only feeds on the frogs in the
trees and bushes, but descends to the ground and even enters the water
in search of food.
From the examples discussed above, the importance of the three
dimensional aspect of the rainforest is apparent. The presence of a
large and diverse habitat above the ground is of great significance in
the rainforest, for of the non-aquatic components of the herpetofauna in
the rainforests of southern El Peten, 42 per cent of the species spend
at least part of their lives in the bushes and trees. Another important
part of the forest is the subterranean level--the rich mulch,
underground tunnels, and rotting subterranean vegetation. Of the 78
species of amphibians and reptiles in southern El Peten, seven are
primarily fossorial, and half-a-dozen others are secondarily fossorial.
Probably the fossorial members of the fauna are the least well
represented in the collection, for such widespread species as _Dermophis
mexicanus mexicanus_, _Rhadinaea decorata decorata_ and _Tantilla
schistosa schistosa_ were expected, but not found.
In the following discussion of the ecological distribution of amphibians
and reptiles in the rainforest I have depended chiefly on my
observations made in southern El Peten, but have taken into
consideration observations made on the same species in other regions,
together with reports from other workers. The reader should keep in mind
that the evidence varies from species to species. Of some species I have
observed only one animal in the field; of others, I have seen scores and
sometimes hundreds of individuals. For species on which I have few
observations or rather inconclusive evidence, the circumstance of
inadequate data is mentioned.
In analyzing the ecological distribution within the forest, it is
convenient to recognize five subdivisions (habitats); each is treated
below as a unit.
1. AQUATIC.--This habitat includes permanent streams and rivers (Pl. 10,
fig. 1), some of which are clear and others muddy. In the rainy season
temporary ponds form in depressions on the forest floor (Pl. 10, fig.
2); these are important as breeding sites for many species of
amphibians. Aquatic members of the herpetofauna are here considered to
be those species that either spend the greatest part of their lives in
the water or usually retreat to water for shelter. Seven species of
turtles and one crocodilian are aquatic. Of these, _Dermatemys mawi_,
_Staurotypus triporcatus_, and _Pseudemys scripta ornata_ inhabit clear
water, whereas _Chelydra rossignoni_, _Claudius angustatus_,
_Kinosternon acutum_, and _K. leucostomum_ inhabit muddy water.
_Crocodylus moreleti_ apparently inhabits both clear and muddy water,
for in the dry season it lives along the clear rivers, but in the rainy
season inhabits flooded areas in the forest as well.
2. AQUATIC MARGIN.--Extensive marshes were lacking in the part of
southern El Peten that I visited; consequently, the aquatic margin
habitat is there limited to the edges of rivers and borders of temporary
ponds. _Bufo marinus_, _Rana palmipes_, and _Rana pipiens_ are
characteristic inhabitants of the aquatic margin, although in the rainy
reason _Bufo marinus_ often is found away from water. Observations
indicate that _Tretanorhinus nigroluteus lateralis_ inhabits the margins
of ponds and streams and actually spends considerable time in the water.
Although _Iguana iguana rhinolopha_ is arboreal, it lives in trees along
rivers, into which it plunges upon being disturbed. Species included in
this category are those that customarily spend most of their lives at
the edge of permanent water. Frogs and toads that migrate to the water
for breeding and the snakes that prey on the frogs at that time are not
assigned to the aquatic-margin habitat.
3. FOSSORIAL.--Characteristic inhabitants of the mulch on the forest
floor are _Bolitoglossa moreleti mulleri_, _Lepidophyma flavimaculatum
flavimaculatum_, _Scincella cherriei cherriei_, _Ninia sebae sebae_,
_Pliocercus euryzonus aequalis_, and _Micrurus affinis apiatus_. Other
species of snakes that spend most of their lives above ground often
forage in the mulch layer; among these are _Coniophanes bipunctatus
biserialis_, _Coniophanes fissidens fissidens_, _Coniophanes imperialis
clavatus_, _Lampropeltis doliata polyzona_, and _Stenorrhina
degenhardti_. Among the amphibians, at least _Hypopachus cuneus
nigroreticulatus_, _Eleutherodactylus rostralis_, and _Syrrhophus
leprus_ are known to seek shelter in the mulch.
4. TERRESTRIAL.--One turtle, _Geoemyda areolata_, is primarily
terrestrial. Among the lizards, conspicuous terrestrial species are
_Anolis humilis uniformis_ and _Ameiva festiva edwardsi_; _Anolis
lemurinus bourgeaei_ and _Basiliscus vittatus_ spend part of their lives
on the ground, but also live on trees and in bushes. _Eumeces
schwartzei_ and _E. sumichrasti_ apparently are terrestrial. The only
terrestrial lizard that is nocturnal is _Coleonyx elegans elegans_,
which by day hides in the leaf litter or below ground. Nocturnal
amphibians that are terrestrial include _Bufo marinus_, _Bufo valliceps
valliceps_, _Eleutherodactylus rugulosus rugulosus_, _Syrrhophus
leprus_, and _Hypopachus cuneus nigroreticulatus_. A large number of
active diurnal snakes are terrestrial; these include _Boa constrictor
imperator_, _Clelia clelia clelia_, _Dryadophis melanolomus laevis_,
_Drymarchon corais melanurus_, _Drymobius margaritiferus
margaritiferus_, _Pseustes poecilonotus poecilonotus_, and _Spilotes
pullatus mexicanus_. Nocturnal terrestrial snakes include three kinds of
_Bothrops_ (_B. atrox asper_, _B. nasutus_, and _B. nummifer nummifer_),
all of which seem to be equally active by day.
5. ARBOREAL.--In this habitat the third dimension (height) of
the rainforest probably is the most complex insofar as the
inter-relationships of species and ecological niches are concerned. I
have attempted to categorize species as to microhabitats within the
arboreal habitat; in so doing, I recognize four subdivisions--bushes,
tree trunks, tree tops, and epiphytes.
Bush inhabitants include several species of lizards and snakes, all of
which have rather elongate, slender bodies, and long tails. Common
bush-inhabitants in southern El Peten are _Anolis limifrons rodriguezi_,
_Basiliscus vittatus_, _Laemanctus deborrei_, _Leptophis mexicanus
mexicanus_, and _Oxybelis aeneus aeneus_. All of these are diurnal, and
all but _Laemanctus_ have been observed sleeping on bushes at night.
Tree-trunk inhabitants include five species of lizards. _Thecadactylus
rapicaudus_ lives on the trunks of large trees; _Sphaerodactylus
lineolatus_ lives beneath the bark on dead trees and on corozo palms.
_Anolis lemurinus bourgeaei_ lives on the bases and buttresses of large
trees, from which it often descends to the ground. _Corythophanes
cristatus_ and _Anolis capito_ were found only on tree trunks and large
vines.
The least information is available for the species living in the tree
tops. The following species were obtained from tops of trees when they
were felled, or have been observed living in the tree tops: _Anolis
biporcatus_, _Iguana iguana rhinolopha_, _Celestus rozellae_,
_Leptodeira septentrionalis polysticta_, _Leptophis ahaetulla
praestans_, _Sibon dimidiata dimidiata_, and _Sibon nebulata nebulata_.
Epiphytes, especially the bromeliads, provide refuge for a variety of
tree frogs and small snakes. Of the tree frogs, _Hyla picta_, _Hyla
staufferi_, _Phyllomedusa callidryas taylori_, _Similisca baudini_, and
_Similisca phaeota cyanosticta_ have been found in bromeliads; other
species probably occur there. Among the snakes, _Imantodes cenchoa
leucomelas_, _Leptodeira frenata malleisi_, _Leptodeira septentrionalis
polysticta_, _Sibon dimidiata dimidiata_, and _Sibon nebulata nebulata_
are frequent inhabitants of bromeliads; all of these snakes are
nocturnal.