(Means Are Given in Parentheses After the Observed Ranges.)
=======+===+==================+==================+================= Stage | N | Total length | Body length | Tail length -------+---+------------------+------------------+----------------- 25 | 6 | 25.5-28.0 (26.1) | 9.0-9.5 (9.3) | 16.2-18.5 (16.7) 33 | 2 | 28.5-30.0 (29.3) | 10.2-10.5 (10.4) | 18.0-19.8 (18.9) 36 | 8 | 29.5-34.5 (32.3) | 10.2-11.7 (10.8) | 19.3-23.0 (21.5) 37 | 7 | 31.6-37.5 (34.6) | 11.0-12.5 (11.5) | 21.6-25.0 (23.2) 41 | 3 | 33.0-37.2 (35.2) | 11.6-12.2 (11.9) | 21.4-25.2 (23.2) 43 | 1 | ---- | 12.4 | ---- 46 | 9 | ---- | 13.1-15.7 (14.9) | ---- -------+---+------------------+------------------+-----------------
_Stages 34_, _36_, _37_, and _38_.--Stage 34, foot paddle-shaped with four toe buds; stage 36, five toe buds; stages 37 and 38, lengthening of toes. In all four stages, spiracle persistent, and pigmentation resembling that of early stages.
_Stage 39._--Metatarsal tubercle present; greatest total length (36.9 mm.) attained.
_Stage 40._--Subarticular tubercles prominent; skin over forelimbs transparent; cloacal tail-piece and spiracle absent; outer tooth-rows degenerating; caudal fins shallower than in preceding stages; distal part of tail nearly straight; size of dark markings on tail decreased; pigment present on hind limb.
_Stage 43._--Forelimbs erupted; larval mouthparts absent; corner of mouth between nostril and eye; transverse bands present on hind limbs; tail greatly reduced (about 8 mm. in length).
_Stage 44._--Sacral hump barely noticeable; tail reduced to a stub; corner of mouth at level of pupil of eye; dorsal surfaces pale olive-green; venter white.
Changes proceed in a definite pattern during the growth and development of tadpoles. Larval teeth are absent in hatchlings; the inner tooth-rows develop first, and the third lower row last. At metamorphosis the third lower row is the first to be lost. The tail increases gradually in length relative to the body. In stage 25 the tail is 52.1 per cent of the total length, and in stage 36, 64.6 per cent. In later stages the tail becomes relatively shorter through resorption. Duellman and Klaas (1964:320) noted a great size-variation in _Triprion_ tadpoles in stage 25. No such variation is apparent in any stage of any of the species of _Smilisca_ studied.
The growth and development of the other species of _Smilisca_ do not differ significantly from that of _S. phaeota_. The tadpoles of _S.
sila_ and _sordida_ from streams have relatively longer tails at hatching. For example, in tadpoles of _S. sordida_ the average length of tail is 64.0 per cent of the body-length in stage 25, and in stage 37, 67.0 per cent.
_Behavior_
The tadpoles of _S. baudini_, _cyanosticta_, _phaeota_, and _puma_ are pelagic inhabitants of shallow ponds. Early stages of _S. baudini_ in which external gills are present have been observed to hang vertically with the gills spread out at the surface of the water, a behavior noted by Zweifel (1964:206) in tadpoles of _Phrynohyas venulosa_, which also develop in warm, standing water having a relatively low oxygen-tension.
When disturbed the pelagic tadpoles usually dive and seek shelter amidst vegetation or in mud on the bottom. This behavior was observed in _S.
baudini_, _cyanosticta_, and _phaeota_ by day and at night. No tadpoles of _S. puma_ were observed by day; those seen at night were near the surface of small water-filled depressions in a gra.s.sy marsh; they responded to light by taking refuge in the dense gra.s.s. Perhaps tadpoles of this species are negatively phototactic and remain hidden by day.
The stream-inhabiting tadpoles of _S. sila_ and _sordida_ live in clear pools in rocky streams, where they were observed to cling by their mouths to rocks in the stream and to seek shelter amidst pebbles or beneath rocks and leaves on the bottom. These tadpoles are not found in shallow riffles.
We have not found tadpoles of two species of _Smilisca_ in the same body of water and therefore cannot offer observations on ecological relationships in sympatric situations.
PHYLOGENETIC RELATIONSHIPS
Identifiable hylid remains are known from the Miocene to the Recent, but these fossils are mostly fragmentary and provide little useful information regarding the phylogenetic relationships of living genera.
Frogs of the genus _Smilisca_ are generalized and show no striking adaptations, either in their structure or in their modes of life history.
Interspecific Relationships
In attempting to understand the relationships of the species of _Smilisca_ we have emphasized osteological characters. The phylogeny suggested by these characters is supported by other lines of evidence, including external morphology, tadpoles, and breeding calls.
Our concept of the prototype of the genus _Smilisca_ is a moderate-sized hylid having: (1) a well-developed frontoparietal fontanelle, (2) frontoparietal lacking lateral processes, (3) no bony squamosal-maxillary arch, (4) a fully ossified ethmoid, (5) paired subgular vocal sac, (6) moderately webbed fingers and toes, (7) relatively few supernumerary tubercles on the digits, (8) eggs deposited in clumps in ponds, (9) anteroventral mouth in tadpoles bordered by one row of l.a.b.i.al papillae, but median part of upper lip bare, (10) tail relatively short and deep in tadpoles, and (11) a breeding call consisting of a series of like notes.
Two phyletic lines evolved from this prototype. The first of these was the stock that gave rise to the _baudini_ group. The evolutionary changes that took place in this line included increase in size, development of a lateral curvature of the maxillary, and an increased amount of cranial ossification, especially in the dermal roofing bones.
This phyletic line retained the larval characters and breeding call of the prototype. The second phyletic line gave rise to the _sordida_ group and diverged from the prototype in the development of an angular maxillary and a breeding call consisting of a primary note followed by secondary notes. The frogs in this phyletic line retained the moderate size of the prototype and did not develop additional dermal bone. Our concept of the phylogenetic relationships is shown graphically in Figure 17.
Within the _baudini_ group one stock retained separate nasals and did not develop a bony squamosal-maxillary arch, but broad lateral processes developed on the frontoparietals. The tadpoles remained unchanged from the primitive type. This stock evolved into _S. phaeota_. In the other stock the nasals became fully ossified and a bony squamosal-maxillary arch developed. One branch of this second stock retained tadpoles having only one row of l.a.b.i.al papillae and did not develop lateral processes on the frontoparietals; this branch evolved into _S. cyanosticta_. The other branch diverged and gave rise to _S. baudini_ by developing relatively shorter hind legs, large lateral processes on the frontoparietals, and tadpoles having two rows of l.a.b.i.al papillae.
Within the _sordida_ group the cranial features remained unchanged in one line, which gave rise to _S. sila_, whereas in a second line the nasals were reduced, and their long axes shifted with the result that they are not parallel to the maxillaries; the amount of ossification of the ethmoid was reduced, and the tadpoles developed two rows of l.a.b.i.al papillae. In this second line one branch retained the pond-breeding habits and gave rise to _S. puma_, whereas a second branch became adapted to stream-breeding and gave rise to _S. sordida_.
_baudini_ _cyanosticta_ / + _phaeota_ _sordida_ / / + puma_ / / _sila_ /_____/ / / / | | Prototype
[Ill.u.s.tration: FIG. 17. Hypothesized phylogenetic relationships of the species of _Smilisca_.]
Certain aspects of this proposed phylogeny warrant further comment.
Features such as the deposition of additional bone that roofs the skull or that forms lateral projections from the frontoparietals, like those in _S. baudini_ and _phaeota_, are minor alterations of dermal elements and not basic modifications of the architecture of the skull.
Consequently, we hypothesize the independent development of these dermal changes in _S. baudini_ and _phaeota_. Similar kinds of dermal modifications have evolved independently in many diverse groups of frogs.
Likewise, we propose the parallel development of stream-adapted tadpoles in _S. sordida_ and _sila_; in both cases the tadpoles adapted to changing environmental conditions (see following section on evolutionary history). Tadpoles of _S. sordida_ already had two rows of l.a.b.i.al papillae before entering the streams; subsequently the tadpoles developed complete rows of papillae, ventral mouths and long tails having low fins. Possibly the tadpoles of _S. sila_ had two rows of l.a.b.i.al papillae prior to their adapting to stream conditions; in the process of adapting they developed ventral mouths and long tails having low fins. Similar modifications in tadpoles have occurred in many diverse groups of Middle American hylids, such as _Plectrohyla_, _Ptychohyla_, the _Hyla uranochroa_ group, and the _Hyla taeniopus_ group.
Our lack of concern about coloration is due to the fact that, with the exception of the blue spots on the flanks and posterior surfaces of the thighs in some species, the coloration of _Smilisca_, consisting of a pattern of irregular dark marks on a paler dorsum and dark transverse bars on the limbs, is not much different from that of many other Neotropical hylids. Blue is a structural color, rare among Amphibia, which is achieved by the absence of lipoph.o.r.es above the guanoph.o.r.es.
Thus, the incident light rays at the blue end of the spectrum are reflected by the guanoph.o.r.es without interference by an overlying yellow lipoph.o.r.e screen. According to n.o.ble (1931), lipoph.o.r.es are capable of amoeboid movement that permits shifts in their positions, between or beneath the guanoph.o.r.es. We do not know whether this behavior of lipoph.o.r.es is widespread and is effected in response to environmental changes, or whether it is a genetically controlled attribute that is restricted in appearance. If the latter is the case we must a.s.sume that the prototype of _Smilisca_ possessed such an attribute which was lost in _S. baudini_, _phaeota_, and _puma_. The development of blue spots is not constant in _S. sordida_ and _S. sila_; in _S. cyanosticta_ the spots range in color from blue to pale green.
The coloration of the tadpoles is not distinctive, except for the presence of dorsal blotches on the tails of _S. sila_ and _sordida_.
However, the similarity in pattern cannot be interpreted as indicating close relationships because nearly identical patterns are present in _Hyla legleri_ and some species of _Prostherapis_. This disruptive coloration seems to be directly a.s.sociated with the pebble-bottom, stream-inhabiting tadpoles.
In the _baudini_ group, _S. phaeota_ and _cyanosticta_ are allopatric, whereas _S. baudini_ occurs sympatrically with both of those species.
The call of _S. baudini_ differs notably from the calls of _S. phaeota_ and _cyanosticta_, which are more nearly alike. Although in the phylogenetic scheme proposed here _S. sila_ is considered to be more distantly related to _S. puma_ than is _S. sordida_, the calls of _S.
sila_ and _puma_ more closely resemble one another than either resembles that of _S. sordida_. _Smilisca sila_ and _puma_ are allopatric, whereas _S. sordida_ is broadly sympatric with both of those species. We a.s.sume that in their respective phyletic lines the differentiation of both _S.
baudini_ and _sordida_ was the result of genetic changes in geographically isolated populations. Subsequently, each species dispersed into areas inhabited by other members of their respective groups. Selection for differences in the breeding calls helped to reinforce other differences in the populations and thereby aided in maintaining specificity.
Evolutionary History
With respect to temporal and spatial aspects of evolution in _Smilisca_, we have tried to correlate the phylogenetic evidence on _Smilisca_ with the geologic data on Middle America presented by Lloyd (1963), Vinson and Brineman (1963), Guzman and Cserna (1963), Maldonado-Koerdell (1964), and Whitmore and Stewart (1965). Likewise, we have borne in mind the evidence for, and ideas about, the evolution of the Middle American herpetofauna given by Dunn (1931b), Schmidt (1943), Stuart (1950, 1964) Duellman (1958, MS), and Savage (MS).
According to Stuart"s (1950) historical arrangement of the herpetofauna, _Smilisca_ is a member of the Autochthonous Middle American Faunal Element, and according to Savage"s (MS) arrangement the genus belongs to the Middle American Element, a fauna which was derived from a generalized tropical American unit that was isolated in tropical North America by the inundation of the Isthmian Link in early Tertiary, that developed _in situ_ in tropical North America, and that was restricted to Middle America by climatic change in the late Cenozoic.
Savage (MS) relied on the paleogeographic maps of Lloyd (1963) to hypothesize the extent and centers of differentiation of the Middle American Faunal Element. According to Lloyd"s concept, Middle America in the Miocene consisted of a broad peninsula extending southeastward to about central Nicaragua, separated from the Panamanian Spur of continental South America by shallow seas. A large island, the Talamanca Range, and remnants of the Guanarivas Ridge formed an archipelago in the shallow sea. The recent discovery of remains of mammals having definite North American affinities in the Miocene of the Ca.n.a.l Zone (Whitmore and Stewart, 1965) provides substantial evidence that at least a peninsula was continuous southeastward from Nuclear Central America to the area of the present Ca.n.a.l Zone in early mid-Miocene time. South America was isolated from Central America by the Bolivar Trough until late mid-Pliocene.
Thus, in the mid-Tertiary the broad peninsula of Nuclear Central America, which consisted of low and moderately uplifted regions having a tropical mesic climate, provided the site for the evolution of _Smilisca_. It is not possible to determine when the genus evolved, but to explain the differentiation of the species it is unnecessary to have the ancestral _Smilisca_ present prior to the Miocene.
We view the Miocene _Smilisca_ as the prototype described in the preceding section, and suppose that it lived in the mesic tropical environment of the eastern part of the Central American Peninsula (in what is now Costa Rica and western Panama). Two stocks differentiated, probably in middle Miocene times; one of these, the ancestral stock of the _baudini_ group, was widespread on the Caribbean lowlands from the Nicaraguan Depression to the Bolivar Trough, and the other, the ancestral stock of the _sordida_ group, was restricted to the Pacific lowlands of the same region. In late Miocene time the ancestral stock of the _baudini_ group dispersed northwestward around the deep embayment in the Nicaraguan depression into upper Central America (in what is now Honduras and Guatemala) and thence into southern Mexico. Apparently differentiation took place on each side of the Nicaraguan Depression; the frogs to the south of the depression evolved into _S. phaeota_, whereas those to the north of the depression represented the stock from which _S. baudini_ and _cyanosticta_ arose. Prior to the uplift of the mountains in the late Miocene and the Pliocene the _baudini-cyanosticta_ stock probably was widespread in northwestern Central America. The elevation of the mountains resulted in notable climatic changes, princ.i.p.ally the development of sub-humid environments on the Pacific lowlands. The frogs living on the Pacific lowlands became adapted to sub-humid conditions and developed into _S. baudini_. The stock on the Caribbean lowlands remained in mesic environments and evolved into _S.
cyanosticta_.
Possibly in the middle Miocene before the Talamanca Range in Costa Rica and western Panama was greatly uplifted, the ancestral stock of the _sordida_ group invaded the Caribbean lowlands of what is now Costa Rica. The subsequent elevation of the Talamanca Range in the Pliocene effectively isolated the ancestral stock of _S. sila_ on the Pacific lowlands from the _puma-sordida_ stock on the Caribbean lowlands. The former was subjected to the sub-humid conditions which developed on the Pacific lowlands when the Talamanca Range was uplifted. It adapted to the sub-humid environment by living along streams and evolving stream-adapted tadpoles. On the Caribbean side of the Talamanca Range the _puma-sordida_ stock inhabited mesic environments. The stock that evolved into _S. puma_ remained in the lowlands as a pond-breeding frog, whereas those frogs living on the slopes of the newly elevated mountains became adapted for their montane existence by developing stream-adapted tadpoles and thus differentiated into _S. sordida_.
Probably the six species of _Smilisca_ were in existence by the end of the Pliocene; at that time a continuous land connection existed from Central America to South America. The climatic fluctuations in the Pleistocene, and the post-Wisconsin development of present climatic and vegetational patterns in Middle America, brought about the present patterns of distribution of the species. From its place of origin on the Caribbean lowlands of lower Central America, _S. phaeota_ dispersed northward into Nicaragua and southward along the Pacific slopes of northwestern South America. Perhaps in the late Pleistocene or in post-Wisconsin time when mesic conditions were more widespread than now, _S. phaeota_ moved onto the Pacific lowlands of Costa Rica. Its route could have been through the Arenal Depression. Subsequent aridity restricted its range on the Pacific lowlands to the Golfo Dulce region.
Climatic fluctuation in northern Central America restricted the distribution of _S. cyanosticta_ to mesic habitats on the slopes of the Mexican and Guatemalan highlands and to certain humid areas on the lowlands. _Smilisca baudini_ was well adapted to sub-humid conditions, and the species dispersed northward to the Rio Grande Embayment and to the edge of the Sonoran Desert and southward into Costa Rica. In southern Mexico and Central America the species invaded mesic habitats.
Consequently, in some areas it is sympatric with _S. cyanosticta_ and _phaeota_.
_Smilisca puma_ dispersed northward onto the Caribbean lowlands of southern Nicaragua. Its southward movements probably were limited by the ridges of the Talamanca Range that extend to the Caribbean coast in the area of Punta Cahuita in Costa Rica. _Smilisca sila_ dispersed along the Pacific lowlands and slopes of the mountains from eastern Costa Rica and western Panama through eastern Panama to northern Colombia. Climatic fluctuation in the Pleistocene evidently provided sufficient alt.i.tudinal shifts in environments in the Talamanca Range to permit _S. sordida_ to move onto the Pacific slopes. From its upland distribution the species followed streams down to both the Caribbean and Pacific lowlands, where it is sympatric with _S. puma_ on the Caribbean lowlands and _S. sila_ on the Pacific lowlands.
The evolution of the species-groups of _Smilisca_ was effected through isolation by physical barriers in the Cenozoic; the differentiation of the species was initiated by further isolation of populations by changes in physiography and climate. Present patterns of distribution resulted from Pleistocene and post-Wisconsin climatic changes. Today, sympatric species have different breeding habits and breeding calls which reinforce the differences in morphology.