EVOLUTION OF OVIDUCTAL GESTATION IN AMPHIBIANS

Oviductal retention of developing embryos, with provision for maternal nutrition after yolk is exhausted (viviparity) and maintenance throug h metamorphosis, has evolved independently in each of the three living orders of amphibians, the Anura (frogs and toads), the Urodela (salam anders and newts), and the Gymnophiona (caecilians). In anurans and ur odeles obligate viviparity is very rare (less than 1% of species); a f ew additional species retain the developing young, but nutrition is yo lk-dependent (ovoviviparity) and, at least in salamanders, the young m ay be born before metamorphosis is complete. However, in caecilians pr obably the majority of the approximately 170 species are viviparous, a nd none are ovoviviparous. All of the amphibians that retain their you ng oviductally practice internal fertilization; the mechanism is cloac al apposition in frogs, spermatophore reception in salamanders, and in tromission in caecilians. Internal fertilization is a necessary but no t sufficient exaptation (sensu Gould and Vrba: Paleobiology 8:4-15, '8 2) for viviparity. The salamanders and all but one of the frogs that a re oviductal developers live at high altitudes and are subject to rigo rous climatic variables; hence, it has been suggested that cold might be a ''selection pressure'' for the evolution of egg retention. Howeve r, one frog and all the live-bearing caecilians are tropical low to mi ddle elevation inhabitants, so factors other than cold are implicated in the evolution of live-bearing. Viviparity might facilitate life in a rigorous environment, but likely is not ''caused'' by such an existe nce. (C) 1993 Wiley-Liss, Inc.