No description available.

There is no abstract available for this thesis.

Normal development of the urostyle is described during late stages of metamorphosis in five species of anurans: Xenopus laevis (Daudin), Bufo americanus Holbrook, Pseudacris triseriata (Wied), Hyla chrysoscelis Cope, and Rana pipiens Schreber.Tadpoles of these five species were reared in the laboratory, preserved in Bouin's fixative at various stages of metamorphosis (Gosner stages 34 to 44), and decalcified in 1% HC1 for 48 hours.A block of tissue containing the urotstyle was removed from the tadpole, embedded in paraffin, serially cross-sectioned, stained according to Mallory's trichromatic staining technique, and permanently mounted in Pro-Texx.The developing urostyle of all five species is composed of essentially the same cartilaginous elements: one pair of basidorsals above the notochord and' he hypochord below. Among the five species there is variation in such details as the number of spinal nerve foramina and the degree of fusion of the basidorsals; however, both the hypochord and basidorsals are quite similar in all five genera examined.The development of the urostyle as observed in this study provides insufficient basis for any conclusions about the phylogenetic trends in the five genera.The contradictory descriptions of the developing urostyle found in the literature appear to result from (1) varied methods of study (alizarin-staining of whole specimens or serial cross-sections), (2) the variety of species examined, and (3) the particular stage of development of the tadpole described by an investigator.

No description available.

No description available.

Many organisms with complex life cycles undergo transition periods associated with increased vulnerability to predation. Several evolutionary adaptations have been proposed as antipredator defenses for organisms during risky transition periods. These include: shortening of the transition period, parental care, cryptic coloration, and synchrony of risky transitions with large numbers of conspecifics. The results of my research support the hypothesis that synchrony of metamorphosis and emergence from the water and aggregation during the period of transformation may be antipredator defenses for the western toad (Bufo boreas). For some anuran species, synchronous metamorphosis may function as an antipredator adaptation by swamping predators during the period of transformation. I examined the levels of synchrony of emergence from the water of metamorphosing western toads (Bufo boreas) in the presence and absence of a live snake predator, the common garter snake (Thamnophis sirtalis) in a laboratory experiment. To compare between the treatments, I measured the time to emergence from the water, the number of metamorphs emerging together, and the level of aggregation (before and during emergence) of the toads in each treatment. There was a difference between the treatments when all three factors were considered. I attributed these differences to a behavioral response in which B. boreas emerged sooner in the presence of the predator, regardless of whether individual toads had reached the point at which they were physically better suited to the terrestrial environment than the larval environment. Since the Pacific treefrog (Hyla regilla) is also preyed upon by T. sirtalis during the vulnerable period of metamorphosis, I conducted a laboratory experiment to test the effects of the presence of T. sirtalis on 1) aggregation of larval and metamorphosing H. regilla, 2) time to metamorphosis, 3) synchrony of metamorphosis, 4) time to emergence from the water and 5) synchrony of emergence from the water. The only significant effect observed in this experiment was a difference between aggregation levels of H. regilla throughout the experiment. There was, however, a strong trend in which the variances around the mean times to metamorphosis and emergence of the frogs in the control treatments were larger than those in the predator treatments. This could indicate a trend toward synchrony of metamorphosis and emergence for H. regilla in the presence of snake predators. / Graduation date: 1998