Graduate Thesis Or Dissertation
 

Breeding habits, embryonic thermal requirements and embryonic and larval development of the Cascade frog, Rana cascadae Slater

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  • The breeding habits and embryonic thermal requirements of the Cascade frog of Oregon, Rana cascadae Slater, are described. The limits of temperature tolerance of the pre-feeding stages are from 6° to 27°C for embryos held at constant temperatures. However, for pre-feeding embryos exposed at different developmental stages for short durations to normally lethal chronic warm and cold temperatures, the tolerance limits broaden as the embryo becomes older. The embryonic rates of development were studied over a wide range of constant temperatures. The pre-gastrular period develops at a slower rate than the post-gastrular period at 10° to 18°C but between 18° to 25°C it develops at a faster rate, The biggest increases in rate (from first cleavage to gill circulation) occur with only relatively small increments of heat at the lower end of the thermal limits. Q₁₀ values range from 25.2 in the 8° to 10°C interval to 2.2 at the 20° to 25°C interval. Field observations of adult breeding behavior, such as selection of spawning sites, initiation of the breeding season and placement, size and number of the egg masses indicate a correlation between breeding habits and embryonic thermal requirements. The overall breeding strategy of the Cascade frog seems to be that of providing an environment for utilization of most available heat during the day to achieve maximal embryonic rates of development, thereby shortening the time to hatching and reducing the chances of mortality from cold night temperatures, drying temporary ponds and potholes, and predation. However, as a result of these tactics R. cascadae suffers significant embryonic losses from freezing temperatures and desiccation, The comparison of breeding habits and correlative embryonic thermal adaptations of R. cascadae with other species of Rana presents a more meaningful picture of embryonic temperature responses and their adaptiveness from the standpoint of physiological and ecological evolution of this species, and also serves to clarify the taxonomic status of R. cascadae within the Ranid complex of the Pacific Northwest. As a by-product of this examination of the breeding habits and embryonic thermal requirements of R. cascadae, considerable data were accumulated describing embryonic and larval development. Forty-six stages of development, from the fertilized egg through metamorphosis, are described. External form, pigment patterns and other descriptive characters, as well as the nature of the mouth parts and certain linear dimensions of the larvae, were recorded and analyzed. The diagrams are intended as a reference series for comparative studies of embryos and tadpoles of the genus Rana in North America. Measurements of tadpoles expressed as absolute values are of limited use as taxonomic criteria, since these dimensions are variable and change through time. However, relative body ratios are constant during much of larval development and may be a more diagnostic feature for taxonomic purposes. The nature of the mouth parts adequately characterizes the tadpoles of R. cascadae, but comparisons with other species of Rana should be made with equivalent growth stages.
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