|Abstract or Summary
- 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
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
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
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.