Incubation Time And Rate Of Embryonic Development In Amphibians:The Influence Of Ovum Size,Temperaturaen, And Reproductive Mode

Incubation Time and Rate of Embryonic Development in Amphibians: The Influence of Ovum Size, Temperature, and Reproductive Mode Author(s): David F. Bradford Reviewed work(s): Source: Physiological Zoology, Vol. 63, No. 6 (Nov. - Dec., 1990), pp. 1157-1180 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30152638 . Accessed: 17/04/2012 10:41
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Time Incubation andRateof Embryonic in The of Development Amphibians: Influence Ovum and Mode Size,Temperature, Reproductive
F. David Bradford
Scienceand EngineeringProgram, School of Public Health, Environmental Los of California, Angeles, California 90024 University Accepted 4/3/90

Abstract
Incubation time in amphibians can be mathematicallypartitioned into two other parameters as follows: Incubation time = rate of embryonic development-' X protraction of development. The latterparameter is important because it reflects the degree ofdevelopment of the embryo at hatching. Protraction of development is defined as the ratio of incubation time to time taken to reach a neurula stage common to all amphibians, and it is directly correlated with Gosner stage at hatching in anurans (n = 30 species). This analysis assesses the influence of two covariates, ovum size and temperature, on the three parameters above for two orders ofamphibians,anurans and caudates. The ANCOVAof55 species in 1 7families shows that incubation time scales to ovum volume0444. e-0 121 temp (multiple R' = 0. 88), rate ofdevelopment scales to ovum volume435 . e'115 temp (multiple R2= 0.90), and protraction of development is not significantly related to either ovum volume or temperature. Ovum volume is inversely related to temperature, although r2 is small(0.26). Incubation time of caudates is 2.47 times that for anurans for a given ovum volume and temperature. This difference can be attributed to both a slower rate of embryonic development in caudates than anurans (factor = 0.71) and a greater protraction of embryonic development (factor = 1. 76). This analysisprovides a basisfor evaluating the influence ofreproductive mode onparameters ofembryonicdevelopment. Examples are drawn from amphibians that breed in ephemeral bodies of water and amphibians that breed on land.

Introduction
Incubation time constitutes a significant ecological and physiological parameterto embryonicamphibians.While inside the egg capsule, the embryo derives some protection from predation, microbial attack,desiccation, and

1990. ofChicago. Physiological63(6):1157-1180. 1 1990 The Zoology by University All reserved. rights 0031-935X/90/6306-8982$02.00

1158 D.F.Bradford other inimical conditions. But until it hatches, the embryo cannot exploit environmentalresources or escape adverse environmentalconditions. The relative importance of these costs and benefits varies among amphibians and is reflected in the wide range of incubation times among species.For example, amphibianembryos in highly ephemeral bodies of water develop and hatch quickly (e.g., less than 12 h in the spadefoot toad, Scaphiopus couchi, at 34' C;Zweifel [1968]). In contrast,terrestrialamphibianswith direct development (i.e., those that hatch as miniatureversions of the adult) hatch only aftera long incubationtime (e.g., up to 9 mo in the salamander, Bolitoglossacompacta, at...