Adaptation of reproductive strategies: Endocrine and ecological factors associated with the evolution of terrestrial breeding in amphibians

The goal of this study was to identify the endocrine and ecological factors associated with the evolution of terrestrial breeding in amphibians. I focused on the redbacked salamander, Plethodon cinereus, a woodland species that has a reproductive strategy typical of terrestrial salamanders, involving direct development, small clutches of large eggs, terrestrial oviposition, and obligate maternal care. This strategy differs from the more ancestral, aquatic breeding amphibians that have a biphasic life cycle, oviposit large clutches of small eggs in ponds, and do not exhibit parental care.; Two physiological adaptations associated with terrestrial breeding were suggested from experiments. First, clutch size is determined at the earliest stages of ovarian development (one year prior to the time of oviposition), as supplemental feeding treatments throughout the foraging period did not increase the numbers of developing oocytes. Second, injections of follicle stimulating hormone (FSH) and luteinizing hormone (LH) were associated with separate effects on oocyte development: FSH controlled oocyte recruitment and caused moderate oocyte growth, and LH primarily controlled oocyte growth and restricted oocyte recruitment. This is the first study to show different roles of FSH and LH in amphibian oocyte development, as both FSH and LH affect oocyte recruitment and growth in aquatic-breeding amphibians.; Ecological experiments using hormone treatments to increase the range of oviposition dates conducted in 1999 and 2000 demonstrated that stabilizing selection might have acted to produce the oviposition times observed in natural populations. In both years, females ovipositing during intermediate times (June) were least likely to abandon their clutches and had the highest fecundity. Body size, egg number, egg size, and oviposition date were shown to affect offspring size at hatching. The specific relationships between these maternal traits and offspring size differed in magnitude and direction between years, probably due to the differences in the environmental conditions. A cross-fostering experiment showed that size-mediated behaviors of mothers during brooding had a greater effect on offspring size than pre-ovipositional maternal traits, such as egg size or yolk quality. These results suggest that maternal phenotypic effects on offspring are complex and are important factors in the ecology and evolution of terrestrial amphibians.