Ecological Aspects of Amphibian Metamorphosis

Abstract

A synthetic theory of the ecology of amphibian metamorphosis is founded on the observation that the large variation in length of larval period and body size at metamorphosis typical of a particular species of amphibian cannot be directly explained by differences in dates of hatching or egg sizes. It is proposed that as development proceeds, variation in exponential growth coefficients causes a trend from a normal distribution to a skewed distribution of body sizes. The degree of skewing increases and the median of the distribution decreases with increasing initial densities of populations. The relative advantages of the largest members of a cohort may arise from a variety of mechanisms including the production of growth inhibitors, interference competition, and size-selective feeding behavior. These mechanisms result in a nonnormal distribution of competitive ability, a possible source of the density-dependent competition coefficient found in systems with many species (1). In our model the ranges of body sizes and dates of metamorphosis are determined by a minimum body size that must be obtained and a maximum body size that will not be exceeded at metamorphosis. Between these two size thresholds the endocrinological initiation of metamorphosis is expected to be related to the recent growth history of the individual larva. Species that exploit uncertain environments will have a wide range of possible sizes at metamorphosis. Species exploiting relatively certain environments will have a narrower range. The evolution of neoteny and direct development logically follow from the application of these ideas to the ecological context of the evolution of amphibian life histories. Species that live in constant aquatic habitats surrounded by hostile environments (desert ponds, caves, high-altitude lakes) may evolve permanent larvae genetically incapable of metamorphosis. Other populations may evolve a facultative metamorphosis such that populations are a mixture of neotenes and terrestrial adults. Direct development results from selection to escape the competition, predation, and environmental uncertainty characteristic of some aquatic habitats and is usually accompanied by parental care. The relation between our ecological model and the physiological mechanisms that initiate metamorphosis can only be suggested and it remains an open problem for developmental biologists.

Retention of low-fitness genotypes over six decades of admixture between native and introduced tiger salamanders

“…Unfortunately, we cannot include larvae from the JCL site in our analyses because they were not reared to the completion of metamorphosis. Variation in hydroperiod has previously been identified as a major selective force in the evolution of salamanders with complex life history pathways [49,50,58,59]. Our data suggest that in this hybrid swarm, environmental variation ( e.g ., pond hydroperiod) is likely important in balancing selection for highly non-native individuals, and thereby maintaining higher frequencies of native genotypes.…”

“…For example, a faster growth rate with early obligate metamorphosis might be extremely advantageous in short-hydroperiod aquatic habitats, particularly in California's Mediterranean climate where there is practically no possibility of summer rainfall to rescue a drying pond. However, in more permanent aquatic habitats, such as the man-made livestock ponds that are now common in California, delaying metamorphosis would allow the attainment of larger sizes and may be an advantageous strategy [49,50]. Furthermore, avoiding the semi-arid, and often highly agricultural terrestrial habitat that has typified the hybrid swarm region since the initial BTS introduction 60 years ago may further increase fitness of paedomorphic ( i.e ., permanently aquatic) salamanders.…”

Hybrid vigor between native and introduced salamanders raises new challenges for conservation

“…Pure native California Tiger Salamanders must metamorphose to reproduce (46), but Barred Tiger Salamanders regularly forgo metamorphosis in perennial ponds and breed as paedomorphs (47,48). Paedomorphs often reach sexual maturity earlier than metamorphs, produce larger clutches, and may breed earlier in a given season; any of these life-history factors may provide introduced genotypes with an advantage in perennial ponds (49,50). The perennial pond populations studied here (BW1 and JCL2) show strong viability selection favoring hybrids during the early part of the larval period (Table 1 and Fig.…”

Locomotion and survival of two sympatric larval anurans, Bufo gargarizans (Anura: Bufonidae) and Rana zhenhaiensis (Anura: Ranidae), after partial tail loss

“…A high level of tail damage was found by BLAIR & WASSERSUG (2000) in a wild population of tadpoles. Tail damage in tadpoles can be caused by vertebrates, such as fish, turtles and salamanders, and by invertebrates, such as crayfish and dragonfly larvae (WILBUR & COLLINS 1973, CALDWELL et al 1980, VAN BUSKIRK & MCCOLLUM 2000a, VAN BUSKIRK et al 2003, WILSON et al 2005.…”