|PROGENESIS AND NEOTENY 319
erations? If so, the micropterous imago is like a solitaria grasshopper and poses no threat to my general hypothesis. A mushroom or a leaf sounds like an ephemeral, superabundant resource, but a well- protected piece of subterranean turf does not.
The facultative paedomorphosis of several Ambystoma species has been known for more than a century—ever since Duméril, having reported (with some surprise) the sexual maturation of axolotls, compounded his astonishment by observing the transformation of other specimens (Chapter 6). In most species with facultative paedomorphosis, the ease and frequency of transformation can be correlated with geographic, climatic, and ecological gradients. These correlations provide a key for determining the adaptive significance of paedomorphosis and for ascertaining whether it represents a progenetic truncation by precocious maturation or a neotenic retardation of somatic development.
Most of the early interpretations were mechanistic and nonevolutionary. In Ambystoma gracile, for example, frequency of paedomorphosis is far greater in cold Rocky Mountain ponds than in lowland habitats, where most specimens transform (Snyder, 1956). Early interpretations included thyroid inhibition by low iodine in cold ponds and retardation of growth rate by low temperatures, causing a loss in sensitivity of tissues to thyroid hormone (Sprules, 1974a). These mechanisms may well apply, but they leave open the issue of whether any adaptive significance can be ascribed to consistent patterns in the frequency of paedomorphosis.
Sprules (1974a) has reviewed the evidence for adaptive significance and has confirmed and extended a widespread consensus in recent literature. Paedomorphosis with larval reproduction is most common in ponds where the surrounding terrestrial environment is harsh— severe fluctuations in temperature, lack of suitable cover or food, and low humidity, for example—and where water is permanent and predators (fish) are rare or absent. For A. gracile, Sprules argues that paedomorphosis is more common in Rocky Mountain ponds because they are more permanent and free from predaceous fish than lowland ponds. Wilbur (1971) records a short larval life with early transformation for A. laterale in transient, vernal ponds. He cites several reports of massive mortality in Ambystoma following the drying up of ponds. In temporary ponds of New Jersey, A. tigrinum metamorphoses in 75 days; larvae are born in the spring and transform in the summer before the ponds dry. In a pond that dried two weeks after the transformation of its larvae, metamorphosis had occurred more