The evolution of adult body size in black-bellied salamanders (Desmognathusquadramaculatuscomplex)

Abstract

We investigated the possible role of environmental variables in determining body size within a complex of salamander species (Desmognathus quadramaculatus). We analyzed data generated from life-history studies on populations from throughout the range of this species complex. We incorporated an alternative-hypothesis framework (sensu Platt) to determine the better predictor of adult body size, age at maturity, or size at metamorphosis. We found that almost 90% of the variation in adult body size was explained by size at metamorphosis, which was determined by a combination of rate of larval growth and length of the larval period. Environmental temperature and moisture level were positively correlated with larval growth rate and length of the larval period, respectively. We propose a simple model of body-size evolution that incorporates both adaptive and plastic components. We suggest that the length of the larval period may adaptively respond to moisture-level predictability. In addition, we suggest that the response of the larval growth rate to temperature may be plastic. Because the selection pressure due to drying-induced mortality is pervasive among species of amphibians, it may have played a role in shaping body-size radiation in desmognathines as well as the ecological structure of Appalachian streamside communities.

Life history of a small form of the plethodontid salamander Desmognathus quadramaculatus

“…A juvenile period of one year for males and two years for females supports a conservativeestimate of 48 months (for males) and 60 months (for females) for age at rst reproduction/maturation(maturation one year earlier may also be possible for larvae metamorphosing after two years). All other estimates of minimal size at rst reproduction/maturation in D. quadramaculatus are larger (Organ, 1961;Bruce, 1988;Castanet et al, 1996;Camp et al, 2000;Bruce and Castanet, 2002), as are all other estimates of minimal size at metamorphosis (Organ, 1961;Bruce, 1988;Austin and Camp, 1992;Camp et al, 2000). Based on these comparisons and other available data (Petranka, 1998;Camp et al, 2000), we consider the population of D. quadramaculatusat Gott Farm Creek to represent a case of body size reduction.…”

“…All other estimates of minimal size at rst reproduction/maturation in D. quadramaculatus are larger (Organ, 1961;Bruce, 1988;Castanet et al, 1996;Camp et al, 2000;Bruce and Castanet, 2002), as are all other estimates of minimal size at metamorphosis (Organ, 1961;Bruce, 1988;Austin and Camp, 1992;Camp et al, 2000). Based on these comparisons and other available data (Petranka, 1998;Camp et al, 2000), we consider the population of D. quadramaculatusat Gott Farm Creek to represent a case of body size reduction. Given the similarity in hatchling size and larval and juvenile growth rates at Gott Farm Creek compared to estimates from other populations, we suggest that the small size at Gott Farm Creek is principally due to precocious metamorphosis and maturation.…”

“…However, larval development and growth are decoupled in plethodontids:variation in growth rate does not result in variation in metamorphic timing (Beachy, 1995b(Beachy, , 1997O'Laughlin and Harris, 2000). Clearly, rapid larval growth can have consequences for increases in metamorphic and maturation size in plethodontids (Ryan, 1998;Camp et al, 2000), but it fails to affect metamorphic timing.…”

“…In contrast, lower temperature is thought to result in later oviposition (Bruce, 1982;Voss, 1993;Ryan, 1998), with a consequent effect of later hatching and later metamorphosis at larger size (Beachy, 1995b). There are few direct temperature data on the habitats occupied by larval plethodontids and the general assumption is a correlation of elevation and temperature (Camp et al, 2000), but Voss (1993) has shown that it is the thermal variation associated with stream order (rather than elevation) that results in variation in age at metamorphosis in plethodontid larvae. Unfortunately, except for Voss (1993), data on speci c stream temperature regimes experienced by plethodontidlarvae are lacking.…”

“…The discovery of the Bald Mountains population represented an opportunity to test the hypothesis that reduction in adult body size in plethodontids is a result of precocious metamorphosis and maturation (Bruce, 1988;Beachy, 1995a). The larval period, size at metamorphosis, and age and size at maturation are known to vary considerably in this species (Organ, 1961;Bruce, 1985Bruce, , 1988Austin and Camp, 1992;Camp et al, 2000). The objective of the present study was to evaluate the "precociousthreshold hypothesis" based on analysis of seasonal variation in the distributions of body sizes and life-history stages in the Bald Mountain population of D. quadramaculatus.…”