Proximate mechanisms determining size variability in natterjack toads

Abstract

In the toad Bufo calamita, among-population variation of size follows roughly a converse Bergmann cline, but populations exist that do not fit this pattern. We propose that latitudinal body size variation is a byproduct of adaptive covariation among the life-history traits juvenile growth rate, longevity and lifetime fecundity. We choose five populations (two in Andalusia, two in Catalonia and one in Rhineland-Palatinate) representing a variation of adult size from 39 mm to 95 mm snout-vent length, a latitudinal gradient from 37 to 501 and an altitudinal gradient from sea level to 420 m. Skeletochronology was used to estimate the age-related life-history traits of 313 toads and their lifetime pattern of growth. At southern latitudes, toads matured and reproduced earlier than those at northern latitudes, but had a reduced potential reproductive lifespan due to lower longevity. Ageadjusted adult size depended mainly on the size achieved between metamorphosis and first hibernation or aestivation, which in turn was influenced by local factors. We propose that first-year size corresponds to the duration of the aboveground activity period, temperature during the activity period and the type of shelter sites and hibernacula available in the habitat. After attaining sexual maturity, the growth rates did not differ among populations. Interactions of multiple environmental factors during the first year of life determine age at maturity, adult size and size variation among populations. Local body size and potential reproductive lifespan covary to optimize lifetime fecundity throughout the geographical range. The presence of a small-sized population in southern Spain does not fit the pattern predicted by a converse Bergmann cline, but is compatible with the hypothesis that body size variation among B. calamita populations may be the evolutionary byproduct of optimized lifetime fecundity.

Adult body size = f (initial size + growth rate × age): explaining the proximate cause of Bergman’s cline in a toad along altitudinal gradients

“…Many low growth rates should be encountered at extremely high latitudes or altitudes where environmental conditions in terms of climate, growth period and food supply are considerably adverse, especially in the cases when animals spend one or more years being in the larval stage. The predominant effect of slow growth rates on reduced body size along geographic gradients has been confirmed in two species exhibiting converse Bergmann's rule: Bufo calamita, a toad with a broad latitudinal range (Sinsch et al 2010), and Nanorana parkeri, a frog with the highest altitude distribution in the world (Ma et al 2009b). A concave pattern of body size of Rana temporaria across large latitudinal gradients may be attributed to different predominance of growth rate and age in determining body size in different ranges of latitude (Laugen et al 2005).…”

“…(1) Two distinct size patterns along latitudinal or altitudinal gradients have been identified among anuran species (review in Ashton 2002): some conform to Bergmann's rule (e.g. Leclair and Laurin 1996;Lai et al 2005;Lu et al 2006;Ma et al 2009a;Liao and Lu 2010a;Gillespie 2011), whereas others do the opposite (Ryser 1996;Eaton et al 2005;Matthews and Miaud 2007;Cvetković et al 2009;Ma et al 2009b;Sinsch et al 2010). This offers opportunities to assess relative roles of the size-related parameters in shaping the direction and degree of geographic size clines.…”

Altitudinal variation in body size and age structure of the Sauter’s frog Rana sauteri in Taiwan

“…It is a general life history strategy for ectothermic organisms that individuals tend to devote more energy into growth before sexual maturity to reach larger body sizes. When they reach maturity, the growth slows down and more energy was devoted into reproduction (Iturra-Cid et al 2010;Sinsch et al 2010). Harsher conditions at higher-altitude sites in terms of food resource, climate, and activity period may constrain reproductive and growth potentials of animals (Dmitriew 2011).…”

“…Also, the tendency of body size variation in male may differ from that in female within species in the frogs Liao et al 2013). Altitudes and latitudes may cause a similar temperature effect on amphibians (Morrison and Hero 2003;Sinsch et al 2010), so it would be necessary to reduce the latitudinal effect in investigating altitudinal effect on body size in amphibians.…”

Body size variation and sexual size dimorphism across climatic gradients in the widespread treefrog Scinax fuscovarius (Anura, Hylidae)

“…Some of them have been mentioned and include the age at maturity, longevity, clutch size, egg size, larval period, food availability, substrate, etc. (Morrison & Hero 2003;Sinsch et al 2010;Oromi et al 2012;Hsu et al 2014;Davenport & Hossack 2016).…”