Document: MAR-3-46-1

Counter-gradient variation for growth and Bergmann's rule in ectotherms: A test using freshwater fishes.

BELK, M.C.* and D.D.HOUSTON

Brigham Young University, Provo, Utah, U.S.A. ¹

Abstract:
Previous authors have suggested that ectotherms follow Bergmann's rule. That is, body size of a given species will be larger in populations found at higher latitudes. Experimentally documented increased growth rate at a given temperature by individuals from higher latitudes compared to individuals from lower latitudes (counter-gradient variation in growth rate) has been suggested as a mechanism for producing Bergmann size clines. However, evidence for counter-gradient variation in growth rate is not sufficient to conclude that larger body size will result at higher latitudes. Bergmann's rule is based on differences in body size (not growth rate), and in spite of persistent claims that most ectotherms show Bergmann-like size clines, few surveys of body size in natural populations of ectotherms across a latitudinal gradient have been published. We used data from the literature for 17 species of common freshwater fish to test for the existence of Bergmann size clines. Secondly, we tested for evidence of counter-gradient variation for growth in each species by comparing growth rate adjusted for length of growing season among populations. Finally, we assessed the potential for differences in body size due to differences in size at maturity or lifespan. None of the 17 species show evidence of larger body size at higher latitudes. All slopes of the regression of body size at ages 1-3 on latitude are negative or zero. The majority of species show evidence of counter-gradient variation in growth rate. Body sizes are larger than expected at higher latitudes based on length of growing season. Differences in body size persist beyond the typical age at maturity, and the relationship between body size and latitude becomes more negative with increasing age. Freshwater fish show no evidence for Bergmann-type size clines, but consistently exhibit counter-gradient variation in growth rates. A systematic survey of body size in several different taxonomic groups is required before we can confidently evaluate the existence of and mechanisms for Bergmann's rule in ectotherms.

Keywords: Bergmann's rule, counter-gradient variation, growth rate, body size, ectotherm, fish

This abstract is being presented at: 10:30 AM in session:
Poster Session #9: Fish, Lakes, Streams and Wetlands.