Detecting trends in body size: empirical and statistical requirements for intraspecific analyses

Attributing biological explanations to observed ecogeographical and ecological patterns require eliminating potential statistical and sampling artifacts as alternative explanations of the observed patterns.Here,we assess the role of sample size,statistical power,and geographic inclusivity on the general validity and statistical significance of relationships between body size and latitude for 3 well-studied species of turtles.We extend those analyses to emphasize the importance of using statistically robust data in determining macroecological patterns.We examined intraspecific trends in body size with latitude in Chelydra serpentina,Chrysemys picta,and Trachemys scripts using Pearson's correlations,diagnostic tests for influential points,and resampling.Existing data were insufficient to ascertain a latitudinal trend in body size for C.serpentina or T.scripts.There was a significant relationship for C.picta,however,resampling analyses show that,on average,16 of the 23 available independent populations were needed to demonstrate a significant relationship and that at least 20 of 23 populations were required to obtain a statistically powerful correlation between body size and latitude.Furthermore,restricting the latitudes of populations resampled shows that body size trends of C.picta were largely due to leveraging effects of populations at the edge of the species range.Our results suggest that broad inferences regarding ecological trends in body size should be made with caution until underlying(intraspecific)patterns in body size can be statistically and conclusively demonstrated.

Endocrine stress response of Eastern Fence Lizards in fire-disturbed landscapes

Landscape disturbances can alter habitat structure and resource availability,often inducing physiological responses by organisms to cope with the changing conditions.Quantifying the endocrine stress response through measurement of glucocorticoids has become an increasingly common method for determining how organisms physiologically respond to challenges imposed by their environment.We tested the hypothesis that Eastern Fence Lizards cope with fire disturbance effects by modulating their secretion of corticosterone(CORT).We measured the baseline and stress-induced plasma CORT of male Eastern Fence Lizards in a chronosequence of fire-altered habitats(recently burned,recovering from burn,and unburned).Although habitat use by lizards differed among burn treatments,including differences in use of canopy cover,leaf litter,and vegetation composition,we did not detect a significant effect of fire-induced habitat alteration on plasma CORT concentration or on body condition.In addition,we found no effect of blood draw treatment(baseline or stress-induced),body temperature,body condition,or time taken to collect blood samples on concentration of plasma CORT.Low intensity burns,which are typical of prescribed fire,may not be a sufficient stressor to alter CORT secretion in Eastern Fence Lizards(at least during the breeding season).Instead,lizards may avoid allostatic overload using behavioral responses and by selecting microsites within their environment that permit thermoregulatory opportunities necessary for optimal performance and energy assimilation.

Every breath you take:assessing metabolic costs of toxin resistance in garter snakes(Thamnophis)

Trait specialization often comes at the expense of original trait function,potentially causing evolutionary tradeoffs that may render specialist populations vulnerable to extinction.However,many specialized adaptations evolve repeatedly,suggesting selection favors specialization in specific environments.Some garter snake(Thamnophis)populations possess specialized mutations in voltage-gated sodium channels that allow them to consume Pacific newts(Taricha)defended by a highly potent neurotoxin(tetrodotoxin).These mutations,however,also decrease protein and muscle function,suggesting garter snakes may suffer evolutionary tradeoffs.We measured a key physiological process,standard metabolic rate(SMR),to investigate whether specialized adaptations in toxin-resistant garter snakes affect baseline energy expenditure.In snakes,skeletal muscles influence metabolism and power ventilation,so inefficiencies of sodium channels in these muscles might impact whole-animal energy expenditure.Further,because sodium channels are membrane-bound proteins,inefficiencies of channel kinetics and performance might be exacerbated at suboptimal temperatures.We measured SMR in 2 species,Thamnophis atratus and Thamnophis sirtalis,that independently evolved tetrodotoxin resistance through unique mutations,providing replicate experiments with distinct underlying genetics and potential physiological costs.Despite our expectations,neither resistance phenotype nor sodium channel genotype affected metabolism and resistant snakes did not perform worse under suboptimal body temperature.Instead,T.atratus and T.sirtalis show nearly identical rates of mass-adjusted energy expenditure at both temperatures,despite differing eco-morphologies,life histories,and distant phylogenetic positions.These findings suggest SMR may be a conserved feature of Thamnophis,and that any organismal tradeoffs may be compensated to retain whole-animal function.