Body size variation in insects: a macroecological perspective

Body size is a key feature of organisms and varies continuously because of the effects of natural selection on the size-dependency of resource acquisition and mortality rates. This review provides a critical and synthetic overview of body size variation in insects from a predominantly macroecological (large-scale temporal and spatial) perspective. Because of the importance of understanding the proximate determinants of adult size, it commences with a brief summary of the physiological mechanisms underlying adult body size and its variation, based mostly on findings for the model species Drosophila melanogaster and Manduca sexta . Variation in nutrition and temperature have variable effects on critical weight, the interval to cessation of growth (or terminal growth period) and growth rates, so influencing final adult size. Ontogenetic and phylogenetic variation in size, compensatory growth, scaling at the intra- and interspecific levels, sexual size dimorphism, and body size optimisation are then reviewed in light of their influences on individual and species body size frequency distributions. Explicit attention is given to evolutionary trends, including gigantism, Cope's rule and the rates at which size change has taken place, and to temporal ecological trends such as variation in size with succession and size-selectivity during the invasion process. Large-scale spatial variation in size at the intraspecific, interspecific and assemblage levels is considered, with special attention being given to the mechanisms proposed to underlie clinal variation in adult body size. Finally, areas particularly in need of additional research are identified.     

There's more to macroecology than meets the eye

Macroecology sits at the junction of, and can contribute to, the fields of ecology, biogeography, palaeontology and macroevolution, using a broad range of approaches to tackle a diverse set of questions. Here, we argue that there is more to macroecology than mapping, and that while they are potentially useful, maps are insufficient to assess macroecological pattern and process. The true nature of pattern can only be assessed, and competing hypotheses about process can only be disentangled, by adopting a statistical approach, and it is this that has been key to the development of macroecology as a respected and rigorous scientific discipline.     

Climatic adaptation and the evolution of basal and maximum rates of metabolism in rodents

Metabolic rate is a key aspect of organismal biology and the identification of selective factors that have led to species differences is a major goal of evolutionary physiology. We tested whether environmental characteristics and/or diet were significant predictors of interspecific variation in rodent metabolic rates. Mass-specific basal metabolic rates (BMR) and maximum metabolic rates (MMR, measured during cold exposure in a He-O2 atmosphere) were compiled from the literature. Maximum (Tmax) and minimum (Tmin) annual mean temperatures, latitude, altitude, and precipitation were obtained from field stations close to the capture sites reported for each population (N = 57). Diet and all continuous-valued traits showed statistically significant phylogenetic signal, with the exception of mass-corrected MMR and altitude. Therefore, results of phylogenetic analyses are emphasized. Body mass was not correlated with absolute latitude, but was positively correlated with precipitation in analyses with phylogenetically independent contrasts. Conventional multiple regressions that included body mass indicated that Tmax (best), Tmin, latitude, and diet were significant additional predictors of BMR. However, phylogenetic analyses indicated that latitude was the only significant predictor of mass-adjusted BMR (positive partial regression coefficient, one-tailed P = 0.0465). Conventional analyses indicated that Tmax, Tmin (best), and altitude explained significant amounts of the variation in mass-adjusted MMR. With body mass and Tmin in the model, no additional variables were significant predictors. Phylogenetic contrasts yielded similar results. Both conventional and phylogenetic analyses indicated a highly significant positive correlation between residual BMR and MMR (as has also been reported for birds), which is consistent with a key assumption of the aerobic capacity model for the evolution of vertebrate energetics (assuming that MMR and exercise-induced maximal oxygen consumption are positively functionally related). Our results support the hypothesis that variation in environmental factors leads to variation in the selective regime for metabolic rates of rodents. However, the causes of a positive association between BMR and latitude remain obscure. Moreover, an important area for future research will be experiments in all taxa are raised under common conditions to allow definitive tests of climatic adaptation in endotherm metabolic rates and to elucidate the extent of adaptive phenotypic plasticity.     

The phylogeny of a species-level tendency: species heritability and possible deep origins of Bergmann's rule in tetrapods

One of the most widely recognized generalizations in biology is Bergmann's rule, the observation that, within species of birds and mammals, body size tends to be inversely related to ambient temperature. Recent studies indicate that turtles and salamanders also tend to follow Bergmann's rule, which hints that this species-level tendency originated early in tetrapod history. Furthermore, exceptions to Bergmann's rule are concentrated within squamate reptiles (lizards and snakes), suggesting that the tendency to express a Bergmann's rule cline may be heritable at the species level. We evaluated species-level heritability and early origination of Bergmann's rule by mapping size-latitude relationships for 352 species onto a tetrapod phylogeny. When the largest available dataset is used, Bergmann's rule shows significant phylogenetic signal, indicating species-level heritability. This represents one of the few demonstrations of heritability for an emergent species-level property and the first for an ecogeographic rule. When species are discretely coded as showing either Bergmann's rule or its converse, parsimony reconstructions suggest that: (1) the tendency to follow Bergmann's rule is ancestral for tetrapods, and (2) most extant species that express the rule have retained this tendency from that ancient ancestor. The first inference also generally holds when the discrete data or size-latitude correlation coefficients are analyzed using maximum likelihood, although the results are only statistically significant for some versions of the discrete analyses. The best estimates of ancestral states suggest that the traditional adaptive explanation for Bergmann's rule-conservation of metabolic heat-was not involved in the origin of the trait since that origin predates the evolution of endothermy. A more general thermoregulatory hypothesis could apply to endotherms and some ectotherms, but fails to explain why salamanders have retained Bergmann's rule. Thus, if thermoregulation underlies the origin of a Bergmann's rule tendency, this trait may have been continuously maintained while its cause changed. Alternatively, thermoregulation may not underlie Bergmann's rule in any tetrapod group. The results also suggest that many extinct groups not included in our analyses followed Bergmann's rule.     

Cytotoxic potency of trialkyltins to C6 glioma cells <Emphasis Type="Italic">in vitro</Emphasis>: impact of exposure conditions

Because of a possible role of astrocytes in trialkyltin-induced neurotoxicity in vivo various studies have been performed using cultures of astrocytes or glioma cells in vitro. With respect to cytotoxic potencies of trialkyltins these studies gave rather divergent results. Therefore the aim of the present study was to clarify whether variations of experimental conditions could be responsible for the differences of the cytotoxic activities of trimethyltin (TMT), triethyltin (TET) and tributyltin (TBT). Experiments were performed with rat C6 glioma cells. Toxicity was determined by measuring the reduction of the cell protein content. Cultures of proliferating and growth-arrested cells did not differ in their sensitivity. Exposure duration (1–72 h) had a strong but differing influence on the cytotoxic potency of the trialkyltins. After short exposure times the potencies differed largely (TMT < TET < TBT), whereas they became more and more similar with increasing exposure duration. The potency–time relationships for TMT and TET could be described by the equation: EC₅₀ = k x t ^–n, while for TBT an incipient value (EC_50,) had to be included: EC₅₀ = EC ₅₀ + k x t ^–n. Addition of serum albumin to the culture medium decreased the cytotoxic potency of the trialkyltins. However, the impact of protein binding on their bioavailability was relatively low. The cytotoxic potency of the alkyltins was not dependent on the concentration of C6 cells. Taken together, neither differences in exposure conditions nor in the proliferative status of the cells are sufficient to account for the discrepancies in published results for trialkyltin cytotoxicity to astrocytes. Instead they may – at least partially – be explained by differing sensitivities of the endpoints used. Furthermore, C6 glioma cells respond considerably more sensitively to trialkytins than primary astrocytes, which questions their applicability as models for astrocyte toxicity.     

Latitudinal variation in the shape of the species body size distribution: an analysis using freshwater fishes

Many taxonomic and ecological assemblages of species exhibit a right-skewed body size-frequency distribution when characterized at a regional scale. Although this distribution has been frequently described, factors influencing geographic variation in the distribution are not well understood, nor are mechanisms responsible for distribution shape. In this study, variation in the species body size-frequency distributions of 344 regional communities of North American freshwater fishes is examined in relation to latitude, species richness, and taxonomic composition. Although the distribution of all species of North American fishes is right-skewed, a negative correlation exists between latitude and regional community size distribution skewness, with size distributions becoming left-skewed at high latitudes. This relationship is not an artifact of the confounding relationship between latitude and species richness in North American fishes. The negative correlation between latitude and regional community size distribution skewness is partially due to the geographic distribution of families of fishes and apparently enhanced by a nonrandom geographic distribution of species within families. These results are discussed in the context of previous explanations of factors responsible for the generation of species size-frequency distributions related to the fractal nature of the environment, energetics, and evolutionary patterns of body size in North American fishes.     

Patterns of postzygotic isolation in Lepidoptera

I present patterns characterizing the evolution of intrinsic postzygotic isolation in Lepidoptera by analyzing data from the literature on genetic distance, strength of hybrid sterility and inviability, biogeography, and natural hybridization. Using genetic distance as a proxy for time, I investigate the time-course of the evolution of postzygotic isolation and the waiting times to particular hybrid fitness problems. The results show that postzygotic isolation increases gradually as species diverge, but that hybrid sterility evolves faster than hybrid inviability. The overwhelming preponderance of female-specific hybrid problems in Lepidoptera shows that Haldane's rule (the preferential sterility or inviability of the heterogametic sex) is well obeyed. Together the rates and patterns characterizing the accumulation of postzygotic isolation allow several tests of the composite theory of Haldane's rule. Interestingly, comparing these data with those from Drosophila reveals that Haldane's rule for sterility evolves as fast (if not faster) in Lepidoptera. Finally, I show that a substantial fraction of sympatric species hybridizes in nature and that the majority of these suffer some level of hybrid sterility or inviability.     

Sequential estimation in line transect surveys

This article considers using sequential procedures to determine the amount of survey effort required in a line transect survey in order to achieve a certain precision level in estimating the abundance of a biological population. Sequential procedures are constructed for both parametric and nonparametric animal abundance estimators. The criterion used to derive the stopping rules is the width of confidence intervals for the animal abundance. For each estimator considered, we develop stopping rules based on the asymptotic distributions and the bootstrap. A sequential analysis on an aerial survey of the southern bluefin tuna indicates substantial saving of survey effort can be made by employment of the proposed sequential procedures. This savings of survey effort is also observed in a simulation study designed to evaluate the empirical performance of the proposed sequential procedures.