Jack P. Hayes

THE EVOLUTION OF ENDOTHERMY: TESTING THE AEROBIC CAPACITY MODEL

One of the most important events in vertebrate evolution was the acquisition of endothermy, the ability to use metabolic heat production to elevate body temperature above environmental temperature. Several verbal models have been proposed to explain the selective factors leading to the evolution of endothermy. Of these, the aerobic capacity model has received the most attention in recent years. The aerobic capacity model postulates that selection acted mainly to increase maximal aerobic capacity (or associated behavioral abilities) and that elevated resting metabolic rate evolved as a correlated response. Here we evaluate the implicit evolutionary and genetic assumptions of the aerobic capacity model. In light of this evaluation, we assess the utility of phenotypic and genetic correlations for testing the aerobic capacity model. Collectively, the available intraspecific data for terrestrial vertebrates support the notion of a positive phenotypic correlation between resting and maximal rates of oxygen consumption within species. Interspecific analyses provide mixed support for this phenotypic correlation. We argue, however, that assessments of phenotypic or genetic correlations within species and evolutionary correlations among species (from comparative data) are of limited utility, because they may not be able to distinguish between the aerobic capacity model and plausible alternatives, such as selection acting directly on aspects of thermoregulatory abilities. We suggest six sources of information that may help shed light on the selective factors important during the evolution of high aerobic metabolic rates and, ultimately, the attainment of endothermy. Of particular interest will be attempts to determine, using a combination of mechanistic physiological and quantitative‐genetic approaches, whether a positive genetic correlation between resting and maximal rates of oxygen consumption is an ineluctable feature of vertebrate physiology.

Individual Variation in Mammals

The study of individual variation offers an underexploited wealth of opportunities for mammalogists. This paper addresses recent developments in the study of both intra- and inter-individual variation. After reviewing several methods (e.g., intraclass correlation, product-moment correlation, and confirmatory factor analysis) for quantifying intra-individual consistency or repeatability, we discuss how these measures of repeatability can serve as guides for appropriately defining traits and how they may be helpful in ensuring that appropriate statistical models are used (e.g., in accounting for measurement errors in regression analyses). We discuss three aspects of inter-individual variation; phenotypic selection, alternative individual strategies and phenotypic integration, and quantitative genetic analyses. The value of these approaches for studying inter-individual variation is illustrated with recent examples from the literature. Finally, we discuss how many field studies of mammals may be well poised to exploit the unique insights that can be gained from studying individual variation.

NATURAL SELECTION ON THERMOGENIC CAPACITY OF HIGH-ALTITUDE DEER MICE

Adaptive explanations that rely on physiological arguments are common, but tests of hypotheses about the significance of whole‐animal physiological performance (e.g., aerobic capacities) are rare. We studied phenotypic selection on the thermogenic capacity (i.e., maximal rate of oxygen consumption [VO2max] elicited via cold exposure) of high‐altitude (~3800 m) deer mice (Peromyscus maniculatus). A high VO2max equates to a high capacity for heat production and should favor survival in the cold environments prevalent at high altitude. Strong directional selection favored high VO2max, at least in one year. The selection for increased VO2max is consistent with predictions derived from incorporating our physiological data into a biophysical model. During another year, we found weak evidence of selection for decreased body mass. Nonlinear selection was not significant for any of the selection episodes we studied. The strong directional selection for VO2max that we observed suggests that—given ample genetic variation—aerobic metabolism and perhaps endothermy may have evolved rapidly on the geological time scale.

Morphometric indices of body condition in birds: a review

Morphometric estimates of body condition are widely used by ornithologists, but which estimates work best is a matter of debate. We review morphometric approaches (body mass, ratio and residual condition indices, predictive regression models, fat scoring, and abdominal profiles) for estimating body condition (defined as fat mass) in birds. We describe the strengths and weaknesses of each approach. Across diverse indices and species (~200 estimates total), the mean r2 relating condition indices to mass of body fat was 0.55, and 64% of the r2 values were greater than 0.50. But despite their generally good performance, condition indices sometimes perform poorly (i.e., r2 is low). The data indicate that: (1) no single index was clearly best, (2) on average body mass alone, fat scores, and predictive multiple regression equations explained slightly more than 50% of the variation in fat content, (3) on average, ratio and residual indices explained slightly less than 50% of the variation in fat content, and (4) body mass alone, a variable that can be easily and reliably measured, is as good or nearly as good an indicator of fat content as any other condition index. We recommend that: (1) morphometric indicators of condition be empirically validated, (2) researchers publish their body composition data in sufficient detail that they can be used in future analyses exploring the relative merits of different condition indices, and (3) multiple regression directly on measured traits be used instead of condition indices whenever the condition index is not empirically validated.ZusammenfassungMorphometrische Parameter zur Indikation der Körperkondition sind unter Ornithologen weit verbreitet, doch welche Parameter am Besten geeignet sind, wird lebhaft diskutiert. Wir stellen hier morphometrische Ansätze zur Indikation der Körperkondition, definiert als Fettmasse, bei Vögeln zusammen (Körpermasse, residuale Konditionsindices, Regressionsmodelle, Fettwert-Schätzungen und abdominales Pofil). Wir beschreiben die Stärken und Schwächen jedes dieser Ansätze. Über die verschiedenen Indices und Arten (insgesamt ~200 Ansätze) hinweg betrug das mittlere Bestimmtheitsmaß R2 zwischen Konditionsindices und Körpermasse 0,55, und 64% der R2-Werte waren größer als 0,50. Aber ungeachtet ihrer grundsätzlich ganz guten Bedeutung sind Konditionsindices manchmal sehr schwach (R2 ist gering). Die Daten deuten an, dass (1) kein Index für sich allein am besten war, (2) insgesamt Körpermasse, Fettwert und Regressionsmodelle etwas mehr als 50% der Variation im Fettgehalt erklärten, (3) im allgemeinen Verhältnisse und residuale Indices weniger als 50% der Variation im Fettgehalt erklärten, und (4) Körpermasse allein, eine leicht und zuverlässig zu bestimmende Größe, nahezu so gut ist als Indikator für den Fettgehalt wie jeder andere Konditionsindex. Wir empfehlen, dass (1) morphometrische Indikatoren der Körperkondition empirisch validiert werden, (2) Forscher ihre Daten zur Körperzusammensetzung so detailliert veröffentlichen, dass sie in zukünftigen Analysen verwendet werden können, um die relative Bedeutung der verschiedenen Konditionsindices überprüfen zu können, und (3) multiple Regressionsanalysen basierend auf den gemessenen Eigenschaften verwendet werden und nicht Konditionsindices, sofern diese nicht empirisch validiert sind.

Individual variation in metabolism and reproduction of Mus : are energetics and life history linked?

The possibility of functional relationships between energetics and life-history characteristics has been of considerable interest to evolutionary ecologists. Among species of mammals, life-history variables generally are not correlated with mass-independent basal metabolic rate, with the possible exceptions of maximal intrinsic rate of increase, litter size and reproductive effort during lactation. Whether this is generally true at the level of variation among individuals within a population (individual variation) is unclear. Therefore, we tested whether basal or maximal metabolic rates of random-bred female mice (Mus domesticus) were correlated with the size of their litters, litter mass, or mean offspring mass. The effects of variation in maternal mass, maternal age, experimental block and duration of fasting (for basal metabolic rate) were removed by calculating residuals from multiple regression equations. Basal and maximal metabolic rate were not significantly correlated with any of the life-history variables we studied. Thus, our results are generally consistent with those from interspecific comparisons of mammals: little evidence suggests necessary associations between metabolic rates and life history.

Sampling Bias in Respirometry

Comparisons of the metabolic rates of species, populations, and treatment groups of animals are common. However, the data used in these comparisons may not be truly equivalent. We report the effects of varying (1) total time from which minimum metabolism is selected and (2) time over which metabolic rate is calculated (calculation interval) on estimates of oxygen consumption for Microtus agrestis and Apodemus sylvaticus. Oxygen consumption was measured at 10°, 20°, and 30° C, using open-circuit respirometry. The lowest 15 min of metabolism were 13% and 65% higher for Microtus and Apodemus, respectively, when selected from a total monitoring period of 30 min than when selected from a period of 6 h. This demonstrates the importance of standardizing the duration of time from which minimal estimates of metabolism are selected (i. e., the amount of time for which metabolism is measured). For Microtus, minimum oxygen consumption was 12% higher when calculated over 60 min than when calculated over 15 min. We suggest that analyses relying on minimal (e.g., basal) metabolic rates include calculation interval as a potential covariate.

QUANTITATIVE GENETICS OF SPRINT RUNNING SPEED AND SWIMMING ENDURANCE IN LABORATORY HOUSE MICE (MUS DOMESTICUS)

We tested the hypothesis that locomotor speed and endurance show a negative genetic correlation using a genetically variable laboratory strain of house mice (Hsd:ICR: Mus domesticus). A negative genetic correlation would qualify as an evolutionary “constraint,” because both aspects of locomotor performance are generally expected to be under positive directional selection in wild populations. We also tested whether speed or endurance showed any genetic correlation with body mass. For all traits, residuals from multiple regression equations were computed to remove effects of possible confounding variables such as age at testing, measurement block, observer, and sex. Estimates of quantitative genetic parameters were then obtained using Shaws (1987) restricted maximum‐likelihood programs, modified to account for our breeding design, which incorporated cross‐fostering. Both speed and endurance were measured on two consecutive trial days, and both were repeatable. We initially analyzed performances on each trial day and the maximal value. For endurance, the three estimates of narrow‐sense heritabilities ranged from 0.17 to 0.33 (full ADCE model), and some were statistically significantly different from zero using likelihood ratio tests. The heritability estimate for sprint speed measured on trial day 1 was 0.17, but negative for all other measures. Moreover, the additive genetic covariance between speeds measured on the two days was near zero, indicating that the two measures are to some extent different traits. The additive genetic covariance between speed on trial day 1 and any of the four measures of endurance was negative, large, and always statistically significant. None of the measures of speed or endurance was significantly genetically correlated with body mass. Thus, we predict that artificial selection for increased locomotor speed in these mice would result in a decrease in endurance, but no change in body mass. Such experiments could lead to a better understanding of the physiological mechanisms leading to trade‐offs in aspects of locomotor abilities.

The Contributions of Local Heating and Reducing Exposed Surface Area to the Energetic Benefits of Huddling by Short-Tailed Field Voles (Microtus agrestis)

Many small mammals save energy by huddling. Huddling reduces the collective surface area exposed to the local environment and may heat the local environment. Consequently, huddling groups save energy, but the relative importance of reducing exposed surface area and local heating is largely undocumented. Oxygen consumption was measured in groups of Microtus agrestis Linnaeus consisting of five individuals, over a range of temperatures below thermoneutrality. Behavior was recorded simultaneously. Each group was measured when allowed to huddle and when prevented from huddling by partitions in the metabolism chamber. Local heating of the environment was correlated with temperature outside the metabolism chamber (outside temperature)for both huddling and separated groups, but local heating by huddling groups was significantly greater. At 0° C, local heating was 11.8° C and 5.8° C for huddling and separated groups, respectively. At the same outside temperature, O₂ consumption of huddling groups was lower than for separated groups (23% lower at 0° C). At 0° C, local heating accounted for about 55% of the total energy savings of the huddling group. Thus, local heating is a major component of the energy savings associated with huddling

Individual Consistency of Maximal Oxygen Consumption in Deer Mice

Individual physiological performance is often assumed to be consistent and repeatable. We examined the individual consistency of maximum oxygen consumption (Vo2max) in deer mice (Peromyscus maniculatus Wagner). Correlations of repeated measures on individuals were studied over time, short-term captivity, long-term cold acclimation to 3 0C, and acclimation to low (340 m) and high (3800 m) altitudes. The latter three treatments are known to induce significant changes in Vo2max. Two measurement protocols, severe cold exposure and intense exercise, were used to elicit Vo2max. The 1Vo2max as elicited by exercise and cold exposure were strongly and significantly correlated for all experimental treatments. Individual correlations across experimental treatments were generally strong for whole-animal Vo2max data, using both the exercise and cold exposure protocols. Correlations were weaker, but usually remained significant, when effects of body mass were removed. Across-altitude correlations were significant in most cases but correlation coefficients were lower than for other treatments. Our results suggest that in many cases, individual Vo2max measurements in one experimental situation are useful indicators of performance relative to the group mean in other situations or acclimation regimes. Key-words: Deer mice, high altitude, maximal aerobic metabolism, phenotypic plasticity, physiological performance, repeatability, within-individual variation

Allometry, Antilog Transformations, and the Perils of Prediction on the Original Scale

Biologists often use allometric equations that take the form of power functions (e.g., \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape