An optimum body size for mammals? Comparative evidence from bats

1. The distribution of body sizes among mammalian species has been modelled by Brown, Marquet & Taper (1993), who suggest that reproductive power (the rate at which energy from the environment is channelled into offspring production) is maximized at a size of 100 g, and the observed size distribution among species reflects the way reproductive power depends on size. The model makes a testable prediction about life-history allometries: namely, that components of reproductive power should not scale linearly with body size but should change sign at the optimum size.
2. A large set of life-history data from a single clade of small mammals, the bats (Order: Chiroptera), was analysed to test this key prediction. The analyses in this study offer no support for the idea that allometries of reproductive power change sign in bats, either at 100 g or at any other size. Furthermore the life-history allometries of bats, which are mostly below the 100 g optimum, were broadly the same as in mammalian taxa larger than the optimum size.
3. These findings together contradict a key prediction of Brown et al. 's (1993) model to explain the skewed body size distribution across mammalian species.     

Insectivore life histories: further evidence against an optimum body size for mammals

1. This study tests a model of the relationship of body mass to reproductive power (the rate of conversion of energy from the environment to an organism's offspring). Specifically tested is the prediction that the regression of life-history variables on body size will change slope and sign about an 'optimum' body mass of 100 g.
2. Life-history data from the mammalian order Insectivora have been collated and analysed using a phylogenetic comparative method to test this prediction.
3. The analyses showed little evidence for significant changes in slope or sign around 100g body mass, or other possible optimal body masses, contradicting the predictions of the model. These findings agree with those of similar analyses on life-history variables of bats.     

Ancestral state reconstruction of body size in the Caniformia (Carnivora, Mammalia): the effects of incorporating data from the fossil record

A recent molecular phylogeny of the mammalian order Carnivora implied large body size as the ancestral condition for the caniform subclade Arctoidea using the distribution of species mean body sizes among living taxa. "Extant taxa-only" approaches such as these discount character state observations for fossil members of living clades and completely ignore data from extinct lineages. To more rigorously reconstruct body sizes of ancestral forms within the Caniformia, body size and first appearance data were collected for 149 extant and 367 extinct taxa. Body sizes were reconstructed for four ancestral nodes using weighted squared-change parsimony on log-transformed body mass data. Reconstructions based on extant taxa alone favored large body sizes (on the order of 10 to 50 kg) for the last common ancestors of both the Caniformia and Arctoidea. In contrast, reconstructions incorporating fossil data support small body sizes (< 5 kg) for the ancestors of those clades. When the temporal information associated with fossil data was discarded, body size reconstructions became ambiguous, demonstrating that incorporating both character state and temporal information from fossil taxa unambiguously supports a small ancestral body size, thereby falsifying hypotheses derived from extant taxa alone. Body size reconstructions for Caniformia, Arctoidea, and Musteloidea were not sensitive to potential errors introduced by uncertainty in the position of extinct lineages relative to the molecular topology, or to missing body size data for extinct members of an entire major clade (the aquatic Pinnipedia). Incorporating character state observations and temporal information from the fossil record into hypothesis testing has a significant impact on the ability to reconstruct ancestral characters and constrains the range of potential hypotheses of character evolution. Fossil data here provide the evidence to reliably document trends of both increasing and decreasing body size in several caniform clades. More generally, including fossils in such analyses incorporates evidence of directional trends, thereby yielding more reliable ancestral character state reconstructions.     

The Probabilistic Niche Model Reveals the Niche Structure and Role of Body Size in a Complex Food Web

The niche model has been widely used to model the structure of complex food webs, and yet the ecological meaning of the single niche dimension has not been explored. In the niche model, each species has three traits, niche position, diet position and feeding range. Here, a new probabilistic niche model, which allows the maximum likelihood set of trait values to be estimated for each species, is applied to the food web of the Benguela fishery. We also developed the allometric niche model, in which body size is used as the niche dimension. About 80% of the links in the empirical data are predicted by the probabilistic niche model, a significant improvement over recent models. As in the niche model, species are uniformly distributed on the niche axis. Feeding ranges are exponentially distributed, but diet positions are not uniformly distributed below the predator. Species traits are strongly correlated with body size, but the allometric niche model performs significantly worse than the probabilistic niche model. The best-fit parameter set provides a significantly better model of the structure of the Benguela food web than was previously available. The methodology allows the identification of a number of taxa that stand out as outliers either in the model''s poor performance at predicting their predators or prey or in their parameter values. While important, body size alone does not explain the structure of the one-dimensional niche.     

Temperature-associated upper limits to body size in terrestrial poikilotherms

We show that the largest tropical species of terrestrial poikilotherms from 25 taxa exceed, in linear body size, the largest representatives of the same taxa from the temperate (e.g. Great Britain) and polar (e.g. Wrangel Island) zones by 3.2 and 5.7 times, respectively. Here we develop a theoretical approach which quantitatively explains the observed body size patterns and relates them to ambient temperature under the assumption that there exists a temperature-independent critical minimum value of mass-specific metabolic rate b_min, a fall below which is incompatible with successful biological performance. This value sets an upper limit to linear body size within a taxon. Mass-specific metabolic rate decreases with increasing body size but, in poikilotherms, grows exponentially with ambient temperature. Such compensation of the size-related drop in mass-specific metabolic rate b by higher ambient temperature extends the permitted range of body sizes for which b≥b_min. As a result, the maximum linear body size grows approximately twofold per each ten degrees of increase in ambient temperature. We also discuss why this prediction does not apply to interspecific comparisons of aquatic poikilotherms, for which an opposite trend is to be expected. We quantify in theory the maximum body size patterns recently reported for benthic gammaridean amphipods.     

Reversed sexual size dimorphism in microtines: Are females larger than males or are males smaller than females?

Summary We analysed sexual size dimorphism for 21 populations of microtine rodents. Female to male size ratio varied considerably among populations from females significantly larger than males (ratio=1.18) to males larger than females (ratio=0.78). In a multiple regression analysis female to male home range size ratio explained 94% of the total variation in body size dimorphism and was the only one of eight independent variables that was selected in a stepwise regression procedure. When females are the larger sex, males have home range sizes much larger than females. We suggest that the relationship between home range size ratio and body weight size dimorphism reflects different selection pressures on males and females in competition for resources and mates.     

Fitness trade-offs and the maintenance of alternative male morphs in the bulb mite (Rhizoglyphus robini)

Alternative reproductive phenotypes (ARPs) occur across a wide range of taxa. Most ARPs are conditionally expressed in response to a cue, for example body size, that reliably correlates with the status of the environment: individuals below the (body size) threshold then develop into one morph, and individuals above the threshold develop into the alternative morph. The environmental threshold model provides a theoretical framework to understand the evolution and maintenance of such ARPs, yet no study has examined the underlying fitness functions that are necessary to realize this. Here, we empirically examined fitness functions for the two male morphs of the bulb mite (Rhizoglyphus robini). Fitness functions were derived in relation to male size for solitary males and in relation to female size under competition. In both cases, the fitness functions of the two morphs intersected, and the resulting fitness trade-offs may play a role in the maintenance of this male dimorphism. We furthermore found that competition was strongest between males of the same morph, suggesting that fitness trade-off in relation to male size may persist under competition. Our results are a first step towards unravelling fitness functions of ARPs that are environmentally cued threshold traits, which is essential for understanding their maintenance and in explaining the response to selection against alternative morphs.     

A test of Rensch's rule in varanid lizards

In a model group of giant reptiles, we explored the allometric relationships between male and female body size and compared the effects of sexual and fecundity selection, as well as some proximate causes, on macroevolutionary patterns of sexual size dimorphism (SSD). Monitor lizards are a morphologically homogeneous group that has been affected by extreme changes in body size during their evolutionary history, resulting in 14‐fold differences among the body sizes of recent species. Here, we analysed data concerning the maximum and/or mean male and female snout–vent lengths in 42 species of monitor lizard from literary sources and supplemented these data with measurements made in zoos. There was a wide scale of SSD from nearly monomorphic species belonging mostly to the subgenus Odatria and Prasinus group of the Euprepriosaurus to apparently male‐larger taxa. The variable best explaining SSD was the body size itself; the larger the species, the higher the SSD. This pattern agrees with the currently discussed Rensch's rule, claiming that the relationship between male and female body size is hyperallometric, i.e. the allometric exponent of this relationship exceeds unity and thus SSD increases with body size in the case of male‐larger taxa. All our estimates of the reduced major axis regression slopes of this relationship ranged from 1.132 to 1.155. These estimates are significantly higher than unity, and thus unequivocally corroborate the validity of Rensch's rule in this reptilian group. In spite of our expectation that the variation in SSD can be alternatively explained by variables reflecting the strength of sexual selection (presence of male combat), fecundity selection (e.g. clutch size and mass) and/or proximate ecological factors (habitat type), none of these variables had consistent effects on SSD, especially when the data were adjusted to phylogenetic dependence and/or body size. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 293–306.     

Interspecific patterns of species richness, geographic range size, and body size among New World venomous snakes

Many higher taxa exhibit latitudinal gradients in species richness, geographic range size, and body size. However, these variables are often interdependent, such that examinations of univariate or bivariate patterns alone may be misleading. Therefore, I examined latitudinal gradients in, and relationships between, species richness, geographic range size, and body size among 144 species of New World venomous snakes [families Elapidae (coral snakes) and Viperidae (pitvipers)]. Both lineages are monophyletic, collectively span 99° of latitude, and are extremely variable in body size and geographic range sizes. Coral snakes exhibit highest species richness near the equator, while pitviper species richness peaks in Central America. Species – range size distributions were strongly right-skewed for both families. There was little support for Bergmann's rule or Rapoport's rule for snakes of either family, as neither body size nor range size increased significantly with latitude. However, range area and median range latitude were positively correlated above 15° N, indicating a possible "Rapoport effect" at high northern latitudes. Geographic range size was positively associated with body size. Available continental area strongly influenced range size. Comparative (phylogenetically-based) analyses revealed that shared history is a poor predictor of range size variation within clades. Among vipers, trends in geographic range sizes may have been structured more by historical biogeography than by macroecological biotic factors.     

Locomotion in some small to medium-sized mammals: a geometric morphometric analysis of the penultimate lumbar vertebra,pelvis and hindlimbs

We assessed the influence of a variety of aspects of locomotion and ecology including gait and locomotor types, maximal running speed, home range, and body size on postcranial shape variation in small to medium-sized mammals, employing geometric morphometric analysis and phylogenetic comparative methods. The four views analyzed, i.e., dorsal view of the penultimate lumbar vertebra, lateral view of the pelvis, posterior view of the proximal femur and proximal view of the tibia, showed clear phylogenetic signal and interesting patterns of association with movement. Variation in home range size was related to some tibia shape changes, while speed was associated with lumbar vertebra, pelvis and tibia shape changes. Femur shape was not related to any locomotor variables. In both locomotor type and high-speed gait analyses, locomotor groups were distinguished in both pelvis and tibia shape analyses. These results suggest that adaptations to both typical and high-speed gaits could explain a considerable portion of the shape of those elements. In addition, lumbar vertebra and tibia showed non-significant relationships with body mass, which suggests that they might be used in morpho-functional analyses and locomotor inferences on fossil taxa, with little or no bias for body size. Lastly, we observed morpho-functional convergences among several mammalian taxa and detected some taxa that achieve similar locomotor features following different morphological paths.     

The geography of body size – challenges of the interspecific approach

Recent compilations of large-scale data bases on the geographical distributions and body sizes of animals, coupled with developments in spatial statistics, have led to renewed interest in the geographical distribution of animal body sizes and the interspecific version of Bergmann's rule. Standard practice seems to be an examination of mean body sizes within higher taxa on gridded maps, with little regard to species richness or phylogeny. However, because the frequency distribution of body sizes is typically highly skewed, average size within grid cells may differ significantly between species-rich and species-poor cells even when the median and modal sizes remain constant. Species richness influences body size patterns because species are not added to communities at random in relation to their size: areas of low diversity are characterized by a higher range of body sizes than is expected by chance. Finally, a consideration of phylogenetic structure within taxa is necessary to elucidate whether patterns in the geography of size result from turnover between or within intermediate taxonomic levels. We suggest that the highest and lowest quantiles of body size distribution be mapped in order to expose possible physiological or ecological limitations on body size.     

The ontogeny of home ranges: evidence from coral reef fishes

The concept of home ranges is fundamental to ecology. Numerous studies have quantified how home ranges scale with body size across taxa. However, these relationships are not always applicable intraspecifically. Here, we describe how the home range of an important group of reef fish, the parrotfishes, scales with body mass. With masses spanning five orders of magnitude, from the early postsettlement stage through to adulthood, we find no evidence of a response to predation risk, dietary shifts or sex change on home range expansion rates. Instead, we document a distinct ontogenetic shift in home range expansion with sexual maturity. Juvenile parrotfishes displayed rapid home range growth until reaching approximately 100–150 mm length. Thereafter, the relationship between home range and mass broke down. This shift reflected changes in colour patterns, social status and reproductive behaviour associated with the transition to adult stages. While there is a clear relationship between body mass and home ranges among adult individuals of different species, it does not appear to be applicable to size changes within species. Ontogenetic changes in parrotfishes do not follow expected mass–area scaling relationships.     

Age, area and avian diversification

Using coarse resolution data on the spatial distribution of the entire New World avifauna, we test for phylogenclic patterns in the mean and total geographic range sizes of taxa. The analyses reveal that (i) the species-range size distribution is only approximately normalized, and remains significantly left-skewed, under logarithmic transformation. Most variance in range sizes is explained at the level of species within genera; (ii) there is no effect of the age of taxa on mean clade range size, although older taxa are more likely to have larger total range sizes; (iii) there is some evidence that taxa comprising more species have larger total range sizes; (iv) there is little or no evidence for a relationship between rate of cladogenesis and range size. The results suggest that geographic range size is a labile trait, at least for New World birds, and that the influence of evolutionary history is only weakly detectable in the range size variation of extant taxa, at least at the scale of analysis used here. In addition to these conclusions, two general and important procedural issues emerge.     

The biogeography of plant reproduction: potential determinants of species' range sizes

Aim A major biogeographical hypothesis, the ‘niche‐breadth’ hypothesis, explains species range sizes based on the extent of a species’ niche (e.g. diversity of habitats occupied; range of environmental conditions tolerated). An alternative hypothesis explains range sizes using metapopulation theory (e.g. colonization dynamics; dispersal ability). Both niche breadth and colonization ability may be related to plant species’ reproductive characteristics. We evaluate both hypotheses by examining the relationship of plant range size to mating system and genomic structure (ploidy status). Location Western North America. Methods Using a data set of 60 taxa in the genus Clarkia (Onagraceae), we use three analytical techniques to examine the effect of reproductive characteristics on range size. We conduct cross‐species analyses of present‐day taxa to examine both mating system and polyploidy in relation to range size. We also conduct a phylogenetically independent contrasts analysis (using caic software) on the relationship of mating system to range size in diploid species. Third, we compare closely related taxon pairs that differ in mating system as an alternative method to control for phylogeny. Results Polyploid species have significantly larger ranges compared with diploid species. When considering only diploid taxa, no significant relationship is found for mating system in both cross‐species and phylogenetically independent contrasts analyses. The diploid pairwise analysis, comparing only the range sizes of direct sister taxa with alternative mating systems, does show a relationship, with outcrossing species having larger ranges than self‐fertilizing species. Main conclusions We argue that genetic diversity, colonization ability, or a combination of both factors may influence plant species’ range sizes. The significant pairwise analysis suggests that both the independent contrast and the cross‐species analysis may be confounded by polytomies of species at the terminal nodes of the phylogeny, indicating the importance of comparing the results of multiple analytical techniques. We propose that the range sizes of self‐fertilizing species have a bimodal distribution, obscuring the effect of mating system on range size, and that a broader survey of plant taxa will resolve the two modes from that of the outcrossing species. Lastly, polyploid species appear to show significantly larger range sizes than diploid species, irrespective of mating system.     

Comparative energetics of the giant hummingbird (Patagona gigas)

Hummingbirds (family Trochilidae) represent an extreme outcome in vertebrate physiological design and are the only birds capable of sustained hovering. The giant hummingbird (Patagona gigas) is the largest trochilid, with a mass of ~20 g, and is found over an altitudinal range from 0 to 4,500 m above sea level. We report here measurements of daily, basal, and hovering rates of oxygen consumption in the giant hummingbird; compare these values with data from smaller hummingbirds; and assess overall metabolic and allometric limits to trochilid body size. The sustained metabolic scope (i.e., the ratio of daily energy expenditure to basal metabolic rate) in the giant hummingbird is higher than that in smaller hummingbirds but lies below a proposed theoretical maximum value for endotherms. Scaling exponents in the allometric relationships for different modes of energetic expenditure were comparable, suggesting that the giant hummingbird, although a clear outlier in terms of body size, does not obviously deviate from metabolic relationships derived from other trochilid taxa.     

Morphometrics of the anterior dentition in strepsirhine primates

Size variations in the anterior dentition were analyzed for 26 species of strepsirhine primates. The upper and lower incisor rows of strepsirhines, like those of anthropoid primates, scale isometrically with body size. Within the order Primates, strepsirhines exhibit the smallest incisors relative to body size, followed in increasing size by tarsiers, platyrrhines, and catarrhines. If the lateral teeth of the indriid toothcomb are interpreted as incisors and not canines, correlations between mandibular tooth size variables and body weight are maximized. The upper incisors of strepsirhines are extremely small and frequently widely separated, most likely to minimize occlusion with the toothcomb. Species deviations for assorted size variables of the anterior dentition generally fail to reflect functional variations in the use of the anterior teeth; some of the variables, however, do reflect taxonomic differences within the Strepsirhini. Although toothcomb size variations among extant strepsirhines are more readily interpreted in terms of gum feeding and bark scraping than they are in terms of grooming, anterior dental morphology as a whole is more easily explained by a grooming hypothesis when existing models of toothcomb origins are considered.     

Patterns of diversity, altitudinal range and body size among freshwater fishes in the Yangtze River basin, China

Aim To document patterns in diversity, altitudinal range and body size of freshwater fishes along an elevational gradient in the Yangtze River basin. Location The Yangtze River basin, China. Methods We used published data to compile the distribution, altitudinal range and body size of freshwater fishes. Correlation, regression, clustering and graphical analyses were used to explore patterns in diversity, altitudinal range and body size of freshwater fishes in 100‐m elevation zones from 0 to 5200 m. Results Species richness patterns across the elevational gradient for total, non‐endemic and endemic fishes were different. The ratio of endemics to total richness peaked at mid elevation. Land area on a 500‐m interval scale explained a significant amount of the variation in species richness. Species density displayed two peaks at mid‐elevation zones. The cluster analysis revealed five distinct assemblages across the elevation gradient. The relationship between elevational range size and the midpoint of the elevational range revealed a triangular distribution. The frequency distribution of log maximum standard length data displayed an atypical right‐skewed pattern. Intermediate body sizes occurred across the greatest range of elevation while small and large body sizes possessed only small elevational amplitudes. The size‐elevation relationship between the two major families revealed a very strong pattern of body size constraint among the Cobitidae with no corresponding elevational constraint and a lot of body size and elevational diversification among the Cyprinidae. Main conclusion The data failed to support either Rapoport's rule or Bergmann's rule.     

Simulation of cell movement and interaction

A mechanical model of cell motion was developed that reproduced the behaviour of cells in 2-dimensional culture. Cell adhesion was modelled with inter-cellular cross-links that attached for different times giving a range of adhesion strength. Simulations revealed an adhesion threshold below which cell motion was almost unaffected and above which cells moved as if permanently linked. Comparing simulated cell clusters (with known connections) to calculated clusters (based only on distance) showed that the calculated clusters did not correspond well across the full size range from small to big clusters. The radial distribution function of the cells was found to be a better measure, giving a good correlation with the known cell linkage throughout the simulation run. This analysis showed that cells were best modelled with a degree of stickiness just under the critical threshold level. This allowed fluidlike motion while maintaining cohesiveness across the population.     

The influence of sex and body size on food habits of a giant tropical snake, Python reticulatus

1. In many animal species, dietary habits shift with body size, and differ between the sexes. However, the intraspecific range of body sizes is usually low, making it difficult to quantify size-associated trophic shifts, or to determine the degree to which sex differences in diet are due to body-size differences. Large snakes are ideal for such a study, because they provide a vast range of body sizes within a single population.
2. More than 1000 Reticulated Pythons ( Python reticulatus ) from southern Sumatra were examined, with specimens from 1·5 to > 6 m in snout–vent length, and from 1 to 75 kg in mass. Females attained much larger body sizes than did conspecific males (maxima of 20 vs 75 kg, 5 vs 7 m), but had similar head lengths at the same body lengths.
3. Prey sizes, feeding frequencies and numbers of stomach parasites (ascarid nematodes) increased with body size in both sexes, and dietary composition changed ontogenetically. Small snakes fed mostly on rats, but shifted to larger mammalian taxa (e.g. pangolins, porcupines, monkeys, wild pigs, mouse deer) at 3–4-m body length.
4. Adult males and females showed strong ecological divergence. For some traits, this divergence was entirely caused by the strong allometry (combined with sexual size dimorphism), but in other cases (e.g. feeding frequency, dietary composition), the sexes followed different allometric trajectories. For example, females shifted from rats to larger mammals at a smaller body size than did conspecific males, and feeding frequencies increased more rapidly with body size in females than in males. These allometric divergences enhanced the degree of sex difference in trophic ecology induced by sexual size dimorphism.     

Herbivorous fishes,ecosystem function and mobile links on coral reefs

Understanding large-scale movement of ecologically important taxa is key to both species and ecosystem management. Those species responsible for maintaining functional connectivity between habitats are often called mobile links and are regarded as essential elements of resilience. By providing connectivity, they support resilience across spatial scales. Most marine organisms, including fishes, have long-term, biogeographic-scale connectivity through larval movement. Although most reef species are highly site attached after larval settlement, some taxa may also be able to provide rapid, reef-scale connectivity as adults. On coral reefs, the identity of such taxa and the extent of their mobility are not yet known. We use acoustic telemetry to monitor the movements of Kyphosus vaigiensis, one of the few reef fishes that feeds on adult brown macroalgae. Unlike other benthic herbivorous fish species, it also exhibits large-scale (>2 km) movements. Individual K. vaigiensis cover, on average, a 2.5 km length of reef (11 km maximum) each day. These large-scale movements suggest that this species may act as a mobile link, providing functional connectivity, should the need arise, and helping to support functional processes across habitats and spatial scales. An analysis of published studies of home ranges in reef fishes found a consistent relationship between home range size and body length. K. vaigiensis is the sole herbivore to depart significantly from the expected home range–body size relationship, with home range sizes more comparable to exceptionally mobile large pelagic predators rather than other reef herbivores. While the large-scale movements of K. vaigiensis reveal its potential capacity to enhance resilience over large areas, it also emphasizes the potential limitations of small marine reserves to protect some herbivore populations.