Body Size Constraints Research Articles

Shortfalls in tracking data available to inform North American migratory bird conservation

Animal tracking has become an effective way to identify where and when migratory species encounter threats throughout their annual cycle. Yet, untracked or poorly tracked species and undiscoverable or inaccessible data for the species that have been tracked mean that gaps in the knowledge of where and when species occur are still an issue for conservation. These gaps in knowledge of species movements have been termed the “movement shortfall”. Here, we quantify the movement shortfall for North American migratory birds by comprehensively reviewing full annual cycle tracking data and identifying gaps and biases in how, where, and what species are tracked with electronic tracking devices. We found 30 species for which tracking is not feasible given body size constraints, no data for 291 trackable species, and restricted or reduced data accessibility for an additional 59 species. Thus, despite the ability to track most species, the movement shortfall remains a constraint to informing conservation strategies for 56 % of North American migratory bird species. The number increases to 65 % when considering species with restricted or reduced data accessibility, further limiting access to this information. Moreover, 23 % of the tracking data stems from low precision tracking technologies reducing the implementation and effectiveness to conservation actions. A lack of species and population data hinders conservation and biases management decisions, ultimately making inefficient use of conservation resources. We encourage researchers to consider these gaps in their decisions about future tracking efforts, conservation management, and data archiving practices.

The ontogenetic dietary shift from non-dangerous to dangerous prey in predator-eating predators under capture risk.

Evaluating the patterns and generality of ontogenetic dietary shifts (ODSs) contributes to understanding prey-predator interactions and food web dynamics. Numerous studies have focused on predators that target distinctively lower trophic-level organisms. However, the ODS of predators that routinely prey on organisms at similar trophic levels (i.e., predator-eating predators) have been neglected in ODS research. The ODS patterns of predator eaters may not fit into conventional frameworks owing to constraints of potential capture risk (e.g., deadly counterattack from prey) and body size. We aimed to reveal the ODS patterns of predator eaters and determine whether the patterns were affected by body size and capture risk. Assuming that capture risk is a significant factor in ODS patterns, we expected: (1) juvenile araneophagic spiders to forage on non-dangerous prey (insects) and capture larger non-dangerous prey more frequently than dangerous prey (spiders); and (2) as they grow, their prey types will shift from non-dangerous to dangerous prey because larger predators will be able to capture dangerous prey as the optimal food. As a result of field observations, we revealed that the major ODS pattern in these spiders changed from a mixed (both insect and spider) to a spider-dominant diet. The model selection approach showed that this diet shift was partly due to predator size, and the relative importance of predator size was higher than the life stage per se and almost equal to species identity. In these spiders, the body size of spider prey tended to be smaller than that of insects when the predators were small, suggesting that capture risk may be a critical factor in determining the ODS patterns of these predators. Therefore, our study adds to the evidence that the capture risk is crucial in comprehensively understanding the mechanisms determining ODS patterns in natural systems.

Predators minimize energy costs, rather than maximize energy gains under warming: Evidence from a microcosm feeding experiment

Abstract Climate warming may alter predator–prey interactions and predator feeding behaviour due to increased metabolic demands. How predators meet these increased demands may depend on trade‐offs in prey energy content and body size, handling time and other functional constraints. We tested hypotheses associated with these trade‐offs with the predatory mite Stratiolaelaps scimitus, and three prey that differed in body size, energy content, and defenses (Folsomia candida, Oppia nitens, and Carpoglyphus lactis). We estimated metabolic rate, predation in choice and no choice feeding trials, movement rate, and lipid and protein content for all four species at 16°C and 24°C. We used these data to estimate the predator's energy demands and compared these to estimated energy intake in the choice feeding trials. Predators had greater metabolic demands at 24°C than at 16°C, but temperature did not affect predator or prey movement rates. Warming decreased lipid content, but not protein content, of all three prey species, leading to lower energy content for C. lactis and O. nitens, but not F. candida. In both feeding trials at 24°C, predators increased their feeding on the smaller, energy‐poor C. lactis, but not the larger, energy‐rich F. candida, resulting in lower estimated energy intake. S. scimitus did not feed on O. nitens at either temperature. Predators increasingly fed on small‐bodied prey under warming and not the large‐bodied prey despite the potential for greater energetic gains from larger prey. We posit that predators minimized energy lost during feeding through lower handling costs associated with C. lactis, rather than maximize energy gain. We conclude that selection of prey based on body size changes with temperature as a trade‐off for predators to balance increased metabolic demands. As predators provide top‐down control and regulate energy flow through the consumption of their prey, changes to predator feeding behaviour with climate warming may affect food web dynamics and ecosystem‐level processes. Read the free Plain Language Summary for this article on the Journal blog.

Ecological Predictors and Functional Implications of Eye Size in Deep-Sea Shrimps

Constraints on energy resources and available light in the deep sea should place strong selection pressure on eye size, a fundamental determinant of visual ability. By examining eye size among 16 species (454 individuals) of deep-sea sergestid shrimps, we show significant differences in intraspecific eye growth rates and species eye-size averages that are correlated to different aspects of ecology and result in variable sighting distance thresholds of bioluminescence, one measure of visual performance. We used linear regressions modeling the lowest and highest bounds of phylogenetic signal to test for ecological correlates of relative and absolute eye size, which indicate the allocation of energetic resources toward eyes and an optical basis of visual capability, respectively. Of the ecological variables tested [mean depth, diel vertical migration (DVM) distance, habitat type, and light organ type], light organ type was the only significant correlate of both relative and absolute eye size, suggesting that bioluminescence plays a particularly important role in the evolution of sergestid vision and that these animals may be reliant on bioluminescent signaling. Our findings also suggest that the DVM imposes visual demands distinct from the average depths occupied by a species. While DVM distance correlated with relative eye size, mean depth correlated with absolute eye size, revealing that eye size increases with depth before 1,000 m, then decreases in bathypelagic (aphotic) zone. By applying measured eye sizes to models of visual performance, we estimated that sergestids can detect a bioluminescent point source from ≤3.77 m away, and that these sighting distance thresholds vary between species by a factor of three. In relative terms, however, all sergestids under the test conditions had a common detection threshold at ∼63.5 body lengths, suggesting that bioluminescence sighting distance is proportional among species and may be related to shared behaviors of swarming and copulation. By considering the effects of evolutionary history, light and nutrient availability, and the constraints of body size, our study reveals new patterns of deep-sea eye size evolution and provides new insights into the visual ecology of this diverse and important deep-sea group.

Predator Environment Does Not Predict Life History in the Morphologically Constrained Fish Alfaro cultratus (Cyprinodontiformes: Poeciliidae)

Predation is known to have a significant effect on life history diversification in a variety of species. However, physical constraints of body shape and size can sometimes limit life history divergence. We test this idea in the Costa Rican livebearing fishAlfaro cultratus. Individuals in this species have a narrow body and keeled ventral surface, and females do not develop a distended abdomen when pregnant like other livebearing fishes. Here, we describe the life history ofA. cultratusfrom 20 different populations across both high-predation and low-predation environments. We found significantly lower reproductive allotment in females from high-predation environments than in females from low-predation environments, but no significant difference in female or male size at maturity, number of offspring produced by females, or size of offspring. We found thatA. cultratusexhibit isometric patterns of allocation for clutch dry mass in relation to female dry mass in high-predation and low-predation environments. Our results suggest that body shape constraints in this species limit the life history divergence we typically see between populations from high-predation and low-predation environments in other species.

Single-cell RNA sequencing of equine mesenchymal stromal cells from primary donor-matched tissue sources reveals functional heterogeneity in immune modulation and cell motility

BackgroundThe efficacy of mesenchymal stromal cell (MSC) therapy is thought to depend on the intrinsic heterogeneity of MSC cultures isolated from different tissue sources as well as individual MSCs isolated from the same tissue source, neither of which is well understood. To study this, we used MSC cultures isolated from horses. The horse is recognized as a physiologically relevant large animal model appropriate for translational MSC studies. Moreover, due to its large size the horse allows for the simultaneous collection of adequate samples from multiple tissues of the same animal, and thus, for the unique collection of donor matched MSC cultures from different sources. The latter is much more challenging in mice and humans due to body size and ethical constraints, respectively.MethodsIn the present study, we performed single-cell RNA sequencing (scRNA-seq) on primary equine MSCs that were collected from three donor-matched tissue sources; adipose tissue (AT), bone marrow (BM), and peripheral blood (PB). Based on transcriptional differences detected with scRNA-seq, we performed functional experiments to examine motility and immune regulatory function in distinct MSC populations.ResultsWe observed both inter- and intra-source heterogeneity across the three sources of equine MSCs. Functional experiments demonstrated that transcriptional differences correspond with phenotypic variance in cellular motility and immune regulatory function. Specifically, we found that (i) differential expression of junctional adhesion molecule 2 (JAM2) between MSC cultures from the three donor-matched tissue sources translated into altered cell motility of BM-derived MSCs when RNA interference was used to knock down this gene, and (ii) differences in C-X-C motif chemokine ligand 6 (CXCL6) expression in clonal MSC lines derived from the same tissue source correlated with the chemoattractive capacity of PB-derived MSCs.ConclusionsUltimately, these findings will enhance our understanding of MSC heterogeneity and will lead to improvements in the therapeutic potential of MSCs, accelerating the transition from bench to bedside.

Ontogenetic allometry and architectural properties of the paravertebral and hindlimb musculature in Eastern cottontail rabbits (Sylvilagus floridanus): functional implications for developmental changes in locomotor performance.

Due to small body size, an immature musculoskeletal system, and other growth-related limits on performance, juvenile mammals frequently experience a greater risk of predation than their adult counterparts. As a result, behaviorally precocious juveniles are hypothesized to exhibit musculoskeletal advantages that permit them to accelerate rapidly and evade predation. This hypothesis was tested through detailed quantitative evaluation of muscle growth in wild Eastern cottontail rabbits (Sylvilagus floridanus). Cottontail rabbits experience high rates of mortality during the first year of life, suggesting that selection might act to improve performance in growing juveniles. Therefore, it was predicted that muscle properties associated with force and power capacity should be enhanced in juvenile rabbits to facilitate enhanced locomotor performance. We quantified muscle architecture from 24 paravertebral and hindlimb muscles across ontogeny in a sample of n=29 rabbits and evaluated the body mass scaling of muscle mass (MM), physiological cross-sectional area (PCSA), isometric force (Fmax ), and instantaneous power (Pinst ), along with several dimensionless architectural indices. In contrast to our hypothesis, MM and PCSA for most muscles change with positive allometry during growth by scaling at and , respectively, whereas Fmax and Pinst generally scale indistinguishably from isometry, as do the architectural indices tested. However, scaling patterns indicate that the digital flexors and ankle extensors of juvenile S.floridanus have greater capacities for force and power, respectively, than those in adults, suggesting these muscle properties may be a part of several compensatory features that promote enhanced acceleration performance in young rabbits. Overall, our study implies that body size constraints place larger, more mature rabbits at a disadvantage during acceleration, and that adults must develop hypertrophied muscles in order to maintain mechanical similarity in force and power capacities across development. These findings challenge the accepted understanding that juvenile animals are at a performance detriment relative to adults. Instead, for prey-predator interactions necessitating short intervals of high force and power generation relative to body mass, as demonstrated by rapid acceleration of cottontail rabbits fleeing predators, it may be the adults that struggle to keep pace with juveniles.

Urbanization-driven changes in web building and body size in an orb web spider.

In animals, behavioural responses may play an important role in determining population persistence in the face of environmental changes. Body size is a key trait central to many life-history traits and behaviours. Correlations with body size may constrain behavioural variation in response to environmental changes, especially when size itself is influenced by environmental conditions. Urbanization is an important human-induced rapid environmental change that imposes multiple selection pressures on both body size and (size-constrained) behaviour. How these combine to shape behavioural responses of urban-dwelling species is unclear. Using web building, an easily quantifiable behaviour linked to body size and the garden spider Araneus diadematus as a model, we evaluated direct behavioural responses to urbanization and body size constraints across a network of 63 selected populations differing in urbanization intensity. We additionally studied urbanization at two spatial scales to account for some environmental pressures varying across scales and to obtain first qualitative insights about the role of plasticity and genetic selection. Spiders were smaller in highly urbanized sites (local scale only), in line with expectations based on reduced prey biomass availability and the Urban Heat Island effect. Web surface and mesh width decreased with urbanization at the local scale, while web surface also increased with urbanization at the landscape scale. The latter two responses are expected to compensate, at least in part, for reduced prey biomass availability in cities. The use of multivariate mixed modelling reveals that although web traits and body size are correlated within populations, behavioural responses to urbanization do not appear to be constrained by size: there is no evidence of size-web correlations among populations or among landscapes, and web traits appear independent from each other. Our results demonstrate that responses in size-dependent behaviours may be decoupled from size changes, thereby allowing fitness maximization in novel environments. The spatial scale at which traits respond suggests contributions of both genetic adaptation (for web investment) and plasticity (for mesh width). Although fecundity decreased with local-scale urbanization, A. diadematus abundances were similar across urbanization gradients; behavioural responses thus appear overall successful at the population level.

The Morphology of the Bovid Calcaneus: Function, Phylogenetic Signal, and Allometric Scaling

Despite its clear functional role in hock (ankle) plantarflexion, the bovid calcaneus has been understudied with respect to the functional constraints imposed by locomotion in differing habitats, the allometric influence of inter-specific body size differences, and phylogenetic signal. This study uses a comparative sample of extant bovid species to shed light on the evolution of bovid calcaneal morphology. I measured eight linear measurements on 204 calcaneus specimens representing 41 extant bovid species. Using a morphological body size proxy validated against published species-mean body mass estimates, I performed Ordinary Least Squares regression to examine the allometric relationships of each measurement with body size. I classified each bovid species to a preferred habitat type based on published literature, and performed Phylogenetic Generalized Least Squares (PGLS) to test for differences in morphology between bovid taxa with different preferred habitats while considering evolutionary relatedness. I visualized morphological differences between taxa using Principal Components Analysis plotted in a phylomorphospace. Results demonstrate that several measurements of the bovid calcaneus have an allometric relationship to body size. The functional length of the calcaneus scales with negative allometry, which likely maintains a comparable safety factor within the calcaneal tuber at larger body sizes. While open-habitat bovids have relatively shorter calcaneal tubers, this difference is not significant when controlling for the influence of body size and phylogenetic signal using PGLS. Among bovid tribes that have a deep evolutionary history of adaptation to open habitats, Antilopini have relatively longer calcaneal tubers than Alcelaphini or Hippotragini, which may reflect the unique importance of stotting behavior in predator avoidance among antelopins. Overall, the morphology of the bovid calcaneus has been shaped by a complex interaction of phylogenetic and body-size constraints as well as adaptation to modes of predator avoidance mediated by preferred habitat.

Assessing metabolic constraints on the maximum body size of actinopterygians: locomotion energetics of Leedsichthys problematicus (Actinopterygii, Pachycormiformes)

AbstractMaximum sizes attained by living actinopterygians are much smaller than those reached by chondrichthyans. Several factors, including the high metabolic requirements of bony fishes, have been proposed as possible body‐size constraints but no empirical approaches exist. Remarkably, fossil evidence has rarely been considered despite some extinct actinopterygians reaching sizes comparable to those of the largest living sharks. Here, we have assessed the locomotion energetics of Leedsichthys problematicus, an extinct gigantic suspension‐feeder and the largest actinopterygian ever known, shedding light on the metabolic limits of body size in actinopterygians and the possible underlying factors that drove the gigantism in pachycormiforms. Phylogenetic generalized least squares analyses and power performance curves established in living fishes were used to infer the metabolic budget and locomotion cost of L. problematicus in a wide range of scenarios. Our approach predicts that specimens weighing up to 44.9 tonnes would have been energetically viable and suggests that similar body sizes could also be possible among living taxa, discarding metabolic factors as likely body size constraints in actinopterygians. Other aspects, such as the high degree of endoskeletal ossification, oviparity, indirect development or the establishment of other large suspension‐feeders, could have hindered the evolution of gigantism among post‐Mesozoic ray‐finned fish groups. From this perspective, the evolution of anatomical innovations that allowed the transition towards a suspension‐feeding lifestyle in medium‐sized pachycormiforms and the emergence of ecological opportunity during the Mesozoic are proposed as the most likely factors for promoting the acquisition of gigantism in this successful lineage of actinopterygians.

Incubation behaviours of oviraptorosaur dinosaurs in relation to body size.

Most birds sit on their eggs during incubation, a behaviour that likely evolved among non-avian dinosaurs. Several 'brooding' specimens of smaller species of oviraptorosaurs and troodontids reveal these non-avian theropods sat on their eggs, although little is known of incubation behaviour in larger theropod species. Here we examine egg clutches over a large body size range of oviraptorosaurs in order to understand the potential effect of body size on incubation behaviour. Eggshell porosity indicates that the eggs of all oviraptorosaurs were exposed in the nest, similar to brooding birds. Although all oviraptorosaur clutches consist of radially arranged eggs in a ring configuration, clutch morphology varies in that the central opening is small or absent in the smallest species, becomes significantly larger in larger species, and occupies most of the nest area in giant species. Our results suggest that the smallest oviraptorosaurs probably sat directly on the eggs, whereas with increasing body size more weight was likely carried by the central opening, reducing or eliminating the load on the eggs and still potentially allowing for some contact during incubation in giant species. This adaptation, not seen in birds, appears to remove the body size constraints of incubation behaviour in giant oviraptorosaurs.

Energetic tradeoffs control the size distribution of aquatic mammals

Four extant lineages of mammals have invaded and diversified in the water: Sirenia, Cetacea, Pinnipedia, and Lutrinae. Most of these aquatic clades are larger bodied, on average, than their closest land-dwelling relatives, but the extent to which potential ecological, biomechanical, and physiological controls contributed to this pattern remains untested quantitatively. Here, we use previously published data on the body masses of 3,859 living and 2,999 fossil mammal species to examine the evolutionary trajectories of body size in aquatic mammals through both comparative phylogenetic analysis and examination of the fossil record. Both methods indicate that the evolution of an aquatic lifestyle is driving three of the four extant aquatic mammal clades toward a size attractor at ∼500 kg. The existence of this body size attractor and the relatively rapid selection toward, and limited deviation from, this attractor rule out most hypothesized drivers of size increase. These three independent body size increases and a shared aquatic optimum size are consistent with control by differences in the scaling of energetic intake and cost functions with body size between the terrestrial and aquatic realms. Under this energetic model, thermoregulatory costs constrain minimum size, whereas limitations on feeding efficiency constrain maximum size. The optimum size occurs at an intermediate value where thermoregulatory costs are low but feeding efficiency remains high. Rather than being released from size pressures, water-dwelling mammals are driven and confined to larger body sizes by the strict energetic demands of the aquatic medium.

Linking fishing pressure with ecosystem thresholds and food web stability on coral reefs

AbstractManaging fisheries for ecosystem resilience is essential, but practical guidance is limited by food‐web complexity. Processes, mechanisms, and thresholds associated with ecosystem overfishing were investigated by combining traditional concepts in fisheries biology with recent advances in food‐web modeling. Diverse coral‐reef food webs were simplified by grouping species into guilds based on the way they capture, store, and transfer energy, rather than taxonomically, as is traditionally done. Biomass fluxes between the guilds were then quantified using an allometric trophic model. The model was calibrated by linking parameters describing growth, predation, and competition with known body size and metabolic constraints, and then adjusting the base rate of parameters to match fish biomass estimates from a “pristine” coral reef system. The calibrated model was then tested by replacing equilibrium fish biomasses with observations from fished systems across the Pacific, spanning nine islands and numerous major‐reef habitats. Encouraging relationships were found between predicted algal accumulation and field observations, and between modelled and observed guild restructuring. In terms of food‐web ecology, “pristine” food webs were characterized by asynchronous population dynamics between the guilds (i.e., offsetting fluctuations), which maximized their persistence and the net accumulation of biomass within food webs. Beneficial, offsetting fluctuations were driven by the contrasting roles of density dependence, apparent competition, and predation. Fishing for predators synchronized the population fluctuations between the guilds, resulting in larger amplitudes (i.e., highs and lows), and a growing dominance of small herbivores. Continued fishing for large herbivores eventually led to an inflection point where algal biomass accumulated exponentially, revealing an ecosystem‐based fisheries benchmark. Management targets that maximized fisheries yields while controlling for algal accumulation required simultaneous exploitation across the guilds; a significant challenge because maximum yields of predators, large herbivores, and small herbivores were magnitudes of order apart. This strategy also represented a departure from modern commercial fisheries policies that place catch quotas on entire fish families taxonomically, committing systems to smaller fish, higher biomass turnover, and undesirable algal accumulation. Moving forward, the model provided a flexible and adaptable framework to consider economic and ecosystem objectives of fisheries simultaneously, ultimately balancing resilient food webs against higher fisheries productivity.

Regional endothermy as a trigger for gigantism in some extinct macropredatory sharks.

Otodontids include some of the largest macropredatory sharks that ever lived, the most extreme case being Otodus (Megaselachus) megalodon. The reasons underlying their gigantism, distribution patterns and extinction have been classically linked with climatic factors and the evolution, radiation and migrations of cetaceans during the Paleogene. However, most of these previous proposals are based on the idea of otodontids as ectothermic sharks regardless of the ecological, energetic and body size constraints that this implies. Interestingly, a few recent studies have suggested the possible existence of endothermy in these sharks thus opening the door to a series of new interpretations. Accordingly, this work proposes that regional endothermy was present in otodontids and some closely related taxa (cretoxyrhinids), playing an important role in the evolution of gigantism and in allowing an active mode of live. The existence of regional endothermy in these groups is supported here by three different approaches including isotopic-based approximations, swimming speed inferences and the application of a novel methodology for assessing energetic budget and cost of swimming in extinct taxa. In addition, this finding has wider implications. It calls into question some previous paleotemperature estimates based partially on these taxa, suggests that the existing hypothesis about the evolution of regional endothermy in fishes requires modification, and provides key evidence for understanding the evolution of gigantism in active macropredators.

Mechanics of the thorax in flies.

Insects represent more than 60% of all multicellular life forms, and are easily among the most diverse and abundant organisms on earth. They evolved functional wings and the ability to fly, which enabled them to occupy diverse niches. Insects of the hyper-diverse orders show extreme miniaturization of their body size. The reduced body size, however, imposes steep constraints on flight ability, as their wings must flap faster to generate sufficient forces to stay aloft. Here, we discuss the various physiological and biomechanical adaptations of the thorax in flies which enabled them to overcome the myriad constraints of small body size, while ensuring very precise control of their wing motion. One such adaptation is the evolution of specialized myogenic or asynchronous muscles that power the high-frequency wing motion, in combination with neurogenic or synchronous steering muscles that control higher-order wing kinematic patterns. Additionally, passive cuticular linkages within the thorax coordinate fast and yet precise bilateral wing movement, in combination with an actively controlled clutch and gear system that enables flexible flight patterns. Thus, the study of thoracic biomechanics, along with the underlying sensory-motor processing, is central in understanding how the insect body form is adapted for flight.

Current Status of Pediatric Ventricular Assist Device Support

The last decade has witnessed significant advancement in the field of ventricular assist device (VAD) support. Although device options for pediatric patients were previously severely limited because of body size constraints, this frustrating situation has gradually been changing, owing to ongoing device miniaturization. Recognition of the superiority of VAD support compared with conventional extracorporeal membrane oxygenation support has spurred enthusiasm for VAD support in children. In this article, we discuss the current status of pediatric VAD support; where do we stand now and where will we be heading? Because this field is rapidly changing, it is anticipated that this article will provide a general overview of what is currently occurring in the field of pediatric VAD support.

Energetic and ecological constraints on population density of reef fishes.

Population ecology has classically focused on pairwise species interactions, hindering the description of general patterns and processes of population abundance at large spatial scales. Here we use the metabolic theory of ecology as a framework to formulate and test a model that yields predictions linking population density to the physiological constraints of body size and temperature on individual metabolism, and the ecological constraints of trophic structure and species richness on energy partitioning among species. Our model was tested by applying Bayesian quantile regression to a comprehensive reef-fish community database, from which we extracted density data for 5609 populations spread across 49 sites around the world. Our results indicate that population density declines markedly with increases in community species richness and that, after accounting for richness, energetic constraints are manifested most strongly for the most abundant species, which generally are of small body size and occupy lower trophic groups. Overall, our findings suggest that, at the global scale, factors associated with community species richness are the major drivers of variation in population density. Given that populations of species-rich tropical systems exhibit markedly lower maximum densities, they may be particularly susceptible to stochastic extinction.

Body size affects individual winter foraging strategies of thick-billed murres in the Bering Sea.

Foraging and migration often require different energetic and movement strategies. Though not readily apparent, constraints during one phase might influence the foraging strategies observed in another. For marine birds that fly and dive, body size constraints likely present a trade-off between foraging ability and migration as smaller bodies reduce flight costs, whereas larger bodies are advantageous for diving deeper. This study examines individual wintering strategies of deep diving thick-billed murres (Uria lomvia) breeding at three colonies in the south-eastern Bering Sea: St Paul, St George and Bogoslof. These colonies, arranged north to south, are located such that breeding birds forage in a gradient from shelf to deep-water habitats. We used geolocation time-depth recorders and stable isotopes from feathers to determine differences in foraging behaviour and diet of murres during three non-breeding periods, 2008-2011. Body size was quantified by a principal component analysis (wing, culmen, head+bill and tarsus length). A hierarchical cluster analysis identified winter foraging strategies based on individual movement, diving behaviour and diet (inferred from stable isotopes). Structural body size differed by breeding island. Larger birds from St Paul had higher wing loading than smaller birds from St George. Larger birds, mainly from St Paul, dove to deeper depths, spent more time in the Bering Sea, and likely consumed higher trophic-level prey in late winter. Three winter foraging strategies were identified. The main strategy, employed by small birds from all three breeding colonies in the first 2 years, was characterized by high residency areas in the North Pacific south of the Aleutians and nocturnal diving. In contrast, 31% of birds from St Paul remained in the Bering Sea and foraged mainly during the day, apparently feeding on higher trophic-level prey. Throat feather stable isotopes indicated that individuals exhibited flexibility in the use of this colony-specific foraging strategy. The third strategy only occurred in 2010/2011, when birds dove more and deeper, suggesting limited prey resources. Foraging strategies partitioned with respect to annual differences, presumably in response to shifts in distribution of prey, and were linked to body size. The presence of a colony-specific wintering strategy suggests the potential for overwinter survival differences between these populations.

Escape behaviour in shore crabs: constraints of body size and available shelter

AbstractThe escape behaviour of organisms that retreat to shelter to avoid predators is influenced by availability and quality (e.g. appropriate size) of shelters. We examined escape behaviour (flight initiation distance, distance fled to shelter) of the crab Leptograpsus variegatus, relative to shelter size and distribution on a shoreline with multiple shelters and on a shoreline with limited shelter. Larger crabs tended to be nearer to larger shelters than smaller crabs and fled sooner and further than smaller crabs. Larger crabs often fled past the nearest shelters if they were too small. Group size increased at the site with fewer shelters, but this did not influence flight behaviour. We conclude that the availability of shelters and body size influences escape behaviour of these crabs.

Species distribution, ecology, abundance, body size and phylogeny originate interrelated rarity patterns at regional scale

The most pervasive macroecological patterns concern (1) the frequency distribution of range size, (2) the relationship between range size and species abundance and (3) the effect of body size on range size. We investigated these patterns at a regional scale using the tenebrionid beetles of Latium (Central Italy). For this, we calculated geographical range size (no. of 10-km square cells), ecological tolerance (no. of phytoclimatic units) and abundance (no. of sampled individuals) using a large database containing 3561 georeferenced records for 84 native species. For each species, we also calculated body mass and its ‘phylogenetic diversity’ on the basis of cladistic relationships. Frequency distribution of range size followed a log-normal distribution as found in many other animal groups. However, a log-normal distribution accommodated well the frequency distribution of ecological tolerance, a so far unexplored issue. Range size was correlated with abundance and ecological tolerance, thus supporting the hypothesis that a positive correlation between distribution and abundance is a reflection of interspecific differences in ecological specialization. Larger species tended to have larger ranges and broader ecological tolerance. However, contrary to what known in most vertebrates, not only small-sized, but also many medium-to-large-sized species exhibited great variability in their range size, probably because tenebrionids are not so strictly influenced by body size constraints (e.g. home ranges) as vertebrates. Moreover, in contrast to other animals, tenebrionid body size does not influence species abundances, probably because these detritivorous animals are not strongly regulated by competition. Finally, contrary to the assumption that rare species should be mainly found among lineages that split from basal nodes, rarity of a tenebrionid species was not influenced by the phylogenetic position of its tribe. However, lineages that split from more basal nodes had lower variability in terms of species geographical distribution, ecological tolerance and abundance, which suggests that lineages that split from more basal nodes are not only morphologically conservative but also tend to have an ecological ‘inertia’.