Growth and patterning in the conodont skeleton

Recent advances in our understanding of conodont palaeobiology and functional morphology have rendered established hypotheses of element growth untenable. In order to address this problem, hard tissue histology is reviewed paying particular attention to the relationships during growth of the component hard tissues comprising conodont elements, and ignoring a priori assumptions of the homologies of these tissues. Conodont element growth is considered further in terms of the pattern of formation, of which four distinct types are described, all possibly derived from a primitive condition after heterochronic changes in the timing of various developmental stages. It is hoped that this may provide further means of unravelling conodont phylogeny. The manner in which the tissues grew is considered homologous with other vertebrate hard tissues, and the elements appear to have grown in a way similar to the growing scales and growing dentition of other vertebrates.     

Morphological conservation of limb natural pendular period in the domestic dog (Canis familiaris): Implications for locomotor energetics

For better understanding of the links between limb morphology and the metabolic cost of locomotion, we have characterized the relationships between limb length and shape and other functionally important variables in the straightened forelimbs and hindlimbs of a sample of 12 domestic dogs (Canis familiaris). Intra-animal comparisons show that forelimbs and hindlimbs are very similar (not significantly different) in natural pendular period (NPP), center-of-mass, and radius of gyration, even though they differ distinctly in mass, length, moment-of-inertia, and other limb proportions. The conservation of limb NPP, despite pronounced dissimilarity in other limb characteristics, appears to be the result of systematic differences in shape, forelimbs tending to be cylindrical and hindlimbs conical. Estimating limb NPP for other species from data in the literature on segment inertia and total limb length, we present evidence that the similarity between forelimbs and hindlimbs in NPP is generally true for mammals across a large size range. Limbs swinging with or near their natural pendular periods will maximize within-limb pendular exchange of potential and kinetic energy. As all four limbs of moderate- and large-size animals swing with the same period during walking, maximal advantage can be derived from the pendular exchange of energy only if forelimbs and hindlimbs are very similar in NPP. We hypothesize that an important constraint in the evolution of limb length and shape is the locomotor economy derived from forelimbs and hindlimbs of similar natural pendular period. J. Morphol. 234:183–196, 1997. © 1997 Wiley-Liss, Inc.     

Standard energetics of phyllostomid bats: the inadequacies of phylogenetic-contrast analyses

The basal rates of metabolism (BMR) of bats belonging to the family Phyllostomidae are re-examined after an earlier correlation with food habits was rejected because it did not take phylogeny into consideration. This rejection was based on an erroneous attribution of food habits and on an analytical method, phylogenetic contrasts, that ignores interactions that occur among character states and preferentially attributes responsibility for character states to phylogeny. The re-examination made here was based on analysis of covariance, which makes no a priori assumptions on the relative impact of factors that influence character states and permits factor interactions to be identified. A resulting model, based on variation in body mass, food habits, occurrence with respect to elevation, and residence on islands or continents, accounts for 99.4% of the variation in the BMR of 30 species of phyllostomids. Basal rate is also correlated with subfamily, but only if food habits are excluded because they are correlated with subfamily affiliation, as is residence on islands and continents, two examples of factor interaction. The preference to assign the effects of food habits and island residence on basal rate to subfamily affiliation (and phylogeny) is not justified. The concept that quantitative physiological characters can be transmitted via phylogeny without regard to the habits of animals and the characteristics of their environments cannot be defended. Phylogeny is the historical context in which the evolution of character states occurs, not the 'cause' of their evolution.     

Aerodynamic corrections for the flight of birds and bats in wind tunnels

Few wind tunnel studies of animal flight have controlled or corrected for distortions to behaviour, physiology or flight aerodynamics representing the difference between flight in the tunnel and flight in free air. Aerodynamic correction factors are derived based on lifting-line theory and the method of images for an animal flying freely within closed- and open-section wind tunnels; the method is very similar to that used to model flight in ground effect, and as in ground effect the corrections to induced drag may be substantial. These correction factors are used to estimate bound wing circulation, drag and mechanical power for comparison with free flight, and to derive testable predictions of optimum flight strategies for an animal in a tunnel. In an open-section tunnel, mechanical power is increased compared to free flight, and the animal should fly at the tunnel centre. In a closed tunnel mechanical power is usually reduced, and substantial savings are available, particularly at low speeds, if the animal flies close to the tunnel roof. Anecdotal observations confirm that birds and bats adopt this strategy. The mechanical power-speed curve in a closed tunnel is flatter than the curve for free flight, and this may explain the flat metabolic power-speed curves for birds and bats obtained in some measurements.     

Feeding mechanisms in bats: variation within the constraints of flight

By any standard, bats are a successful group of mammals andthe evolution of flight and echolocation were certainly keyinnovations behind their success. That is only part of the story,however. Bats have diversified into trophic niches that rangefrom insectivory to feeding on blood, fruit, or nectar. Whileflight places fundamental constraints on the shape of the postcranialskeleton, skull shape in bats is remarkably diverse. Morphologicalstudies of individual families and sympatric assemblages demonstratethat variation in skull shape is clearly associated with trophicspecialization. Field experiments demonstrate that species-specificbiting behaviors during feeding are common and analyses indicatethat the evolution of cranial morphology and feeding behaviorare correlated. Modeling experiments further suggest that feeding(loading) behaviors and skull shape are functionally linked.If the skulls of bats are under selective pressure for minimalmass because of the energetic demands of flight, then they maybe more "optimized" to meet mechanical demands than are theskulls of other mammals. This would make bats a unique modelsystem for studying the evolution of diversity in skull shapeand its functional implications for the evolution of feedingstrategies in mammals.     

Locomotor energetics in primates: gait mechanics and their relationship to the energetics of vertical and horizontal locomotion

All primates regularly move within three-dimensional arboreal environments and must often climb, but little is known about the energetic costs of this critical activity. Limited previous work on the energetics of incline locomotion suggests that there may be differential selective pressures for large compared to small primates in choosing to exploit a complex arboreal environment. Necessary metabolic and gait data have never been collected to examine this possibility and biomechanical mechanisms that might explain size-based differences in the cost of arboreal movement. Energetics and kinematics were collected for five species of primate during climbing and horizontal locomotion. Subjects moved on a treadmill with a narrow vertical substrate and one with a narrow horizontal substrate at their maximum sustainable speed for 10–20 min while oxygen consumption was monitored. Data during climbing were compared to those during horizontal locomotion and across size. Results show that climbing energetic costs were similar to horizontal costs for small primates (<0.5 kg) but were nearly double for larger species. Spatio-temporal gait characteristics suggest that the relationship between the cost of locomotion and the rate of force production changes between the two locomotor modes. Thus, the main determinants of climbing costs are fundamentally different from those during horizontal locomotion. These new results combining spatiotemporal and energetic data confirm and expand on our previous argument (Hanna et al.: Science 320 (2008) 898) that similar costs of horizontal and vertical locomotion in small primates facilitated the successful occupation of a fine-branch arboreal milieu by the earliest primates.     

Standard energetics of leaf-nosed bats (Hipposideridae): its relationship to intermittent- and protracted-foraging tactics in bats and birds

Basal rates of metabolism within the insectivorous genera Hipposideros and Ascelliscus, Old World leaf-nosed bats (Hipposideridae), ranged from 58% to 77% of the mammalian standard. The larger species, Hipposideros diadema and Hipposideros maggietaylori, effectively thermoregulated at ambient temperatures down to 9 degrees C, whereas two smaller species, Hipposideros galeritus and Hipposideros cervinus, occasionally permitted body temperatures to fall below 32 degrees C. The low basal rates of metabolism in hipposiderids correlated with a predatory life-style characterized by intermittent flight from a perch to capture insects, a correlation similar to that found in nonpasserine birds. Intermittent-foraging bats and nonpasserines collectively had basal rates of metabolism that averaged 75% of those that pursue insects during protracted flight. However, no difference in basal rate was found between protracted- and intermittent-foraging passerines, which had basal rates 1.8- and 2.4-times those of protracted-foraging and intermittent-foraging bats and nonpasserines, respectively. Bats, swifts, and caprimulgids that enter torpor have basal rates that are 85% of those of similar species that do not enter torpor. Body mass, order affiliation, foraging mode, and propensity to enter into torpor collectively account for 97% of the variation in basal rate of metabolism in insectivorous bats and birds. Foraging style therefore appears to be a factor contributing to the diversity in endotherm energetics. Minimal thermal conductance in the genus Hipposideros ranged from 75% to 102% of the mammalian standard. Birds have minimal thermal conductances that are 75% of mammals and intermittent foragers have minimal conductances that are 78% of protracted foragers.     

The evolution of flight and echolocation in bats: another leap in the dark

The earliest known complete bats, from the Eocene (49–53 Mya), were already capable of flapping flight and echolocation. In the absence of direct fossil evidence there have been many speculative scenarios advanced to explain the evolution of these behaviours and their distributions in extant bats. Theories assuming chiropteran monophyly have generally presumed the ancestral pre‐bat was nocturnal, arboreal and insectivorous. Following this assumption hypotheses can be divided into the echolocation first, flight first and tandem development hypotheses, all of which assume that flight evolved only once in the lineage. In contrast, the chiropteran diphyly hypothesis suggests that flight evolved twice. Evidence supporting and refuting the different hypotheses are reviewed. It is concluded that there are significant problems attached to all the current models. A novel hypothesis is advanced, which starts from the assumption that bats are monophyletic and the ancestral pre‐bat was arboreal, but diurnal and frugivorous. After the evolution of flight it is suggested that these animals were driven into the nocturnal niche by the evolution of raptorial birds, and different groups evolved either specialised nocturnal vision (megachiropterans) or echolocation (microchiropterans). A block on sensory modality transfer has retained this distribution of perceptual capabilities ever since, despite some Megachiroptera evolving rudimentary echolocation, and the dietary convergence of some Microchiroptera with the Megachiroptera. The new hypothesis overcomes many of the problems identified in previous treatments.     

Sunset-related timing of flight activity in neotropical bats

Summary The times of onset and completion of the hunting flights of three colonies of neotropical bats, each comprising 100–200 individuals, were observed for nine months. The colonies were of different species: Molossus ater (M.a.) and Molossus molossus (M.m.) of the Molossidae, and Myotis nigricans (My. n.) of the Vespertilionidae. Individuals of Phyllostomus hastatus (P.h., Phyllostomidae) were also observed. All the bats roosted in a building near Restrepo, Colombia (4°16N, 73°34W). Times of emergence in the evening and the return of the last animals in the morning were recorded on 2 to 3 successive days each month. For all bats, the emergence time changed in parallel with that of sunset, and the return paralleled sunrise (Fig. 1). Accordingly, the duration of the activity period is positively correlated with the duration of the night. No annual periodic changes in phase (re sunset/sunrise) of the onset and end of flight activity could be demonstrated, but there was a close relationship between the timing of activity and particular light intensities during twilight (Fig. 4). The first flyers of M.a. appear at the highest intensity (30–300 lx) and those of My. n. at the lowest (0.1–5 lx); the last flyers to return appear in the opposite sequence. For each species, the return to the roost usually occurs at a lower intensity than the departure. These findings, made with four neotropical bat species, differ from those of Subbaraj and Chandrashekaran (1977) with the emballonurid bat Taphozous that they studied at 9°58 N in India. The ecological factors that may play a role in timing the flight activity of tropical bats are discussed. Sunset-related timing, based on the combined effect of (a) the circadian oscillation in arousal and (b) the transition during twilight to a light-intensity range with reduced inhibition of activity (lightsampling behavior), tends to be the rule in tropical bats; time-of-day-related timing is the exception.Supported by the Deutsche Forschungsgemeinschaft (Er 59/1-3+6)     

BMPR-II is dispensable for formation of the limb skeleton

Initiation of BMP signaling is dependent upon activation of Type I BMP receptor by constitutively active Type II BMP receptor. Three Type II BMP receptors have been identified; Acvr2a and Acvr2b serve as receptors for BMPs and for activin-like ligands whereas BMPR-II functions only as a BMP receptor. As BMP signaling is required for endochondral ossification and loss of either Acvr2a or Acvr2b is not associated with deficits in limb development, we hypothesized that BMPR-II would be essential for BMP signaling during skeletogenesis. We removed BMPR-II from early limb mesoderm by crossing BMPR-II floxed mice with those carrying the Prx1-Cre transgene. Mice lacking limb expression of BMPR-II have normal skeletons that could not be distinguished from control littermates. From these data, we conclude that BMPR-II is not required for endochondral ossification in the limb where loss of BMPR-II may be compensated by BMP utilization of Acvr2a and Acvr2b.     

Some proposals in cardiac muscle mechanics and energetics

Based on A. V. Hill's three-component model, mechanical properties of the contractile element (CE), such as velocity and tension, were determined as muscle shortening and loads in quick-release or afterloaded isotonic contraction. The method is applicable for studying cardiac mechanics, to obtain force-velocity data of the same CE length at varous afterloads. Analysis of the energetics of cardiac muscle was based on simulation studies of cardiac mechanics (Wong 1971, 1972). By proper derivation, the conventional contractile element work (CEW) was found to be a minor energy determinant. The tension-time integral and tension-independent heat (Ricchiuti and Gibbs, 1965) represent energy utilization for activation and maintenance of tension, the primary energy determinant.     

The evolution of flight in bats: narrowing the field of plausible hypotheses

The evolution of flapping flight in bats from an arboreal gliding ancestor appears on the surface to be a relatively simple transition. However, bat flight is a highly complex functional system from a morphological, physiological, and aerodynamic perspective, and the transition from a gliding precursor may involve functional discontinuities that represent evolutionary hurdles. In this review, I suggest a framework for a comprehensive treatment of the evolution of complex functional systems that emphasizes a mechanistic understanding of the initial state, the final state, and the proposed transitional states. In this case, bats represent the final state and extant mammalian gliders are used as a model for the initial state. To explore possible transitional states, I propose a set of criteria for evaluating hypotheses about the evolution of flight in vertebrates and suggest methods by which we can advance our understanding of the transition from gliding to flapping flight. Although it is impossible ever to know with certainty the sequence of events landing to flapping flight, the field of possibilities can be narrowed to those that maintain the functional continuity of the wing and result in improved aerodynamic performance across this transition. The fundamental differences between gliding and flapping flight should not necessarily be seen as evidence that this transition could not occur; rather, these differences point out compelling aspects of the aerodynamics of animal wings that require further investigation.     

Evolution and development of the primate limb skeleton

The order Primates is composed of many closely related lineages, each having a relatively well established phylogeny supported by both the fossil record and molecular data. 1 Primate evolution is characterized by a series of adaptive radiations beginning early in the Cenozoic era. Studies of these radiations have uncovered two major trends. One is that substantial amounts of morphological diversity have been produced over short periods of evolutionary time. 2 The other is that consistent and repeated patterns (variational tendencies 3 ) are detected. Taxa within clades, such as the strepsirrhines of Madagascar and the platyrrhines of the Neotropics, have diversified in body size, substrate preference, and diet. 2 , 4 - 6 The diversification of adaptive strategies within such clades is accompanied by repeated patterns of change in cheiridial proportions 7 , 8 (Fig. 1) and tooth‐cusp morphology. 9 There are obvious adaptive, natural‐selection based explanations for these patterns. The hands and feet are in direct contact with a substrate, so their form would be expected to reflect substrate preference, whereas tooth shape is related directly to the functional demands of masticating foods having different mechanical properties. What remains unclear, however, is the role of developmental and genetic processes that underlie the evolutionary diversity of the primate body plan. Are variational tendencies a signature of constraints in developmental pathways? What is the genetic basis for similar morphological transformations among closely related species? These are a sampling of the types of questions we believe can be addressed by future research integrating evidence from paleontology, comparative morphology, and developmental genetics.     

Multiple roles for four-jointed in planar polarity and limb patterning

Insect cuticles have been a model system for the study of planar polarity for many years and a number of genes required for this process have been identified. These genes organise the polarised arrangement of hairs on the legs, wings, thorax, and abdomen of adult Drosophila. It has previously been shown that four-jointed is involved in planar polarity decisions in the eye as well as proximal distal leg and wing development. We now present evidence that four-jointed is expressed in a gradient through the developing wing and show that it is required for planar polarity determination in both the wing and the abdomen. Clones of cells either lacking or ectopically expressing four-jointed cause both autonomous and nonautonomous repolarisation of hairs in these tissues. We propose that the inferred four-jointed expression gradient is important for planar polarity establishment and that local inversions of the gradient by the clones are the probable cause of the observed polarity phenotypes. In addition we observe defects in wing vein development. The subtle phenotypes of mutant flies, and the diverse patterning processes in which it is involved, suggest that four-jointed may act as a modifier of the activity of multiple other signalling factors.