Morphometric relationships of take-off speed in anuran amphibians

Locomotory speed correlates with muscle mass (determining force and stride rate), limb length (stride rate and distance), and laterally compressed body trunk (force and stride distance). To delineate generalization of the locomotory-morphometric relationships specifically in anuran amphibians, we investigated take-off speed and the three morphological variables from seven species, Rana nigromaculata, R. rugosa, and Bombina orientalis, Eleuthrodectilus fitzingeri, E. diastema, Bufo typhonius, Colostethus flotator and Physalaemus pustulosus. The fastest jumper E. fitzingeri (3.41 m s(-1)) showed 2.49-fold greater speed than the slowest B. typhonius. Take-off speed correlated well with both thigh muscle mass relative to body mass and hindlimb length relative to snout-vent length (HL/SVL), but poorly correlated with the inter-ilial width relative to SVL. The best morphological predictor was HL/SVL (speed=-3.28+3.916 HL/SVL, r=0.968, P<0.0001), suggesting that anuran take-off speed is portrayed well with high gear and acceleration distance characterized by hindlimbs.     

Thermal relationships and exercise physiology in anuran amphibians: integration and evolutionary implications

Thermal and water balance are coupled in anurans, and species with particularly permeable skin avoid overheating more effectively than minimizing variance of body temperature. In turn, temperature affects muscle performance in several ways, so documenting the mean and variance of body temperature of active frogs can help explain variation in behavioral performance. The two types of activities studied in most detail, jumping and calling, differ markedly in duration and intensity, and there are distinct differences in the metabolic profile and fiber type of the supporting muscles. Characteristics of jumping and calling also vary significantly among species, and these differences have a number of implications that we discuss in some detail throughout this paper. One question that emerges from this topic is whether anuran species exhibit activity temperatures that match the temperature range over which they perform best. Although this seems the case, thermal preferences are variable and may not necessarily reflect typical activity temperatures. The performance versus temperature curves and the thermal limits for anuran activity reflect the thermal ecology of species more than their systematic position. Anuran thermal physiology, therefore, seems to be phenotypically plastic and susceptible to adaptive evolution. Although generalizations regarding the mechanistic basis of such adjustments are not yet possible, recent attempts have been made to reveal the mechanistic basis of acclimation and acclimatization.     

Control of breathing in anuran amphibians

The primary role of the respiratory system is to ensure adequate tissue oxygenation, eliminate carbon dioxide and help to regulate acid-base status. To maintain this homeostasis, amphibians possess an array of receptors located at peripheral and central chemoreceptive sites that sense respiration-related variables in both internal and external environments. As in mammals, input from these receptors is integrated at central rhythmogenic and pattern-forming elements in the medulla in a manner that meets the demands determined by the environment within the constraints of the behavior and breathing pattern of the animal. Also as in mammals, while outputs from areas in the midbrain may modulate respiration directly, they do not play a significant role in the production of the normal respiratory rhythm. However, despite these similarities, the breathing patterns of the two classes are different: mammals maintain homeostasis of arterial blood gases through rhythmic and continuous breathing, whereas amphibians display an intermittent pattern of aerial respiration. While the latter is also often rhythmic, it allows a degree of fluctuation in key respiratory variables that has led some to suggest that control is not as tight in these animals. In this review we will focus specifically on recent advances in studies of the control of ventilation in anuran amphibians. This is the group of amphibians that has attracted the most recent attention from respiratory physiologists.     

Hormones and acoustic communication in anuran amphibians

Circulating hormone levels can mediate changes in the quality of courtship signals by males and/or mate choice by females and may thus play an important role in the evolution of courtship signals. Costs associated with shifts in hormone levels of males, for example, could effectively stabilize directional selection by females on male signals. Alternatively, if hormone levels affect the selection of mates by females, then variation in hormone levels among females could contribute to the maintenance of variability in the quality of males' signals. Here, I review what is known regarding the effects of hormone levels on the quality of acoustic signals produced by males and on the choice of mates by females in anuran amphibians. Surprisingly, despite the long history of anuran amphibians as model organisms for studying acoustic communication and physiology, we know very little about how variation in circulating hormone levels contributes to variation in the vocal quality of males. Proposed relationships between androgen levels and vocal quality depicted in recent models, for example, are subject to the same criticisms raised for similar models proposed in relation to birds, namely that the evidence for graded effects of androgens on vocal performance is often weak or not rigorously tested and responses seen in one species are often not observed in other species. Although several studies offer intriguing support for graded effects of hormones on calling behavior, additional comparative studies will be required to understand these relationships. Recent studies indicate that hormones may also mediate changes in anuran females' choice of mates, suggesting that the hormone levels of females can influence the evolution of males' mating signals. No studies to date have concurrently addressed the potential complexity of hormone-behavior relationships from the perspective of sender as well as receiver, nor have any studies addressed the costs that are potentially associated with changes in circulating hormone levels in anurans (i.e., life-history tradeoffs associated with elevations in circulating androgens in males). The mechanisms involved in hormonally induced changes in signal production and selectivity also require further investigation. Anuran amphibians are, in many ways, conducive to investigating such questions.     

Retina and lens regeneration in anuran amphibians

Anuran amphibians can regenerate the retina through differentiation of stem cells in the ciliary marginal zone and through transdifferentiation of the retinal pigmented epithelium. By contrast, the regeneration of the lens has been demonstrated only in larvae of species belonging to the Xenopus genus, where the lens regenerates through transdifferentiation of the outer cornea. Retinal pigmented epithelium to neural retina and outer cornea to lens transdifferentiation processes are triggered and sustained by signaling molecules belonging to the family of the fibroblast growth factor. Both during retina and lens regeneration there is a re-activation of many of the genes which are activated during development of the eye, even though the spatial and temporal pattern of gene expression is not a simple repetition of that found in development.     

Evolutionary transformations of ontogenesis in anuran amphibians

A review of the recent published data on ontogenesis of direct developing and marsupial frogs. The development of these representatives of anuran amphibians seems to be evolutionary advanced and considerably differs from the development of species traditionally used in amphibian embryology.     

Active and resting metabolism in birds: allometry, phylogeny and ecology

Variation in resting metabolic rate is strongly correlated with differences in body weight among birds. The lowest taxonomic level at which most of the variance in resting metabolic rate and body weight is evident for the sample is among families within orders. The allometric exponent across family points is 0.67. This exponent accords with the surface area interpretation of metabolic scaling based on considerations of heat loss. Deviations of family points from this allometric line are used to examine how resting metabolic rates differ among taxa, and whether variation in resting metabolic rate is correlated with broad differences in ecology and behaviour. Despite the strong correlation between resting metabolic rate and body weight, there is evidence for adaptive departures from the allometric line, and possible selective forces are discussed.
The allometric scaling of active metabolic rate is compared with that of resting metabolic rate. The allometric exponents for the two levels of energy expenditure differ, demonstrating that active small-bodied birds require proportionately more energy per unit time above resting levels than do active large-bodied birds. No consistent evidence was found to indicate that the different methods used to estimate active metabolic rate result in systematic bias. Birds require more energy relative to body size when undertaking breeding activities than at other stages of the annual cycle.     

Diversity of cortical bone morphology in anuran amphibians

The cortical bones of mammals, birds, and reptiles are composed of a complex of woven bone and lamellar bone (fibrolamellar bone) organized into a variety of different patterns; however, it remains unclear whether amphibians possess similar structures. Importantly, to understand the evolutionary process of limb bones in tetrapods, it is necessary to compare the bone structure of amphibians (aquatic to terrestrial) with that of amniotes (mostly terrestrial). Therefore, this study compared the cortical bones in the long bones of several frog species before and after metamorphosis. Using micro-computed tomography (CT), we found that the cortical bones in the fibrolamellar bone of Xenopus tropicalis (Pipoidea superfamily) and Lithobates catesbeianus (Ranoidea superfamily) froglets are dense, whereas those of Ceratophrys cranwelli (Hyloidea superfamily) are porous. To clarify whether these features are common to their superfamily or sister group, four other frog species were examined. Histochemical analyses revealed porous cortical bones in C. ornata and Lepidobatrachus laevis (belonging to the same family, Ceratophryidae, as C. cranwelli). However, the cortical bones of Dryophytes japonicus (Hylidae, a sister group of Ceratophryidae in the Hyloidea superfamily), Microhyla okinavensis (Microhylidae, independent of the Hyloidea superfamily), and Pleurodeles waltl, a newt as an outgroup of anurans, are dense with no observed cavities. Our findings demonstrate that at least three members of the Ceratophryidae family have porous cortical bones similar to those of reptiles, birds, and mammals, suggesting that the process of fibrolamellar bone formation arose evolutionarily in amphibians and is conserved in the common ancestor of amniotes.     

The external gills of anuran amphibians: Comparative morphology and ultrastructure

The external gills of anuran amphibians are transient structures, covered by the development of the operculum and regressing soon afterwards. Their functional role has been regarded as equivocal. However, detailed morphological analysis has been limited. Analysis of 21 species from six families using scanning and transmission electron microscopy revealed diversity at the anatomical and cellular levels in extent and length of gill filaments, numbers of surface ciliated cells, width of water‐blood barrier distance, and evidence of gill motility. The most highly developed external gills were found in species with delayed hatching, such as Phyllomedusa trinitatis, or in species in which hatchlings hang from the surface film of temporary ponds, such as Phrynohyas venulosa in which gills added 26–38% to body surface area. In one family, the bufonids, all four species examined had poorly developed gills, but in other families where we examined several species, the hylids and leptodactylids, there was considerable diversity of external gills, suggesting flexible adaptation to incubation and hatching environment. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Evolutionary causes and consequences of sequential polyandry in anuran amphibians

Among anuran amphibians (frogs and toads), there are two types of polyandry: simultaneous polyandry, where sperm from multiple males compete to fertilize eggs, and sequential polyandry, where eggs from a single female are fertilized by multiple males in a series of temporally separate mating events, and sperm competition is absent. Here we review the occurrence of sequential polyandry in anuran amphibians, outline theoretical explanations for the evolution of this mating system and discuss potential evolutionary implications. Sequential polyandry has been reported in a limited number of anurans, but its widespread taxonomic and geographic distribution suggests it may be common. There have been no empirical studies that have explicitly investigated the evolutionary consequences of sequential polyandry in anurans, but species with this mating pattern share an array of behavioural, morphological and physiological characteristics, suggesting that there has been common sexual selection on their reproductive system. Sequential polyandry may have a number of adaptive benefits, including spreading the risk of brood failure in unpredictable environments, insuring against male infertility, or providing genetic benefits, either through good genes, intrinsic compatibility or genetic diversity effects. Anurans with sequential polyandry provide untapped opportunities for innovative research approaches that will contribute significantly to understanding anuran evolution and also, more broadly, to the development of sexual‐selection and life‐history theory.     

Dental morphology and diet in anuran amphibians from south India

The morphology of the jaw and palatine surfaces of eight species of metamorphosed anuran amphibians ( Microhyla ornata, M. rubra, Uperodon systoma, Tomopterna rolandae, Polypedates maculatus, Rana cyanophlycris, R. crassa and R. hexadactyla ) from a locality in south India, were examined by scanning electron microscopy. A relationship was observed between dentition (or its absence) and diet. In large prey feeders, there is a strong tendency towards the development of large secondary (or even tertiary) cusps, while myrmecophagous and termitophagous species lack teeth.
In the largely folivorous adults of Rana hexadactyla , secondary cusps are reduced to faint ridges and the tooth is cylindro-conical (as opposed to the recurved teeth with apices oriented lingually or distally in the insectivorous species). Although the phylogenetic relationships within members of the community are largely unknown, the oral armature is reflective of diet, and may represent adaptive suites.     

Control and interaction of the cardiovascular and respiratory systems in anuran amphibians

In anuran amphibians, respiratory rhythm is generated within the central nervous system (CNS) and is modulated by chemo- and mechanoreceptors located in the vascular system and within the CNS. The site for central respiratory rhythmogenesis and the role of various neurotransmitters and neuromodulators is described. Ventilatory air flow is generated by a positive pressure, buccal force pump driven by efferent motor output from cranial nerves. The vagus (cranial nerve X) also controls heart rate and pulmocutaneous arterial resistance that, in turn, affect cardiac shunts within the undivided anuran ventricle; however, little is known about the control of central vagal motor outflow to the heart and pulmocutaneous artery. Anatomical evidence indicates a close proximity of the centers responsible for respiratory rhythmogenesis and the vagal motoneurons involved in cardiovascular regulation. Furthermore, anurans in which phasic feedback from chemo- and mechanoreceptors is prevented by artificial ventilation exhibit cardiorespiratory interactions that appear similar to those of conscious animals. These observations indicate interactions between respiratory and cardiovascular centers within the CNS. Thus, like mammals and other air-breathing vertebrates, the cardio-respiratory interactions in anurans result from both feedback and feed-forward mechanisms.