Neuromast topography in anuran amphibians

Generalized anuran tadpoles across families exhibit a similar neuromast morphology on their heads, as follows: (1) all neuromast lines known for anurans are present; (2) within these lines total neuromast number ranges from about 250 to 320; (3) neuromasts form linear stitches composed of two to three, but sometimes up to five, neuromasts; (4) neuromast linear dimensions are ? 10 μm; and (5) neuromasts contain ? 15 hair cells. Compared with generalized forms, stream, arboreal, carnivorous, and desert-pond forms have fewer neuromasts but they contain more hair cells. They do not, however, form stitches. Obligate midwater suspension-feeding forms, including Xenopus (Pipidae), Rhinophrynus (Rhinophyrnidae), and Phrynomerus (Microhylidae), form stitches that contain > six, but potentially up to 18 or more, loosely aggregated neuromasts. Xenopus and Rhinophrynus have large neuromasts (up to 40 μm across). Chiasmocleis (Microhylidae) tadpoles form stiches that are linearly arranged with up to ten neuromasts. Whereas urodeles can have more than one neuromast row per line and may form both linear and transverse stitches, anurans have only one row of neuromasts per line and form only transverse stitches. Neuromasts in anurans tend to be smaller and more circular than in urodeles and positioned flush with the epidermal surface. A greater percentage of anurans form stitches, and anurans have greater intrafamilial variation in stitch formation than do urodeles.     

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.     

Sound source perception in anuran amphibians

Sound source perception refers to the auditory system's ability to parse incoming sensory information into coherent representations of distinct sound sources in the environment. Such abilities are no doubt key to successful communication in many taxa, but we know little about their function in animal communication systems. For anuran amphibians (frogs and toads), social and reproductive behaviors depend on a listener's ability to hear and identify sound signals amid high levels of background noise in acoustically cluttered environments. Recent neuroethological studies are revealing how frogs parse these complex acoustic scenes to identify individual calls in noisy breeding choruses. Current evidence highlights some interesting similarities and differences in how the auditory systems of frogs and other vertebrates (most notably birds and mammals) perform auditory scene analysis.     

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.     

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.     

Developmental changes in interrenal responsiveness in anuran amphibians

Basal activity of the hypothalamo-pituitary-interrenal (HPI)axis changes over development in larval amphibians, but developmentof the responsiveness of this axis to an external stressor hasnot been studied. We compared developmental changes in whole-bodycorticosterone content of two anuran amphibian species, Ranapipiens (family Ranidae) and Xenopus laevis (family Pipidae).We also examined developmental changes in the responsivenessof the HPI axis by subjecting tadpoles of different developmentalstages to a laboratory shaking/confinement stress and to ACTHinjection. We measured whole-body corticosterone content asan indicator of the activity of the HPI axis. Whole-body corticosteronecontent of R. pipiens remained low during premetamorphosis andprometamorphosis but increased dramatically at metamorphic climaxand remained elevated in juvenile frogs. By contrast, whole-bodycorticosterone content of X. laevis was highest during premetamorphosis,declined at the onset of prometamorphosis, increased at metamorphicclimax and remained at climax levels in juvenile frogs. Premetamorphicand prometamorphic tadpoles of both species showed strong corticosteroneresponses to both shaking stress and ACTH injection. The magnitudeand pattern of response differed among developmental stages,with premetamorphic tadpoles of both species showing greaterresponsiveness to stress and ACTH. Our results show that interrenalresponsiveness is developed in premetamorphic tadpoles, suggestingthat at these stages tadpoles are capable of mounting an increasein stress hormone production in response to changes in the externalenvironment. Our results also highlight the importance of comparativestudies in understanding the development of the stress axis.     

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 dependence of behavioural performance of anuran amphibians

The thermal dependence of performance capacity was assessed in two anuran amphibians: Bufo boreas (western toad) and Rana pipiens (leopard frog). Quantitative measurements of performance showed that Bufo could sustain slow rates of walking for 10 min and cover greater distances than Rana, which initially jumped more vigorously but fatigued within 5 min. Changes in performance with changes in body temperature were virtually instantaneous, and performance exhibited no acclimation over 7 days. Within the range of temperatures studied, performance capacity increased with increasing body temperature and reached a maximum at 28 C in Bufo and 20 to 29 C in Rana. Performance capacity and the underlying metabolic processes had a similar thermal dependence within a species. The behavioural capacity for activity is apparently maximal for both species at body temperatures normally encountered in the field. Anuran behaviours requiring sustained activity (migration to breeding sites, mating, foraging) must therefore be markedly temperature-sensitive.     

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.     

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.