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Microhylid frogs have the unusual ability to aim their tongues independently of the head during prey capture. The tongue is elongated hydrostatically, which requires a novel protractor muscle (m. genioglossus) morphology. This novel morphology and demand for fine motor control raised the question: Are there unusual aspects to the innervation of the tongue muscles in microhylids? Retrograde labeling of the hypoglossal nerve revealed an exceptionally large number of motor neurons distributed more caudally in the brainstem when compared to other anurans. In addition, denervation and EMG experiments have demonstrated that the m. genioglossus is the primary muscle responsible for aiming the tongue within the Microhylidae. Thus, an increase in motor neurons may have resulted from an increase in the innervation of the m. genioglossus required to produce hydrostatic elongation and aim the tongue. The speed of tongue protraction is highly variable between species (30 n 200 cm/s). Preliminary results suggest that microhylids with slower protraction speeds will have a larger number of motor neurons innervating the m. genioglossus, which allows more control over the tongue than microhylids that protract their tongues at faster speeds. This hypothesis suggests a trade-off between the speed of protraction and the degree of control over the tongue in three dimensions. This trade-off may also reflect a frogis feeding strategy and ecological niche.

Parental choice of early offspring environments can have important fitness consequences for their young, especially in taxa with otherwise limited parental investment. The pathways by which site variables influence fitness will determine the strength of selection for site choice. In 1995 and 1996, I investigated the direct and indirect effects of rearing site choice in Dendrobates pumilio, a frog with extended parental care. Female D. pumilio deposit their tadpoles in Dieffenbachia and bromeliad axils that vary in age, water volume, pH, mean temperature, and temperature variance. The results of this study indicate that female D. pumilio actively discriminate among abundantly available rearing sites when depositing their tadpoles and that these site choices may have both direct and indirect consequences for tadpole survival. Females consistently chose younger rearing sites with greater water volumes in both years, and selected on mean temperature in 1995. Although the fitness consequences of water volume and axil age could not be assessed in this study, these axil characters are expected to have direct effects on offspring survival. Variance in axil temperature had an indirect effect on tadpole fitness via its effect on tadpole growth in 1996; however, females showed no preference for temperature variance when depositing their tadpoles. These results are consistent with the prediction that mothers showing extended parental care will more likely discriminate among sites (if they discriminate at all) based on characters having direct effects on offspring survival than among sites based on characters with only indirect effects.

Several vertebrate species have shown ontogenetic adjustments in the form and material properties of the growing musculoskeletal system to allow the same level of locomotor performance in juveniles and adults. The rate of growth and amount of time the musculoskeletal system has to develop may affect the amount of compensatory change needed to produce the same level of performance; for example, the faster the growth rate and/or the shorter the amount of time for development, the greater the need for these adjustments. To test these predictions, individuals of Hyla chrysoscelis were raised at 28fC and 18fC to evaluate the effects of developmental duration and growth rate on bone and muscle morphology, bone material properties, and jumping performance. Although two-fold differences in developmental duration and larval growth rates were found between frogs raised at 28fC and 18fC, little or no difference was found in hind limb morphology, bone shape, bone mineral content, bone material properties, relative muscle mass or jumping performance at the end of metamorphosis. However, during the first nine weeks of post-metamorphic growth in both groups, absolute jumping performance, relative proportion of hind limb muscle mass, and bone material properties changed with age. These results suggest that an increase in the relative proportion of hind limb muscle mass, as well as an increase in the strength and stiffness of the hind limb skeleton, contribute to the improvement of jumping performance during post-metamorphic growth. Supported by a Howard Hughes Predoctoral Fellowship and NSF (IBN-9701121).

Leptodactylid frogs exhibit diversity of the tongue and jaw mechanics during prey capture. Two mechanisms of prey capture have been seen in leptodactylids: inertial elongation and mechanical pulling. Phylogenetic analyses suggest that inertial elongation has evolved independently from mechanical pulling several times within leptodactylids. The goal of this study is to examine the evolution of feeding mechanisms in this diverse group of frogs. Feeding mechanisms of frogs can be categorized based on patterns of kinematics. The primary kinematic difference between inertial elongation and mechanical pulling is the movements of the tongue and jaw. During inertial elongation, the movements of the tongue and jaw are precisely coordinated, whereas in mechanical pulling, these movements are uncoupled. Leptodactylids that are considered to be inertial elongators tend to have short gape cycles (~11 ms), a high velocity of the lower jaw during mouth opening, and a great increase in the length of the tongue (180% of resting length). In contrast, leptodactylid frogs that are defined as mechanical pullers generally have longer gape cycles (~28 ms), slower velocity of the lower jaw, and a decrease in the length of the tongue during protraction (40% of resting length). Thus, we expect to find relative differences in the lever system of the jaw between inertial elongators and mechanical pullers. Inertial elongators should have proportionally shorter in-levers in order to maximize the speed of mouth opening, while mechanical pullers should have proportionally longer in-levers in order to maximize the force of mouth closing. The variability in kinematics and morphology among species of leptodactylids may reflect differences in the foraging ecology between species.

To study the spatio-temporal behaviour of swimming cycles in frogs, swimming bouts of Rana esculenta were video-taped. Durations and displacements of the propulsive, glide and recovery phase (i.e. the six spatio-temporal variables) were determined and instantaneous velocity fluctuations and average locomotor speed were calculated. None of these six correlate with average speed. This points either at the absence of coordination or at the presence of a complex control system attuning the spatio-temporal variables to each other to realize any intentional speed. Instantaneous velocities at the transitions between successive phases, however, are highly correlated with the average locomotor speed. Mathematical simulations prove that such strong correlations emerge automatically from randomly chosen propulsive, glide and recovery characteristics. Apparently, the overall swimming performance is mainly determined by the magnitude of the propulsive impulse, whereas glide and recovery characteristics have only minor effects. In this respect, swimming can be regarded as `jumping in water' and it is postulated that the strategy to control swimming performance (speed) is retained from the evolutionary primary saltatorial mode.

Historically, functional morphologists have used mostly single joint models such as the lever arm or four-bar linkage in biomechanical studies. While these models have been useful in understanding single joint systems such as the human elbow, they are less useful for understanding systems in which torques produced at one joint are propagated to other joints. In these multijoint systems, such as the entire arm, a more complex approach is needed. Here, we report on our efforts to use multijoint models to understand patterns of muscle activation in the jaws and tongue that frogs employ to capture prey. Preliminary simulations have yielded the following: 1) small changes in muscle activation have a large effect on the movement trajectory; 2) muscles may exert substantial torques at joints far from their origin and insertion; and 3) it appears that rapid mouth opening is facilitated by a catch mechanism; when realistic values are used in the model, the mouth opens more slowly than in the animals. These results suggest that, despite their computational complexity, multijoint models provide an important tool for understanding muscle activation patterns in biomechanics and functional morphology.

A critical step in the evolution of flight is developing the ability to control direction of flight. Aerodynamic stability affects both maintaining direction of flight, and maneuvering. Aerodynamic stability permits a gliding animal to maintain its direction of flight without actively steering. Aerodynamic stability also resists intentional disturbances to direction of flight such as the initiation of turning maneuvers. Gliding has originated independently in 2 families of frogs: Rhacophoridae and Hylidae. Gliding tree frogs possess a suite of morphologies and behaviors associated with gliding such as enlarged, extensively webbed hands and feet. Among Phyllomedusinae, a subfamily of Hylidae, gliding has evolved in the genus Agalychnis. To assess how stability changes with gliding ability, I compared the aerodynamic stability of 3 Agalychnis species. The species chosen represent 3 stages in the development of gliding within this genus: 1) Agalychnis callidryas, a frog with minimal webbing on its hands and feet, 2) A. calcarifer, a frog with an intermediate amount of webbing on its hands and feet, and 3) A. spurrelli, a frog with extensively webbed hands and feet. Aerodynamic stability and maneuverability were assessed using aerodynamic forces and torques measured from physical models of the frog species in a wind tunnel.

Anurans inhabiting northern locales survive the winter by avoiding cold through aquatic hibernation or burrowing, or by tolerating freezing of their tissues. This study describes the overwintering strategy of the most aquatic of the North American hylids, the northern cricket frog (Acris crepitans). Laboratory choice tests indicated that cricket frogs choose terrestrial hibernacula, especially under leaf litter in shallow depressions, when faced with decreasing temperatures. However, unlike Bufo and some Pseudacris, cricket frogs did not burrow when air temperatures dropped, even when faced with desiccating conditions. Surprisingly, cricket frogs are not freeze tolerant like most other nearctic hylids that hibernate in terrestrial sites. Given these data, field searches were conducted and cricket frogs were found hibernating in crayfish burrows and other cavities along the pond's edge - sites which provide full protection from freezing. In conclusion, cricket frogs can neither burrow nor survive freezing, and therefore require pre-existing subterranean sites to successfully overwinter. Thus, I suggest that habitat quality and winter severity be explored as possible causes of this species' decline in the northern portion of its range.

Polyploidy is relatively common in amphibians. In anurans, differences in ploidy within species groups are always associated with differences in species-specific calls. It is unknown, however, whether alteration of calls is a direct effect of polyploidy or a result of subsequent evolutionary modifications. We produced artificial, first-generation polyploids of the South African clawed frog (Xenopus laevis) and recorded their vocal behavior to address this question. Tetraploid Xenopus were created using the French press and raised to maturity. We used tactile stimulation to elicit the release call type from male diploid and tetraploid Xenopus and calls were analyzed with ILS. This call type was given singly with a call duration of about 0.23 s, about 13 pulses per call, and a subsequent pulse rate of approximately 57 pulses/s. Mean fundamental and dominant frequencies were 751 and 2532 Hz, respectively. There were no significant differences in release call characteristics between diploid and tetraploid animals. This suggests that polyploidy alone does not have an obligatory effect on vocal behavior of frogs. Supported by NSF #IBN95-14305.

Among Anura, different jumping abilities (in height, distance and speed) have been attributed to differences in hindlimb segment lengths (Zug, 1978). Differences in extensor muscle mass have also been cited (Emerson, 1985) to explain differences in performance between hoppers. In this study, whole muscle contractile properties (isometric force, contraction speed, fatigability) were compared between the plantaris longus (PL) and peroneus (Per) of Rana pipiens (long-distance hopper) and Bufo marinus (short hopper). We found that maximum tetanic tension in PL scaled directly with muscle mass between the two species so that PL force/g of muscle mass was the same. Per was larger in Bufo, and also produced more force/g than in Rana. Contraction and half-relaxation times were the same in PL, but slower in Bufo Per than in Rana Per. PL in Rana was more fatigable than in Bufo, but Per was more fatigable in Bufo than in Rana. These results suggest that the fiber-type composition of PL in both species is similar, but that Bufo Per has more larger and slower motor units than Rana. Supported in part by a grant from Sigma Xi.

In the chicken embryo, neural crest cells contribute to the connective tissue components of cranial muscle. Furthermore, a 'cryptic segmentation' exists, in which mesodermal and neural crest components of a given muscle are derived from the same visceral arch. Larval anurans have a highly apomorphic head. Unique features include, for example, second (hyoid) arch muscles that both originate and insert on cartilages from the first (mandibular) arch. This makes anurans an excellent testing ground for the hypothesis that cryptic segmentation is a conserved vertebrate trait that persists even in clades that have undergone drastic evolutionary changes in cranial morphology. Vital dye labelling and ablation of selected regions of migrating neural crest cells in Bombina orientalis indicate that neural crest contributes to connective tissue components at the attachment sites of several cranial muscles. Crest ablation severely disrupts cranial muscle patterning. Preliminary data indicate that cryptic segmentation is conserved in frogs. Supported by NSF (to J.H.) and by the Wenner-Gren Center Foundations and the Swedish Institute (to L.O.).

Social signals are essential in the coordination of reproductive behavior, and increasingly, social cues are being shown to have a profound effect on reproductive physiology. Previous studies demonstrated that conspecific mating calls influence testicular morphology in frogs. However, these studies did not assess changes in circulating sex steroid levels. We examined the effects of a conspecific mating chorus on androgen levels in male Southern leopard frogs, Rant sphenocephala. For 10 consecutive nights, we exposed male frogs to recordings of either a conspecific mating chorus (n = 6) or a chorus that had been spectrally shifted by 500 Hz (n = 6). Mean plasma androgen levels were significantly higher in the normal chorus group (17.1 [+ or -] 5.6 ng/ml plasma) compared to the altered chorus group (4.6 [+ or -] 2.1 ng/ml plasma) (t(10)=2.4, p[is less than]0.05; log transformed data). Some males in both groups called in response to the auditory stimuli. In spite of this behavioral similarity, sex steroid levels were dramatically different. This suggests that the difference in hormone levels was due to the influence of auditory stimulation by the chorus and not a result of the animal's own calling behavior. Supported by R01 MH45696 to W.W.