Amphibian lipid levels at metamorphosis correlate to post-metamorphic terrestrial survival

In organisms that have complex life cycles, factors in the larval environment may affect both larval and adult traits. For amphibians, the postmetamorphic transition from the aquatic environment to terrestrial habitat may be a period of high juvenile mortality. We hypothesized that lipid stores at metamorphosis may affect an animal’s success during this critical transition period. We examined variation in total lipid levels among years and sites in recently metamorphosed individuals of two pond-breeding salamander species, the marbled salamander (Ambystoma opacum) and the mole salamander (A. talpoideum), with limited data for one anuran species (southern leopard frog, Rana sphenocephala). Lipid levels were allometrically related to body size and ranged from 1.9 to 23.8% of body dry mass. The two salamander species differed in lipid allocation patterns, with A. opacum apportioning a higher percentage of total lipid reserves into fat bodies than A. talpoideum. Species differences in lipid allocation patterns may primarily reflect that large metamorphs will mature as one-year olds, and, regardless of species, will alter lipid compartmentalization accordingly. We used mark–recapture data obtained at drift fences encircling breeding ponds for 13 A. opacum cohorts to estimate the proportion of postmetamorphic individuals that survived to breed (age 1–4) and the mean age at first reproduction. Regression models indicated that size-corrected lipid level at metamorphosis (i.e., lipid residuals), and to a lesser extent rainfall following metamorphosis, was positively related to adult survival. Snout-vent length at metamorphosis was negatively related to age at first reproduction. We suggest that lipid stores at metamorphosis are vital to juvenile survival in the months following the transition from aquatic to terrestrial habitat, and that a trade-off shaped by postmetamorphic selection in the terrestrial habitat exists between allocation to energy stores versus structural growth in the larval environment.     

Development and morphological variation of the axial and appendicular skeleton in hylidae (Lissamphibia,Anura)

The axial and appendicular skeleton, the associated musculature and tendons form a functional system related to specific modes of locomotion in anurans. Many transformations in the structures linked with the locomotor function of the adult occur during larval stages and metamorphosis. In this study, we present the larval ontogeny and adult morphology of the axial and appendicular skeletons of 14 species of frogs in the family Hylidae with different locomotor modes and habitat uses. Among Hylidae, a diversity of shapes, locomotory types occurs (e.g., walker, swimmer, jumper, hopper) and different habitat types occupied (shrubby, terrestrial, aquatic, arboreal). Many elements complete differentiation at the end of metamorphosis; others, such as sesamoids, still show an incomplete development at that stage. Sixty seven characters were scored and optimized in an available phylogeny. Nine characters of developmental timing and adult osteology are optimized as synapomorphies of specific groups. Some characters appear to be related to the locomotor type (e.g., the sacro‐urostyle region configuration is highly linked with the jumping mode; nonexpanded diapophyses would related to aquatic habitat use). Nevertheless, the functional interpretations are quite particular to this family. Monophyletic clades are also groups with shared locomotory modes or habitat uses. Hence, the hypothesis of common ancestry or adaptation can be evaluated, taking into account the analysis level of the phylogenetic context, so that, when a character is inherited via common ancestry, it necessarily means that functional constraints could also be inherited. Here, we outline the basis for further work on: postmetamorphic development as a fundamental period for the complete differentiation of structures related to a full locomotor functionality; the biomechanical performance in relationship to the variation in ligaments and sesamoids; the importance of analyzing these topics within the frame of heterochrony. J. Morphol. 277:786–813, 2016. © 2016 Wiley Periodicals, Inc.     

Triturus newts defy the running-swimming dilemma

Conflicts between structural requirements for carrying out different ecologically relevant functions may result in a compromise phenotype that maximizes neither function. Identifying and evaluating functional trade-offs may therefore aid in understanding the evolution of organismal performance. We examined the possibility of an evolutionary trade-off between aquatic and terrestrial locomotion in females of European species of the newt genus Triturus. Biomechanical models suggest a conflict between the requirements for aquatic and terrestrial locomotion. For instance, having an elongate, slender body, a large tail, and reduced limbs should benefit undulatory swimming, but at the cost of reduced running capacity. To test the prediction of an evolutionary trade-off between swimming and running capacity, we investigated relationships between size-corrected morphology and maximum locomotor performance in females of ten species of newts. Phylogenetic comparative analyses revealed that an evolutionary trend of body elongation (increasing axilla-groin distance) is associated with a reduction in head width and forelimb length. Body elongation resulted in reduced maximum running speed, but, surprisingly, also led to a reduction in swimming speed. The evolution of longer tails was associated with an increase in maximal swimming speed. We found no evidence for an evolutionary trade-off between aquatic and terrestrial locomotor performance, probably because of the unexpected negative effect of body elongation on swimming speed. We conclude that the idea of a design conflict between aquatic and terrestrial locomotion, mediated through antagonistic effects of body elongation, does not apply to our model system.     

Fitness consequences of larval traits persist across the metamorphic boundary

Metamorphosis is thought to provide an adaptive decoupling between traits specialized for each life-history stage in species with complex life cycles. However, an increasing number of studies are finding that larval traits can carry-over to influence postmetamorphic performance, suggesting that these life-history stages may not be free to evolve independently of each other. We used a phenotypic selection framework to compare the relative and interactive effects of larval size, time to hatching, and time to settlement on postmetamorphic survival and growth in a marine invertebrate, Styela plicata. Time to hatching was the only larval trait found to be under directional selection, individuals that took more time to hatch into larvae survived better after metamorphosis but grew more slowly. Nonlinear selection was found to act on multivariate trait combinations, once again acting in opposite directions for selection acting via survival and growth. Individuals with above average values of larval traits were most likely to survive, but surviving individuals with intermediate larval traits grew to the largest size. These results demonstrate that larval traits can have multiple, complex fitness consequences that persist across the metamorphic boundary; and thus postmetamorphic selection pressures may constrain the evolution of larval traits.     

Influence of introduced fish Percottus glenii (Odontobutidae, Pisces)

Monitoring of 28 waterbodies has been carried out since 1994 in the region of reserve "Lake Glubokoe" (Moscow region, Russia). It was revealed that species diversity as well as abundance of larval amphibians correlate negatively with presence of introduced fish, rotan, Perccottus glenii (Odontobutidae). Newts (Triturus cristatus, T. vulgaris) and frogs (Rana temporaria, R. arvalis, R. lessonae) as a rule are not capable to breed in waterbodies colonised by rotan. In contrast, toads (Bufo bufo) breed successfully in such sites. Larvae of toads are comparatively less edible for rotan and pass their metamorphosis. Persistence of amphibians to predation of rotan decreases in the row: B. bufo (R. temporaria, R. arvalis and R. lessonae), T. vulgaris, T. cristatus. The Crested newt (T. cristatus) is the most endangered species and could extinct in next years.     

Morphological integration and ontogenetic niche shift: a study of crested newt limbs

This study deals with the ontogenetic and evolutionary aspects of integration patterns in the limbs of crested newt species, which, like most amphibians, have a biphasic life history with two morphologically distinct stages (larval vs. juvenile and adult) that occupy different environments (aquatic vs. terrestrial). We analyzed the structure and pattern of correlation between limb skeletal elements at three ontogenetic stages (larval, juvenile, and adult) of four closely related species that differ in their preferences of aquatic habitats (more terrestrial and more aquatic). We found dynamic changes in the pattern of morphological integration between successive ontogenetic stages, as well as changes over the course of crested newt phylogeny. Generally, equivalent ontogenetic stages of different species of crested newts show higher concordance in the correlation pattern than successive ontogenetic stages within species. Among species, two opposing correlation patterns were observed: in more terrestrial species, homologous limb elements are less correlated and within-limb elements are more correlated; in aquatic species, the reverse pattern occurs. These results indicate that the function seems to be the covariance-generating factor, which has shaped the patterns of morphological integration of crested newt limbs.     

Retrofitting larval neuromuscular circuits in the metamorphosing frog

Maturation of vertebrate neuromuscular systems typically occurs in a continuous, orderly progression. After an initial period of developmental adjustment by means of cell death and axonal pruning, relatively stable relationships, with only subtle modifications, are maintained between motoneurons and their appropriate targets throughout life. However, among a restricted group of vertebrates (amphibians and especially the anuran amphibians) the sequential maturation of neuromuscular systems is altered by an abrupt reordering of the basic body plan that encompasses cellular changes in all tissues from skeleton to nervous system. Many anuran amphibians possess neuromuscular circuits that are remarkable by virtue of their complete reorganization during the brief span of metamorphosis. During this period motor systems initially designed for the behavioral patterns of aquatic tadpoles are adjusted to meet the drastically different motor activities of postmetamorphic terrestrial life. This adjustment involves the deletion of neural elements mediating larval specific activities, the accelerated maturation of neural circuits eliciting adult-specific activities and the retrofitting of larval neuromuscular components to serve postmetamorphic behaviors. This review focuses on the cellular events associated with the neuromuscular adaptation in the jaw complex during metamorphosis of the leopard frog, Rana pipiens. As part of the metamorphic reorganization of the jaw apparatus there is a complete turnover of the myofiber complement of the adductor mandibulae musculature. Trigeminal motoneurons initially deployed to the larval myofibers are redirected to new muscle fibers. Simultaneously the cellular geometry and synaptic input to these motoneurons is revamped. These changes suggest that trigeminal neuromuscular circuitry established during embryogenesis is updated during metamorphosis and reused to provide the basis for adult jaw motor activity that is far different than its larval counterpart.     

The ontogenetic shift in thermoregulatory behaviour of newt larvae: testing the 'enemy-free temperatures' hypothesis

Growth and development affect life-history traits, and consequently organismal fitness. The inevitable increase in body size during ontogeny is associated with changes in both resource use and predation risk, which results in the ontogenetic shift in habitat preferences. In this study, we examined whether the shift in preferred body temperatures ( T _ps) of newt larvae Triturus alpestris increases the T _ps deviation of the most vulnerable stages (after hatching and during metamorphosis) from the T _ps range of their main predator, dragonfly larvae Aeshna cyanea – the 'predator-free temperatures' hypothesis. Analyses of thermoregulatory behaviour in the laboratory thermal gradient showed that freshly hatched newt larvae maintained lower water temperatures than larvae in later stages, whose T _ps largely fell into the T _ps range of dragonfly larvae. With respect to the thermal quality of natural habitat, the anti-predator effectiveness of the T _ps shift decreased during development. Water temperatures in natural habitats were located largely below the preferred body temperature range of both newt and dragonfly larvae, which limits their potential thermal niche partitioning. We conclude that factors other than predator avoidance drive the ontogenetic T _ps shift in our model system.     

The effects of perceived predation risk on pre- and post-metamorphic phenotypes in the common frog

Where organisms undergo radical changes in habitat during ontogeny, dramatic phenotypic reshaping may be required. However, physiological and functional interrelationships may constrain the extent to which an individual's phenotype can be equally well adapted to their habitat throughout the life cycle. The phenotypic response of tadpoles to the presence of a predator has been reported for several species of anuran but the potential post-metamorphic consequences have rarely been considered. We reared common frog Rana temporaria tadpoles in the presence or absence of a larval odonate predator, Aeshna juncea , and examined the consequences of the resulting phenotypic adjustment in the aquatic larval stage of the life cycle for the terrestrial juvenile phenotype. In early development tadpoles developed deeper tail fins and muscles in response to the predator and, in experimental trials, swam further than those reared in the absence of a predator. While the difference in swimming ability remained significant throughout the larval period, by the onset of metamorphosis we could no longer detect any differences in the morphological parameters measured. The corresponding post-metamorphic phenotypes also did not initially differ in terms of morphology. At 12 weeks post-metamorphosis, however, froglets that developed from predator-exposed tadpoles swam more slowly and less far than those that developed from tadpoles reared in the absence of predators, the opposite trend to that observed in the larval stage of the life cycle, and had narrower femurs. These results suggest that there may be long-term costs for subsequent life-history stages of tailoring the larval phenotype to prevailing environmental conditions.     

Testing the evolutionary constraints of metamorphosis: The ontogeny of head shape in Triturus newts

In vertebrates with complex, biphasic, life cycles, larvae have a distinct morphology and ecological preferences compared to metamorphosed juveniles and adults. In amphibians, abrupt and rapid metamorphic changes transform aquatic larvae to terrestrial juveniles. The main aim of this study is to test whether, relative to larval stages, metamorphosis (1) resets the pattern of variation between ontogenetic stages and species, (2) constrains intraspecific morphological variability, and (3) similar to the “hour‐glass” model reduces morphological disparity. We explore postembryonic ontogenetic trajectories of head shape (from hatching to completed metamorphosis) of two well‐defined, morphologically distinct Triturus newts species and their F1 hybrids. Variation in head shape is quantified and compared on two levels: dynamic (across ontogenetic stages) and static (at a particular stage). Our results show that the ontogenetic trajectories diverge early during development and continue to diverge throughout larval stages and metamorphosis. The high within‐group variance and the largest disparity level (between‐group variance) characterize the metamorphosed stage. Hence, our results indicate that metamorphosis does not canalize head shape variation generated during larval development and that metamorphosed phenotype is not more constrained relative to larval ones. Therefore, metamorphosis cannot be regarded as a developmental constraint, at least not for salamander head shape.     

The evolution of metamorphosis in temnospondyls

Temnospondyls, possible relatives of extant amphibians and crudely similar to recent salamanders, are known from larval, neotenic and metamorphosed stages. Here, ontogenetic data of various temnospondyl taxa are analysed in order to recognize metamorphosis. Here, metamorphosis is strictly defined as a shift from an aquatic to a terrestrial existence. Following a check-list of criteria, the most likely metamorphosis-induced changes are proved in three temnospondyl lineages: eryopids, zatrachydids and dissorophoids. In a few other, unrelated taxa, terrestrial adults are known but no larval or metamorphosing forms. The distribution of metamorphosis among the Temnospondyli does not strictly correlate with phylogeny, which highlights the widespread occurrence of neoteny. In each group, characteristic patterns of metamorphosis are described and compared. Among temnospondyls, dissorophoids had the most intensive type of metamorphosis, characterized by a condensed ontogeny and a relatively small body size. The result was a distinct transformed morphotype with far-reaching terrestrial adaptations.     

The ontogeny of the olfactory system in ceratophryid frogs (Anura,Ceratophryidae)

The aquatic‐to‐terrestrial shift in the life cycle of most anurans suggests that the differences between the larval and adult morphology of the nose are required for sensory function in two media with different physical characteristics. However, a better controlled test of specialization to medium is to compare adult stages of terrestrial frogs with those that remain fully aquatic as adults. The Ceratophryidae is a monophyletic group of neotropical frogs whose diversification from a common terrestrial ancestor gave rise to both terrestrial (Ceratophrys, Chacophrys) and aquatic (Lepidobatrachus) adults. So, ceratophryids represent an excellent model to analyze the morphology and possible changes related to a secondary aquatic life. We describe the histomorphology of the nose during the ontogeny of the Ceratophryidae, paying particular attention to the condition in adult stages of the recessus olfactorius (a small area of olfactory epithelium that appears to be used for aquatic olfaction) and the eminentia olfactoria (a raised ridge on the floor of the principal cavity correlated with terrestrial olfaction). The species examined (Ceratophrys cranwelli, Chacophrys pierottii, Lepidobatrachus laevis, and L. llanensis) share a common larval olfactory organ composed by the principal cavity, the vomeronasal organ and the lateral appendix. At postmetamorphic stages, ceratophryids present a common morphology of the nose with the principal, middle, and inferior cavities with characteristics similar to other neobatrachians at the end of metamorphosis. However, in advanced adult stages, Lepidobatrachus laevis presents a recessus olfactorius with a heightened (peramorphic) development and a rudimentary (paedomorphic) eminentia olfactoria. Thus, the adult nose in Lepidobatrachus laevis arises from a common developmental ‘terrestrial’ pathway up to postmetamorphic stages, when its ontogeny leads to a distinctive morphology related to the evolutionarily derived, secondarily aquatic life of adults of this lineage.     

Water Turbidity Affects the Development of Sexual Morphology in the Palmate Newt

In theory, animal signals are designed to optimize transmission across a specific habitat. However, sexual signals characteristics often reflect habitat quality, a feature that does not necessarily match habitat structure. Besides, many species exploit a particular habitat for breeding so that the growth of sexual signals can depend on the additive effects of breeding and non‐breeding habitats. We combined field and experimental data to investigate the relative effect of terrestrial and aquatic habitat on the development of sexual ornaments in the palmate newt, Triturus helveticus. This species exploits a large ecological range of habitats. Like many amphibians, it spends the breeding season in water and the rest of year on land. We tested the influence of terrestrial (forest cover) and aquatic habitat variables (turbidity, organic matter, pH, nitrate and chloride) on male sexual morphology. Neither terrestrial nor aquatic habitat variables accounted for body size variation. In contrast, the size of male sexual traits decreased with water turbidity, suggesting that the expression of visual signals matched the local conditions of signal transmission. We provide experimental evidence that this pattern is not caused by reduced foraging efficiency in turbid water. We propose alternative mechanisms to account for the relationship between turbidity and visual sexual signals, and discuss the consequences of small scale environmental variation on mate choice.     

Trade-offs across life stages: does predator–induced hatching plasticity reduce anuran post-metamorphic performance?

In species with complex life cycles hatching plasticity can provide an effective escape from egg predators, but theoretical studies predict a predation-risk trade-off across egg and larval stages. In this study, we examine whether the presence of an egg predator can alter the timing of hatching in an anuran, Rana temporaria, and the consequences of hatching plasticity after transition to the terrestrial habitat. Predator cues induced earlier hatching, and hatchlings were smaller, less developed and had relatively shorter and deeper tails than control hatchlings. The predator–induced differences in developmental time were compensated throughout the larval period; there was no predator effect on metamorph age or size. Surprisingly, the effects of egg predators were perceptible after metamorphosis. Juveniles emerging from the predator and the no-predator treatments differed in several size-adjusted morphological dimensions. Seemingly these morphological differences were not large enough to give rise to suboptimal growth or locomotor performance after metamorphosis. Thus, our results suggest only a short-term effect on juvenile phenotype, but not a trade-off between hatching time and juvenile performance.     

Thermal acclimation of swimming performance in newt larvae: the influence of diel temperature fluctuations during embryogenesis

1.  Thermal acclimation is one of the basic strategies by which organisms cope with thermal heterogeneity of the environment. Under predictable variation in environmental temperatures, theory predicts that selection favours acclimation of thermal performance curves over fixed phenotypes.
2.  We examined the influence of diel fluctuations in developmental temperatures on the thermal sensitivity of the maximal swimming capacity in larvae of the alpine newt, Triturus alpestris .
3.  We incubated newt eggs under three thermal regimes with varying daily amplitudes (1, 5 and 9 °C) and similar means (17·6–17·9 °C), and accordingly we measured the swimming speed of hatched larvae at three experimental temperatures (12, 17 and 22 °C), which they would normally experience in their natural habitat.
4.  Embryonic development under low and middle temperature fluctuations produced larvae with similar swimming speeds across experimental temperatures. In contrast, the most fluctuating regime induced development of phenotypes, which at 12 °C swam faster than larvae developed under moderate diel fluctuations.
5.  Our results provide evidence that diel temperature fluctuations induce acclimation of thermal dependence of locomotor performance. In ectotherms experiencing diel cycles in environmental temperatures, this plastic response may act as an important pacemaker in the evolution of thermal sensitivity.     

Landscape context influences the abundance of amphibians and the strength of their food web interactions in small ponds

Many species, including most amphibians, undergo an ontogenetic niche shift (ONS) from an aquatic larval stage to a terrestrial adult stage. We use the ratio of aquatic to terrestrial habitat in a landscape as a tool to understand the influence of landscape context on the population growth of ONS species. The aquatic to terrestrial ratio (ATR) of habitats can be viewed as an analog to the influence of resource ratios on the population growth of consumers and depends on the degree to which each habitat type limits the growth of a given population. Population growth rates of shorter‐lived species tend to be more limited by demographic rates in early (aquatic) life stages. As a result, increasing the ATR should lead to a higher total population size in the landscape (and higher densities in the terrestrial habitat), but have little influence on the density of individuals in any given aquatic habitat. Alternatively, population growth rates of longer‐lived species tend to be more limited by demographic rates in later (terrestrial) life stages and increasing the ATR should have little influence on the total population size in the landscape, but decrease the density of individuals in any given aquatic habitat. We show that among‐landscape variation in the breeding‐pond densities of three widespread amphibians with contrasting life histories is consistent with this framework. Within‐pond densities of Pseudacris crucifer, a species with short‐lived adults, were not influenced by ATR, whereas within‐pond densities of Hyla versicolor, a longer‐lived member of the same family (Hylidae), declined as ATR increased. Ambystoma maculatum, a long‐lived salamander, also had lower densities in ponds with higher ATR. Because A. maculatum larvae are important predators in ponds, we use structural equation modeling to show that landscape context (ATR) can moderate community structure via direct (amphibian abundances) and indirect (prey species richness) effects.     

Patterns of natural selection on size at metamorphosis in water frogs

Strategies for optimal metamorphosis are key adaptations in organisms with complex life cycles, and the components of the larval growth environment causing variation in this trait are well studied empirically and theoretically. However, when relating these findings to a broader evolutionary or ecological context, usually the following assumptions are made: (1) size at metamorphosis positively relates to future fitness, and (2) the larval growth environment affects fitness mainly through its effect on timing of and size at metamorphosis. These assumptions remain poorly tested, because data on postmetamorphic fitness components are still rare. We created variation in timing of and size at metamorphosis by manipulating larval competition, nonlethal presence of predators, pond drying, and onset of larval development, and measured the consequences for subsequent terrestrial survival and growth in 1564 individually marked water frogs (Rana lessonae and R. esculenta), raised in enclosures in their natural environment. Individuals metamorphosing at a large size had an increased chance of survival during the following terrestrial stage (mean linear selection gradient: 0.09), grew faster and were larger at maturity than individuals metamorphosing at smaller sizes. Late metamorphosing individuals had a lower survival rate (mean linear selection gradient: -0.03) and grew more slowly than early metamorphosing ones. We found these patterns to be consistent over the three years of the study and the two species, and the results did not depend on the nature of the larval growth manipulation. Furthermore, individuals did not compensate for a small size at metamorphosis by enhancing their postmetamorphic growth. Thus, we found simple relationships between larval growth and postmetamorphic fitness components, and support for this frequently made assumption. Our results suggest postmetamorphic selection for fast larval growth and provide a quantitative estimate for the water frog example.     

Moving in two worlds: aquatic and terrestrial locomotion in sea snakes (Laticauda colubrina,Laticaudidae)

Yellow‐lipped sea kraits (Laticauda colubrina) are amphibious in their habits. We measured their locomotor speeds in water and on land to investigate two topics: (1) to what degree have adaptations to increase swimming speed (paddle‐like tail etc.) reduced terrestrial locomotor ability in sea kraits?; and (2) do a sea krait’s sex and body size influence its locomotor ability in these two habitats, as might be expected from the fact that different age and sex classes of sea kraits use the marine and terrestrial environments in different ways? To estimate ancestral states for locomotor performance, we measured speeds of three species of Australian terrestrial elapids that spend part of their time foraging in water. The evolutionary modifications of Laticauda for marine life have enhanced their swimming speeds by about 60%, but decreased their terrestrial locomotor speed by about 80%. Larger snakes moved faster than smaller individuals in absolute terms but were slower in terms of body lengths travelled per second, especially on land. Male sea kraits were faster than females (independent of the body‐size effect), especially on land. Prey items in the gut reduced locomotor speeds both on land and in water. Proteroglyphous snakes may offer exceptional opportunities to study phylogenetic shifts in locomotor ability, because (1) they display multiple independent evolutionary shifts from terrestrial to aquatic habits, and (2) one proteroglyph lineage (the laticaudids) displays considerable intraspecific and interspecific diversity in terms of the degree to which they use terrestrial vs. aquatic habitats.     

Metamorphosis of the amphibian eye

For many metamorphosing amphibians, the visual system must remain functional as the animal changes from an aquatic to a terrestrial habitat. Thyroid hormone, the trigger for metamorphosis, brings about changes at all levels of the animal, and profoundly alters the visual system, from cellular changes within the eye to new central connections subserving the binocular vision that develops during metamorphosis in some species. I will survey the alterations in the visual system in the metamorphosis of several Amphibian groups, and consider the role of thyroid hormone in bringing about these transformations through action at the molecular level.     

From swimming to walking: a single basic network for two different behaviors

 In this paper we consider the hypothesis that the spinal locomotor network controlling trunk movements has remained essentially unchanged during the evolutionary transition from aquatic to terrestrial locomotion. The wider repertoire of axial motor patterns expressed by amphibians would then be explained by the influence from separate limb pattern generators, added during this evolution. This study is based on EMG data recorded in vivo from epaxial musculature in the newt Pleurodeles waltl during unrestrained swimming and walking, and on a simplified model of the lamprey spinal pattern generator for swimming. Using computer simulations, we have examined the output generated by the lamprey model network for different input drives. Two distinct inputs were identified which reproduced the main features of the swimming and walking motor patterns in the newt. The swimming pattern is generated when the network receives tonic excitation with local intensity gradients near the neck and girdle regions. To produce the walking pattern, the network must receive (in addition to a tonic excitation at the girdles) a phasic drive which is out of phase in the neck and tail regions in relation to the middle part of the body. To fit the symmetry of the walking pattern, however, the intersegmental connectivity of the network had to be modified by reversing the direction of the crossed inhibitory pathways in the rostral part of the spinal cord. This study suggests that the input drive required for the generation of the distinct walking pattern could, at least partly, be attributed to mechanosensory feedback received by the network directly from the intraspinal stretch-receptor system. Indeed, the input drive required resembles the pattern of activity of stretch receptors sensing the lateral bending of the trunk, as expressed during walking in urodeles. Moreover, our results indicate that a nonuniform distribution of these stretch receptors along the trunk can explain the discontinuities exhibited in the swimming pattern of the newt. Thus, separate limb pattern generators can influence the original network controlling axial movements not only through a direct coupling at the central level but also via a mechanical coupling between trunk and limbs, which in turn influences the sensory signals sent back to the network. Taken together, our findings support the hypothesis of a phylogenetic conservatism of the spinal locomotor networks generating axial motor patterns from agnathans to amphibians. Received: 12 October 2001 / Accepted in revised form: 16 May 2002 Correspondence to: T. Bem (e-mail: tiaza.bem@ibib.waw.pl)