Absence of atrial smooth muscle in the heart of the loggerhead sea turtle (Caretta caretta): a re-evaluation of its role in diving physiology.

Contraction of atrial smooth muscle in the hearts of semi-aquatic emydid turtles regulates ventricular filling, and it has been proposed that it could regulate stroke volume during characteristic rapid transitions in cardiac output associated with diving. For this hypothesis to be supported, atrial smooth muscle should be widely distributed in diving Testudines. To further understand the putative function and evolutionary significance of endocardial smooth muscle in Testudines, we studied the hearts of loggerhead sea turtles, Caretta caretta (n=7), using immunohistochemistry and histology. Surprisingly, we found no evidence of prominent atrial smooth muscle in C. caretta. However, smooth muscle was readily identified in the sinus venosus. Our results suggest that atrial smooth muscle does not contribute to the diving capabilities of C. caretta, indicating that the possible roles of smooth muscle in emydid turtle hearts require a re-evaluation. In sea turtles, the sinus venosus may instead contribute to regulate cardiac filling.

Turning turtle: scaling relationships and self-righting ability in Chelydra serpentina.

Testudines are susceptible to inversion and self-righting using their necks, limbs or both, to generate enough mechanical force to flip over. We investigated how shell morphology, neck length and self-righting biomechanics scale with body mass during ontogeny in Chelydra serpentina, which uses neck-powered self-righting. We found that younger turtles flipped over twice as fast as older individuals. A simple geometric model predicted the relationships of shell shape and self-righting time with body mass. Conversely, neck force, power output and kinetic energy increase with body mass at rates greater than predicted. These findings were correlated with relatively longer necks in younger turtles than would be predicted by geometric similarity. Therefore, younger turtles self-right with lower biomechanical costs than predicted by simple scaling theory. Considering younger turtles are more prone to inverting and their shells offer less protection, faster and less costly self-righting would be advantageous in overcoming the detriments of inversion.

Scaling of axial muscle architecture in juvenile Alligator mississippiensis reveals an enhanced performance capacity of accessory breathing mechanisms.

Quantitative functional anatomy of amniote thoracic and abdominal regions is crucial to understanding constraints on and adaptations for facilitating simultaneous breathing and locomotion. Crocodilians have diverse locomotor modes and variable breathing mechanics facilitated by basal and derived (accessory) muscles. However, the inherent flexibility of these systems is not well studied, and the functional specialisation of the crocodilian trunk is yet to be investigated. Increases in body size and trunk stiffness would be expected to cause a disproportionate increase in muscle force demands and therefore constrain the basal costal aspiration mechanism, necessitating changes in respiratory mechanics. Here, we describe the anatomy of the trunk muscles, their properties that determine muscle performance (mass, length and physiological cross-sectional area [PCSA]) and investigate their scaling in juvenile Alligator mississippiensis spanning an order of magnitude in body mass (359 g-5.5 kg). Comparatively, the expiratory muscles (transversus abdominis, rectus abdominis, iliocostalis), which compress the trunk, have greater relative PCSA being specialised for greater force-generating capacity, while the inspiratory muscles (diaphragmaticus, truncocaudalis ischiotruncus, ischiopubis), which create negative internal pressure, have greater relative fascicle lengths, being adapted for greater working range and contraction velocity. Fascicle lengths of the accessory diaphragmaticus scaled with positive allometry in the alligators examined, enhancing contractile capacity, in line with this muscle's ability to modulate both tidal volume and breathing frequency in response to energetic demand during terrestrial locomotion. The iliocostalis, an accessory expiratory muscle, also demonstrated positive allometry in fascicle lengths and mass. All accessory muscles of the infrapubic abdominal wall demonstrated positive allometry in PCSA, which would enhance their force-generating capacity. Conversely, the basal tetrapod expiratory pump (transversus abdominis) scaled isometrically, which may indicate a decreased reliance on this muscle with ontogeny. Collectively, these findings would support existing anecdotal evidence that crocodilians shift their breathing mechanics as they increase in size. Furthermore, the functional specialisation of the diaphragmaticus and compliance of the body wall in the lumbar region against which it works may contribute to low-cost breathing in crocodilians.

Examining the accuracy of trackways for predicting gait selection and speed of locomotion.

BACKGROUND: Using Froude numbers (Fr) and relative stride length (stride length: hip height), trackways have been widely used to determine the speed and gait of an animal. This approach, however, is limited by the ability to estimate hip height accurately and by the lack of information related to the substrate properties when the tracks were made, in particular for extinct fauna. By studying the Svalbard ptarmigan moving on snow, we assessed the accuracy of trackway predictions from a species-specific model and two additional Fr based models by ground truthing data extracted from videos as the tracks were being made. RESULTS: The species-specific model accounted for more than 60% of the variability in speed for walking and aerial running, but only accounted for 19% when grounded running, likely due to its stabilizing role while moving faster over a changing substrate. The error in speed estimated was 0-35% for all gaits when using the species-specific model, whereas Fr based estimates produced errors up to 55%. The highest errors were associated with the walking gait. The transition between pendular to bouncing gaits fell close to the estimates using relative stride length described for other extant vertebrates. Conversely, the transition from grounded to aerial running appears to be species specific and highly dependent on posture and substrate. CONCLUSION: Altogether, this study highlights that using trackways to derive predictions on the locomotor speed and gait, using stride length as the only predictor, are problematic as accurate predictions require information from the animal in question.

Inferring cost of transport from whole-body kinematics in three sympatric turtle species with different locomotor habits.

Chelonians are mechanically unusual vertebrates as an exoskeleton limits their body wall mobility. They generally move slowly on land and have aquatic or semi-aquatic lifestyles. Somewhat surprisingly, the limited experimental work that has been done suggests that their energetic cost of transport (CoT) are relatively low. This study examines the mechanical evidence for CoT in three turtle species that have differing degrees of terrestrial activity. Our results show that Apolone travels faster than the other two species, and that Chelydra has higher levels of yaw. All the species show poor mean levels of energy recovery, and, whilst there is considerable variation, never show the high levels of energy recovery seen in cursorial quadrupeds. The mean mechanical CoT is 2 to 4 times higher than is generally seen in terrestrial animals. We therefore find no mechanical support for a low CoT in these species. This study illustrates the need for research on a wider range of chelonians to discover whether there are indeed general trends in mechanical and metabolic energy costs.

XROMM kinematics of ventilation in wild turkeys (Meleagris gallopavo).

The avian ribcage is derived relative to other amniotes, and is hypothesised to be constrained in its movements during ventilation. The double-headed ribs form two articulations with the vertebrae, and are thought to rotate about a strict anatomical axis. However, this costovertebral joint constraint has not been demonstrated empirically and was not found in other taxa with double-headed ribs (i.e. crocodilians). Here, we used X-ray reconstruction of moving morphology (XROMM) to quantify rib rotation in wild turkeys (Meleagris gallopavo) during breathing. We demonstrate that, as predicted from anatomy, the ribs do rotate in a hinge-like manner about a single axis. There is also evidence for elliptical motion of the sternum, as has been reported in other taxa. The evolution of the avian ribcage is closely related to the co-evolution of ventilation and flight, and these results are important for how we model ventilation mechanics in living and fossil birds.

A novel accessory respiratory muscle in the American alligator ( Alligator mississippiensis).

The muscles that effect lung ventilation are key to understanding the evolutionary constraints on animal form and function. Here, through electromyography, we demonstrate a newly discovered respiratory function for the iliocostalis muscle in the American alligator ( Alligator mississippiensis). The iliocostalis is active during expiration when breathing on land at 28°C and this activity is mediated through the uncinate processes on the vertebral ribs. There was also an increase in muscle activity during the forced expirations of alarm distress vocalizations. Interestingly, we did not find any respiratory activity in the iliocostalis when the alligators were breathing with their body submerged in water at 18°C, which resulted in a reduced breathing frequency. The iliocostalis is an accessory breathing muscle that alligators are able to recruit in to assist expiration under certain conditions.

Thermoregulation in rapid growing broiler chickens is compromised by constraints on radiative and convective cooling performance.

Broiler chickens are selected to undergo a rapid six-week hatch-to-slaughter growth phase to attain large body and muscle mass. Broilers have relatively high resting and locomotor metabolic costs suggesting that adaptive thermoregulatory mechanisms are required to dissipate excess heat. Using thermal imaging in the growing broiler we characterised the trajectory of radiative and convective cooling in still air across broiler development. Scaling of head, tarsus and toe surface area did not deviate from body mass2/3 while torso area increased with positive allometry, body mass0.82, reflecting increased feather coverage and/or disproportionate abdominal/thoracic growth. Despite relatively increased area, the body became less effective for heat transfer presumably due to increasing feather coverage. Conversely, the magnitude of heat exchange from the distal hindlimbs was improved in larger birds. Overall capacity to transfer heat by convection and radiation in still air was attenuated over development, since the proportion of resting metabolic rate accounted for decreased in standing and sitting postures. This physiological constraint could be ameliorated by increased latent heat transfer or provision of environmental ventilation, which we modelled according to industrial guidelines. Based on models, higher airspeeds coincided with improved convective cooling that assisted in maintaining the proportion of RMR accounted for by convective and radiative heat transfer. These data highlight the potentially adverse thermoregulatory effects of rapid growth rate and body mass increases, which may contribute to the increased sedentary resting and decreased locomotor behaviour observed in large broilers.

Rib kinematics during lung ventilation in the American alligator (Alligator mississippiensis): an XROMM analysis.

The current hypothesis regarding the mechanics of breathing in crocodylians is that the double-headed ribs, with both a capitulum and tuberculum, rotate about a constrained axis passing through the two articulations; moreover, this axis shifts in the caudal thoracic ribs, as the vertebral parapophysis moves from the centrum to the transverse process. Additionally, the ventral ribcage in crocodylians is thought to possess additional degrees of freedom through mobile intermediate ribs. In this study, X-ray reconstruction of moving morphology (XROMM) was used to quantify rib rotation during breathing in American alligators. Whilst costovertebral joint anatomy predicted overall patterns of motion across the ribcage (decreased bucket handle motion and increased calliper motion), there were significant deviations: anatomical axes overestimated pump handle motion and, generally, ribs in vivo rotate about all three body axes more equally than predicted. The intermediate ribs are mobile, with a high degree of rotation measured about the dorsal intracostal joints, especially in the more caudal ribs. Motion of the sternal ribs became increasingly complex caudally, owing to a combination of the movements of the vertebral and intermediate segments. As the crocodylian ribcage is sometimes used as a model for the ancestral archosaur, these results have important implications for how rib motion is reconstructed in fossil taxa, and illustrate the difficulties in reconstructing rib movement based on osteology alone.

Variety, sex and ontogenetic differences in the pelvic limb muscle architectural properties of leghorn chickens (Gallus gallus domesticus) and their links with locomotor performance.

Leghorn (layer) chickens (Gallus gallus domesticus) differ in locomotor morphology and performance due to artificial selection for standard (large) and bantam (small) varieties, sexual dimorphisms and ontogenetic stage. Here, the hind limb skeletal muscle architectural properties of mature and juvenile standard breeds and mature bantams are compared and linked to measures of locomotor performance. Mature males possessed greater relative muscle physiological cross-sectional areas (PCSAs) than their conspecific females, indicative of greater force-generating capacity, and in line with their greater maximum sustainable speeds compared with females. Furthermore, some of the relative fascicle lengths of the pennate muscles were greater in mature males than in mature females, which may permit greater muscle contractibility. Immature standard leghorns, however, did not share the same dimorphisms as their mature forms. The differences in architectural properties between immature and mature standard males indicate that with the onset of male sexual maturity, concomitant with increasing muscle mass in males, the relative fascicle lengths of pennate muscles and the relative PCSAs of the parallel-fibred muscles also increase. The age-related differences in standard breed male muscle architecture are linked to the presence and absence of sex differences in maximum aerobic speeds. Males of bantam and standard varieties shared similar muscle proportions (% body mass), but exhibited intrinsic muscle differences with a tendency for greater force-generating capabilities in bantams and greater contractile capabilities in standards. The metabolic costs associated with the longer fascicle lengths, together with more crouched limbs in standard than in bantam males may explain the lack of allometry in the minimum metabolic cost of transport between these birds of different size.

Differential sex-specific walking kinematics in leghorn chickens (Gallus gallus domesticus) selectively bred for different body size.

The differing limb dynamics and postures of small and large terrestrial animals may be mechanisms for minimising metabolic costs under scale-dependent muscle force, work and power demands; however, empirical evidence for this is lacking. Leghorn chickens (Gallus gallus domesticus) are highly dimorphic: males have greater body mass and relative muscle mass than females, which are permanently gravid and have greater relative intestinal mass. Furthermore, leghorns are selected for standard (large) and bantam (small) varieties and the former are sexually dimorphic in posture, with females having a more upright limb. Here, high-speed videography and morphological measurements were used to examine the walking gaits of leghorn chickens of the two varieties and sexes. Hindlimb skeletal elements were geometrically similar among the bird groups, yet the bird groups did not move with dynamic similarity. In agreement with the interspecific scaling of relative duty factor (DF, the proportion of a stride period with ground contact for any given foot) with body mass, bantams walked with greater DF than standards, and females walked with greater DF than males. Greater DF in females than in males was achieved via variety-specific kinematic mechanisms, associated with the presence/absence of postural dimorphism. Females may require greater DF in order to reduce peak muscle forces and minimise power demands associated with lower muscle to reproductive tissue mass ratios and smaller body size. Furthermore, a more upright posture observed in the standard, but not bantam, females, may relate to minimising the work demands of being larger and having proportionally larger reproductive tissue volume. Lower DF in males relative to females may also be a work-minimising strategy and/or due to greater limb inertia (as a result of greater pelvic limb muscle mass) prolonging the swing phase.

Intraspecific scaling of the minimum metabolic cost of transport in leghorn chickens (Gallus gallus domesticus): links with limb kinematics, morphometrics and posture.

The minimum metabolic cost of transport (CoTmin; J kg(-1) m(-1)) scales negatively with increasing body mass (∝Mb (-1/3)) across species from a wide range of taxa associated with marked differences in body plan. At the intraspecific level, or between closely related species, however, CoTmin does not always scale with Mb. Similarity in physiology, dynamics of movement, skeletal geometry and posture between closely related individuals is thought to be responsible for this phenomenon, despite the fact that energetic, kinematic and morphometric data are rarely collected together. We examined the relationship between these integrated components of locomotion in leghorn chickens (Gallus gallus domesticus) selectively bred for large and bantam (miniature) varieties. Interspecific allometry predicts a CoTmin ∼16% greater in bantams compared with the larger variety. However, despite 38% and 23% differences in Mb and leg length, respectively, the two varieties shared an identical walking CoTmin, independent of speed and equal to the allometric prediction derived from interspecific data for the larger variety. Furthermore, the two varieties moved with dynamic similarity and shared geometrically similar appendicular and axial skeletons. Hip height, however, did not scale geometrically and the smaller variety had more erect limbs, contrary to interspecific scaling trends. The lower than predicted CoTmin in bantams for their Mb was associated with both the more erect posture and a lower cost per stride (J kg(-1) stride(-1)). Therefore, our findings are consistent with the notion that a more erect limb is associated with a lower CoTmin and with the previous assumption that similarity in skeletal shape, inherently linked to walking dynamics, is associated with similarity in CoTmin.

The effects of season and sex upon the morphology and material properties of keratin in the Svalbard rock ptarmigan (Lagopus muta hyperborea).

The material properties and morphologies of the modified integumentary organs of birds (the keratinous bills, claws and feathers) have evolved to withstand the variety of mechanical stresses imposed by their interaction with the environment. These stresses are likely to vary temporally in seasonal environments and may also differ between the sexes as a result of behavioural dimorphism. Here we investigate the morphology and material properties of the claws of male and female Svalbard ptarmigan (Lagopus muta hyperborea) during the summer and winter using nanoindentation. Despite differences in locomotor demands between the sexes and pronounced seasonal differences in environmental conditions, like ground substrate, ambient temperature and day length, there was no significant difference in Young׳s modulus or hardness between the seasons for each sex. However, when comparing males and females, female claws were significantly harder than those of males and both sexes had significantly wider claws during winter. We propose that wider claws may follow winter claw moulting as the claws are regrown and form an important part of the ptarmigan׳s snowshoe-like foot that is an adaptation to locomotion on snow. Future work focusing on growth rates and more broad measures of material properties in both captive and wild birds is required to determine the extent of seasonal and sex differences in the material properties of their keratinous structures.

Shell shape does not accurately predict self-righting ability in hatchling freshwater turtles.

Flat hydrodynamic shells likely represent an evolutionary trade-off between adaptation to an aquatic lifestyle and the instability of more rounded shells, thought beneficial for self-righting. Trade-offs often result in compromises, this is particularly true when freshwater turtles, with flatter shells, must self-right to avoid the negative effects of inverting. These turtles, theoretically, invest more biomechanical effort to achieve successful and timely self-righting when compared to turtles with rounded carapaces. This increase in effort places these hatchlings in a precarious position; prone to inversion and predation and with shells seemingly maladapted to the act of self-righting. Here, we examine hatchling self-righting performance in three morphologically distinct freshwater turtle species (Apalone spinifera, Chelydra serpentina and Trachemys scripta scripta) that inhabit similar environmental niches. We demonstrate that these hatchlings were capable of rapid self-righting and used considerably less biomechanical effort relative to adult turtles. Despite differences in shell morphology the energetic efficiency of self-righting remained remarkably low and uniform between the three species. Our results confound theoretical predictions of self-righting ability based on shell shape metrics and indicate that other morphological characteristics like neck or tail morphology and shell material properties must be considered to better understand the biomechanical nuances of Testudine self-righting.

Testing the form-function paradigm: body shape correlates with kinematics but not energetics in selectively-bred birds.

A central concept of evolutionary biology, supported by broad scale allometric analyses, asserts that changing morphology should induce downstream changes in locomotor kinematics and energetics, and by inference selective fitness. However, if these mechanistic relationships exist at local intraspecific scales, where they could provide substrate for fundamental microevolutionary processes, is unknown. Here, analyses of selectively-bred duck breeds demonstrate that distinct body shapes incur kinematic shifts during walking, but these do not translate into differences in energetics. A combination of modular relationships between anatomical regions, and a trade-off between limb flexion and trunk pitching, are shown to homogenise potential functional differences between the breeds, accounting for this discrepancy between form and function. This complex interplay between morphology, motion and physiology indicates that understanding evolutionary links between the avian body plan and locomotor diversity requires studying locomotion as an integrated whole and not key anatomical innovations in isolation.

The metabolic cost of incline locomotion in the Svalbard rock ptarmigan (Lagopus muta hyperborea): the effects of incline grade and seasonal fluctuations in body mass.

In a terrestrial environment animals must locomote over varying terrain; despite this, the majority of studies of animal locomotion focus on level locomotion. The influence moving up an inclined surface has on the metabolic cost of locomotion and the efficiency with which animals perform positive work against gravity is still not well understood. Generally speaking, existing data sets lack consistency in the use of grades, further compounded by differences between species in terms of morphology and locomotor gait. Here we investigated the metabolic cost of locomotion using respirometry in the Svalbard ptarmigan (Lagopus muta hyperborea). The Svalbard ptarmigan provides a unique opportunity to investigate the cost of incline locomotion as it undergoes a seasonal fluctuation in body mass, which doubles in winter, meaning the requirement for positive mechanical work also fluctuates with season. We demonstrate that at the same degree of incline, the cost of lifting 1 kg by 1 vertical metre remains relatively constant between seasons despite the large differences in body mass from summer to winter. These findings are consistent with the notion that positive mechanical work alone dictates the cost of lifting above a certain body mass. However, our data indicate that this cost may vary according to the degree of incline and gait.

Understanding sex differences in the cost of terrestrial locomotion.

Little is known regarding the physiological consequences of the behavioural and morphological differences that result from sexual selection in birds. Male and female Svalbard rock ptarmigans (Lagopus muta hyperborea) exhibit distinctive behavioural differences during the breeding season. In particular, males continuously compete for and defend territories in order to breed successfully, placing large demands on their locomotor system. Here, we demonstrate that male birds have improved locomotor performance compared with females, showing both a lower cost of locomotion (CoL) and a higher top speed. We propose that the observed sex differences in locomotor capability may be due to sexual selection for improved male performance. While the mechanisms underlying these energetic differences are unclear, future studies should be wary when pooling male and female data.

The metabolic cost of walking on gradients with a waddling gait.

Using open-flow respirometry and video footage (25 frames s(-1)), the energy expenditure and hindlimb kinematics of barnacle geese, Branta leucopsis, were measured whilst they were exercising on a treadmill at gradients of +7 and -7 deg, and on a level surface. In agreement with previous studies, ascending a gradient incurred metabolic costs higher than those experienced on level ground at comparable speeds. The geese, however, are the first species to show an increased duty factor when ascending a gradient. This increased duty factor was accompanied by a longer stance time, which was probably to enable the additional force required for ascending to be generated. Contrary to previous findings, the geese did not experience decreased metabolic costs when descending a gradient. For a given speed, the geese took relatively shorter and quicker strides when walking downhill. This 'choppy' stride and perhaps a lack of postural plasticity (an inability to adopt a more crouched posture) may negate any energy savings gained from gravity's assistance in moving the centre of mass downhill. Also contrary to previous studies, the incremental increase in metabolic cost with increasing speed was similar for each gradient, indicating that the efficiency of locomotion (mechanical work done/chemical energy consumed) is not constant across all walking speeds. The data here suggest that there are species-specific metabolic responses to locomotion on slopes, as well as the established kinematics differences. It is likely that a suite of factors, such as ecology, posture, gait, leggedness and foot morphology, will subtly affect an organism's ability to negotiate gradients.

Mediterranean Spur-Thighed Tortoises (Testudo graeca) Have Optimal Speeds at Which They Can Minimise the Metabolic Cost of Transport, on a Treadmill.

Tortoises are famed for their slow locomotion, which is in part related to their herbivorous diet and the constraints imposed by their protective shells. For most animals, the metabolic cost of transport (CoT) is close to the value predicted for their body mass. Testudines appear to be an exception to this rule, as previous studies indicate that, for their body mass, they are economical walkers. The metabolic efficiency of their terrestrial locomotion is explainable by their walking gait biomechanics and the specialisation of their limb muscle physiology, which embodies a predominance of energy-efficient slow-twitch type I muscle fibres. However, there are only two published experimental reports of the energetics of locomotion in tortoises, and these data show high variability. Here, Mediterranean spur-thighed tortoises (Testudo graeca) were trained to walk on a treadmill. Open-flow respirometry and high-speed filming were simultaneously used to measure the metabolic cost of transport and to quantify limb kinematics, respectively. Our data support the low cost of transport previously reported and demonstrate a novel curvilinear relationship to speed in Testudines, suggesting tortoises have an energetically optimal speed range over which they can move in order to minimise the metabolic cost of transport.