Eric J. McElroy

Tuataras and salamanders show that walking and running mechanics are ancient features of tetrapod locomotion

The lumbering locomotor behaviours of tuataras and salamanders are the best examples of quadrupedal locomotion of early terrestrial vertebrates. We show they use the same walking (out-of-phase) and running (in-phase) patterns of external mechanical energy fluctuations of the centre-of-mass known in fast moving (cursorial) animals. Thus, walking and running centre-of-mass mechanics have been a feature of tetrapods since quadrupedal locomotion emerged over 400 million years ago. When walking, these sprawling animals save external mechanical energy with the same pendular effectiveness observed in cursorial animals. However, unlike cursorial animals (that change footfall patterns and mechanics with speed), tuataras and salamanders use only diagonal couplet gaits and indifferently change from walking to running mechanics with no significant change in total mechanical energy. Thus, the change from walking to running is not related to speed and the advantage of walking versus running is unclear. Furthermore, lumbering mechanics in primitive tetrapods is reflected in having total mechanical energy driven by potential energy (rather than kinetic energy as in cursorial animals) and relative centre-of-mass displacements an order of magnitude greater than cursorial animals. Thus, large vertical displacements associated with lumbering locomotion in primitive tetrapods may preclude their ability to increase speed.

Tail Autotomy, Tail Size, and Locomotor Performance in Lizards*

The effect of tail autotomy on locomotor performance has been studied in a number of lizard species. Most of these studies (65%) show that tail autotomy has a negative effect on sprint speed, some studies (26%) show no effect of autotomy on sprint speed, and a few (9%) show a positive effect of autotomy on sprint speed. A variety of hypotheses have been proposed to explain the variation across these studies, but none has been tested. We synthesize these data using meta-analysis and then test whether any of four variables explain the variation in how tail autotomy impacts sprint speed: (1) differences in methodology in previous studies, (2) phylogeny, (3) relative tail size, and (4) habitat use. We find little evidence that methodology or habitat use influences how sprint speed changes following tail autotomy. Although the sampling is phylogenetically sparse, phylogeny appears to play a role, with skinks and iguanids showing fairly consistent decreases in speeds after autotomy and with lacertids and geckos showing large variation in how autotomy impacts speed. After removing two outlying species with unusually large and long tails (Takydromus sp.), we find a positive relationship between relative tail size and sprint speed change after autotomy. Lizards with larger tails exhibit a greater change in speed after tail loss. This finding suggests that future studies of tail autotomy and locomotor performance might profitably incorporate variation in tail size and that species-specific responses to autotomy need to be considered.

Getting Up to Speed: Acceleration Strategies in the Florida Scrub Lizard, Sceloporus woodi

Small animals typically rely on quick bursts and intermittent pauses when moving in the wild. Hence, the study of acceleration capacity is important for understanding the ecology and evolution of locomotor performance. In this study, we investigate intraspecific variation in the acceleration capacity of a small lizard (Sceloporus woodi). To quantify animal acceleration performance, the momentum‐impulse theorem is applied to data collected from high‐speed video recordings of individuals accelerating from a standstill and over a subsequent distance of 0.4 m. Unlike earlier studies, the momentum‐impulse approach allows one to directly and precisely quantify the per step contribution to acceleration capacity. Like other small vertebrates, we show that S. woodi is capable of accelerating to near maximum speeds (∼2 m s−1) within ∼0.4 m and needs only a few steps (at least five) to achieve maximum speed. However, considerable intraspecific variation in acceleration capacity exists; individuals take different numbers of steps (two to five steps) over the first 0.4 m, and only some individuals (10 of 19) reach their maximum speed over the first 0.4 m. Only acceleration performance in steps 1 and 2 is predictive of running speed at 0.4 m; accelerations in steps 3, 4, and 5 are not related to individual differences in speed. Individual variation in acceleration strategy is considerable, with individuals using one of three strategies to reach maximum speed. Muscle mass‐specific power during acceleration approaches the maximum power output measured for lizard hindlimb musculature (∼900 W kg−1), suggesting that S. woodi accelerations approach the limit of their musculoskeletal system. This study highlights the utility of the momentum‐impulse approach to study acceleration performance and the importance of elucidating the per step contribution to acceleration capacity.

Host Performance as a Target of Manipulation by Parasites: A Meta-Analysis

Abstract: The mechanisms underlying parasite-altered host behavior and fitness remain largely unanswered. The purpose of this review is to provide a perspective that has not been fully incorporated into the debate on how parasites manipulate their hosts. We argue that performance capacity is an important target of parasitic manipulation, and we aim to integrate the study of performance with that of parasitic manipulations of host behavior and fitness. We performed a meta-analysis from the published literature of 101 measures of the effect of parasites on host performance capacity to address the following questions. (1) Do parasites exert an important effect on host performance capacity? (2) Is that effect routinely to decrease or enhance performance capacity? And, (3) what factors explain variation in the effect sizes that have been quantified? Although negligible–small effect sizes were detected in 40/101 measures, host performance capacity was overall affected by parasitic infection, with a negative direction and medium–large magnitude in 58/101 measures and an increase in performance capacity in 3/101 measures. Host age, type of host performance, the host tissue infected by the parasite, and whether the study was experimental or based on natural infections each explained a significant amount of the variation in effect size. The significance of each factor is briefly discussed in light of the potential adaptive character of host manipulations by parasites.

Symmetrical gaits and center of mass mechanics in small-bodied, primitive mammals

Widely accepted relationships between gaits (footfall patterns) and center of mass mechanics have been formulated from observations for cursorial mammals. However, sparse data on smaller or more generalized forms suggest a fundamentally different relationship. This study explores locomotor dynamics in one eutherian and five metatherian (marsupials) mammals-all small-bodied (<2 kg) with generalized body plans that utilize symmetrical gaits. Across our sample, trials conforming to vaulting mechanics occurred least frequently (<10% of all trials) while bouncing mechanics was obtained most commonly (60%); the remaining trials represented mixed mechanics. Contrary to the common situation in large mammals, there was no evidence for discrete gait switching within symmetrical gaits as speed increased. This was in part due to the common practice of grounded running. The adaptive advantage of different patterns of center-of-mass motion and their putative energy savings remain questionable in small-bodied mammals.

Effects of a muscle-infecting parasitic nematode on the locomotor performance of their fish host

The southern flounder Paralichthys lethostigma, host to the nematode Philometroides paralichthydis that is embedded in place of the inclinator muscles of the dorsal and anal fin elements, is hypothesized to impair two aspects of locomotor performance (swimming and burying capacity). Peak swimming acceleration and both measures of burying performance did not differ between infected and uninfected fish, whereas swimming velocity of infected fish was significantly lower than that of uninfected fish. Smaller infected fish swam at significantly slower speeds than smaller uninfected fish, whereas there was no difference among larger fish. Neither the location nor the number of worms affected either swimming or burying performance. The decrease in swimming velocity observed in smaller infected fish may be sufficient in rendering them more vulnerable to predation and environmental stressors.

The correlation between locomotor performance and hindlimb kinematics during burst locomotion in the Florida scrub lizard, Sceloporus woodi

SUMMARY Burst locomotion is thought to be closely linked to an organisms ability to survive and reproduce. During the burst, animals start from a standstill and then rapidly accelerate to near-maximum running speeds. Many previous studies have described the functional predictors of maximum running speed; however, only recently has work emerged that describes the morphological, functional and biomechanical underpinnings of acceleration capacity. Herein we present data on the three-dimensional hindlimb kinematics during burst locomotion, and the relationship between burst locomotor kinematics and locomotor performance in a small terrestrial lizard (Sceloporus woodi). We focus only on stance phase joint angular kinematics. Sceloporus woodi exhibited considerable variation in hindlimb kinematics and performance across the first three strides of burst locomotion. Stride 1 was defined by larger joint angular excursions at the knee and ankle; by stride 3, the knee and ankle showed smaller joint angular excursions. The hip swept through similar arcs across all strides, with most of the motion caused by femoral retraction and rotation. Metatarsophalangeal (MTP) kinematics exhibited smaller maximum angles in stride 1 compared with strides 2 and 3. The significant correlations between angular kinematics and locomotor performance were different across the first three strides. For stride 1, MTP kinematics predicted final maximum running speed; this correlation is likely explained by a correlation between stride 1 MTP kinematics and stride 2 acceleration performance. For stride 3, several aspects of joint kinematics at each joint predicted maximum running speed. Overall, S. woodi exhibits markedly different kinematics, performance and kinematics-performance correlations across the first three strides. This finding suggests that future studies of burst locomotion and acceleration performance should perform analyses on a stride-by-stride basis and avoid combining data from different strides across the burst locomotor event. Finally, the kinematics-performance correlations observed in S. woodi were quite different from those described for other species, suggesting that there is not a single kinematic pattern that is optimal for high burst performance.

Abdominal muscle function in ventilation and locomotion in new world opossums and basal eutherians: Breathing and running with and without epipubic bones.

All tetrapods have the same four basic abdominal hypaxial muscle layers that wrap around the abdomen between the pelvis, ribcage, and spine. However, the marsupials and our immediate mammalian ancestors have epipubic bones extending anteriorly into the ventral hypaxial layers with two additional muscles connecting them to the ventral midline and femur. Studies of two marsupials have shown that all of the abdominal hypaxials play a part bilaterally in resting ventilation and during locomotion there is an asymmetrical pattern of activity as the hypaxial muscles form a cross‐couplet linkage that uses the epipubic bone as a lever to provide long‐axis support of the body between diagonal limb couplets during each step. The cross‐couplet epipubic lever system defines the earliest mammals and is lost in placental mammals. To expand our understanding of the evolution of mammalian abdominal muscle function and loco‐ventilatory integration we tested the generality of the cross‐couplet system in marsupials and conducted the first formal studies of hypaxial abdominal motor patterns in generalized placental mammals focusing on a representative rodent and insectivore. These new data reveal 1) that continuous abdominal muscle tonus during resting ventilation and a 1:1 breath to step cycle during locomotion appear to be the basal condition for mammals, 2) that the loss of epipubic bones in eutherians is associated with a shift from the cross‐couplet dominated motor pattern of marsupials to a shoulder‐to‐pelvis system with unilateral activation of abdominal muscles during locomotion and 3) that hypaxial function in generalized eutherians is more similar to marsupials than cursorial mammals. J. Morphol. 2009.

Locomotory Fatigue During Moderate and Severe Hypoxia and Hypercapnia in the Atlantic Blue Crab, Callinectes sapidus

The Atlantic blue crab, Callinectes sapidus (Rathbun), is a highly mobile crustacean that must locomote to find food, evade predators, find mates, and avoid adverse conditions such as hypoxia. In this study we tested the effects of two levels of hypoxia (10.4 kPa, 50% air saturation = moderate hypoxia; 4 kPa, 20% air saturation = severe hypoxia) and hypercapnic hypoxia (50% air saturation O2 with Pco2 = 2 kPa) on fatigue during sustained continuous exercise. Fatigue was induced by an exercise trial that entailed continuous sideways hexapedal walking on an underwater treadmill. Fatigue was quantified using two methods: (1) a pull force test that measures the holding strength of the legs, and (2) the number of fatigue-resisting behaviors (180° turns and stopping). Fatigue was defined as a pull force of 67% or less of the initial pre-exercise pull force and was reached after 6.12 h of walking for crabs in well-aerated normoxic seawater, 4 h in 50% air saturation, 2.07 h in 20% air saturation, and 4.58 h in 50% air saturation and hypercapnia. The number of fatigue-resisting behaviors increased with walking time in all treatments. Performance decreased in hypoxia, with fatigue being reached more quickly as the level of hypoxia intensified. Hypercapnia in moderate hypoxia did not have a deleterious influence on behavior and lengthened slightly the time it took crabs to fatigue. In addition, severe hypoxia exacerbated changes in gait kinematics as crabs became fatigued, by significantly increasing stride length and decreasing stride frequency.

Sequential analyses of foraging behavior and attack speed in ambush and widely foraging lizards

Food acquisition mode in lizards (i.e., ambush vs. widely searching) has been intensely scrutinized for the past decade to identify correlations between food acquisition mode, diet, sprint speed, and other aspects of phenotypic diversity. To begin to understand these correlations, we studied foraging mode variation in natural foraging behavior and attack speed in three ambush predators and two widely foraging species in the field. Sequential analyses revealed considerable variation in the temporal structure of behavioral repertoires associated with acquiring food. Ambush and wide-foraging species use unique combinations of behaviors prior to prey attack with differences among and between food acquisition modes. Attack speeds were well below maximum sprint speed for these species. Thus, the widely demonstrated correlation between food acquisition mode and sprint speed is not related to prey capture per se. The striking variation in prey capture repertoires in these model ambush and wide foragers shows that we have a long way to go before we will understand the ecological relevance of many performance and phenotypic traits that are related to foraging mode in lizards.