Limb preference and skeletal asymmetry in the cane toad, Bufo marinus (Anura: Bufonidae)

Laterality in Horse Training: Psychological and Physical Balance and Coordination and Strength Rather Than Straightness

Simple Summary

For centuries, straightening a horse has been considered a key element in achieving its responsiveness and suppleness and has been a traditional goal in training. However, body asymmetry (natural crookedness), motor laterality (the preference for limbs on one side) and sensory laterality (the preference for sensory organs on one side) are naturally occurring phenomena. In humans, the forced correction of these imbalances, for example, forcing left-handed children to write with their right hands, has been shown to lead to psychological imbalance. In view of this, lateral asymmetries in horses should be accepted, and training should focus on psychological and physical balance, coordination and equal strength on both sides, instead of enforcing “straightness”. To explore this, we conducted a review of the literature on motor and sensory laterality in horses and found that the evidence suggests that enforcing straightness may be stressful and may even be counterproductive by causing psychological and physical imbalance relative to a horse, making it tense and uncooperative. In general, body asymmetry has been shown to have little impact on performance, but increases in motor and sensory laterality can indicate insufficiencies in housing, handling and training. We, therefore, propose that laterality should be recognized as a welfare indicator and that straightness in a horse should be achieved by conducting training focused on balance, coordination and equal strength on both sides.

Abstract

For centuries, a goal of training in many equestrian disciplines has been to straighten the horse, which is considered a key element in achieving its responsiveness and suppleness. However, laterality is a naturally occurring phenomenon in horses and encompasses body asymmetry, motor laterality and sensory laterality. Furthermore, forcibly counterbalancing motor laterality has been considered a cause of psychological imbalance in humans. Perhaps asymmetry and laterality should rather be accepted, with a focus on training psychological and physical balance, coordination and equal strength on both sides instead of enforcing “straightness”. To explore this, we conducted a review of the literature on the function and causes of motor and sensory laterality in horses, especially in horses when trained on the ground or under a rider. The literature reveals that body asymmetry is innate but does not prevent the horse from performing at a high level under a rider. Motor laterality is equally distributed in feral horses, while in domestic horses, age, breed, training and carrying a rider may cause left leg preferences. Most horses initially observe novel persons and potentially threatening objects or situations with their left sensory organs. Pronounced preferences for the use of left sensory organs or limbs indicate that the horse is experiencing increased emotionality or stress, and long-term insufficiencies in welfare, housing or training may result in left shifts in motor and sensory laterality and pessimistic mentalities. Therefore, increasing laterality can be regarded as an indicator for insufficiencies in housing, handling and training. We propose that laterality be recognized as a welfare indicator and that straightening the horse should be achieved by conducting training focused on balance, coordination and equal strength on both sides.

Skeletal radiographic anatomy of echidnas: insights into unusual mammals

Long-beaked echidnas (Zaglossus spp.) have received less attention in the literature than Short-beaked echidnas (Tachyglossus aculeatus). Their natural history, anatomy, and physiology are poorly known. To improve our understanding of this unique group, we undertook a radiographic study of the Eastern long-beaked echidna (Zaglossus bartoni), and provide a comparative analysis of its radiographic skeletal anatomy with that of T. aculeatus. We examine conventional radiography and computed tomographic images of Zaglossus and Tachyglossus, describe the anatomical features of Zaglossus, and compare those with Tachyglossus. We provide evidence of epicoracoid overlap in echidnas, a feature not well documented in monotremes. The significance of epicoracoid overlap requires further investigation. Our study is intended as a reference for the radiographic anatomy of Z. bartoni.

Lateralized motor behaviour in the righting responses of the cane toad (Rhinella marina)

This paper reports a series of tests for fore- and hind-limb preferences used by cane toads, Rhinella marina, to assist returning to the righted position after being overturned. We confirm the strong and significant right-handedness reported in this species, which under certain conditions exceeded 90% right-hand preference at the group level. Toads were tested under a variety of conditions including horizontal and inclined surfaces, with and without the opportunity for the forelimbs to grasp a support, in order to assess the effects of different vestibular and proprioceptive input on the strength and direction of fore- and hind-limb preferences. A range of behavioural strategies indicated learning effects; however, the strength or direction of limb preferences did not increase significantly with experience, even in toads retested multiple times. Comparisons with the mammalian condition for limb preferences are discussed with relevance to practice effects and established limb preferences, and to effects associated with arousal or stress. In contrast to the expectation that handedness in toads represents intentional or voluntary preferences, the presence of lateralized central pattern generators in the toads is postulated to explain the different forms of lateralization revealed by our tests.

The biological significance of acoustic stimuli determines ear preference in the music frog

Behavioral and neurophysiological studies support the idea that right ear advantage (REA) exists for perception of conspecific vocal signals in birds and mammals. Nevertheless, few studies have focused on anuran species that typically communicate through vocalization. The present study examined the direction and latencies of orientation behaviors in Emei music frogs (Babina daunchina) produced in response to six auditory stimuli emitted by a speaker placed directly behind the subjects. The stimuli included male advertisement calls produced from within burrow nests, which have been shown to be highly sexually attractive (HSA), calls produced from outside burrows, which are of low sexual attractiveness (LSA), screech calls produced when frogs are attacked by snakes, white noise, thunder and silence. For all sound stimuli except the screech, the frogs preferentially turned to the right. Right ear preference was strongest for HSA calls. For the screech and thunder stimuli, there was an increased tendency for subjects to move further from the speaker rather than turning. These results support the idea that in anurans, right ear preference is associated with perception of positive or neutral signals such as the conspecific advertisement call and white noise, while a left ear preference is associated with perception of negative signals such as predatory attack.

Directional asymmetry in the limbs, skull and pelvis of the silver fox (V. vulpes)

Directional asymmetry (DA) is a characteristic of most vertebrates, most strikingly exhibited by the placement of various organs (heart, lungs, liver, etc.) but also noted in small differences in the metrics of skeletal structures such as the pelvis of certain fish or sauropsids. We have analyzed DA in the skeleton of the fox (V. vulpes), using ∼1,000 radiographs of foxes from populations used in the genetic analysis of behavior and morphology. Careful measurements from this robust data base demonstrate that: 1) DA occurs in the limb bones, the ileum, and ischium and in the mandible; 2) regardless of the direction of the length asymmetry vector of a particular skeletal unit, the vectorial direction of length is always opposite to that of width; 3) with the exception of the humerus and radius, there is no correlation or inverse correlation between vectorial amplitudes or magnitudes of bone asymmetries. 4) Postnatal measurements on foxes demonstrate that the asymmetry increases after birth and continues to change (increasing or decreasing) during postnatal growth. 5) A behavior test for preferential use of a specific forelimb exhibited fluctuating asymmetry but not DA. None of the skeletal asymmetries were significantly correlated with a preferential use of a specific forelimb. We suggest that for the majority of fox skeletal parameters, growth on the right and left side of the fox are differentially biased resulting in fixed differences between the two sides in either the rate of growth or the length of the period during which growth occurs. Random effects around these fixed differences perturb the magnitude of the effects such that the magnitudes of length and width asymmetries are not inversely correlated at the level of individual animals.

Chimpanzee facial symmetry: a biometric measure of chimpanzee health

This paper reports a study of fitness indicator theory in chimpanzees. First, it establishes a theoretical perspective for the study of fitness indicator theory and the relationships among indicators of fitness in humans and other animals. Second, it describes a methodology for assessing facial fluctuating asymmetry (FA) in a sample (N = 21) of zoo chimpanzees (Pan troglodytes). Third, associations among chimpanzee facial FA and health are described. FA was positively associated with negative health symptoms, and negatively associated with general health. Results are discussed under the framework of good genes theory.

Assessing and interpreting lateralised behaviours in anuran larvae

We review here what is known about lateralised behaviours in tadpoles, focusing on turning biases and methods for testing them. Two testing protocols have been used that are specific for explosive turns which are likely to be Mauthner cell-mediated. They involve recording the turns made by tadpoles: (1) upon descent after they take a breath of air, and (2) after they are mechanically startled. Turning bias has also been explored for tadpoles: (3) in a T-maze, (4) as they exit a tube into a larger arena, and (5) as they turn away from a barrier or object brought towards their snout. Slower turns observed under the last three protocols may not be regulated by the same neuromotor pathways as the faster turns elicited in the first two protocols. Turning biases do not occur in tadpoles of all species. In species where turning biases have been observed during explosive turns, the biases are typically towards the left side. The majority of tadpoles have a single asymmetric left-sided spiracle, yet lateralised behaviours in these tadpoles do not appear to be obligatorily linked to this morphological asymmetry. Tadpole turning biases are usually of the order of 60-90% towards the preferred side and are rarely absolute for individuals, and never for populations. Turning biases, when present, usually appear shortly after hatching and disappear before metamorphosis. Turning biases that appear during metamorphosis may depend on appendicular rather than axial muscle, and thus are fundamentally different behaviours; i.e., involving different neuromuscular components. Lateralised behaviours may occur in anurans before they reach the free-living larval stage. This could include bias in the side towards which a frog embryo coils its tail within the egg case, or the side towards which a newly hatched tadpole leans when lying on the bottom How lateralised behaviours of anurans that change with ontogeny relate to each other is not known Although it has been suggested that turning biases are absent in the more archaic frog genera only two 'archaic' genera have been examined Xenopus and Bombina and the data are too few to discern any clear phylogenetic patterns Lastly we present a model suggesting why tadpoles might have turning biases in situations where maximum speed is paramount This model assumes that the fastest neuromotor programs will be 'hardwired' in the central nervous system The model predicts that the amount of morphological asymmetry in the Mauthner neurons will be directly proportional to the amount of turning bias seen in tadpoles.

Asymmetries in amphibians: a review of morphology and behaviour

Morphological and behavioural asymmetries in amphibians are reviewed. Among the characteristics considered are: (1) the asymmetry of the shoulder girdle (epicoracoid overlap); (2) the distribution of the left and right variants of its structure in amphibian populations; (3) asymmetry in the position of the spiracle(s); (4) asymmetric order of forelimb emergence from opercular chambers in tadpoles; and (5) preferential forelimb use in adult amphibians. I show that there are no direct cause-and-effect relationships between these characteristics, which would explain their development. Other asymmetries, such as asymmetry of the visceral organs, turning behaviour of tadpoles, asymmetries in the length and weights of the long bones, and some neuromorphological traits, also show few examples of relationships. However, the simultaneous absence of many asymmetries in some amphibians and their presence in others suggests a common cause, which affects all of these asymmetries indirectly, presumably very early in ontogenesis.

Left and right in the amphibian world: which way to develop and where to turn?

The last decade has seen a dramatic increase in studies on the development, function and evolution of asymmetries in vertebrates, including amphibians. Here we discuss current knowledge of behavioral and anatomical asymmetries in amphibians. Behavioral laterality in the response of both adult and larval anurans to presumed predators and competitors is strong and may be related, respectively, to laterality in the telencephalon of adults and the Mauthner neurons of tadpoles. These behavior lateralities, however, do not seem to correlate with visceral asymmetries in the same animals. We briefly compare what is known about the evolution and development of asymmetry in the structure and function of amphibians with what is known about asymmetries in other chordate and non-chordate groups. Available data suggest that the majority of asymmetries in amphibians fall into two independent groups: (1) related to situs viscerum and (2) of a neurobehavioral nature. We find little evidence linking these two groups, which implies different developmental regulatory pathways and independent evolutionary histories for visceral and telencephalic lateralizations. Studies of animals other than standard model species are essential to test hypotheses about the evolution of laterality in amphibians and other chordates.

Lateralised brain function in anurans: Comparison to lateralisation in other vertebrates

In recent years researchers have begun to investigate lateralisation of behaviour in amphibians. Given the mounting evidence of lateralisation in birds and mammals, and even reptiles, over the past two or more decades, it is not surprising that amphibians have attracted attention in this context. In particular, the evidence for lateralisation in fish has provided a strong basis for this research. This paper summarises the currently available information on lateralisation in anuran amphibians and discusses it in comparison to lateralisation in other vertebrate species, beginning with examples of motor lateralisation and then discussing functional asymmetries that occur between the left and right sides of the brain. The latter are manifested as side biases in responding to different stimuli or, in a number of non-amphibian species, revealed by monocular testing. Most of the examples discussed refer to lateralisation present at the level of the forebrain hemispheres, and so represent hemispheric specialisation. Lateralisation usually refers to examples in which there is a population bias for the majority of individuals in a population to be lateralised in the same direction. In other words, there is a significant skew in the frequency distribution. Such population biases in lateralisation are now known to be widespread among the vertebrates and, as shown, there are some surprisingly similar patterns of lateralisation in those species of fish, amphibians, reptiles, birds, and mammals that have been studied so far. It is also noted that, despite their ubiquity in vertebrates, far from all forms of lateralisation develop solely, or even largely, according to genetic determinants. In fact, the clear and powerful influences of environmental stimulation on development of some kinds of lateralisation in birds provide a basis for similar investigations in anurans.