Low thermal dependence of the contractile properties of a wing muscle in the bat Carollia perspicillata

A chemo-mechanical constitutive model for muscle activation in bat wing skins

Birds, bats and insects have evolved unique wing structures to achieve a wide range of flight capabilities. Insects have relatively stiff and passive wings, birds have a complex and hierarchical feathered structure and bats have an articulated skeletal system integrated with a highly stretchable skin. The compliant skin of the wing distinguishes bats from all other flying animals and contributes to bats' remarkable, highly manoeuvrable flight performance and high energetic efficiency. The structural and functional complexity of the bat wing skin is one of the least understood although important elements of the bat flight anatomy. The wing skin has two unusual features: a discrete array of very soft elastin fibres and a discrete array of skeletal muscle fibres. The latter is intriguing because skeletal muscle is typically attached to bone, so the arrangement of intramembranous muscle in soft skin raises questions about its role in flight. In this paper, we develop a multi-scale chemo-mechanical constitutive model for bat wing skin. The chemo-mechanical model links cross-bridge cycling to a structure-based continuum model that describes the active viscoelastic behaviour of the soft anisotropic skin tissue. Continuum models at the tissue length-scale are valuable as they are easily implemented in commercial finite element codes to solve problems involving complex geometries, loading and boundary conditions. The constitutive model presented in this paper will be used in detailed finite element simulations to improve our understanding of the mechanics of bat flight in the context of wing kinematics and aerodynamic performance.

Application of a novel deep learning-based 3D videography workflow to bat flight

Studying the detailed biomechanics of flying animals requires accurate three-dimensional coordinates for key anatomical landmarks. Traditionally, this relies on manually digitizing animal videos, a labor-intensive task that scales poorly with increasing framerates and numbers of cameras. Here, we present a workflow that combines deep learning-powered automatic digitization with filtering and correction of mislabeled points using quality metrics from deep learning and 3D reconstruction. We tested our workflow using a particularly challenging scenario: bat flight. First, we documented four bats flying steadily in a 2 m3 wind tunnel test section. Wing kinematic parameters resulting from manually digitizing bats with markers applied to anatomical landmarks were not significantly different from those resulting from applying our workflow to the same bats without markers for five out of six parameters. Second, we compared coordinates from manual digitization against those yielded via our workflow for bats flying freely in a 344 m3 enclosure. Average distance between coordinates from our workflow and those from manual digitization was less than a millimeter larger than the average human-to-human coordinate distance. The improved efficiency of our workflow has the potential to increase the scalability of studies on animal flight biomechanics.

A comparison of thermal sensitivities of wing muscle contractile properties from a temperate and tropical bat species

Endotherms experience temperature variation among body regions, or regional heterothermy, despite maintaining high core body temperatures. Bat forelimbs are elongated to function as wings, which makes them vulnerable to heat loss and exaggerates regional heterothermy. A tropical bat species, Carollia perspicillata, flies with distal wing muscles that are substantially (>10°C) cooler than proximal wing muscles and significantly less temperature sensitive. We hypothesized that the difference between proximal and distal wing muscles would be even more extreme in a temperate bat species that is capable of flight at variable environmental temperatures. We measured the contractile properties of the proximal pectoralis muscle and distal extensor carpi radialis muscle at a range of temperatures in the big brown bat, Eptesicus fuscus, and compared their thermal dependence with that of the same muscles in C. perspicillata. We found that, overall, temperature sensitivities between species were remarkably similar. The sole exception was the shortening velocity of the pectoralis muscle in E. fuscus, which was less temperature sensitive than in C. perspicillata. This decreased temperature sensitivity in a proximal muscle runs counter to our prediction. We suggest that the relative lability of body temperature in E. fuscus may make better pectoralis function at low temperatures advantageous.

How thermal challenges change gene regulation in the songbird brain and gonad: implications for sexual selection in our changing world

In a rapidly warming world, exposure to high temperatures may impact fitness, but the gene regulatory mechanisms that link sublethal heat to sexually selected traits are not well understood, particularly in endothermic animals. Our experiment used zebra finches (Taeniopygia guttata), songbirds that experience extreme temperature fluctuations in their native Australia. We exposed captive males to an acute thermal challenge (43°C) compared with thermoneutral (35°C) and lower (27°C) temperatures. We found significantly more heat dissipation behaviors at 43°C, a temperature previously shown to reduce song production and fertility, and more heat retention behaviors at 27°C. Next, we characterized transcriptomic responses in tissues important for mating effort - the posterior telencephalon, for its role in song production, and the testis, for its role in fertility and hormone production. Differential expression of hundreds of genes in the testes, but few in the brain, suggest the brain is less responsive to extreme temperatures. Nevertheless, gene network analyses revealed that expression related to dopaminergic signaling in the brain co-varied with heat dissipation behaviors, providing a mechanism by which temporary thermal challenges may alter motivational circuits for song production. In both brain and testis, we observed correlations between thermally sensitive gene networks and individual differences in thermoregulatory behavior. Although we cannot directly relate these gene regulatory changes to mating success, our results suggest that individual variation in response to thermal challenges could impact sexually selected traits in a warming world.

Body Temperature Frequency Distributions: A Tool for Assessing Thermal Performance in Endotherms?

There is increasing recognition that rather than being fully homeothermic, most endotherms display some degree of flexibility in body temperature. However, the degree to which this occurs varies widely from the relatively strict homeothermy in species, such as humans to the dramatic seasonal hibernation seen in Holarctic ground squirrels, to many points in between. To date, attempts to analyse this variability within the framework generated by the study of thermal performance curves have been lacking. We tested if frequency distribution histograms of continuous body temperature measurements could provide a useful analogue to a thermal performance curve in endotherms. We provide examples from mammals displaying a range of thermoregulatory phenotypes, break down continuous core body temperature traces into various components (active and rest phase modes, spreads and skew) and compare these components to hypothetical performance curves. We did not find analogous patterns to ectotherm thermal performance curves, in either full datasets or by breaking body temperature values into more biologically relevant components. Most species had either bimodal or right-skewed (or both) distributions for both active and rest phase body temperatures, indicating a greater capacity for mammals to tolerate body temperatures elevated above the optimal temperatures than commonly assumed. We suggest that while core body temperature distributions may prove useful in generating optimal body temperatures for thermal performance studies and in various ecological applications, they may not be a good means of assessing the shape and breath of thermal performance in endotherms. We also urge researchers to move beyond only using mean body temperatures and to embrace the full variability in both active and resting temperatures in endotherms.

A proximal-distal difference in bat wing muscle thermal sensitivity parallels a difference in operating temperatures along the wing

Flight is a demanding form of locomotion, requiring fast activation and relaxation in wing muscles to produce the necessary wingbeat frequencies. Bats maintain high body temperatures during flight, but their wing muscles cool under typical environmental conditions. Because distal wing muscles are colder during flight than proximal muscles, we hypothesized that they would be less temperature sensitive to compensate for temperature effects, resulting in proximal-distal differences in temperature sensitivity that match differences in muscle operating temperature. We measured contractile rates across temperatures in the proximal pectoralis muscle and an interosseous in the handwing of Carollia perspicillata, a small neotropical fruit bat, and compared their thermal dependence with that of a forearm muscle measured in a previous study. We found that the contractile properties of the pectoralis were significantly more temperature sensitive than those of the distal muscles. This suggests that cooling of the distal wing muscles imposes a selective pressure on muscle contractile function which has led to shifts in temperature sensitivity. This study is the first to demonstrate differences in temperature sensitivity along the length of a single limb in an endotherm and suggests that temperature variation may be underappreciated as a determinant of locomotor performance in endotherms generally.

Do endotherms have thermal performance curves?

Temperature is an important environmental factor governing the ability of organisms to grow, survive and reproduce. Thermal performance curves (TPCs), with some caveats, are useful for charting the relationship between body temperature and some measure of performance in ectotherms, and provide a standardized set of characteristics for interspecific comparisons. Endotherms, however, have a more complicated relationship with environmental temperature, as endothermy leads to a decoupling of body temperature from external temperature through use of metabolic heat production, large changes in insulation and variable rates of evaporative heat loss. This has impeded our ability to model endothermic performance in relation to environmental temperature as well as to readily compare performance between species. In this Commentary, we compare the strengths and weaknesses of potential TPC analogues (including other useful proxies for linking performance to temperature) in endotherms and suggest several ways forward in the comparative ecophysiology of endotherms. Our goal is to provide a common language with which ecologists and physiologists can evaluate the effects of temperature on performance. Key directions for improving our understanding of endotherm thermoregulatory physiology include a comparative approach to the study of the level and precision of body temperature, measuring performance directly over a range of body temperatures and building comprehensive mechanistic models of endotherm responses to environmental temperatures. We believe the answer to the question posed in the title could be 'yes', but only if 'performance' is well defined and understood in relation to body temperature variation, and the costs and benefits of endothermy are specifically modelled.

The need for speed: functional specializations of locomotor and feeding muscles in Anolis lizards

Muscles often perform diverse mechanical roles within an organism. Tuning of contractile properties may therefore provide an opportunity for muscles to better perform their different roles and impact their associated whole-organism performance. Here, we examined the muscle contractile physiology of a jaw and a leg muscle in five Anolis species to determine whether consistent physiological differences are found in these muscles. We found that these jaw and leg muscles exhibited consistent patterns of variation across species, which may be related to the functional use of each muscle. In particular, we found that each muscle had differentially increased different measures of muscle speed. Although the jaw muscles had faster peak contractile velocities than the leg muscles, the leg muscles had faster twitch times and faster contractile velocities under intermediate loads. We also found that the jaw muscles exerted higher specific tensions and had a greater curvature to their force-velocity relationship. The consistent patterns across five species suggest that these jaw and leg muscles have specialized in different ways. Examination of these contractile property variations may help illuminate important features relating to performing their individual functional roles.

Warm bodies, cool wings: regional heterothermy in flying bats

Many endothermic animals experience variable limb temperatures, even as they tightly regulate core temperature. The limbs are often cooler than the core at rest, but because the large locomotor muscles of the limbs produce heat during exercise, they are thought to operate at or above core temperature during activity. Bats, small-bodied flying mammals with greatly elongated forelimbs, possess wings with large surfaces lacking any insulating fur. We hypothesized that during flight the relatively small muscles that move the elbow and wrist operate below core body temperature because of elevated heat loss. We measured muscle temperature continuously in the small fruit bat Carollia perspicillata before and during wind tunnel flights, and discretely in diverse bats at rest in Belize. We found that bats maintained high rectal temperatures, but that there was a steep proximal-to-distal gradient in wing muscle temperature. Forearm muscles were 4-6°C cooler than rectal temperature at rest and approximately 12°C cooler during flights at an air temperature of 22°C. These findings invite further study into how bats and other endotherms maintain locomotor performance in variable environments, when some muscles may be operating at low temperatures that are expected to slow contractile properties.

The likely effects of thermal climate change on vertebrate skeletal muscle mechanics with possible consequences for animal movement and behaviour

Climate change can involve alteration in the local temperature that an animal is exposed to, which in turn may affect skeletal muscle temperature. The underlying effects of temperature on the mechanical performance of skeletal muscle can affect organismal performance in key activities, such as locomotion and fitness-related behaviours, including prey capture and predator avoidance. The contractile performance of skeletal muscle is optimized within a specific thermal range. An increased muscle temperature can initially cause substantial improvements in force production, faster rates of force generation, relaxation, shortening, and production of power output. However, if muscle temperature becomes too high, then maximal force production and power output can decrease. Any deleterious effects of temperature change on muscle mechanics could be exacerbated by other climatic changes, such as drought, altered water, or airflow regimes that affect the environment the animal needs to move through. Many species will change their location on a daily, or even seasonal basis, to modulate the temperature that they are exposed to, thereby improving the mechanical performance of their muscle. Some species undergo seasonal acclimation to optimize muscle mechanics to longer-term changes in temperature or undergo dormancy to avoid extreme climatic conditions. As local climate alters, species either cope with the change, adapt, avoid extreme climate, move, or undergo localized extinction events. Given that such outcomes will be determined by organismal performance within the thermal environment, the effects of climate change on muscle mechanics could have a major impact on the ability of a population to survive in a particular location.