The floor is open for biomechanics

01 November 2017 - By: Sophie Regnault

The floor is open for biomechanics

By Sophie Regnault, Royal Veterinary College

The Open Biomechanics session at the SEB Annual Meeting showcases a diversity of research questions, techniques and species. The latest event, in Gothenburg, was no exception, with oral presentations highlighting previously unappreciated complexity in animal anatomy and behaviours.

The rich inner workings of the lizard inner ear

In lizards, the outer ear is absent and the ear canals are linked to the throat and to each other in an open system. In some species, the linked inner ears allow you to look straight through the head! Although there isn’t a lot to see, the hearing and balance organs of the inner ear are more complex than they seem. Bruce Young and Dawei Han (A.T. Still University and Truman State University, USA) specifically focused on the hearing capabilities of the Asiatic water monitor lizard (Varanus salvator), and were surprised to find that its auditory system is more dynamic and responsive than previously thought. They found several features that show monitor lizards are able to control and change how sound is perceived by the ear. The ear drum itself contains a muscle that can be voluntarily contracted to alter its vibrational characteristics. The drum is also divided into upper and lower regions with different mechanical properties, with the stiffer lower portion up to 12 times more sensitive to certain sounds. These features are similar to those found in the ear drum of humans and other mammals, suggesting lizards have independently evolved a similar sophisticated setup.

In addition, the lizards are also able to pump air through the throat and pressurise the shared canal system as another way of altering how the ear drum processes sound waves. “Our experimental results challenge some of the basic assumptions of previous ear models”, says Young. Several other SEB Open Biomechanics presentations explored how the lizard inner ear correlates with size or ecology, and how it aids the body’s stabilisation on uneven ground. The newly-appreciated complexity of the lizard ear may prove relevant to other research questions and fields.

Dissecting multifunctional muscle mechanics

Continuing the theme of anatomical and functional complexity, Chris Tijs (Harvard University, USA) and colleagues explored the mechanical properties of pennate muscles. These are muscles whose contractile fibres attach to the tendon at a slanting angle, rather than parallel to it. When a pennate muscle contracts, the fibre angle increases – and the larger the change in angle, the faster the whole muscle can shorten as it contracts. Previously, estimates of the changes in fibre angle that are used to characterise different pinnate muscles have been based on measurements from only one part of the muscle, and so assume that the whole muscle has the same mechanical properties. However some muscles have regions with different muscle fibre arrangements.

The medial gastrocnemius is one example; the large muscle of the inner calf is split into upper and lower portions with fibres of different lengths and attachment angles. Tijs asked “How do these regions differ?” By stimulating the muscle in anaesthetised rats, the researchers found that that the upper, shorter fibres underwent greater overall change in their angle of attachment. Tijs thinks that these regional variations in muscle fibre properties might be satisfying different mechanical needs within the muscle. “The calf muscle has less leverage at the level of the knee joint, compared to the level of the ankle. Having shorter contractile fibres means more of them can be packed into this region, increasing muscle force and compensating for the lower leverage here.” The results suggest that the mechanical properties of pinnate muscles rely on a complex interplay of factors, not reflected by measurements taken from just one region.

Chewing it over

Some presentations showcased familiar behaviours in unfamiliar species. Egon Heiss (Friedrich-Schiller University of Jena) and colleagues investigated chewing in amphibians, challenging perceptions in these species. These animals supposedly do not chew, except for some salamanders – mechanical analyses suggest the stresses would be too great for their jaws – but the researchers noticed that the newt T. carnifex displayed chewinglike behaviours after catching its maggot prey. It would bob its head up and down, repeatedly open and close its mouth and move its tongue. Was this some form of chewing? And if so, how were these newts processing the maggots, given their weak jaws? Heiss and colleagues used high speed x-ray videos to study the behaviour. Their videos reveal that the newts were not simply repositioning food; the hapless maggot remained in the same place, pushed against the roof of the mouth by the tongue.

In fact, these newts possess small teeth actually embedded in the roof of the mouth, and the cyclical movements of their tongue act to grind the food object against these teeth. Maggots had grazes and lesions in their outer cuticle, suggesting newts’ chewing prepares these tough titbits for easier digestion. Chewing is well-studied in mammals, birds, reptiles, and fish and results of this most recent experiment suggest chewing behaviour may be even more widespread than thought. “I was very surprised how well the tiny palatal teeth worked to rasp prey.” Heiss says, “Further studies will show whether chewing might have evolved much earlier in a shared ancestor, perhaps as part of the process of coming to live on land.”

Frogs’ hips don’t lie… but do deceive

Other researchers revealed unexpected functional versatility between animals with different anatomy and lifestyles. The pelvic bones of frogs are generally one of three recognised shapes, each correlated with walking-, jumping- or swimming-specialists. However, some frogs are capable of a range of behaviours. The red-legged running frog (Kassina maculata) is just one such animal, famed for its walking and running, but also as good a jumper as any specialist. Its ‘walkingtype’ pelvis rolls from side to side, allowing the frog to take large strides, but this doesn’t happen during jumping. Amber Collings and colleagues (Royal Veterinary College, UK) asked why Kassina isn’t constrained by its walking-type pelvis and how it manages to achieve such multi-functionality.

Other researchers revealed unexpected functional versatility between animals with different anatomy and lifestyles. The pelvic bones of frogs are generally one of three recognised shapes, each correlated with walking-, jumping- or swimming-specialists. However, some frogs are capable of a range of behaviours. The red-legged running frog (Kassina maculata) is just one such animal, famed for its walking and running, but also as good a jumper as any specialist. Its ‘walkingtype’ pelvis rolls from side to side, allowing the frog to take large strides, but this doesn’t happen during jumping. Amber Collings and colleagues (Royal Veterinary College, UK) asked why Kassina isn’t constrained by its walking-type pelvis and how it manages to achieve such multi-functionality.

The team recorded the frogs’ movement and fed the data into their anatomical computer model to understand how the muscles were contributing to each type of behaviour. Collings says “Being such radically different movements, requiring very different postures, we were expecting the muscles to change position in ways that enhanced leverage for either walking or jumping.” But they were surprised to find that this was not the case. Instead, it seems that the secret is all in timing of pelvic muscle contractions. By only contracting muscles on one side, the pelvis can roll freely during walking, whereas coordinated bilateral contractions stabilise the pelvis to allow such impressive jumping.

Next year’s SEB Open Biomechanics sessions will continue to provide a presentation venue for rich and varied biomechanics research, with 1.5 days of oral presentations scheduled at the Annual Meeting in Florence, 3–6 July 2018.

 

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Sophie Regnault

I graduated as a veterinary surgeon from the Royal Veterinary College in 2012. During my final year, I collaborated with the Structure and Motion Lab to produce a small research project on osteopathology found in the feet of rhinoceroses. Following graduation, I spent some time in practice, working at several PDSA hospitals in London. I also spent this time expanding my undergraduate research project into a published paper. In July 2013, I returned to the RVC to start my PhD, entitled "Comparative Mechanobiology and Evolution of Patellar Sesamoids".