In focus: Do slow-moving animals have stiff backs?

This week’s post is from Michael Granatosky, a postdoctoral scholar at the University of Chicago in the department of Organismal Biology and Anatomy (Figure 1). If you would like to write for Anatomy to You, get in touch via Facebook or Twitter.

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Figure 1. Michael Granatosky (left) collecting data in Brazil on the comparative energetic costs of feeding and locomotion in capuchin monkeys (right).

I describe myself professionally as a comparative evolutionary biomechanist, which essentially means I am interested in the ways that animals move and how this has changed through evolutionary time. When I first started graduate school, I became fascinated by the animals that take it slow as they locomote through the trees (Figure 2). Instead of leaping and running, sloths, lorises and other careful arboreal quadrupeds slowly bridge, cantilever and cryptically walk to navigate their complex three-dimensional environment.

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Figure 2. Sloths are careful arboreal quadrupeds that prefer a more slow and steady approach to moving through the trees. And yes, they are much beloved by many humans because they are adorable!

Beyond the remarkable behaviors of these animals is their morphology, and no part of their anatomy got me more excited than their axial skeleton. The axial skeleton consists of the vertebrae and the ribs. For a fast-moving animal, the ribs and the vertebral bodies tend to be long and thin, and the bony projections, sticking out of the vertebrae, more formally known as spinous and transverse processes, are long and oriented at obtuse angles (Figure 3a).

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Figure 3. Contrasting axial morphology between a fast-moving arboreal squirrel (A) compared to the careful arboreal silky anteater (Cyclopes didactylus) (B).

Taken together, these features are thought to allow for fast and powerful movements of the back that likely help these animals increase their leaping distance or running speed. In contrast, the vertebral morphology of careful arboreal quadrupeds includes expanded ribs, stocky vertebral bodies, and short transverse and spinous processes (Figure 4). The exemplar for this morphological suite, and admittedly the animal that sparked my initial interest in vertebral morphology, is the silky anteater (Cyclopes didactylus) (Figure 3b). Having these features associated with a stiff axial skeleton likely allows to whole back to remain rigid during locomotion and increases the ability of an animal to bridge, cantilever, and move effectively among terminal branches.

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Figure 4. Demonstrating variation in vertebral morphology among various arboreal species, in 3 different views. The scale bars are 10 mm. The bones are scaled to the same anterior mediolateral width of the body. “Inverted quadrupeds”, as the name implies, move upside down, like sloths. “Anti-pronograde” animals habitually hold their limbs in tension, such as underneath branches (like many monkeys do). “Pronograde” animals move their limbs in a (parasagittal) plane parallel to the body, like most mammals do. The various letterings (“PI, BR, GC”, etc.) indicate different species. Figure is from Granatosky et al., (2014).

While initially reading through the scientific literature for this project, it quickly became apparent that most prior reports were largely speculative about how anatomy and behavior were correlated for the axial skeleton of careful-locomotors. As such, I wanted to figure out whether having rigid vertebral morphology really did help careful arboreal quadrupeds accomplish behaviors like bridging or cantilevering. I compared the locomotor behavior of careful arboreal quadrupeds (wooly opossums and slender lorises) and fast-moving arboreal quadrupeds (short-tailed opossums and fat-tailed dwarf lemurs) as they foraged through a simulated arboreal environment with thin compliant branches. During all trials, the careful arboreal quadrupeds were able to more effectively forage in the thin terminal branches and they commonly used behaviors that required rigid back postures. Thus my hypothesis was supported! Science for the win!

This project was one of my early studies in graduate school, and allowed me to explore the relationship between form and function in a system that has received relatively little attention, at least when compared to the limbs. Furthermore, this work inspired my eventual PhD research focused on how arboreal animals change their limb-loading behavior during suspensory locomotion.

 

Refererence:

Granatosky, M. C., Miller, C. E., Boyer, D. M. and Schmitt, D. (2014). Lumbar vertebral morphology of flying, gliding, and suspensory mammals: implications for the locomotor behavior of the subfossil lemurs Palaeopropithecus and Babakotia. J. Hum. Evol. 75, 40–52.

We do regret that we only showed one sloth picture in this post. The internet might, maybe, have more.

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