Using X-rays to learn what walking rats can teach us about early placental mammal locomotion

In paleontology, we often infer the habits and behaviors of fossil vertebrates by reference to their skeletal shape. However, it is often difficult to appreciate what those shapes are telling us: how well does shape correlate with motion?

We are members of the eutheiran branch of the mammal family tree. Among many questions concerning mammal evolution, one is how did the earliest eutherian (so-called “placental”) mammals use their forelimbs? This question has important implications for how our earliest relatives got around. The earilest known members of our group are small and are often hypothesized to be scansorial (Luo et al., 2003, 2011), meaning that they are at home on the ground as well as clambering up trees.These inferences are drawn in large part from the form of the fossilized forelimb bones and their presumed functions.

If you’ve been following this blog, you know that I have been immersed in learning XROMM (X-ray Reconstruction of Moving Morphology), a technique that combines video fluoroscopy (X-ray movies) with registration of three-dimensional bone models to yield 3-D moving X-rays.

I am happy to report that my colleagues, two Stockton undergraduates (Radha Varadharajan and Corey Gilbert), and I have published an Open Access article in PLoS One that, for the very first time, reconstructs the three-dimensional movements of the long bones (humerus, radius, ulna) in the forelimb of rats. Why rats? Rats have a forelimb anatomy that is very similar in many ways to those of the earliest eutherian mammals, and as a plus, rats are scansorial. Rats are also relatively easy mammals to work with in the lab (although some days they out-clever the humans) and can be trained. As a fun side note, we named two of the rats Pink and Floyd.

Our setup was straightforward — at the C-arms XROMM lab at Brown University, the rats walked along a plank of wood to a darkened hide box. While traversing the plank, they made their X-ray cameos in two fluoroscopes connected to hi-speed cameras filming at 250 frames per second (your iPhone camera films at 30 frames per second in normal mode). When we were finished collecting our data, the rats were CT-scanned so that we could have exact three-dimensional models of their limb bones. The most painstaking part was the several months it took to digitize each of our good trials. That is, using animation software, we had to match the bone models up to their X-ray shadows in the two calibrated fluoroscope movies. Once this was accomplished, our task turned to watching how the bones moved in three-dimensional space as well as analyzing the joint angle data that was generated.

Our basic setup for the XROMM study -- rats were trained to walk across a plank towards a dark hide box, leading them between the two videofluoroscopes.

Our basic setup for the XROMM study — rats were trained to walk across a plank towards a dark hide box, leading them between the two videofluoroscopes.

What we found both confirms previous work on small mammal locomotion, but added some interesting new insights as well. As a general rule, small mammals have a crouched posture where the elbows and knees are bent. This type of posture may aid small mammals in maneuvering around objects and keeping a lower center of gravity, which would enhance stability, especially on branches and other narrow perches. Not surprisingly and given previous work on rat locomotion, we see that these mammals do indeed walk on crouched limbs — the elbow angle, for example, never exceeded 123 degrees in full extension. By way of comparison, your elbow can be extended to 180 degrees.

A figure from my book, The Bare Bones. Note how the rat has a more crouched posture whereas the cat is more upright.

A figure from my book, The Bare Bones. Note how the rat has a more crouched posture whereas the cat is more upright.

However, we often get the impression that mammal locomotion is similar at different scales. From cats and dogs on up, it appears that the forelimbs and hindlimbs function very much as glorified pendulums. In essence, eutherian mammal locomotion is understood as mostly two-dimensional. Although rats are small and have a crouched posture, their limb bones would be presumed to follow the pendulum model.

But what the bones were doing in three-dimensions was fascinating. Both the humerus (upper arm bone) and radius (the forearm bone that aligns with your thumb) showed they were capable of long-axis rotation. Long-axis rotation is similar to the way a lathe or axle spins. Our rats’ bones certainly weren’t spinning on their long-axis, but they did show a non-trivial range of movement. A step cycle consists of a stance phase (when the hand is on the ground and forelimb is supporting the body) and a swing phase (when the hand is off the ground and the forelimb is swinging back to support the body for the next step). We found that during stance, the humerus both moves toward the midline (adducts) and rotates on its long axis towards the body. These combined movements appear to ensure that the elbow points backwards so that the forearm maintains an upright posture. During swing, the humerus moves away from the body midline (abducts) and rotates on its long axis away from the body. These combined movements seem to allow the forelimb to clear the rat’s body as the limb is brought forward to start a new step.

Lateral, ventral, and radioulnar joint views of the humerus (sea green), radius (black), and ulna (red) in a typical step cycle in Rattus norvegicus. Long-axis rotation (LAR) of the radius about the ulna (radius pronation) is shown in cranial view from the perspective of the ulna (the ulna appears to be stationary in the radioulnar joint view relative to the humerus and radius). Note radius (black) LAR relative to the ulna (red). Percentages = portion of the step cycle. Black bar in ventral view = body midline based on sternum.

Lateral (side), ventral (belly), and radioulnar joint (at the elbow) views of the humerus (sea green), radius (black), and ulna (red) in a typical step cycle in Rattus norvegicus. Long-axis rotation (LAR) of the radius about the ulna (radius pronation) is shown in cranial view (the rat is walking toward you) from the perspective of the ulna (the ulna appears to be stationary in the radioulnar joint view relative to the humerus and radius). Note radius (black) LAR relative to the ulna (red). Percentages = portion of the step cycle. Black bar in ventral view = body midline based on sternum.

MOVIE 1 – All the rats walking betwixt the fluoroscopes with their CT-scanned bones registered to the frames.

MOVIE 2 – One of our rats, “Floyd,” demonstrating a typical step cycle.

What was particularly exciting to me was that we saw, for the first time in rats, the radius pivot about the ulna! In humans, we take these movements for granted: our radius pivots around our ulna with ease, directing our palms either downward (pronated) when its shaft cross over the ulna, or upward (supinated) when its shaft rotates into parallel with the ulna. Up until now, it has been unclear if the radius could move in this way to flip the hand palm-side down in rats, or whether their hand posture was maintained via positioning of the limb in general. We now know that, indeed, the radius does move and does appear to be correlated with hand placement in rats. These movements are much more subtle than in you and I (in our rats a range of 10-30 degrees of rotation), but they appear to be correlated with pronation of the hand.

MOVIE 3 – One of our rats, “Floyd,” showing how the radius pivots on the ulna during a step cycle.

Our research has two messages. The first message is that given the similarities in the forelimb skeletons of the earliest known eutherian mammals (Juramaia and Eomaia) to those of rats, it is likely that a similar range of movements were possible in these distant relatives on our family tree. Paleontologists studying these fossils, such as Zhe-Xi Luo and colleagues (Luo et al., 2003, 2011), have already suggested these early eutherian mammals were scansorial, and our data bolster their hypothesis. These sorts of insights are helpful in constraining when particular locomotor behaviors and movements became possible and how that might have effected mammalian evolution.

The second message is that small mammal locomotion is probably not as similar to those of larger mammals as we often think, a sentiment echoed by the late Farish Jenkins (e.g., Jenkins, 1971) and by Martin Fischer and his colleagues (Fischer et al. 2002; Fisher and Blickman, 2006). Moreover, our rat data show that, at least for the forelimb, long-axis rotation plays a role in normal overground movement.

We hope our study provides another perspective on small mammal locomotion and encourages new and fruitful research in our furry friends past and present.

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I am grateful to my colleagues and former students for their help and work on this project. I want especially to thank Elizabeth Brainerd (Brown University). She has been a source of encouragement and a patient teacher to an old dinosaur learning new tricks, and her help with learning XROMM and on designing the experiment which led to this paper (my first foray into animal kinematics) was invaluable.

The authors of the paper (* indicates a Stockton University undergraduate)

Matthew F. Bonnan (Stockton University, Biology)
Jason Shulman (Stockton University, Physics)
*Radha Varadharajan (Stockton University, Biology)
*Corey Gilbert (Stockton University, Physics)
Mary Wilkes (Stockton University, Biology)
Angela Horner (California State University, San Berardino)
Elizabeth Brainerd (Brown University)

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References

Fischer, M. S., and R. Blickhan. 2006. The tri-segmented limbs of therian mammals: kinematics, dynamics, and self-stabilization—a review. Journal of Experimental Zoology Part A: Comparative Experimental Biology 305A:935–952.

Fischer, M. S., N. Schilling, M. Schmidt, D. Haarhaus, and H. Witte. 2002. Basic limb kinematics of small therian mammals. The Journal of Experimental Biology 205:1315–38.

Jenkins, F. A. 1971. Limb posture and locomotion in the Virginia opossum (Didelphis marsupalis) and in other non-cursorial mammals. Journal of Zoology, London 165:303–315.

Luo, Z.-X., Q. Ji, J. R. Wible, and C.-X. Yuan. 2003. An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution. Science 302:1934–1940.

Luo, Z.-X., C.-X. Yuan, Q.-J. Meng, and Q. Ji. 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476:442–445.

Leaping lizards and running rats

Given the positive feedback and interest in our POV of a ferret running on a treadmill, we’ve upped the ante here at the Best Feet Forward lab.  We proudly present two more GoPro POV movies of our magnificent animals running for science.  Would you like to see a running Bearded Dragon (Pogona vitticeps) and lab rat (Rattus norvegicus)?  Of course you would.

Above you see Greenbeard running for science.  We’re shaking a tasty bucket of crickets off-camera to get him to run.

Above you see one of our lab rats, Frank, also running for science.  If you look closely you can see the reflective beads attached to him that we follow with the infrared OptiTrack camera system.

Bridget Kuhlman is once again thanked for her brilliant camera work.

Why do we do what we do?

Forelimb kinematics research off and running in the BFF Lab

Just a brief note: our forelimb kinematics research on lizards and mammals is off and running (pun intended) in the BFF Locomotion Lab.  This semester, several teams of undergrads from biology and physics are working with myself and Dr. Jason Shulman (Physics) on a variety of projects to explore the typical range of motion and posture in lizard and mammal forelimbs.

Corey Barnes (left) and Alex Lauffer are working with a bearded dragon lizards to determine the typical range of motion in their forelimbs.

Corey Barnes (left) and Alex Lauffer are working with bearded dragon lizards to determine the typical range of motion in their forelimbs.

A close up of one of our bearded dragons, decked out with optical tracking markers.

A close up of one of our bearded dragons, decked out with optical tracking markers.

Undergrad Bridget Kuhlman coaxing one of our ferrest, "Mocha," with ferret treats to walk on the treadmill.

Undergrad Bridget Kuhlman coaxing one of our ferrets, “Mocha,” with ferret treats to walk on the treadmill.

The BFF Lab is thriving thanks to the help of NAMS lab staff.  We particularly want to thank Justine Ciraolo, Chrissy Schairer, Bill Harron, Mike Farrell, and Mike Santoro for their invaluable help in acquiring lab space and with technical assistance, and Deanne Gipple for help with student safety and animal welfare training.  None of this would occur without the assistance and animal care provided by John Rokita and the animal lab staff and volunteers.  We also thank NAMS Dean Dennis Weiss and the Biology and Physics programs for their continued support and assistance with our research endeavors.  Finally, we give a special “shout out” to the Stockton Federation of Teachers for their strong encouragement of faculty research “without walls.”  Thanks everyone!

New students … same old rats

Just a short post to introduce you to some of the “newer” students in the Bonnan Lab: Kelsey Gamble and Caleb Bayewu.

Kelsey Gamble in Lab

Kelsey Gamble with Peter the rat, showing off the vest she designed for tracking our furry friends.

Undergraduate Caleb Bayewu with another rat we dubbed Jabba.

Undergraduate Caleb Bayewu with another rat we dubbed Jabba.

Today we were working with some Sprague-Dawley rats to track how much their forelimb is abducted at the elbow (pulled away from the side of the body) during locomotion.  We use an apparatus called the OptiTrack V120 which consists of 3 integrated infrared cameras that send out rapid pulses of IR light.  The rats wear a vest with two markers on the back which gives us the position of their body’s mid-line, and another small marker is affixed to their elbow (with the equivalent of eyelash glue) … with tender loving care, of course.

Peter the rat walking along his track, showing off his tracking vest and the tracking marker on his elbow.

Peter the rat walking along his track, showing off his tracking vest and the tracking marker on his elbow.

Peter the rat was more interested in exploring the lab than being measured for science.

Peter the rat was more interested in exploring the lab than being measured for science.

You know you’re a scientist when after months of trial and error and fiddling with the equipment, we literally jumped for joy today when we successfully recorded all five walking trials!  Why are we doing this?  Stay tuned …

The “Rat Pack” Succeeds

A lot has happened in the Bonnan Lab at Stockton these past few months.

First, the “Rat Pack” as I fondly call them (Evan Drake, Kadeisha Pinkney, and Radha Varadharajan), presented their research proposals for 3-D rat locomotion and kinematics to the Northeastern Regional Vertebrate Evolution Symposium (NERVES) on March 22nd, 2013, at the New York Institute of Technology (NYIT) College of Osteopathic Medicine.  Their talks were very well received and we had excellent suggestions from colleagues and scholars.  I was especially proud of these undergraduates because they were able to give technical talks to a scientific audience having only worked with me for a few months on their projects. Bravo!

A special thank you to the Symposium’s organizers, Drs. Brian Beatty and Matthew Mihlbachler!

You can read their NERVES talk titles below.

Next, they put their collective heads together and, with my input, created a very nice poster for the 2013 NAMS Research Symposium at Stockton.

You can read their NAMS research abstract below as well.

The "Rat Pack" presented their preliminary research on rat locomotion to the 2013 NAMS Research Symposium.  Left to right: Kadeisha Pinkney, Radha Varajharadan, and Evan Drake.

The “Rat Pack” presented their preliminary research on rat locomotion to the 2013 NAMS Research Symposium. Left to right: Kadeisha Pinkney, Radha Varadharajan, and Evan Drake.

Kadeisha (left) and Radha (right) explain the joys of rat locomotion to interested students.

The “Rat Pack” attracted quite a crowd.

Last, but not least, Radha Varadharajan received the first Robert L. Fines scholarship awarded at Stockton, April 26, 2013, for her work on this research and her future career goals in veterinary medicine.  Dr. Fines is a former Stockton alumnus (1975) and is one of the premiere M.D. researchers successfully fighting pancreatic cancer.  He is the Herbert Irving Associate Professor of Medicine in the Division of Medical Oncology at the Columbia University College of Physicians & Surgeons in New York, New York.  I am honored and proud that Radha has received this award from such a prestigious alumnus.

The scholarship will allow Radha to travel with me to Brown University the week of May 20-24, where we will work with Dr. Elizabeth Brainerd and colleagues X-ray filming the rats walking, running, and landing at their XROMM C-arms facility.  Stay tuned and we’ll keep you posted on our time and research activities while at Brown.

Finally, I must acknowledge the help of our campus veterinarians, Drs. Ralph Werner and Mary Wilkes, for their efforts in helping me with the rats, as well as our animal caretaker, John Rokita, for his constant help and suggestions on rodent protocols and biology.

I feel truly grateful to have made such a jump to a new college and to already be surrounded by supportive faculty, eager students, and the chance to pursue 3-D kinematics research.

NERVES Talk Titles

Varadharajan, Radha and Bonnan, Matthew F. 2013. Exploring 3-D long bone kinematics in the White Rat (Rattus norvegicus) as a model for inferring forelimb posture in early mammals: Contribution of the scapula.>

Pinkney, Kadeisha and Bonnan, Matthew F. 2013. Exploring 3-D long bone kinematics in the White Rat (Rattus norvegicus) as a model for inferring forelimb posture in early mammals: Contribution of the humerus.

Drake, Evan and Bonnan, Matthew F. 2013. Exploring 3-D long bone kinematics in the White Rat (Rattus norvegicus) as a model for inferring forelimb posture in early mammals: Contribution of the radius and ulna.

NAMS RESEARCH SYMPOSIUM ABSTRACT

Forelimb movements in Rattus norvegicus (white rat) and their relationship to pronation: implications for early mammal forelimb posture

Varadharajan, Radha; Pinkney, Kadeisha; Drake, Evan; and Bonnan, Matthew F.

 

Rattus norvegicus (the white rat) is a therian mammal with a forelimb morphology similar to that of early non-cursorial mammals. Currently, early mammal limb posture is controversial, with reconstructions ranging from sprawling to parasagittal. With this current ambiguity, the study of forelimb shape and movements in R. norvegicus may provide a model to infer the locomotor patterns of earlier mammals.  Previous research, most notably by Jenkins (1971, 1974), indicates that the forelimb posture of rats does not follow simple, pendulum-like mechanics but rather a more complex, less-upright range of movement. For the first time, we will study the 3-D morphology and kinematics of the forelimb in R. norvegicus by utilizing three-dimensional moving X-ray animations generated through the XROMM (X-ray Reconstruction of Moving Morphology) technique.  Specifically, we will focus on the three-dimensional movements of the scapula, humerus, and antebrachium (radius and ulna), and their combined contribution to pronation (placing the hand palm-side down).  To this end, we will test three interdependent hypotheses on the contribution of each of these limb segments to pronation. For the scapula, we examined the serratus anterior, supraspinatous, infraspinatous, spinotrapezius, acromiotrapezius, and rhomboids major and minor. Data gathered on the rat scapula through literature and dissection lead to the hypothesis that this element contributes in a significant way to pronation. Specific features of the humerus distinguish the parasagittal from the sprawling stance in early fossil mammals: degree of torsion, condylar structures of the elbow joint, width of the intertubercular groove, and the relative sizes of the lesser and greater tubercles.  These features are associated with major locomotor muscles such as the pectoralis major and minor, deltoids, and pronator teres. We hypothesize that the humerus will contribute in a significant way to the pronation of the hand in the white rat. In humans, the radius can rotate about the ulna to pronate the hand because these elements are bowed, creating the space necessary to allow such movements. The pronator teres and pronator quadratus pull on the radius and rotate it about the ulna, whereas the supinator and biceps brachii act as antagonists to return the bones to a parallel position. Unlike humans, the radius and ulna of Rattus norvegicus fit tightly together like two spoons stacked together, with little, if any, space available in which the radius can rotate about the ulna.  Moreover, the pronator quadratus has not yet been described or identified in our rat dissections. Instead, the radius and ulna appear “fused” by the interosseous membrane and rendered incapable of supination. Rats have little need to supinate the forelimb because the forelimbs are primarily used in locomotion. It is therefore hypothesized that all pronation and supination occur in the humerus and scapula in rats because the radius and ulna are in such close proximity to each other that we infer they participate little, if any, to pronation.  After we capture the three-dimensional movements of these bones with XROMM, we will test our hypotheses and perhaps gain insight into the posture of early mammals.