A day in the life of an equine anatomist: Ribs, rabbit holes & why I have a bone to pick with Sid the sloth…

I have a bone to pick with Sid the Sloth and his kin. When other mammals were evolving to a certain pattern, these lazy little freeloaders sidestepped the gatekeeper in their DNA and decided to misbehave. How is this relevant to horses I hear you ask? Well, with my tongue firmly in cheek… If God was showing off when he created the horse, he was drunk when sculpting the neck of the sloth… So what kind of hallucinogen was he on when he confused the two?! *

I must confess, I’m a total square and therefore, not familiar with the hallucinogenic kind, but I’ll admit that I am prone to finding myself down some epic Google rabbit holes. My favourite variety? Time travelling through the scientific literature, and filing interesting bits and pieces away in my memory palace and hard drive for future reference.

 

 

My academic rabbit hole adventures during early 2014 generally followed two common threads relating to the congenital malformation of the 6th and 7th cervical vertebrae many students of equine anatomy and biomechanics expert, Sharon May-Davis, would be familiar with (you can read more about it in the Congenitally Crooked Neck piece) – my Masters research project on the neurological implications of the malformation, and the ever-evolving paper Sharon invited me to write with her, addressing the associated muscular, skeletal and postural changes in affected horses.

For my project, I was hunting for any information that might elucidate the apparent dysfunction in the forelimbs of affected horses – starting with alterations to the vertebral canal that may result in compression of the spinal cord, and working my way down the leg to identify other potential explanations to consider, whether a significant change was detected in the CT scans of the vertebral canals I was examining or not.

 

 

For our research paper, I was tasked with locating studies describing changes to muscular or soft tissue attachments in association with skeletal malformations and potential functional implications, while later, a greater focus on proprioception started to emerge (enter my faithful Denoix text, Physical Therapy and Massage for the Horse, and one of my favourite quotes of all time!).

And so, somewhere between giraffes, human club feet, and malformation syndromes featuring all kinds of vertebral and associated defects in all kinds of species, I found myself at the Royal Veterinary College in Edinburgh in 1901, staring at something rather fascinating in Professor Bradley’s anatomy class.

Fascinating why? Because Bradley comments on the altered attachment of the scalenus muscle, in association with a rudimentary, or incompletely developed, first rib in a horse – exactly the kind of example I was looking for.

 

“The scaleni muscles, failing to find a sufficiently ample area for attachment on the rudimentary rib and its fibrous continuation, were largely inserted into the cephalic border of the 2nd rib.

For a similar reason the pectoral prolongation of the rectus abdominis was attached to the 2nd rib instead of the 1st…

The altered attachment of muscles in these cases gives an indication of how a muscle may be lengthened or shortened because of its having to seek a new and sufficiently firm foothold when its usual origin or insertion is no longer physiologically possible.”

~ Professor O. Charnock Bradley (1901)

 

(Feb 2016 update: I wasn’t kidding about the giraffes, by the way… Check out this new paper on how anatomical shifts in the neck of giraffes functionally change the cervicothoracic boundary to see what happens to longus colli in giraffes!)

While several of the horses in our study did have various rib malformations, we didn’t end up including the scalenus aspect in the longus colli paper as the word count got too high. However, it came in handy later in 2014, as I faced the task of uncovering my own horse’s secrets on the dissection table… Because when I got down to the cervicothoracic junction (where the neck meets the ribcage) and found something strange where his first rib should have been [if you’re keen, you can see it here – dissection warning!], I instantly knew what I was seeing, and that I needed to carefully check how the soft tissue was arranged on each side. And boy, was it a mess – because scalenus had also changed on the side with a fully formed first rib, with disastrous consequences after taking a wrong turn.

That’s a story for another day, but Professor Bradley’s specimen and I would meet again – in a paper about sloths, of all things! I was studying an animal growth and development subject at the time, and while researching an assignment on the potential developmental mechanisms behind the C6/C7 malformation, the title “Breaking Evolutionary and Pleiotropic Constraints” caught my eye.

While the spines of mammals come in all shapes and sizes, the cervical spine is always comprised of seven cervical vertebrae – or at least it should be – which brings me back to Sid, God and hallucinogens. Why? Because sloths defy God – or Darwin, rather.

 

The only mammals which have evolved different numbers of neck vertebrae without any apparent problems are sloths and manatees. Two-toed sloths (Choloepus) have 5-7 neck vertebrae while three-toed sloths (Bradypus) have 8 or 9.

Other skeletal abnormalities were found in sloths including fusion of vertebrae, defective production of cartilage, ossification of sternum and pelvic girdle, abnormal fibrous bands connected to rudimentary ribs, and asymmetric ribs, which can only be explained as side effects of homeotic alteration.

“The defects seen are most similar to mice with Hox mutations… We think that it is the slow lifestyle and low metabolic rate which has allowed evolution to alter the neck length of sloths without any of the side effects seen for other mammals. Their low metabolic rates protect them from cancer and their low activity rates protect them from thoracic outlet syndrome.” ~ Dr Frietson Galis

Sticking their necks out for evolution: Why sloths and manatees have unusually long (or short) necks

 

When I opened the aforementioned paper (which you can read in full here), I was floored by the similarities between Sid’s relatives and the horses presenting with the malformation of C6 and C7 – and the authors had also made the link between Professor Bradley’s mare and the rudimentary ribs identified in their own research. Another point of interest? The reference to thoracic outlet syndrome – Dr Janeen Kleine, Sharon’s co-author on this paper and one of my dissection dream team stars that weekend, wondered if thoracic outlet syndrome could explain some of my horse’s symptoms.

But that wasn’t all. The sloth researchers had a potential explanation for the phenomenon they were seeing in the vertebrae of sloths and manatees: confused identity, a.k.a. homeotic transformation.

 

“In sloths, manatees and natural mammalian mutants with an exceptional number of cervical vertebrae, homeotic changes are almost always incomplete and asymmetric.”

 

Let’s take a look at one of the horses… Are the words “incomplete and asymmetric” ringing any bells here?

 

 

The characteristic feature of the malformation we were seeing in the horses was that, in varying degrees, the affected vertebrae were taking on the appearance of their more cranial (closer to the head) neighbour. Was it possible that the horses’ vertebrae were not just mimicking their neighbours, but pulling a Bradypus move and morphing into them… messing with the gatekeeper’s golden rule of seven?

The answer to that question is yet to be determined, so I guess you’ll have to stay tuned!

 

* As the daughter of two church ministers, I’m being facetious… not with the intent to offend, but to avoid turning this into an alarmist piece – because it’s a genuine question: if sloths only get away with this because they are lazy, how the heck did we let this perpetuate in a species seemingly incompatible with such an evolutionary blip?! If we were laying bets, mine would be on this being the unfortunate consequence of humans confusing ‘fastest’ with ‘fittest’ when interfering with the evolution of a beautiful animal… The only way to find out more? Keep researching!

 

References & further reading:

Bradley, O. C. (1901). A case of rudimentary first thoracic rib in a horse. Journal of anatomy and physiology, 36(Pt 1), 54.

May-Davis, S. (2014). The occurrence of a congenital malformation in the sixth and seventh cervical vertebrae predominantly observed in Thoroughbred horses. Journal of Equine Veterinary Science, 34(11), 1313-1317.

May-Davis, S., & Walker, C. (2015). Variations and Implications of the Gross Morphology in the Longus colli Muscle in Thoroughbred and Thoroughbred Derivative Horses Presenting With a Congenital Malformation of the Sixth and Seventh Cervical Vertebrae. Journal of Equine Veterinary Science.

Sticking their necks out for evolution: Why sloths and manatees have unusually long (or short) necks (BIOMED CENTRAL) http://www.eurekalert.org/pub_releases/2011-05/bc-stn050411.php

Varela-Lasheras, I., Bakker, A. J., van der Mije, S. D., Metz, J., van Alphen, J., & Galis, F. (2011). Breaking evolutionary and pleiotropic constraints in mammals: On sloths, manatees and homeotic mutations. EvoDevo, 2(1), 11.