Two papers recently have come out that I have been involved in, and am thankful to Marcela Randau (the primary author) for giving up time between finishing a PhD, postdoc hunting and preparing for lecturing to write this post. If anyone reads whatsinjohnsfreezer.com regularly, you will see an identical post there as Marcela, understandably, didn't have time to write two similar but different posts for us both as we clambered for her expertise in felid vertebral columns.
The cat’s back.
by Marcela Randau (m.randau@ucl.ac.uk)
It is often said that all cats are very similar in terms of
their skeletal morphology (“a cat is a cat is a cat”). But is this really the
case? It may be if only gross, qualitative anatomy is taken into consideration,
i.e., if you just eyeball the skeletons of tigers and lions you might find
yourself not knowing which one is which. But with huge advances in technology
that allows for extracting detailed shape information off a structure (e.g., a
skull) and for analysing this information (‘Geometric Morphometrics’), it has
become more and more possible to distinguish between relatively similar forms –
which may be from distinct species, separate sexes, or even just different
populations of the same taxon.
And it is reasonable to think that cat skeletons might be a
lot more different than what meets the eye, as for a lineage of apparently similarly
built animals, with not that much variation in diet (all cats are
hypercarnivores) there is substantial variation in body mass (over 300-fold
just in living species!) and in ecology across cat species. From the cursorial
cheetah to the arboreal clouded leopard, felids present a wide range of
locomotory adaptations. Yes, all cats can climb, but some do it better than
others: think lion versus margay (yes, they do descend trees head-first). As
hypercarnivores, all cats are meat specialists, but they also change with
regards to how big their prey is, with a general and
sometimes-not-so-black-and-white three-tier classification into small, mixed
and large prey specialists. The rule of thumb is ‘if you are lighter than
~20-25kg, hunt small stuff. If you are heavier than that, hunt BIG BIG things,
much bigger than yourself. And if you are in the middle ground, hunt some
small-ish things, some big-ish things, and things about your size. Well, -ish’
– their prey size preference has a lot to do with energetic constraints (have a look at Carbone et al. 1999; and Carbone et al. 2007, if you're
interested in this).
But the fun bit here is that form sometimes correlates quite strongly with
function, so we should be able to find differences in some of their bones that
carry this ecological signal.
Indeed, for a while now,
we have known that the shape of the skull and limbs of felids can tell us a lot
about how they move and how big their prey is (Meachen-Samuels and Van Valkenburgh 2009, 2009),
but a large proportion of their skeleton has been largely ignored: we don’t
know half as much about ecomorphology and evolution of the vertebral column. Well,
it was time we changed this a bit! As the PhD student in the Leverhulme-funded
‘Walking the cat back’ (or more informally, “Team Cat”) project, I’ve spend a
big chunk of my first two years travelling around the world (well, ok, mainly
to several locations in the USA) carrying a heavy pellet case containing my
working tool, a Microscribe, to collect 3-D landmarks (Fig. 1) across the presacral
vertebral column of several cat species. And some of first results are just
out! Check them out by reading our latest paper, “Regional differentiation of felid
vertebral column evolution: a study of 3D shape trajectories” in the Organisms
Diversity and Evolution journal (Randau, Cuff, et al. 2016).
 | |
| Fig. 1: Different vertebral morphologies and their respective three-dimensional landmarks. Vertebral images are from CT scans of Acinonyx jubatus (Cheetah, USNM 520539). |
Building from results based on our linear vertebral data
from the beginning of the year (Randau, Goswami, et al. 2016), the 3-D vertebral
coordinates carry a lot more information and we were able to describe how this complex
shape-function relationship takes place throughout the axial skeleton (in cats
at least) in much better detail than our prior study did. One of the
difficulties in studying serial structures such as the vertebral column is that
some clades present variation in vertebral count which makes it less
straightforward to compare individual vertebrae or regions across species.
However, mammals are relatively strongly constrained in vertebral count, and
Felidae (cats; living and known fossils) show no variation at all, having 27
presacral vertebrae. So adaptation of the axial skeleton in mammals has been
suggested to happen by modification of shape rather than changes in vertebral
number.
Using a variety of geometric morphometric analyses, under a
phylogenetically informative methodology, we have shown that there is clear
shape and functional regionalisation across the vertebral column, with
vertebrae forming clusters that share similar signal. Most interestingly, the
big picture of these results is a neck region which is either very conservative
in shape, or is under much stronger constraints preventing it from responding
to direct evolutionary pressures, contrasting with the ‘posteriormost’ post-diaphragmatic
tenth thoracic (T10) to last lumbar (L7) vertebral region, which show the
strongest ecological correlations.
We were able to analyse shape change through functional
vertebral regions, rather than individual vertebrae alone, by making a novel
application of a technique called the ‘Phenotypic Trajectory Analysis’, and
demonstrated that the direction of vertebral shape trajectories in the
morphospace changes considerably between both prey size and locomotory
ecomorphs in cats, but that the amount of change in each group was the same. It
was again in this T10-L7 region that ecological groups differed the most in
vertebral shape trajectories (Fig. 2)
 | |
| Figure 2: Phenotypic trajectory analysis (PTA) of vertebrae in the T10 – L7 region grouped by prey size (A) and locomotory (B) categories. |
So in the postcranial morphology of cats can be
distinguished, changing its anatomy in order to accommodate the different
lifestyles we see across species. But the distinct parts of this structure
respond to selection differently. The next step is figuring out how that might
happen and we are working on it.
While Team Cat continues to investigate other biomechanical
and evolutionary aspects of postcranial morphology in this interesting family,
we’ve been able to discuss some of these and other results in a recent outreach
event organised by the University College of London Grant Museum of Zoology and
The Royal Veterinary College. We called it “Wild Cats Uncovered: movement
evolves”. Check how it went here: (https://blogs.ucl.ac.uk/museums/2016/11/17/cheetah-post-mortem/)
and here (http://www.rvc.ac.uk/research/research-centres-and-facilities/structure-and-motion/news/wild-cats-uncovered),
with even more pics here (https://www.flickr.com/photos/144824896@N07/sets/72157676695634065/
).
References used here:
Carbone, C., Mace,
G. M., Roberts, S. C., and Macdonald, D. W. 1999. Energetic constaints on the
diet of terrestrial carnivores. Nature
402:286-288.
Carbone, C., Teacher, A., and Rowcliffe, J. M. 2007. The costs of
carnivory. PLoS biology 5 (2):e22.
Meachen-Samuels, J. and Van Valkenburgh, B. 2009. Craniodental
indicators of prey size preference in the Felidae. Biol J Linn Soc 96 (4):784-799.
———. 2009. Forelimb indicators of prey-size preference in the
Felidae. Journal of morphology 270
(6):729-744.
Randau, M., Cuff, A. R., Hutchinson, J. R., Pierce, S. E., and
Goswami, A. 2016. Regional differentiation of felid vertebral column evolution:
a study of 3D shape trajectories. Organisms
Diversity and Evolution Online First.
Randau, M., Goswami, A., Hutchinson, J. R., Cuff, A. R., and Pierce,
S. E. 2016. Cryptic complexity in felid vertebral evolution: shape
differentiation and allometry of the axial skeleton. Zoological Journal of the Linnean Society 178 (1):183-202.
