Felid field work (part 1)

As part of the ongoing felid research grant I am involved in for my postdoc, we have been studying felid scaling. The literature shows that cats maintain the same standing posture at all body sizes (whether small or big), but we wanted to find out what the felids are doing whilst under steady state locomotion (walking, trotting or galloping/running at a constant speed). John and colleagues had collected some data previously from domestic cat, ocelot, caracal, serval, leopard, and tigers from Amazing Animals, and a couple of locations in the USA. However, with the 40ish species of felids, this covers only a tiny diversity of those. As such, we managed to put together a BSc project at the Royal Veterinary College (RVC) for a student in the bioveterinary sciences degree programme to go collect some more data and analyse the "historical" dataset.

We first went to Colchester Zoo last year (2015) to see if we could work with their felid collection. Having done a recon. of the site we figured we could work easily with their tigers and cheetah. But we needed a platform to put the forceplate within allowing the cats to walk steadily (not up or down hill or up/down steps).

Force plate platform

Platform setup showing how I put together the platform, with and undercut on one side allowing the cables to exit so the cats didn't chew on them. Red is the forceplate

So I designed what was a very simple system for the Colchester enclosures. Basically layers of marine plywood stacked on top of each other and secured together. Fortunately they were the same thickness of our forceplate, so I was able design it so that we could sneak the cables out one side so the cats wouldn't chew on them. The first platform was built, and shipped by courier to Colchester. Somehow during the trip (all of 2hrs up the road), the forceplate platform turned into a box of books... or at least that is what was delivered to Colchester. Due to this, we had to rebuild the platform and try again so we got delayed by almost a month, but finally it got there.

Platform (II) with plate installed. Cables were run out the left (my right in picture) and buried under wood 

Data collection
So we had two days with the tigers and cheetah collecting data, although in truth the data collection only lasts an hour or two as we don't want to stress the cats.

Setting up the forceplate whilst being watched by the test subjects

One thing that we would come to quickly learn, is that scaredy-cat works as a description for most species of felids. The tigers were initially less than happy to go onto the plates, even with food bribes.

The tigers would scrape the food back off the platform/plate and eat it. I have to say I was impressed by their dexterity and picking up food with a paw to get to their mouths, but it didn't help the data collection. We got them moving across the plate, but not how we wanted:

Eventually, after much coaxing, we did get them to walk over the plate. We (the incredibly helpful keepers) even managed to get them to manage a trot from time to time.

Tiger with right front foot on the plate (marked by the grey duct tape in the middle)

Luckily Jack the cheetah was much easier. He has an area that he regularly paces when he is off exhibit between being fed and being let back out. As such we stuck out platform right there. We had the funny realisation though that after the first go, we needed to stick a log on the side nearest the fence otherwise Jack would continually walk down the side and not on the plate.

Jack being a beautiful test subject with a nice right front foot on the plate

Results
Whilst we do not have a full set of results to show you, I thought it would be useful to show what the output of forceplate is for those of you who don't know:

Force trace from one of the trials for a tiger

For walking felids what we get is a more or less m-shaped major peak in forces corresponding to the vertical force. The first peak of the m is caused by the foot impacting the ground and the second peak caused as the animal pushes off the ground. The front-back force trace tends to look like a sin-x graph (although the one above is messy). This is again as the foot impacts the ground it causes a deceleration force (negative), before transitioning to an acceleratory one (positive). The remaining line is the mediolateral forces (side to side). This force is generally minor but is linked to any number of joint rotational forces (e.g. pronation/supination). These forces and shapes of the traces all change depending on the speed of the animal (with the m becoming n shaped at running speeds). Combined with the video data we are able to calculate running speeds and posture (or at least joint angles) and see how they change across an animal at different speeds, as well as across different species/body sizes.

End of part 1
So two days of work, we had a lot of data and a very busy student. Unfortunately due to the delays, we were unable to do anymore in 2015. However, that is not the end, and part two has actually just finished now so will appear imminently on the blog. It includes a lot more pretty pictures of exotic cats, as well as more successes, and some terrible failures.