MachineMachine /stream

The background to this research

Everybody loves giraffes, and god knows they've been covered on Tet Zoo enough times (see the links below). And something that's been mentioned many times is the alleged inability of giraffes to swim, or even to float. There are several specific comments on this in the literature (e.g., Shortridge 1934, Goodwin 1954, MacClintock 1973, Wood 1982); Crandall (1964) mentioned a case where a captive giraffe escaped from a carrying crate, fell off the end of a jetty, and immediately sank in the Hudson River (incidentally, dead giraffes have apparently been in the Hudson more than once: it is alleged that the river has, on occasion, been used as a dump for the corpses of zoo animals).

Why should giraffes be unable to swim? I'm extremely sceptical of such claims, given that other animals sometimes said to be unable to swim - like giant tortoises, pigs, rhinos and camels - actually swim just fine or even very well. Nevertheless, giraffes are such a strange shape that one can just about believe that their swimming and/or floating behaviour differs from that of other quadrupedal mammals.

And while it's possible to find reports and even images of giraffes wading [photos above below © Jeff Arnold] (Jonathan Kingdon notes that giraffes "will wade quite deep rivers" (Kingdon 1989, p. 325)), reports that describe them swimming don't exist, so far as I can tell. A few years ago, the excellent BBC series Big Cat Diary featured a scene where a group of giraffes tried to cross the Mara while it was in flood. The giraffes got about half-way across before turning back [two of them are at the turning-back phase in the adjacent still, © BBC], and at one stage were in water that submerged them right up to the bases of their necks. I thought at the time that these giraffes must have been swimming: I was so confident that I even mentioned this on the Dinosaur Mailing List back in 2000 (I now think it more likely that those Kenyan giraffes still had their feet on the riverbed: you can see this for yourself, as the footage is online here*). Several writers and researchers who have googled 'swimming giraffes' during the course of research have discovered my comment, and I've since seen it paraphrased in a few places (such as here at Telegraph.co.uk and on Focus magazine's Q&A page).

* Thanks to Dartian for finding this video for me.

After seeing the Focus magazine piece (this was in 2008, I think), I came up with an idea. I knew that my august colleague Donald Henderson of the Royal Tyrrell Museum of Palaeontology - well known for his work on what you might call mathematic palaeontology - had produced a digital model of a giraffe for a previous project (Henderson 1999), and had been testing the buoyancy of some of his models in 'digital water' (Henderson 2003a, b, 2006). Could, I wonder, Don take his digital giraffe and drop it into digital water, and thereby test the hypothesis that giraffes cannot float, or cannot swim? This, my friends, is how papers are born...

Modelling a digital giraffe

To start with, it's clear that we wouldn't actually be able to fully test the swimming abilities of giraffes (given that we couldn't accurately replicate or estimate all the details of aquatic movement in a giraffe), but we could test the floating abilities, and hence indirectly test the potential swimming abilities.

Giraffes are complicated objects, and modelling them digitally is fraught with difficulty. A 3D giraffe model was generated via the digital slicing method described by Henderson (1999) (our model isn't definitely meant to depict any of the Giraffa taxa in particular; yes, we're well aware of the suggestion that the several 'subspecies' conventionally included in G. camelopardalis might warrant species status). Several things were done to make the model more like a real giraffe: ears, ossicones and the fleshy part of the tail were created, and a synthesized reticulated pattern was created because it made the model look so much better. Don has actually pioneered a very interesting technique for generating regularly spaced polygons and/or blotches - it has some really interesting implications that I can't discuss here (for more information, see Appendix A in Henderson & Naish (2010)).

Two details of a digital model are particularly important when testing simulated buoyancy: density and lung volume.

Giraffe limb bones are slightly thicker than those of other artiodactyls (van Schalwyk et al. 2004), so their density was set as being…