One of the most significant papers ever published in the annals of science appeared recently; it deals, for the first time ever, with one of the biggest scientific questions ever faced by the scientific community, and uses cutting-edge technology and awesome powers of deductive reasoning and logic to reach shocking, paradigm-shifting conclusions. I refer, of course, to Don Henderson and Darren Naish’s Journal of Theoretical Biology article ‘Predicting the buoyancy, equilibrium and potential swimming ability of giraffes by computational analysis’. I’m sure you’ve already read it and have joined the Facebook group, but for the few of you that aren’t familiar with this sure-to-win-awards marvel of the peer-review system, here’s the low-down…
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 and 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 somewhat higher than the rest of the body (1050 g/l vs 1000 g/l). The giraffe’s most notable feature – its neck – was set at 850 g/l, as is typical for hoofed mammals. As we’ll see, the high density of the distal parts of the limbs and low density of the neck and head seem to have implications for the floating and swimming behaviour of giraffes.
The lungs of giraffes are peculiar in size and shape (as you’ll know if you watched Inside Nature’s Giants), and exactly how large they are has long been a point of controversy: estimates of their volume in an ‘average’ giraffe (of 1.1 tons) have ranged from a low of 10 litres (Patterson et al. 1957) to a high of 47 litres (Robin et al. 1960). By scaling up from a horse to a big, male, 1.6 ton giraffe, an enormous projected lung volume of 141 litres was obtained (lung volume is about 6 litres in humans). Projections of this size have long been thought to be erroneous, however (Stahl 1967). A probable lung size about eight times that of a human is generally thought about right… in which case about 48 litres is correct for a 1.1 ton giraffe; this scales to 74 litres for a 1.6 ton giraffe. Obviously, there’s much uncertainty here and more data is needed.
Don also modelled a horse Equus caballus [shown here] as a control for the experiment.
Can a giraffe float?
Given that the horse model seems to accurately predict the real-life buoyancy of floating and swimming horses (Henderson & Naish 2010), we’re fairly confident that the giraffe model mimics the real thing. By rising the simulated water level around the giraffe model [as shown in the figure below], it was found that an adult giraffe would start to float at a water depth of about 2.8 m. It seems that the hindlimbs would leave the substrate before the forelimbs, raising the possibility that giraffes in deep water might be able to pole themselves along with their forelimbs alone.
But if the water becomes deeper still, what happens when the giraffe lifts off the substrate and begins to float? In a simulated floating posture, the hips are higher than the shoulders and the heavy forelimbs and short body mean that the foreparts are angled downwards relative to the horizontal. In turn, this means that the neck is rotated downwards and has to be held near-horizontally and just at, or under, the water surface: so far as we can tell, it doesn’t seem possible for giraffes to swim with the neck held erect out of the water. The horizontal neck posture then means that the head has to be held at a very awkward, upward-angled posture (assuming, of course, that the animal wants to keep its eyes and nostrils above the surface).
It’s well known that the giraffe head – particularly that of old males – is dense-boned and fairly heavy, but the cavities in the head and the long trachea and oesophagus make the head and neck rather less dense than the rest of the animal (850 g/l vs 1000 g/l or more). The neck and head account for about 10.8% of the animal’s total mass, but it seems that their low density actually prevents the animal’s anterior part from being even further down in the water (Henderson & Naish 2010). The overall density of the giraffe is higher than that of the horse, making the animal closer to being negatively buoyant and also making it sit lower in the water than the horse model.
In conclusion, it seems that giraffes can float: there’s no reason to assume that they might ‘sink like stones’, nor was there any indication from the model that it was particularly unstable and prone to capsizing. But the model’s posture in the water is low and hardly ideal, and looks downright uncomfortable. What might this mean for swimming ability?
Bad at swimming, or really bad at swimming?
The high mean density of the giraffe and the peculiar and difficult horizontal-necked posture the model adopts in water suggest that giraffes would not perform well in water. A large amount of drag and rotational inertia also afflict the model: in fact, it suffered from 13.5% more frictional drag than the horse model (Henderson & Naish 2010). We also suggest that the unusual gait practised by giraffes may well impair their swimming abilities. Giraffes on land use a synchronous movement of the neck and limbs where backwards and forwards movement of the neck is closely linked to the limb movements. But, with its neck constrained to a sub-horizontal posture where little or no backwards and forwards movement is possible, a swimming giraffe cannot adopt a gait like the one it uses on land.
Quadrupedal mammals – like the horse – typically swim with a trot like that used on land, so it seems reasonable to assume that swimming giraffes should try and swim with a ‘terrestrial’-style gait too. However, note that this argument isn’t completely conclusive, because the swimming gaits of some quadrupedal mammals are very different from their walking or running gaits. Nevertheless, I still think that our assumption is a reasonable one.
Positioned in the water in an uncomfortable pose, afflicted with a relatively high mean density, suffering from substantially high frictional drag, and unable to raise and lower its neck and hence unable to adopt a synchronous gait, we conclude that giraffes would be very poor swimmers, and that it might be assumed that they would avoid this activity if at all possible. Looked at another way, we conclude that giraffes can swim, but not at all well [image below from wikipedia].
Does this have any implications whatsoever for anything?
Making predictions about the floating and swimming abilities of giraffes is fun, but does it have any wider implications? For one thing, it perhaps help make the point that we’re increasingly able to test questions in biology using computational models and simulation, rather than experimentation on real animals. In the case of the question “Can giraffes swim?”, we just aren’t able to use real giraffes, so our approach is – at the moment – the only one we can use to test it.
If giraffes do perform poorly in water – so much so that they avoid crossing large bodies of water should they need do – has this had any impact on their biogeography? If you look at maps of giraffe distribution – and specifically at the ranges of the different giraffe taxa – you’ll see that, in places, rivers and lakes form what appear to be giraffe-proof barriers. The Zambezi and its tributaries might form a southern barrier to Angolan and Thornicroft’s giraffes (Spinage 1968), for example, the Niger and Benue might have prevented the southern spread of the Nigerian giraffe (Hassanin et al. 2007), while the Nile and Great Lakes might have prevented gene flow between Kordofan and Nigerian giraffes (Hassanin et al. 2007) [figure below - from Brown et al. (2007) - shows the approximate ranges and pelage patterns of the extant giraffe taxa].
Unfortunately, we don’t really know enough to be sure whether these distributional limits actually have anything to do with the ability or inability of giraffes to cross water. As we note (Henderson & Naish 2010), big rivers and lakes are formidable barriers to everything terrestrial, not just to clumsy swimmers. Furthermore, the biogeographical history of giraffes extends over many millions of years (and involves Eurasia, too), meaning that climatic changes, changes in drainage patterns, and the terrestrial movement of giraffe populations make it difficult to definitively link any aquatic barriers to giraffe distribution. Others have made similar comments (Cramer & Mazel 2007) [image below: awesome male Masai Giraffe G. camelopardalis tippelskirchi (or G. tippelskirchi), photographed at Lake Manyara National Park, Tanzania. From wikipedia].
Finally, this project is one of several I’ve been involved in that’s gotten a bit of media attention; this attention means that the paper is going to be the subject of more content-free discussion on websites than is usual for peer-reviewed research. Let me assure all concerned readers that this research did not involve the frittering away of pennies otherwise allotted to cancer research or the detection of Earth-killing asteroids. As I’ve said before (and, given that this article is appearing on ScienceBlogs, I’m preaching to the choir anyway), it isn’t widely realised how much work scientists do FOR FREE and FOR FUN, IN THEIR SPARE TIME. Rant over.
Oh, and having mentioned Inside Nature’s Giants, it is my duty to inform you that series 2 starts on June 8th. Episode 1 looks at the anatomy of Carcharodon carcharias, the white shark. This is incredible news; I’ll be writing about the series in due time.
For previous Tet Zoo articles on giraffes, see…
- Dammit, and I sooo loved the ‘necks for sex’ hypothesis
- Tet Zoo picture of the day # 6 (on giraffe skulls)
- Giraffe vs plane
- Giraffe-killing lions exploit paved roads
- Yet another bizarre and unfortunate giraffe death
- Sleep behaviour and sleep postures
- Death by lightning for giraffes, elephants, sheep and cows
- Inside Nature’s Giants part IV: the incredible anatomy of the giraffe
Refs – -
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Cramer, M. D. & Mazel, A. D., 2007. The past distribution of giraffe in KwaZulu-Natal. South African Journal of Wildlife Research 37, 197-201.
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