Variation in walking gaits in ungulates, part 1: why some hoofed mammals cross-walk, whereas others amble

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Ungulates are defined by their feet, which bear hooves. Given this pedal specialisation, one might expect all ungulates to walk with similar gaits.

But this is not the case.

In this Post, I classify, illustrate, and explain the walking gaits of ungulates, which vary considerably and in ways related to habitats and niches.

Let us begin by referring to one of the most relatable of non-ungulate mammals, similar in body size to fairly small ungulates, and sharing some of their habitats, viz. baboons (genus Papio).

Baboons provide a useful starting point because they are anthropoid primates, walking with a simple gait that the human observer can easily understand.

Baboons cross-walk, moving the legs approximately in diagonal pairs (https://www.youtube.com/watch?v=L9mSdjW0SRY and https://www.shutterstock.com/video/clip-1016965867-baboon-walking-on-road and https://www.storyblocks.com/video/stock/a-baboon-walks-through-the-grass-in-africa-rvzv5putliq9ns8jl and https://elements.envato.com/a-baboon-walking-EVQ5HP8).

Humans walk with a similar diagonal pattern of swinging the limbs, except that our bipedality means that our hands remain off the ground (https://www.youtube.com/watch?v=HypJY1XWkWY).

The gait of baboons, viz. the cross-walk, is a stable gait, because it spreads the weight over left and right sides.

The main disadvantage of the cross-walk is that the hind foot risks colliding with the corresponding fore foot, unless placed to its side. This means a slight sideways swing in the gait, detracting from forward propulsion.

In summary so far, the cross-walk is stable but somewhat inefficient. It is not a particularly smooth gait for any fairly long-legged mammal.

AN UNGULATE THAT CROSS-WALKS LIKE A BABOON

Now, returning from primates to ungulates:

Hippopotamidae are technically hoofed mammals, despite the fact that their feet are only slightly unguligrade.

Hippopotamus cross-walks (https://www.youtube.com/watch?v=jqIjyqtSSzo and https://www.youtube.com/watch?v=OLrzqwb8M6o and https://www.youtube.com/watch?v=A8oZ8WRJ3ZM and https://www.youtube.com/watch?v=5jgES8ccMPI).

Furthermore, this is more fully a cross-walk than in baboons, in which the fore foot swings forward noticeably before the opposite hind foot does.

In Hippopotamus, there is no risk of interference between hind and fore feet, because the legs are short relative to the long body.

The pygmy hippo (Hexaprotodon) walks surprisingly differently from its large relative. The synchrony between fore and diagonally opposite hind is lost, so that the stride pattern resembles 'one foot at a time' (thttps://www.youtube.com/watch?v=uYTFo0AGqDw and https://www.alamy.com/stock-photo-pygmy-hippopotamus-choeropsis-liberiensis-adult-walking-on-grass-111494278.html and https://www.alamy.com/stock-photo-pygmy-hippopotamus-choeropsis-liberiensis-hexaprotodon-liberiensis-17801717.html and scroll in https://abc7news.com/baby-pygmy-hippo-melbourne-zoo-cute-animals/795280/).

The difference within Hippopotamidae, between species differing ten-fold in body mass, seems to confirm that, in ungulates, the cross-walk is associated with a premium on maintaining stability.

For the rest of this Post, I will refer to the gait exemplified by the pygmy hippo as a 'semi cross-walk'.

FROM ONE EXTREME TO THE OTHER: HIPPOPOTAMUS TO GIRAFFE

From the least unguligrade of ungulates, let us now turn to some of the most unguligrade, viz. Giraffa.

Part of the reason for the evolution of hooves is increased locomotory efficiency and speed, at some expense of stability.

This is taken to extremes in Giraffa, in which

• the length of the legs means increased efficiency in walking, without particular regard to stability,
• the spread of weight between left and right is compromised, and
• balance is maintained partly by means of the cantilevering effect of the long, massive neck.

The result is an ambling gait, in which the legs are swung not in diagonal pairs but instead in like-sided pairs (https://www.youtube.com/watch?v=Vh2yveXTKaU and https://www.youtube.com/watch?v=DX-PbjeeB6o and https://www.youtube.com/watch?v=xMfn_7I76kI and https://www.youtube.com/watch?v=2lxa9LJTz9I).

To summarise so far:

Ungulates are like 'tetrapods on stilts', to varying degrees. No ungulate with legs proportionately longer than those of Hippopotamus uses a full cross-walk. This is presumably because such a gait would trade off necessary efficiency for superfluous stability, diminishing the advantage of being unguligrade in the first place.

Those ungulates - beyond Hippopotamidae - emphasising stability tend, presumably, to be associated with

• unstable terrain,
• clutter of vegetation/rocks,
• the intermittent, furtive locomotion associated with a tactic of hiding from predators, and
• a tendency suddenly to 'freeze' when alarmed, often with one foot suspended mid-stride (https://www.alamy.com/curious-roe-deer-cervus-elaphus-buck-in-summer-at-sunset-watching-with-one-leg-lifted-in-the-air-roebuck-with-vivid-warm-colors-with-positive-senti-image242003830.html?imageid=F691EBBC-74FD-4C6D-B9E7-24F6805C62EA&p=788497&pn=1&searchId=70ac798c647adbbdaf10a03667d93407&searchtype=0 and https://www.istockphoto.com/photo/rare-meneliks-bushbuck-standing-in-the-forest-gm1145425223-308304724?phrase=bushbuck and https://www.alamy.com/female-bushbuck-tragelaphus-scriptus-looking-at-camera-one-leg-lifted-image62385488.html?imageid=89B00857-A21B-4F64-B081-C49DED4EDFC4&p=191737&pn=1&searchId=70ac798c647adbbdaf10a03667d93407&searchtype=0).

Those ungulates emphasising locomotory efficiency tend instead to be associated with

• firm, smooth ground,
• open vegetation, and
• cursoriality, with a tactic of fleeing from predators with speed and endurance.

The cross-walk, semi cross-walk, and amble constitute a continuum, in which the walking gaits represent gradations rather than a dichotomy. In the intermediate zone, where is the dividing line between the categories of semi cross-walk and amble?

Please bear in mind that the term 'amble' has been used ambiguously in the case of the horse (Equus caballus), which has been selectively bred for extreme versatility in its running gaits (https://en.wikipedia.org/wiki/Ambling_gait).

There are two ways to distinguish the semi cross-walk from the amble, as follows:

• a criterion based on avoidance of collision of hind with fore hoof, and
• a criterion based on lifting of hind hoof relative to landing of opposite fore hoof

CRITERION BASED ON AVOIDANCE OF COLLISION OF HIND WITH FORE HOOF

When the hind hoof swings forward, the fore hoof on the same side lifts either

• after the hind hoof touches the ground (semi cross-walk), or
• before it touches the ground (amble, see https://www.youtube.com/watch?v=ZwKdHJdCMgg).

This subtle distinction is significant, for the following reason. The former means the placement of the hind hoof behind the track of the fore hoof, thus limiting the length of the stride and, correspondingly, the efficiency of locomotion, whereas the latter allows the hind hoof to be placed in or in front of the track of the fore hoof - thus not limiting the length of the stride as the animal goes from slow walking to rapid walking.

Semi cross-walk:

Alces https://www.deviantart.com/nini1965/art/A-Young-Bull-Moose-Walking-in-a-Field-917220578

Alces https://www.shutterstock.com/image-photo/profile-bull-moose-walking-onto-dirt-116525839

Taurotragus https://www.pexels.com/photo/common-eland-walking-on-grass-field-13681228/

Taurotragus https://www.shutterstock.com/image-photo/male-eland-walking-field-cattle-egret-2103184676

Strepsiceros https://www.wallpaperflare.com/deer-walking-on-grass-kudu-male-stripes-elegant-wildlife-wallpaper-ultkf

Strepsiceros https://www.vecteezy.com/photo/900750-kudu-antelopeb)

Amble:

Alcelaphus https://www.alamy.com/lelwel-hartebeest-alcelaphus-buselaphus-lelwel-from-sweetwaters-kenya-image473718879.html?imageid=F9D1E7C0-A169-412E-ADD5-AA3ABB0FEA6D&p=1910266&pn=1&searchId=2b347b49caec7dd90088988bdc87ef46&searchtype=0

Connochaetes https://www.alamy.com/stock-photo-blue-wildebeest-brindled-gnu-white-bearded-wildebeest-connochaetes-76132056.html?imageid=3B36039E-5BEF-4327-8FDE-5C6983B3F4C7&p=68577&pn=1&searchId=c09541d4cdf08595e4ddbcc42292c722&searchtype=0

Oryx https://www.alamy.com/stock-photo-gemsbok-oryx-gazella-gemsbuck-walking-on-arid-ground-etosha-national-128920240.html?imageid=A44A96FA-2F58-472D-B380-37C2206EF55D&p=304631&pn=1&searchId=1055078c46fa76c387809690b15dee96&searchtype=0

To illustrate this distinction even more clearly:

Please carefully follow the feet in these footages of walking in Camelus, which ambles:

https://www.shutterstock.com/video/clip-1067425-camel-walking---slow-motion

https://www.pond5.com/stock-footage/item/93961265-camel-walking-dromedary-one-humped-somali-arabian-camel-gree?

...and, in turn, please note the configuration in the following of specialists in rocky terrain, which is the converse of Camelus in that they use a relatively full cross-walk. I refer to klipspringers (Oreotragus):

Oreotragus https://www.alamy.com/male-klipspringer-oreotragus-oreotragus-a-dry-country-antelope-with-thick-coat-walking-on-tip-toe-on-rocky-lava-in-tsavo-west-np-kenyaafrica-image241664074.html

Oreotragus https://www.dreamstime.com/klipspringer-ewe-oreotragus-walking-rocks-image155290074

Oreotragus https://www.alamy.com/male-klipspringer-oreotragus-oreotragus-a-dry-country-antelope-with-thick-coat-walking-on-tip-toe-on-rocky-lava-in-tsavo-west-np-kenyaafrica-image241664075.html

CRITERION BASED ON LIFTING OF HIND HOOF RELATIVE TO LANDING OF OPPOSITE FORE HOOF

Semi cross-walk:

The following show the semi cross-walk of Alces. Please note that, by the time that the fore is about to be placed, the hind has already been lifted, for long enough that the lower leg has swung to the vertical.

Alces https://a57.foxnews.com/global.fncstatic.com/static/managed/img/fn2/video/876/493/edge_moose_052014.jpg?ve=1&tl=1https://www.istockphoto.com/photo/bull-moose-gm1055977806-282201912

Alces https://fineartamerica.com/featured/walking-bull-moose-randy-straka.html

Alces https://www.alamy.com/bull-moose-denali-national-park-preserve-alaska-united-states-of-america-a-unique-optimised-version-of-an-image-by-nps-ranger-jw-frank-credit-npsjacob-w-frank-image454466690.html?imageid=DC229BEB-798B-4807-AFC7-245D832C911F&p=1837101&pn=1&searchId=acbc6be48e9684687a1dabcb59018a60&searchtype=0

Alces https://pixels.com/featured/bull-moose-crossing-river-jack-bell.html?product=wood-print

The following, correspondingly, show the semi cross-walk of the largest-bodied of all antelopes.

Taurotragus https://www.kimballstock.com/popuppreview.asp?db=a&image=AFW+36+MH0004+01&itemw=4&itemf=0001&itemstep=1&itemx=1

Taurotragus https://www.istockphoto.com/photo/young-eland-walking-on-the-skyline-gm670061476-122489693

Taurotragus https://www.alamy.com/common-or-southern-eland-taurotragus-oryx-de-hoop-nature-reserve-western-cape-south-africa-image555774369.html?imageid=B8DBE331-4F0A-4C2E-9AA5-68B247944D26&p=824482&pn=1&searchId=bebe53939638886546a1f3e44729a7e9&searchtype=0

Amble:

The following, of Alcelaphus, shows the difference between the semi cross-walk of tragelaphins (including Taurotragus) and the amble of alcelaphins (including Alcelaphus). The hind hoof is lifted only once the fore hoof has touched the ground.

Alcelaphus https://www.istockphoto.com/photo/wild-red-hartebeest-during-the-summer-in-beautiful-pilanesberg-national-park-south-gm1358864733-432370852?phrase=hartebeest

The following shows that Camelus, although similar to Alces in the proportional length of the legs, is similar in walking gait to alcelaphins, and different from Alces.

https://www.gettyimages.com.au/detail/photo/side-view-of-camel-walking-royalty-free-image/1146381538

DISCUSSION

The horse has been both boon and bane, in the appreciation by naturalists of how ungulates walk.

Boon, because its gaits have been thoroughly studied and illustrated.

Bane, because

• in the horse the term 'amble' has been transferred to specialised running gaits (e.g. the tolt), while
• in both the horse and wild equids (e.g. see Estes, page 237), the normal walking gait has slipped to being nameless.

The reason for this namelessness is that the normal walking gait of the horse and other equids (https://www.youtube.com/watch?v=t4zdTn02PWQ) is exactly intermediate between a semi cross-walk and an amble.

(This applies also to e.g. the African savanna buffalo, see comment below.)

In the case of Hippopotamus and Giraffa, the walking gaits are - unlike the case of equids - obvious. The former cross-walks, while the latter ambles, despite both of these ungulates being megaherbivores.

This difference is unsurprising, given that

• in Giraffa, a cross-walk would produce excessive interference between fore and hind feet, and
• in Hippopotamus, the body would rapidly tip to the side if both legs on that side were simultaneously lifted.

However, the relative weakness of the legs in Hippopotamus also plays a part. Rrhinoceroses resemble Hippopotamus in that the hind foot does not reach where the fore foot left off (https://www.youtube.com/watch?v=2u8c0TLPOHo), yet these megaherbivores walk similarly to equids. This is presumably because the muscularity of the legs aids in maintaining balance.

Given that both Giraffa and Camelus amble, it is easy to assume that all relatively long-legged ungulates amble.

However, this is not so. Alces has legs proportionately longer than in any other cervid, yet it cross-walks, much like the pygmy hippopotamus.

The adaptive explanation is that Alces relies on locomoting rapidly and efficiently over fallen logs and through deep snow.

As Valerius Geist has pointed out, "when fleeing from enemies they can clear obstacles about...1 m...high without jumping, thanks to their large body and long legs. A wolf or bear on the trail of a moose, however, has to jump to get over such an obstacle...Smaller predators following the moose therefore become fatigued much sooner than the moose. On uneven ground a running moose can pass a horse easily".

The above quote refers to the trotting gait of Alces, which is basically a speeded-up version of the cross-walk, in which all four feet are simultaneously off the ground for part of the cycle. However, the efficiency applies also to cross-walking - with feet lifted high - through cluttered terrain, while Alces forages and commutes.

Another instructive comparison is between the pygmy hippo and klipspringers, which are so extremely specialised for locomotion on rocks that they walk not only on their hooves but on the tips of their hooves (https://animals.sandiegozoo.org/animals/klipspringer and https://upload.wikimedia.org/wikipedia/commons/9/9f/Klipspringer_%28Oreotragus_oreotragus%29_feet.jpg).

Despite the pygmy hippo being hardly unguligrade and klipspringers being 'hyperunguligrade', both semi cross-walk.

This is presumably because - for different reasons - both emphasise stability over efficiency of locomotion. Klipspringers risk stumbling and injuring themselves on the rocks, while the pygmy hippo risks slipping on mud and tripping on ground-level clutter in rainforest.

A basic difference between baboons (https://www.inaturalist.org/observations/159515288) and ungulates is:

when the former walks, two feet are off the ground at the same time; when the latter walks, there is only one foot off the ground at any moment, for most of the stride-cycle.

The instability inherent in having two feet off the ground at the same time is compensated for, in the case of cross-walkers such as baboons, by the decidedly diagonal pattern involved.

to be continued in https://www.inaturalist.org/journal/milewski/85716-variation-in-walking-gaits-in-ungulates-why-some-hoofed-mammals-cross-walk-whereas-others-amble-part-2-the-moose-walks-like-the-pygmy-hippo-while-warthogs-walk-like-giraffes#...

Publicado el 30 de septiembre de 2023 03:21 por milewski