Locomotory and postural peculiarities of impalas (Aepyceros), part 2

...continued from https://www.inaturalist.org/posts/49366-locomotory-and-postural-peculiarities-of-impalas-part-1#

When it comes to locomotion, impalas (Aepyceros) truly are full of surprises.

I now realise that impalas, additionally, are unusual in using a gait called the pace (https://en.wikipedia.org/wiki/Horse_gait#Pace).

I have mentioned previously that the impalas are odd in how little they trot, given that trotting is perhaps the most normal of all running gaits for antelope-size quadrupeds (think of the domestic dog).

In the video clip https://www.youtube.com/watch?v=deTFxRWrnKM , what one sees is that when a male individual of the impala, in Kruger National Park, suddenly approaches the female individual he is courting, in the manner of an eager overture, he clearly uses the pace to cover the few strides needed to reach her.

The best way to observe this footage is to go into the settings on the clip and change the speed to 0.25 instead of normal. This does not slow down the footfalls enough to overcome the confusion of the blur, but it allows one to stop the run at intervals, freeze the footfall sequence, and work out exactly what the gait is.

A reminder: in the pace, a quadruped moves its limbs in a categorically different way from that seen in the trot. Instead of moving diagonal limbs simultaneously, it moves limbs on the same side simultaneously.

Few types of mammals pace. This is because this is an awkward gait, mainly associated with camels, but occurring also in saiga antelopes (Saiga) and the striped hyena (Hyaena hyaena). The use of the pace by the domestic horse (Equus caballus) occurs only with selective breeding and/or training. Pacing in camels and the striped hyena is best explained by the odd conformation in which the hind limbs are shorter than the fore. Saiga is a really odd case that nobody has ever explained. Its limbs are evenly proportioned, and it migrates long distances, seemingly without trotting.

Either impalas pace only in courtship behaviour, or what I previously took for trotting was actually pacing, but I could not tell the difference because of the poor quality of the footage. I should go back and check my facts w.r.t. trotting.

However, assume for a moment that I was right that impalas do trot in a strictly transitional way when milling around under the eye of predators.

Then, what this present discovery means is that impalas are even odder, in terms of its repertoire of gaits, than I thought. If this were a gazelle or any other African bovid with a similarly level back, then there is little doubt that the gait used to cover this short distance in the excitement of courtship would have simply been a trot.

As the male individual in question starts to approach the female individual, one can clearly see that he lifts right fore together with right hind: https://www.youtube.com/watch?v=deTFxRWrnKM. This is the opposite of what he would be doing if trotting: right fore and left hind. This pacing gait is how camels and the striped hyena run, not how gazelles and antelopes run.

As he proceeds, one can see that he has planted the hooves on the right-hand side and now he has just lifted left fore and left hind. Again, this synchrony between limbs on one side makes this the clearest and most unambivalent case of a pace, as opposed to a trot.

One can then see that, when he has almost reached the female, the two limbs on the left side have been planted, and the two limbs on the right side are being swung forward.

This footfall sequence is the same as that of the normal walking gait of impalas, namely the amble. However, what makes this a pace is that the animal is locomoting so rapidly that during part of the footfall-sequence all four feet have been off the ground.

In a walk, there is never a time when at least one foot is on the ground; in a run there is at least a moment in the sequence when the body is suspended in the air.

I conclude that the pace observed in the video below really is a distinctive running gait, as opposed to merely a speeded-up walk.

I realise that it is hard to catch the ardent male impala, below, with all four hooves off the ground as he approaches this female. This is because

the sequence is so brief, and
the run so slow that the hooves are not lifted high above the ground.

However, I assume that this qualifies as a run, i.e. a gait with a brief moment when all four limbs are off the ground. Supporting this is the footfall-sequence, which is more like-sided than in the walking gait of impalas. This male has not just speeded up his normal amble to produce a rapid version of the amble, as seen in elephants. What he has done is to make the limbs on one side (right or left) MORE synchronous than in his walking gait.

If this is pacing, however brief, then this is a new discovery in mammalogy. We can add a new gait to the locomotory repertoire of the impala. This seems significant, because I cannot recall any other ungulate, indigenous to Africa, that ever paces. (The dromedary, Camelus dromedarius, is probably not indigenous, although living in Africa today).

We now know that, in certain circumstances, impalas run in the same sense and using the same gait as elephants run.

When elephants make haste, the footfall-sequence that they use is similar to that shown in the frames below. This is a ‘sped-up walk’, where the walk is an amble. In both elephants and impalas, one could argue that no running gait is actually achieved, because the suspension phase is so extremely brief (and in elephants restricted to juveniles).

Just as an ambling elephant can speed up the same footfall-sequence to something far more hasty than a walk, so impalas do the same under a few circumstances, one of which is courtship.

So, we have something quite unexpected:
Despite the extreme peculiarity of elephants in terms of locomotion and postures, there is at least one point of evolutionary convergence between impalas and elephants in terms of hasty gaits.

Also see https://www.inaturalist.org/posts/56052-bipedal-versatility-in-ruminants-part-2#

Posted on June 26, 2022 10:29 PM by

milewski

Comments

PUZZLING OBSERVATION SUGGESTING EXTREME STRETCHABILITY OF GULAR FLAG IN IMPALA

The first photo below shows a greatly expanded patch of white at the crook-of-throat of an individual male impala. Below this is what seems to be his larynx, greatly lowered from its usual position near the crook-of-throat, as if the larynx has moved far during powerful vocalisation. On spotting this, my first thought was ‘wow, I’ve just discovered that the impala has a stretchy while ‘flag’ on his throat, which provides a visual emphasis to some sort of vocal signal’.

Surprised by this, I searched for further evidence on the web. I failed, as the remaining screenshots attest.

So what is going on? Is this stretchability restricted to males?

Please follow the captions below. Although this was not my search-image on this occasion, many of these photos also nicely illustrate the accentuation of the white ‘chin’ which I hypothesise to be part of a rumination display.

Aepyceros melampus mature males, with one individual showing a crook-of-throat white patch larger than I have ever seen before, as he ostensibly vocalises in male rivalry:
https://speakzeasy.files.wordpress.com/2014/12/impala-male-0030.jpg

The following shows the usual limited extent of the crook-of-throat white patch. If one looks closely for the small larynx, one will see that the white patch includes the small bump of the larynx, but only just. This photo is also unusual in clearly showing the ‘unfolding’ of the tail tassel, a feature that I think is unique to the impala:

http://l7.alamy.com/zooms/d5d34511c13e4789a00a8e37b2051f29/impala-grazing-from-behind-kenya-fb9bra.jpg

Again, the white patch extends only barely beyond the crook-of-throat, just far enough to include the small bump of the larynx:

https://images.robertharding.com/preview/RF/RH_RF/HORIZONTAL/764-297.jpg

Ditto.
http://4.bp.blogspot.com/-wngUt6BQBS0/T5iagxP5NcI/AAAAAAAAAOM/oTOlcgeOnlI/s1600/DSCF0859-Impala+bucks+fighting.JPG

There seems to be some individual variation in the visibility of the crook-of-throat white patch in full profile. In these individuals the white is clearly visible, but again it only barely extends far enough to include the small bump of the larynx.

https://images.robertharding.com/preview/RM/RH/HORIZONTAL/764-323.jpg

Likewise for these females:

http://pixdaus.com/files/items/pics/2/21/311221_f0f914273463f7af60303a992f78ce23_large.jpg

Here the bump of the larynx seems to have been drawn down the neck, perhaps in vocalisation, but the white is not even visible:

http://n7.alamy.com/zooms/d61aa468c74a4b9e8235f483394e1dc9/imapla-aepyceros-melampus-wanting-to-fight-kruger-national-park-mpumalanga-bwfxky.jpg

The following is about the most extended that the crook-of-throat white patch gets in the impala, in my previous experience:

https://thumb7.shutterstock.com/display_pic_with_logo/559429/291588239/stock-photo-impala-in-savanna-national-reserved-south-africa-kenya-291588239.jpg

Here the white patch does seem rather extended, and intriguingly the small bump of the larynx is also depressed. What is going on?

http://medias.photodeck.com/711c84da-1b5c-11e0-b3f4-f1806c0a4227/Mozambique-1007-0710_large.jpg

This is what I regard as ‘normal’ in the female:

https://www.istockphoto.com/photo/impala-doe-head-close-up-portrait-with-lovely-colours-gm890626718-246736011

This rutting male is vocalising but where is the bump of his larynx?

http://jamalamadikwe.com/wp-content/uploads/2013/07/rutting-1600x1200.jpg

In these sparring males, nothing special seems to be going on at the crook-of-throat:

http://3.bp.blogspot.com/-fJ-MU28Fiak/UWQYnbTUv1I/AAAAAAAACV4/6eMQvur9YFU/s1600/impala_01.jpg

Nor here:
http://1.bp.blogspot.com/_0bpOzvhZygs/S91csIX5FRI/AAAAAAAAAOk/zA0iu491IMQ/s1600/impala+dougie+cunningham.jpg

Nor here:
http://i.dailymail.co.uk/i/pix/2015/10/16/11/2D77ECFA00000578-0-image-a-41_1444990404415.jpg

In this individual female, the crook-of-throat white patch is so invisible in full profile that one could be forgiven for assuming that it is absent altogether. Are individuals really so variable?

http://3.bp.blogspot.com/-T5Ss55yYZs8/VUn2LaBD3aI/AAAAAAAADKw/6rO55tIM6xw/s1600/Mavela%2BLodge_657.JPG

Again, nothing strange here although the perspective is revealing:

http://www.sas-safaris.com/p7lsm_img_1/fullsize/ImpalaRams-fighting_fs.jpg

Posted by milewski almost 2 years ago

@tonyrebelo @oviscanadensis_connerties @tandala

Part of the postural specialisation of the African bush elephant is an ability to forage bipedally, in contrast to impalas.

See https://s-media-cache-ak0.pinimg.com/236x/77/00/f6/7700f64cc9e83ca46e2e98d6a9040053.jpg

Elephants are not so much scaled-up versions of smaller mammals, but rather locomotorily and posturally peculiar mammals (see https://www.inaturalist.org/posts/56094-the-basic-nature-of-elephants-part-1#) that happen as well to include the largest land animals today.

This helps to explain why modern elephants are the brainiest of all ‘ungulates’, and indeed the brainiest of all specialised herbivores on Earth. The rationale is that, because elephants rely on a proboscis for foraging and drinking instead of just having a longer neck and more flexible limbs, they need to have the neurological hardware for great dexterity.

There is, in a way, a parallel with impalas in this view of elephants. Impalas are not so much deer-like bovids as locomotorily peculiar artiodactyls that happen to be deer-like in their body proportions as well.

The first thing one notices about elephants is their sheer size, and the mind makes the mistake of therefore assuming that the peculiarities of posture and locomotion in elephants are predictable by their great size. I.e. it is too easy to assume that elephants are the natural result of allometric principles. Similarly, the first thing one notices about impalas is how deer-like they are in their even proportions. And so it is too easy to assume that their various locomotory differences from deer are owing to the somewhat longer-necked and longer-limbed configuration of impalas cf deer. Instead, I suggest, elephants would have their locomotory and postural versatility even if far smaller-bodied (possibly borne out by the extremely diminutive, extinct elephants of Mediterranean islands). And similarly impalas would still have their locomotory modes even if far less similar to deer in other ways too.

Another similar photo illustrates an aspect of the postural peculiarity of elephants. The individual shown below, being a mature male, has a probable body mass of 6 tonnes. Yet it can forage bipedally as proficiently as can any ungulate except the gerenuk and possibly the domestic goat. Considering that elephants cannot jump, even as juveniles, it is remarkable that these massive animals can stand so upright. Admittedly, the hind legs remain somewhat bent. However, photos of baboons in bipedal stance show that their hind legs are bent just as much as this. I suggest that it is not so much the case that elephants are able to adopt bipedal stances despite their great mass; rather it is that graviportal hind limbs and the associated pecularities of the pelvic girdle actually facilitate such a stance. I.e. that elephants are in a way specialised for occasional bipedality, and this specialisation is such that even extreme body masses do not preclude it.

At the same time, impalas are in a sense opposite to elephants: they never adopt bipedal stances while foraging (even though such an ability would seem likely to make the difference between survival and starvation during drought); and impalas can hardly swim whereas elephants swim extremely well.

Loxodonta africana mature male:
https://s-media-cache-ak0.pinimg.com/236x/67/44/03/67440326353e6b899b77cbe8f8a77b66.jpg

Posted by milewski almost 2 years ago

@beartracker

The common impala ambles.

By contrast, a coexisting bovid, namely the nyala (Nyala angasii), semi cross-walks.

Walking gaits form a continuum from cross-walking through semi cross-walking to ambling and 'hyperambling'. However, one can draw a qualitative, i.e. categorical, distinction between 'diagonal gaits' and 'parallel gaits', based on crucial moments in the footfall-sequence.

The following photos show clearly that, unlike the common impala, the nyala falls on the cross-walking side of the continuum.

At first glance, the following may seem identical to what is seen in the common impala. However, both left fore and hind right hooves are just off the ground, i.e. in the air. Because these are diagonal limbs, this means that this gait is a semi cross-walk: https://thumb1.shutterstock.com/display_pic_with_logo/1121969/475785688/stock-photo-side-view-of-a-female-nyala-tragelaphus-angasii-walking-475785688.jpg.

The same comment applies to this male: http://thumb9.shutterstock.com/display_pic_with_logo/1061684/215233366/stock-photo-a-male-nyala-tragelaphus-angasii-walking-against-a-blurred-natural-background-in-hluhluwe-game-215233366.jpg.

Now let us see whether the nyala ‘oversteps’ as the common impala does. The answer is no. As you can see, when right hind is swung forward, it has landed before right fore is lifted. If this were the common impala, the right fore would have lifted before the right hind landed, leading to a brief moment when both were in the air, swinging forward, together. What confirms the difference is that in the nyala the right hind has had to be planted to the side of the right fore to avoid a collision between the hooves. Because there is no moment, however brief, when both limbs on the right side are in the air together, this walking gait cannot be an amble: http://l7.alamy.com/zooms/1a14b6de9ccf431c8415192161961259/nyala-female-walking-affgfy.jpg.

Once again, the following clearly shows that right hind is lifted before left fore is planted. This does not occur in the common impala: https://thumbs.dreamstime.com/x/nyala-female-tragelaphus-angasii-walking-against-blurred-natural-setting-hluhluwe-game-reserve-south-africa-44075625.jpg.

The following clearly shows the lack of ‘overstepping’ in the nyala. Instead of the right fore already lifting on the approach of the right hind, as happens in the common impala, the right fore waits until the right hind is planted before it lifts. This makes this walking gait a semi cross-walk: https://pbs.twimg.com/media/CsUljL-XEAA0xfd.jpg.

The same crucial moment is shown in the next photo. Note that right hind is already planted when right fore is lifted. Because this risks collision of the two limbs, the right hind is planted to the side of the right fore, something unnecessary in the common impala because the hind ‘oversteps’ the fore and this displacement avoids any collision: https://www.istockphoto.com/photo/male-nyala-walking-gm514755667-47689818

Posted by milewski almost 2 years ago

The impala seems odd in the following ways, w.r.t. locomotion.

Firstly, it rarely stots, but when it does kick-stot (chased by African hunting dog, Lycaon pictus) it seems to maintain the stot during the height of the chase. This contrasts with other ruminants (probably including the fallow deer, Dana dama), which perform the stot mainly during the initial encounter, and then run in the most efficient way, for their lives, if their stot fails to work and they are actually selected by the cursorial predator.

Secondly, as mentioned in part 1 of this series of Posts, the impala is peculiarly poor at swimming. Most or all deer swim proficiently, including the fallow deer. However, the impala swims so poorly that it seems to get rapidly exhausted when, in sheer desperation, it takes to the water to escape the African hunting dog. Not only does the impala seem poor at keeping its head above water (its wonderful, almost avian lightness on land somehow turning to leadenness in the water), but it seems to use a peculiar swimming gait judging by the jerkiness of its head as it swims. (By contrast, the heads of swimming deer are steady, indicating the ‘doggy paddle’ normal for quadrupeds when swimming). Even a female (hornless) impala seems barely able to keep its head above water, whereas a male fallow deer with heavy antlers does so with no apparent effort.

The ineptitude of the impala in swimming is part of a pattern in which ungulates which one would expect to swim do so poorly in the case of Africa. Two other good examples of this are both species of African rhinos, and both species of warthogs.

The following video clips, although not ideal, are illustrative enough given how hard it is to capture footage of the points elaborated above.

For the impala, please see http://www.discovery.com/tv-shows/planet-earth/videos/chimpanzees/ and https://www.youtube.com/watch?v=jB1xIT0j52k and http://video.nationalgeographic.com/video/news/141111-wild-dogs-impala-hunt-vin and http://www.dailymotion.com/video/x171a2h_swimming-impala-brutally-killed-by-african-wild-dog-pack_animals.

Note that the impala, filmed from the air for the Attenborough series, actually kick-stots spectacularly as it flings itself in desperation into water, out of its depth. This is a rare glimpse of a peculiar behaviour.

For the fallow deer swimming, please see https://www.youtube.com/watch?v=LS-hKX3xS84.

Posted by milewski almost 2 years ago

The impala (Aepyceros melampus) is comparable with the fallow deer (Dama dama). The adult female in both cases has body mass ca 45 kg. Both species are well-proportioned, and have black-and-white vertical patterns on tail and rump. Both have a panelled pattern on the flanks, with a pale fawn band ventral to the dark fawn back and dorsal to the white belly. Just as the impala is the ‘common buck’ in Kruger Park, so the fallow deer is the ‘common deer’ in Europe. Indeed, when South Africans call impala and other antelopes ‘buck’, the etymological origin of this term is with the fallow deer as much as any other species in Europe (buck being a male deer).

Despite the obvious overall convergence between impala and fallow deer, there are many intriguing differences. Here I illustrate one such difference.

When alarmed, the fallow deer stots (illustrated below in both a video clip and several photos) in a bouncing, stiff-legged way. This is probably a display of individual fitness, conforming to the idea of a self-imposed handicap. The animal manages to maintain fair speed while executing this gait – i.e. it combines flight with advertisement, which seems like a sensible combination.

When the fallow deer stots, it hits the ground with all four feet at once, which means that the running gait is neither a gallop, nor a canter, nor a bound. Although it propels itself into the air with all four feet simultaneously, it still manages to propel itself forward at fair speed, something I have not noticed in any bovid. While stotting thus, the fallow deer uses a caudal flagfor emphasis, by wagging the tail up and down. The tail, thus wagged during stotting, gives the general impression of whiteness. The head is held stiffly erect throughout, and so are the ears. The fallow deer certainly uses this stotting gait when disturbed by humans.

I have never heard of the impala stotting in response to the appearance of humans. The only predator to which I have seen the impala reacting in the kick-stot (illustrated in the videos and photos below) is the African hunting dog (Lycaon pictus). The impala uses the same gait in social play. If adults of the fallow deer stot in play as well, I have yet to hear of that.

When the impala kick-stots, the gait used is quite different from that described above for the fallow deer.

Instead of landing on all four feet at once, the kick-stotting impala lands with a footfall-sequence comparable with the gallop. However, what is odd about this gait is that the hindquarters are thrown up extremely high relative to the forequarters, while keeping the neck (but not the ears, strangely enough) as vertical as possible. At the same time the tail, which has a foldable tassel unknown in any deer, is ‘unfurled’ to create a conspicuous white ‘false-tassel’. This display of the tail does not involve the rhythmic vertical wagging seen in the fallow deer, but it accentuates the display to at least the same degree by virtue of its conspicuous flag-like whiteness.

While the stot of the fallow deer serves its purpose by showing clearly that the animal has force and vigour in its run, the kick-stot of the impala – which can likewise be executed at speed – seems even more effortless in terms of springiness...

(please see next comment for continuation)

Posted by milewski almost 2 years ago

(continued from previous comment)

...The real nature of the display in the case of the impala seems to be more in a kind of ‘dexterity’ or, perhaps, balance. This is because the hindquarters are thrown up so high that the animal seems to risk somersaulting! The show seems to emphasise the coordination of the impala rather than its bounciness, bearing in mind that the impala is a slenderer animal than the fallow deer (the ratio of leg length to body mass is greater in impala than in fallow deer). Because the impala is relatively light and leggy, it should be easy for it to stot as the fallow deer does. Instead it ‘tests itself’ in a way unknown in any deer, by flipping the hindquarters into what almost seems like a ‘running handstand’ from which it manages to recover, and keep its stride, with every footfall-cycle.

While stotting in the fallow deer seems simple to interpret, kick-stotting in the impala remains poorly understood. This is partly because the behaviour is less predictable and not as clearly associated with fear of predation, and partly because it only seems to be used for certain cursorial predators (when not being used socially). Another difference seems to be that stotting is often seen as a joint display of mother and juvenile, whereas I have not noticed such synchrony in the case of the impala, which seems far more capricious in its deployment of the kick-stotting gait.

The following blog mentions this kind of stotting in the impala as it flees from the African hunting dog: http://www.edwardselfephotography.com/wild-dogs-hunt-impala.

The following video clips show the unique stotting gait of the impala: https://www.youtube.com/watch?v=ieRqIIWHi6U and http://www.arkive.org/impala/aepyceros-melampus/video-06.

The following video clip shows the stotting gait of the fallow deer.

Dama dama, adult female and juvenile both stotting, shown for a few seconds from about 1 minute 40 seconds onwards:
https://www.youtube.com/watch?v=A_h2Cjcm6FM

The following five photos all show the stotting gait of Dama dama:

http://media.gettyimages.com/photos/fallow-deer-mother-and-young-stotting-picture-id161816504?s=170667a