Locomotory and postural peculiarities of impalas (Aepyceros), part 1

Everyone knows that impalas (https://en.wikipedia.org/wiki/Impala) bound in a striking way (https://upload.wikimedia.org/wikipedia/commons/8/8f/Impala_AdeFrias.jpg and https://www.storytrender.com/114774/antelope-jumps-so-high-it-reaches-the-height-of-an-elephant/).

However, how many realise that this genus - looking like a normal antelope but with an ancient and distinctive origin - is more aberrant in other aspects of its locomotion and postures?

TROTTING

Impalas are puzzlingly reluctant to trot.

This standard gait is

One of the few times when impalas trot - and then for only a few steps - is when a courting male approaches a female over a short distance (see https://www.youtube.com/watch?v=deTFxRWrnKM).

The reluctance of impalas to trot is more odd than their bounding.

This is because an ecological counterpart in India, the blackbuck (Antilope cervicapra, https://en.wikipedia.org/wiki/Blackbuck), frequently trots (https://www.dreamstime.com/black-buck-adult-male-portrait-close-up-green-bucks-resident-species-gujarat-india-found-many-places-big-image184881075), in addition to bounding high and far (see https://www.gettyimages.com.au/detail/video/pronking-blackbuck-females-run-and-leap-on-indian-stock-video-footage/1B02605_0001 and https://www.reddit.com/r/NatureIsFuckingLit/comments/ax7t1s/jumping_skills_of_this_black_buck_is_on_point/).

KICK-STOTTING

What truly is distinctive of impalas is a gait that I call kick-stotting (https://www.youtube.com/watch?v=JOAGylDP18g and https://www.alamy.com/stock-photo-impalas-aepyceros-melampus-leaping-16555737.html).

Many types of antelopes and deer stot (e.g. https://www.dreamstime.com/black-buck-baby-jumping-mid-air-greenery-bucks-resident-species-gujarat-india-found-many-places-big-groups-image184881477 and https://www.birdsoutsidemywindow.org/2014/06/17/stotting/ and https://www.shutterstock.com/nb/video/clip-5775500-hartebeest-pronking-side-view) in response to the approach of predators. These include

However, the kick-stotting of impalas differs in form and has yet to be explained in function.

As they runs, impalas fling their hind legs high in unison - in some cases so high that they seem to risk somersaulting - while waving the tail high as well (see https://www.youtube.com/watch?v=xjb6hStBahg and https://www.youtube.com/watch?v=0PFq4l_v1iI).

Many naturalists have watched kick-stotting in social play, but few have seen it in serious situations. Since social play is rehearsal, there is presumably a real, life-or-death purpose to stotting in the impala as in other species.

I have noticed that another of the few times when impalas trot is in slowing down to a halt after a bout of playful kick-stotting (see https://www.youtube.com/watch?v=X6Gtjcl6sm4).

When charged by most types of predator, impalas do not stot. The limited evidence hints that kick-stotting in earnest may be reserved for the African hunting dog (Lycaon pictus, https://en.wikipedia.org/wiki/African_wild_dog).

M Burton, in an article titled 'Impala behaviour' (Black Lechwe 4(4), pp. 46-48) states: "Impala sometimes use a similar action (to kick-stotting), as when one is chased by a dog. This it soon outdistances, and then it will proceed for a short distance bouncing on stiff legs before resuming the normal method of progression...the conspicuous black and white markings on the rump...are more prominently displayed in moments of excitement".

SWIMMING

All bovids and deer can swim.

However, impalas are among the most inept in the water (see https://www.youtube.com/watch?v=onAE9aJi9qU and https://www.youtube.com/watch?v=DXQc_v5qjS4).

This was first noticed in the mission to rescue animals stranded on islands during the filling of Kariba Dam on the Zambezi River (https://www.facebook.com/watch/?v=204822168221559 and https://m.facebook.com/watch/?v=2910628739265279&_rdr).

Impalas often live along river banks, where they must risk being chased into the water by predators.

So, it seems odd that gazelles that spend their lives far from rivers can - if needs be - swim more confidently than do impalas (e.g. https://tenor.com/view/gazelle-swimming-escape-gazelles-croc-gif-9565007).

The maximum competence of impalas when immersed can be seen in https://www.youtube.com/watch?v=Yp4P3mxhomc and https://www.youtube.com/watch?v=WjEmeqrka88.

BIPEDALITY

Impalas seem unwilling to rise on their hind legs to forage, even in drought when the only remaining food is high on branches.

The blackbuck specialises more on herbaceous plants and is thus less likely than the impala to seek the foliage of shrubs for food. Yet females of the blackbuck sometimes rear up on their hind legs to flail at each other with their hooves, which has not been observed in impalas.

KNEELING

Impalas are reluctant to kneel, whether while drinking or while suckling.

There are many photos on the Web, showing that impalas tend to splay at water's edge, somewhat like giraffes (Giraffa spp.).
https://www.dreamstime.com/stock-photo-imala-ram-drinking-water-chobe-river-botswana-impala-image93972145
https://www.alamy.com/stock-photo-black-faced-impala-drinking-aepyceros-melampus-petersi-etosha-national-137111020.html?imageid=98E3DAB9-5BA5-4C22-823D-CD73479108ED&p=2080&pn=12&searchId=7dc11c78361bd5df1ff35974e5ef97a8&searchtype=0

The few photos showing kneeling in such situations tend to be where the water is >20 cm below ground level (https://www.dreamstime.com/royalty-free-stock-photo-impala-ram-down-his-knees-drinking-water-sunset-small-pool-image37157805).

Once the suckling juvenile reaches a certain size, it needs either to kneel or to splay its fore legs to reach the teats.

In impalas, the posture adopted is splaying (http://www.africaimagelibrary.com/media/29045c02-d8e0-480f-af5d-8c67d32dc7c4-impala-aepyceros-melampus-lake-mburo-national-park-uganda) - which is unremarkable because various bovids and cervids do the same.

However, this posture undermines the idea that impalas are related to alcelaphins (https://www.canstockphoto.com/red-hartebeest-and-suckling-calf-56774538.html), which kneel while suckling - in common with hippotragins and e.g. the nilgai (https://www.youtube.com/watch?v=IteLEYGUKAU).

Finally: even in the case of lying down to chew the cud (https://www.inaturalist.org/observations/97303706), impalas seem odd.

Most other antelopes and deer are easy enough to spot lying down by day.

However, adults of impalas tend to remain standing during its midday rest, reserving their recumbency for the secrecy of night - which they tend to spend in certain open places away from vegetation.

Perhaps this explains why there are few photos in iNaturalist of impalas in a lying position?

to be continued in https://www.inaturalist.org/journal/milewski/67632-locomotory-and-postural-peculiarities-of-impalas-part-2#...

Publicado el abril 12, 2021 02:03 TARDE por milewski milewski

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In contrast to impalas, the red deer (Cervus elaphus) not only quarrels bipedally, but has a 'hopping' gait while doing so: https://www.youtube.com/watch?v=G2WKCSgjVT0.

Note that these are males with the antlers in the growing condition, which would risk permanent damage by even the gentlest sparring with the head.

Publicado por milewski hace más de 2 años

Impalas give birth by alternately lying and standing: https://www.wildtomorrowfund.org/blog/impalabirth.

Publicado por milewski hace más de 2 años

According to M V Jarman (1979), Beihefte Z. Tierpsychol. 21: 1-92, territorial males of impalas, while herding females, occasionally use two unusual gaits, namely prancing/goose-stepping and bipedal walking. The ability of males to adopt bipedal postures in sexual behaviour makes the general lack of such postures in impalas all the more intriguing.

Publicado por milewski hace más de 2 años

Stotting in the mule deer (Odocoileus hemionus hemionus) consists mainly of a bouncing gait (https://www.youtube.com/watch?v=Mo4mVlP2Pa0). Stotting in the springbok (Antidorcas marsupialis) also consists of bouncing, but of a specialised kind (https://www.youtube.com/watch?v=0sTB0mvPYBs). Neither of these gaits is seen in the impala.

Publicado por milewski hace más de 2 años

Where photos of the kob are mislabelled as impalas, a giveaway can be that the juvenile is kneeling, not splaying, under the mother (https://www.flickr.com/photos/davidbygott/14999216039/).

Publicado por milewski hace más de 2 años
Publicado por milewski hace alrededor de 2 años

The following video footage of impalas https://www.youtube.com/watch?v=pqcoiHN-pZE is worth close viewing.
 
Kim Wolhuter’s commentary shows how little we know about this behaviour.
 
It makes no sense that this alarm-snorting is to ‘surprise’ the opponent. That would only work if done occasionally, but here we see it done routinely within a single antagonistic bout.
 
This leads me to notice a pattern in impalas: this genus seems to mix up alarm behaviour and intraspecific play seamlessly. We have seen this in the case of 'kick-stotting’, for which most photos and videos refer to play, but which also occurs in the deadliest of circumstances when the impala is chased by the African hunting dog.
 
These are my thoughts about this confusion.
 
Stotting is, in a way, a competition not as much between prey and predator as between prey and conspecific prey. I.e. it is essentially an intraspecific competition, even though directed at the predator.

Although the stotting proclaims to the predator ‘look how fit I am’, the way it works is by various members of the herd all proclaiming this simultaneously, i.e. competing with each other to look fittest, so that the predator can choose the weakest.
 
Furthermore, snorting behaviour is also, in a sense, a form of advertisement based on the handicap principle. The usual interpretation is that the impala a snorts to warn other members of its group. However, it is equally likely that it is telling the predator how fit it is, albeit more subtlely than by gross locomotion. The posture and flared nostrils and alert demeanour of the snorting individual would be noticed by the scanning predator.
 
Once one understands these basic relationships, it becomes easier to understand why impalas might incorporate alarm behaviours into rivalry of the sexual kind as well. What these males are saying to each other, when snorting repeatedly as if to a predator, is ‘this is how good I can look to a predator, how about you, can you do better?’
 
Kim Wolhuter seems to ‘shoehorn’ the ‘false-alarm’ behaviour, seen as rival males snort again and again at non-existent predators, into some kind of deception. However, I suspect that it is the opposite of deception: an honest demonstration of fitness, directed at each other.
 
This footage is also worth looking at carefully for the gaits used. Although these males never take more than a few steps forward or backward, it would be interesting to analyse the footfall sequences involved. When one male approaches the other, does he use a trot or a pace? And how does the reverse gear relate to forward gear when a male retreats backwards?

Publicado por milewski hace más de 1 año

A ordinary looking antelope, but is probably the most unique species in its family. It's been basically unchanged since the Pliocene.

Publicado por dmantack hace más de 1 año

@dmantack Many thanks for your comment.

Publicado por milewski hace más de 1 año

@beartracker @davidbygott @dmantack @grinnin @geichhorn

Going through my field notes from August 2000, made during a visit to Ithala Game Reserve in Zululand, I find the following entry:

"5 pm, just before dusk, as I drive back to my lodgings, I see a group of 15 females of the impala, with juveniles, grazing on the short green clover-lawn in the grounds of the lodgings. As my car approaches, the whole group runs off, leaping over the fence, which is only 0.75 m high. The fence-crossing is done in single file, so that one individual leaps after another. Although the barrier is low, each individual (juvenile as well as adult), leaps at least 1.2 m high - as if unable to leap lower. They thus clear this fence with feet, not inches, to spare. At the start of dusk, at 5.15 pm, I see them all grazing the equally lawned but less-green football field."

This implies another subtle locomotory peculiarity of the impala: its leaping can be applied to the clearing of obstacles such as fences, but is somewhat 'hardwired' for display at a certain minimum height, rather than being mainly a form of measured negotiation of obstacles.

Looking at this another way: Strepsiceros strepsiceros, which widely coexists with impalas, jumps over a fence by walking up to it and then, from a standing start, clearing it precisely, with a centimetre to spare (https://www.youtube.com/watch?v=HCKojJ4jvIw and https://www.youtube.com/watch?v=cGB-HmlIyV0). I doubt that impalas ever jump over a fence in this way. Instead, to the degree that they are able to clear fences, they do so while running, and their leaps are imprecise.

The greater kudu is capable of a running leap similar to that of impalas (https://www.youtube.com/watch?v=Ca-GKcMnTmU). However, the impalas seem incapable of a standing jump similar to that of the greater kudu.

Publicado por milewski hace más de 1 año

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