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?

WALKING

Impalas walk by ambling, not cross-walking or semi cross-walking.

In this way, impalas differ from

• all deer (Cervidae), including those most similar in body size and proportions,
• reduncins (Bovidae: Reduncini), and
• 'plains game' (e.g. Antilocapridae, Alcelaphini, Hippotragini).

Large gazelles (e.g. Nanger) require further scrutiny in this context.

The walking gait of impalas supports the view of impalas that they are 'plains game gone cover-dependent', in a sense (discussed elsewhere).

TROTTING

Impalas are puzzlingly reluctant to trot.

This standard gait is

• taken for granted in most unguligrade and digitigrade mammals (e.g. https://www.gettyimages.com.au/detail/video/with-grants-gazelle-trotting-along-shoreline-with-stock-video-footage/691461859 and https://www.youtube.com/watch?v=V1mlwzMH4kM), and
• exaggerated, in some of them, into a form of stotting called 'style-trotting' (e.g. https://www.youtube.com/watch?v=-8R2Lp_S9Es).

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

• the kob (Kobus kob, https://www.youtube.com/watch?v=80d1K5FSilg), which ecologically replaces the impala north of the equator, and
• gazelles (e.g. https://www.reddit.com/r/Awwducational/comments/2mzdfk/stotting_is_a_behavior_amongst_gazelles_in_this/).

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.inaturalist.org/observations/205374514
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
https://www.dreamstime.com/stock-photo-nyala-ram-image1279390

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 12 de abril de 2021 por milewski