Why cross-walking gaits seem unrecognisably different in ruminants and like-size terrestrial monkeys

@matthewinabinett @jeremygilmore @tonyrebelo @variani18 @christiaan_viljoen @paradoxornithidae @beartracker @chewitt1 @gareth_bain

Please also see the following

• https://www.inaturalist.org/journal/milewski/85161-variation-in-walking-gaits-in-ungulates-why-some-hoofed-mammals-cross-walk-whereas-others-amble-part-1#
• https://www.inaturalist.org/journal/milewski/91548-walking-gaits-in-cervidae-deer-tend-to-cross-walk-as-opposed-to-the-ambling-typical-of-many-bovids-part-1#
• https://www.youtube.com/watch?v=nh9e1xoUJ4Q

INTRODUCTION

A cross-walk is a diagonal walking gait, in which left fore tends to move together with right hind, and right fore tends to move together with left hind.

Among ungulates, a 'perfect' example is Hippopotamus amphibius (https://www.pond5.com/stock-footage/item/56467947-hippo-walking-isolated-hippopotamus-video-includes-alpha-cha).

Cross-walking occurs in certain small (body mass less than 35 kilograms) ruminants. More particularly, I refer to cover-dependent, nocturnal, solitary species with inconspicuous colouration (https://www.inaturalist.org/journal/milewski/91630-walking-gaits-in-cervidae-deer-tend-to-cross-walk-as-opposed-to-the-ambling-typical-of-many-bovids-part-2-odocoileinae#).

However, a naturalist can observe these ruminants attentively without noticing that the gait is a cross-walk.

Furthermore, baboons (Papio spp.), macaques (Macaca spp.), and the proboscis monkey (Nasalis larvatus) - all of which habitually cross-walk on the ground - seem to have yet another different action (https://www.gettyimages.com.au/detail/photo/proboscis-monkey-walking-through-mangrove-royalty-free-image/527127928?phrase=proboscis+monkey&adppopup=true and https://www.gettyimages.com.au/detail/photo/proboscis-monkey-walking-through-mangrove-swamp-royalty-free-image/114995835?phrase=proboscis+monkey&adppopup=true).

However, here again, the gait is a cross-walk.

So, how can these disparate impressions be reconciled?

A NOTE ON TECHNICAL TERMS

A problem in studying gaits is confusion of terms.

What I call a cross-walk is alternatively called a 'walking trot' or 'diagonal-sequence walk' (https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.1330260203#:~:text=Diagonal%2Dsequence%20gaits%20have%20the,opposite%20side%20of%20the%20body).

'Diagonal-couplets gaits' (see the reference above) include both a running gait (trot) and a walking gait (which I call a cross-walk).

What I call an amble is alternatively called a 'walking pace' or 'lateral-sequence walk'. In my terminology, just as a trot is the running version of a cross-walk, so a pace is the running version of an amble.

I have invented the term 'semi cross-walk' because

• the 'normal' or 'default' walking gaits in unguligrade mammals are intermediate between 'diagonal-sequence' and 'lateral-sequence' gaits, and
• there has been no apt term for this in the literature - the detailed work of Innis Dagg (https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2907.1973.tb00179.x and https://originalwisdom.com/wp-content/uploads/bsk-pdf-manager/2019/03/Dagg_1973_Gaits-in-mammals.pdf and https://uwspace.uwaterloo.ca/handle/10012/14529) and Hildebrand (https://www.originalwisdom.com/wp-content/uploads/bsk-pdf-manager/2019/04/Hildebrand_1977_Analysis-of-Asymmetrical-Gaits.pdf and https://academic.oup.com/jmammal/article-abstract/58/2/131/892924?login=false) notwithstanding.

AIMS

The aim of this Post is to explain why the walking gaits seem so different in ruminants and monkeys that it took me decades to realise that both kinds of mammals cross-walk.

RESULTS AND DISCUSSION

Please compare

• Mazama https://www.inaturalist.org/observations/184029864 and https://www.inaturalist.org/observations/148448279 with
• Papio https://www.inaturalist.org/observations/197758249 and Nasalis https://www.naturepl.com/stock-photo-proboscis-monkey-nasalis-larvatus-male-walking-on-the-mudflats-of-a-image01352001.html

Which reader would have known that all of these photos illustrate cross-walking?

There are six main reasons why cross-walking in ruminants and monkeys seems to consist of categorically different gaits.

These are as follows.

In the ruminants,

• the hind foot is placed after the opposite fore foot is placed (https://www.inaturalist.org/observations/148448278); by contrast, in the monkeys, the hind foot is placed before the opposite fore foot is placed (https://www.youtube.com/watch?v=EZhO2dhlIFI and https://www.sciencephoto.com/media/1223177/view/chacma-baboon-walking);
• the hind foot superimposes on the print of the fore foot on the same side; by contrast, in the monkeys the hind foot lands to the side of the fore foot (https://depositphotos.com/photo/male-hamadryas-baboon-77413598.html);
• cross-walking tends to occur only in furtive/halting locomotion; by contrast, in the monkeys, cross-walking terrestrially tends to occur with a bold demeanour;
• the feet tend to be lifted high off the ground, as if to avoid making rustling sounds against low plants; by contrast, in the monkeys, the feet swing low;
• some spp. may tend to bob the head back and forth in synchrony with their limb-movements; by contrast, all the monkeys hold the head steady; and
• the figure intermittently 'freezes' vigilantly/suspiciously with one or two feet suspended off the ground (https://www.inaturalist.org/observations/145704914 and https://www.inaturalist.org/observations/143222891 and https://www.inaturalist.org/observations/173412545 ); by contrast, in the monkeys, there is no sudden cessation of movement in an attempt to blend into the environment in concealment from predators.

Part of the reason why ruminants and monkeys deviate, in opposite directions, from the synchronous placement of the diagonally-opposite feet may be

• their different body-proportions, and
• the fact that the former are unguligrade, whereas the latter are plantigrade (hind) and facultatively digitigrade (fore, https://mammals-locomotion.com/walking.html).

In the ruminants in question, the rump is higher than the withers (https://www.inaturalist.org/observations/83544190). This is part of a 'hunched' conformation in which - presumably to boost acceleration when predators pounce - the hind legs are longer and springier than the fore legs.

In the monkeys in question, the rump tends to be lower than the withers (https://es.123rf.com/photo_126109982_a-monkey-walking-in-the-street-on-the-sunny-day.html).

This is because

• there is an 'ape-like' conformation, with relatively long fore legs that facilitate climbing, and
• the hind feet are plantigrade (https://en.wikipedia.org/wiki/Plantigrade).

Both the ruminants and the monkeys deviate from Hippopotamus amphibius, in which fore and hind legs are similar in length (https://www.shutterstock.com/image-photo/rare-sighting-hippo-walking-out-water-60570064).

Therefore,

• the proportionately long hind leg of the ruminants takes a long time to swing fully from lifting to placement, thus landing well after the diagonally-opposite fore leg, whereas
• the proportionately short hind leg of the monkeys takes a short time to perform the analogous swing.

Finally, two relevant differences between the ruminants and the monkeys are that

• in adults of the latter, cross-walking is categorically the only terrestrial walking gait; by contrast, most/all of the ruminants that cross-walk are capable of grading into a semi cross-walk when walking rapidly; and
• when speeding up from walking to running, the former trot, whereas the latter immediately canter/gallop; indeed, no primate is known to trot.

Posted on April 2, 2024 02:27 AM by milewski