Bipedal versatility in ruminants, part 2

In all of the ruminant families other than deer, the only photographic evidence of free-standing bipedally is in a few species of bovids: the gerenuk (Litocranius walleri), the dama gazelle (Nanger dama), and goats (Capra spp.).

The gerenuk is unlike any deer in being specialised for bipedal foraging. The dama gazelle has an unusually long neck and legs but is not as specialised as the gerenuk (https://www.dreamstime.com/stock-photo-dama-gazelle-standing-eating-leaves-image42716737).

The gazelles Ammodorcas clarkei and Gazella arabica (https://www.alamy.com/arabian-gazelle-gazella-gazella-feeding-on-acacia-oman-november-image263048634.html and https://www.gettyimages.com.au/detail/photo/gazelle-under-a-tree-royalty-free-image/1278742020?adppopup=true) are probably also capable of free-standing bipedally. All of these species have habitats drier than that of any deer.

The gerenuk is in some sense 'the exception that proves the rule'. Free-standing bipedally in deer is seasonal and occasional, and associated with generalised form and diet.

By contrast, in the gerenuk it is associated with an extreme specialisation: a diminutive muzzle picks out a staple diet of small leaves among the spines of acacias (https://www.dreamstime.com/royalty-free-stock-photos-impala-eating-image26157188). Any quarreling among females of the gerenuk is enacted on all fours, merely by making poking gestures with the head.

There is one spillover of free-standing bipedally into social/sexual behaviour in the gerenuk. Estes (1991, page 89) states: "Gerenuk copulation is the most spectacular example of the unique antilopine technique: the 2-m-tall male walking bolt upright with forelegs dangling behind the moving female and managing intromission without falling over backwards" (see https://www.zoochat.com/community/media/gerenuk-attempted-mating.81724/).

What emerges is that, overall, the gerenuk lacks the bipedal versatility seen in deer.

In the case of goats, the situation is ambivalent. Firstly, foraging bipedally has been photographed only in the case of the domestic species. Secondly, free-standing bipedally in wild goats has only been photographed in males, where it is part of masculine rivalry and a prelude to horn-butting. I infer that in most cases the posture is maintained only briefly before plunging down and forwards.

Capra ibex

https://www.dreamstime.com/alpine-ibex-capra-pasturing-mating-dueling-slovenian-alps-typical-horned-animal-high-mountains-image174579628

https://www.dreamstime.com/capricorn-alpine-ibex-capra-mountain-swiss-alps-goats-rock-top-hill-silhouette-isolated-white-blue-sky-wild-animal-image193739711

https://www.stockunlimited.com/image/three-rearing-alpine-ibexes_1892091.html

https://www.dreamstime.com/alpine-ibex-alpine-ibex-standing-rock-image109593037

https://www.dreamstime.com/stock-photo-alpine-ibex-capra-rocks-background-image54840479

https://www.dreamstime.com/nubian-ibex-capra-nubiana-males-fighting-image194417167

Capra walie

https://www.alamy.com/rare-walia-ibex-fighting-capra-walia-rarest-ibex-in-world-standing-on-hind-legs-prepare-for-fight-only-about-500-individuals-survived-in-simien-m-image381257500.html

Capra hircus

https://www.schochphoto.com/animals/h1B6D3ECC#h1b6d3ecc

https://www.alamy.com/stock-photo-nanny-goat-standing-on-hind-legs-while-eating-olive-branches-from-131010033.html