How the steenbok (Raphicerus campestris) manages to be uniquely diminutive among the ungulates of semi-desert dunes and plains

Ungulates (https://en.wikipedia.org/wiki/Ungulate) span an extreme range of body sizes.

They also occur in many different climates, with the most drought-adapted forms being able to forgo drinking for long periods.

However, the most diminutive species of hoofed mammals

• are too small to follow rain nomadically, and
• cannot survive without the cover of plants and rocks.

For these reasons, no ungulate smaller than 15 kilograms in adulthood has penetrated semi-desert on loose sand or flat terrain, where the exposure to predators as well as sun and wind is prohibitive.

Some 35 species of ruminants - including primitive forms such as chevrotains and musk deer and advanced forms such as neotragine bovids and brocket deer - can be categorised as having body mass less than 15 kilograms.

These have evolved independently in four families, viz.

• Bovidae,
• Cervidae,
• Tragulidae, and
• Moschidae.

This extremely small size warrants the scientific adoption of the previously informal word ‘bambis’.

Bambis occur, beyond Africa, in Eurasia (including China) and Central and South America. However, none penetrates semi-deserts in these areas.

This Post is about an exceptional species, viz. the steenbok (Raphicerus campestris, particularly the extremely large-eared subspecies steinhardti, https://www.inaturalist.org/observations?taxon_id=1417602), with body mass about 10 kilograms.

Although all bambis are vulnerable to aridity owing to their small size, the steenbok is globally unique. This is because some southern African populations of this bambi reside permanently in semi-desert dunes and plains.

Within Africa, bambis do occur under arid conditions in the Horn of Africa – where the steenbok is replaced by genera dependent on clumps of shrubs or broken terrain. However, all bambis are absent from the Maghreb and the Sahel.

The steenbok inhabits dunes in

• the southwest Kalahari,
• Namaqualand (and along the southwestern Cape coast generally), and
• perhaps also parts of the Namib (sensu lato).

This is significant because dikdiks (Madoqua spp. do not enter dunes. I refer to

• the Damara dikdik (Madoqua damarensis, https://www.inaturalist.org/taxa/1430934-Madoqua-damarensis) of Namibia, and
• the various species of dikdiks on the Horn of Africa (https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=42358).

None of the other desert and semi-desert areas of the world, supports any species of bambi or, as far as we know, has done so in the past. I refer to e.g. the Sonoran, Chihuahan, Arabian, Taklimakan, Tahr, Gobi, Monte, Atacama, Simpson, or Gibson deserts.

The steenbok is able to survive despite the extreme scarcity of cover. This is by virtue of the availability of forms of shelter and food which are not immediately apparent in the other semi-deserts.

Unlike the few other bambis that penetrate arid climates, the steenbok does not need broken terrain. I refer to

• the Cape klipspringer (Oreotragus oreotragus, https://www.inaturalist.org/observations?place_id=7140&subview=map&taxon_id=42384) and the Damara dikdik, which overlap its distribution at the edge of the Namib, and
• the beira and several species of dikdik, on the Horn of Africa.

In the driest habitats of the steenbok, plants and rocks are too small and scattered to provide sufficient cover for the sympatric dikdik and klipspringer.

Uniquely for an ungulate unable to excavate with its head (as done by warthogs, Phacochoerus spp.), the steenbok is known to evade predators by bolting down holes made by the aardvark (Orycteropus afer, https://www.inaturalist.org/taxa/47062-Orycteropus-afer).

The aardvark has body mass exceeding 50 kg, making it

• up to eight-fold more massive than the steenbok, and
• the largest termite- and ant-eating mammal on Earth.

The steenbok is also known to hide and suckle its infants below ground level.

Given its propensity for utilising burrows, the steenbok probably also seeks refuge underground from the sun.

In terms of food, small succulents and edible geophytes (i.e. tuberous forms of herbaceous plants) are surprisingly reliable in the Karoo, in the Kalahari, and at the edge of the Namib.

This distinguishes the Namib from not only the Atacama and Mojave Deserts and their edges but also the semi-deserts north of the equator in Africa and Arabia

The crucial plants, which supply the steenbok with enough water to compensate partly for the lack of shade,

• are inconspicuous, and
• store water during drought.

Yet, these plants lack

• spines, or
• concentrated sodium in their leaves, stems or tubers.

The ability to excavate geophytes adds a further layer of intrigue to the remarkable ability of this bambi to defy aridity: the small hooves and spindly legs of the steenbok are oddly adept at digging.

The steenbok spends only a tiny fraction of its time underground. However, it is in various ways more subterranean than any other bambi. (This includes the habit – odd for an ungulate - of burying its faeces.)

The steenbok has access to subterranean resources for two main reasons.

Firstly it has a remarkable ability to use gracile forelimbs as digging implements. Indeed, the steenbok has the most cursorial anatomy of all bambis, with exceptionally long metacarpals and metatarsals, yet manages to some extent to emulate fossorial animals with short robust limbs.

And secondly, it can use excavations provided by the aardvark, a termite- and ant-eating mammal remarkably common in southern African semi-deserts and lacking precise counterparts in even mesic climates on other continents.

Our new insight is that the extraordinary penetration by the steenbok of arid dunes and plains in southern Africa reflects forms of life below the soil surface, that fail to reach the edges of most deserts on Earth.

These forms are

• fossorial mammals - particularly the aardvark - far larger than the steenbok, and
• tubers, which remain turgid even in drought.

Posted on June 13, 2022 08:07 AM by milewski