An integrated interpretation of the diet and foraging behaviour of the sable antelope (Hippotragus niger)

The trophic (https://www.merriam-webster.com/dictionary/trophic) ecology of the sable antelope (Hippotragus niger, https://www.inaturalist.org/taxa/42334-Hippotragus-niger) - a ruminant combining glamour and vulnerability to extermination - has been studied intensively over the last half-century.

However, as far as I know, no author has yet provided a satisfactory overall interpretation - which I attempt in this Post.

The best summary so far published of the diet and foraging behaviour of this hippotragin bovid is that of Estes (1991, pages 123-124):

The sable antelope is "closely identified with the well-watered Miombo Woodland Zone...The most distinctively different race is H. n. niger...found south of the Zambezi...this race inhabits the driest savanna, undertakes seasonal movements of up to 50 km, and forms the largest herds (up to 200 head and sometimes even 300 in Zimbabwe)...The sable favours a mosaic arrangement of woodland and grassland. The woods have to be open enough to support an understorey of grasses, which are utilised in the rainy season. Sable herds range the open grasslands in the dry season in search of green plants, including the forbs and foliage that make up c. 20% of their diet. Termite mounds, which support lusher growth than the surrounding leached, ancient soil, have many of the grasses and browse plants they like best. Dry-season movements depend on the availability of water and food. Forage quality is in turn closely dependent on annual, manmade fires that burn off the tall, dead grasses within a month or two after the rains end. Greenflush comes up along the drainage lines with their heavier clay soils while the droughtier woodland soils remain blackened and lifeless until woody plants put out new leaves in the miombo spring, a good month before the rains begin, attracting the sable back to the woods. Sables regularly visit salt licks, typically situated at the bases of termite mounds; and where soils are particularly poor, they may visit the sites of old kills to chew bones, presumably to acquire calcium and phosphorus."

Also:
In a recent series of three Posts, I have revisited information on the diet of the sable antelope (https://www.inaturalist.org/journal/milewski/87034-diet-of-the-sable-antelope-part-1-hippotragus-niger-roosevelti-with-special-mention-of-osteophagy#).

The following is my synthesis of the trophic ecology, based on a combination of integration and lateral thinking.

RELATIONSHIP TO PHYLOGENY

Hippotragin bovids are all adapted to trophic poverty. That is to say: in the context of ruminants in general, their habitats produce food in limited quality, or quantity, or a combination of these.

• In the case of Hippotragus, the vegetation is copious (and luxuriant in the case of two subspecies of the sable antelope), but generally unpalatable owing to its fibrousness.
• In the case of most spp. of Oryx, the vegetation is sparse albeit generally of good quality because the soils are fairly nutrient-rich.
• In the case of Addax and Oryx gazella, there is a combination of sparse vegetation (owing to aridity) and generally nutrient-poor soils.

The sable antelope is, among all hippotragins, the species for which adaptation to poverty is least apparent.

Please consider the following:

The habitat of the sable antelope, although overlapping with that of Hippotragus equinus (https://www.inaturalist.org/taxa/42332-Hippotragus-equinus), has generally the most copious (and thus fibrous) vegetation inhabited by any hippotragin species.

This is partly because H. n. variani lives under mean annual rainfall of as much as 1400 mm (https://en.wikipedia.org/wiki/Cangandala_National_Park).

However,

• the sable antelope is the species most dependent on ecotones (https://en.wikipedia.org/wiki/Ecotone), vegetational mosaics, and small-scale concentrations of nutrients, particularly in the mounds of termites, and
• the nominate subspecies (Hippotragus niger niger), in several regions, is particularly associated with basalt, a nutrient-rich substrate (https://scholar.sun.ac.za/items/abfaa061-2801-435c-a008-b7b9308fb0e4 and https://scholar.sun.ac.za/server/api/core/bitstreams/2bd8032f-9a81-4172-822e-c202962d557a/content).

RELATIONSHIP TO CLIMATE

The various spp. of hippotragins span a wide range of climates, from desertic (https://www.iucn.org/content/saharan-addax-antelope-faces-imminent-extinction-0) to mesic.

Of all hippotragins, it is the sable antelope that penetrates the rainiest climates. This is the only one of these spp. that

• forages, in places, at the edge of rainforest (https://www.istockphoto.com/photo/sable-antelope-with-red-billed-oxpecker-gm93473071-5072289?phrase=sable+antelope&searchscope=image%2Cfilm and https://travel2unlimited.com/kenya-shimba-hills-national-reserve-eastern-sable-antelopes/), and
• lives in proximity, in some locations, to forest-specialised bovids such as the forest buffalo (Syncerus nanus, https://www.inaturalist.org/observations?taxon_id=199289), the light-backed duiker (Cephalophus sylvicultor ruficrista, https://www.inaturalist.org/taxa/42294-Cephalophus-silvicultor), and the blue duiker (Philantomba monticola, https://www.inaturalist.org/taxa/74870-Philantomba-monticola).

The main relevance w.r.t. diet and foraging ecology is that plant matter tends, other factors being equal, to be more fibrous, the rainier the climate.

This may help to explain why

• in all its habitats, the sable antelope is particularly associated with ecotones, and
• H. n. variani had a naturally restricted distribution disjunct from that of H. n. niger, leaving an extensive area of miombo in central and eastern Angola unoccupied by the species (https://www.inaturalist.org/journal/milewski/54390-why-was-angola-the-empty-quarter-for-african-large-mammals#).

RELATIONSHIP TO THE GEOGRAPHIC CATENA (https://teara.govt.nz/en/diagram/12340/catena-soil-pattern#:~:text=A%20catena%20is%20the%20sequence,to%20accumulate%20near%20the%20bottom.)

The sable antelope tends to spend the wet season in woodland, and the dry season in vegetation in which trees and shrubs are relatively sparse. This is puzzling, based on the assumption that open vegetation is treeless mainly owing to a relative lack of water.

However, the puzzle is resolved by realising that dambos (https://en.wikipedia.org/wiki/Dambo) may be treeless owing to shortage of not water but rather certain nutrients. A lack of boron relative to other nutrient elements may militate against woody growth.

Because trees and tall shrubs demand more water than do grasses, the soils in dambos retain enough water in the dry season for some green growth of the grasses to continue in the dry season.

RELATIONSHIP TO DISTINCTION BETWEEN GRAZING AND BROWSING

Hippotragins all eat mainly grasses. The sable antelope conforms to this generalisation.

However, dicotyledonous plants, ranging from foliar-spinescent herbaceous Acanthaceae (e.g. Blepharis bainesii, https://www.inaturalist.org/taxa/581151-Blepharis-bainesii) through leguminous lianes (e.g. Dolichos and Mucuna) to shrubs/trees, contribute significantly to the diet. This contribution is likely to be disproportionately great in terms of the supply of crucial micronutrients, particularly copper and cobalt (https://www.researchgate.net/publication/229774801_Why_are_very_large_herbivores_absent_from_Australia_A_new_theory_of_micronutrients).

Does the sable antelope differ from Hippotragus equinus in the extent/degree to which it takes dicotyledonous plants (https://www.sciencedirect.com/science/article/abs/pii/S0140196308003625)?

At the southern limit of its distribution, the sable antelope penetrated

• well beyond the miombo biome, and
• slightly beyond the Tropic of Capricorn (https://www.flickr.com/photos/vanderschelden/5870726792 and https://www.alamy.com/stock-photo-tropic-of-capricorn-sign-kruger-national-park-south-africa-75140599.html?imageid=D3A6CBD6-2E6A-4074-A01C-91A55BB2C991&p=12455&pn=1&searchId=c86aec833cc409beabdbaff08c501c5d&searchtype=0 and https://www.dreamstime.com/tropic-capricorn-information-sign-kruger-national-park-south-africa-next-to-road-image181555220).

Here, it may have found a niche by accepting grasses rejected by the various other ungulates. Evidence for this comes from a reintroduced population, which treats as a staple a species of grass (Chrysopogon serrulatus, http://redlist.sanbi.org/species.php?species=1358-2) ignored/avoided by other species including the plains zebra (https://journals.co.za/doi/abs/10.10520/EJC117282).

RELATIONSHIP TO HEIGHT ABOVE GROUND

The sable antelope is unusual, among non-bovin grazers, in seldom attempting to reach ground level with its mouth.

By the same token, it seems not to have been recorded

• foraging at the maximum height of its neck, let alone adopting any bipedal posture in foraging, or
• using its horns to break down foliage, in the way recorded for certain tragelaphins.

There are several photos on the Web of the sable antelope grazing short, green grass. However, all are in zoos (https://stock.adobe.com/images/closeup-shot-of-a-hippotragus-niger-eating-grass/449894041?prev_url=detail and https://stock.adobe.com/images/sable-antelope-hippotragus-niger/86211163?prev_url=detail).

RELATIONSHIP TO GROWTH-FORM OF GRASSES

In the wild, the sable antelope, unlike various other ruminants, seems never to graze - let alone form - lawns (https://books.google.com.au/books?hl=en&lr=&id=g78qDgAAQBAJ&oi=fnd&pg=PA135&dq=Lawn-formation+by+ungulates&ots=hzPMmAYVC6&sig=VUtFXfIjKHxBnmEhfEYGSUN1Nlo#v=onepage&q=Lawn-formation%20by%20ungulates&f=false).

Instead, this species takes grass

• at considerable height above ground, and
• in relatively large bites.

One lawn-forming grass is fairly frequently eaten by the sable antelope, viz. the cosmopolitan species Cynodon dactylon (https://www.inaturalist.org/taxa/58371-Cynodon-dactylon). However, I assume that this occurs where the species has not been subject to lawn-formation, and has grown above 20 cm high.

The fact that the sable antelope - like all hippotragins - has scant relationship with lawns is consistent with its combination of

• sparse populations, in which the number of individuals per unit area would be insufficient to maintain lawns in the first place, and
• the relative lack of coexisting grazers, particularly in miombo vegetation.

The mouths of alcelaphins are adapted to grazing on lawns, in that the gape is so narrow that each bite is small and must be detached by the relatively weak pressure of the lower incisors against the toothless palatal pad. By contrast, in the sable antelope the gape is so wide that the animal can easily bite off substantial items with its sharp-edged premolars (http://www.vivo.colostate.edu/hbooks/pathphys/digestion/pregastric/cowpage.html and https://stock.adobe.com/images/africa-namibia-windhoek-okapuka-ranch-close-up-of-sable-antelope/284362123?prev_url=detail).

In the following, the premolars are the anterior-most cheek-teeth: https://www.shutterstock.com/image-photo/closeup-sable-antelope-skull-against-blurred-1403998679.

The following is a comparable view of the skull of Alcelaphus caama. The dentition of the upper jaw is similar to that of the sable antelope.

However, the alcelaphin is incapable of biting any item off by means of its premolars, because the tight skin at the junction of lips and cheek constrains both

• the angle to which the jaws can be opened, and
• the stretch of the slightly-opened mouth along the diastema (https://moodle.beverleyhigh.net/mod/resource/view.php?id=6107&forceview=1).

RELATIONSHIP TO SEXUAL SEGREGATION AND SOCIAL BEHAVIOUR

In general among gregarious ruminants, there is a correlation between sexual dimorphism in size (of the body and the horns/antlers) and sexual segregation in foraging (https://academic.oup.com/jmammal/article/85/6/1039/878781?login=false).

Hippotragins show minimal sexual dimorphism and minimal sexual segregation.

Among the hippotragins, the sable antelope is

• the most sexually dimorphic species,
• the only species in which adult females can readily be distinguished from adult males at some distance, and
• the most gregarious species, given the aforementioned seasonal congregations of H. n. niger.

However, any differences in diet and foraging ecology between the sexes remain subtle enough that they have yet to be demonstrated.

The following excerpt from Estes and Estes (1970) seems relevant:

"Along the Zambezi River above Victoria Falls [in June-July 1969] most sable had dull, 'staring' coats...But adult males, inexplicably, looked as glossy as ever...The Combretum/mopane scrub grassland was absolutely infested with larval ticks...in Matopos National Park, where tick birds have been virtually eliminated..., some sable were seriously infested...These were...the only other sable that appeared in generally poor condition...and had dull, staring coats (again excepting adult males)...it further resembled the Big Herd [mentioned above] in frequenting a home range of less than two square miles, and remaining on the same pasture day after day."

Estes and Estes did not offer any explanation for how the adult males, although visibly somewhat thin, managed to maintain the gloss of their pelage under stresses that affected females and juveniles more visibly.

Territoriality is less-developed in hippotragins than in alcelaphins, possibly because the environmental poverty of hippotragins means that home-ranges tend to be larger than can practically be patrolled by mature males. Furthermore, hippotragins tend not to form 'bachelor herds'.

I have the impression, based on the literature, that the sable antelope is the only strictly territorial species of hippotragin; and, even in this species, bachelor herds tend to be somewhat nebulous.

RELATIONSHIP TO FIRE

Fire, much of it anthropogenic, occurs routinely in the miombo biome, including the dambos (relatively treeless drainage lines subject to seasonal waterlogging).

In this loose sense, fire seems intrinsic to the foraging ecology of the sable antelope.

On one hand, the sable antelope is attracted to the post-fire green flush more than is the roan antelope.

On the other hand, the sable antelope is not as dependent on the post-fire green flush as much as is sympatric Alcelaphus lichtensteini (https://www.inaturalist.org/observations?taxon_id=132659) is.

Please note that

• the sable antelope, although presumably eating many of the same grasses as Lichtenstein's hartebeest, consumes grasses - plus sundry dicotyledonous plants - once they have grown >20 cm, and up to 1 m, high in the regenerative cycle associated with combustion; and
• the large mounds of Macrotermes (https://www.inaturalist.org/observations?place_id=7145&taxon_id=346639&view=species), with which H. n. variani is particularly associated, tend to be exempt from fires that routinely sweep the matrix among the mounds.

RELATIONSHIP TO TERMITES

The sable antelope occurs both patchily in the miombo biome, and somewhat beyond this biome.

Within the miombo biome, the species - more than H. equinus - may be at least indirectly dependent for its sustenance on the large mounds of termites, particularly fungus-culturing termites (Macrotermitinae).

The crucial trophic role of fungus-culturing termites is likely to be manifold, including the following.

Large mounds tend

• to contain accumulations of nutrients, including selenium (https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-7998.2008.00544.x), some of which are worth eating directly via geophagy (https://en.wikipedia.org/wiki/Geophagia),
• to support plants more nutrient-demanding - and therefore more nutritious as forage - than are those in the miombo matrix (https://www.researchgate.net/publication/259094145_Do_the_large_termite_mounds_of_Macrotermes_concentrate_micronutrients_in_addition_to_macronutrients_in_nutrient-poor_African_savannas and https://d1wqtxts1xzle7.cloudfront.net/43697719/Termite_mounds_as_islands_woody_plant_as20160313-1683-19koitq-libre.pdf?1457918886=&response-content-disposition=inline%3B+filename%3DTermite_mounds_as_islands_woody_plant_as.pdf&Expires=1702290882&Signature=BUc82zW5V-unlElO7T6TjYdIi6DOPJLpa7XU0VInMqZ5p5EkPHVhpFQipfZMZHQ2VNJqdYZeyPfjAOV2rJAEsxA6~KrhKrzQ5u2A9N1ZwhFAXpnpMKkvU0KtvxgfpuajfW0jYIk3BhP0utkVJNgDBqtuZ4OyjJesFCmeZz-cRmSpqvsdnETogKQNZ0fQFSZHaNiGhOvea8X02did-x1w0HiOkhpdW43YwgHdqecXGRi5Jzzvm4GZxB6~bKBkTMRFxT5RhFSRkyPxnvKonfdxZkeuViBSGVAir8zvMM9bid3jtGkhSpM9LhYgKIvak-bieao7SDMePlvXWt8WdOeK-Q__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA), and
• to be exempt from depletion of certain nutrients by volatilisation during the grass fires that sweep, every year or two, through the matrix.

RELATIONSHIP TO HORTICULTURE

In a sense, the whole of the miombo biome is anthropogenic, via a system of slash-and-burn cultivation (https://www.profor.info/sites/profor.info/files/Miombo_web_0.pdf).

This horticultural regime is

• intense enough to be ecologically profound, but
• scattered enough in space and time that an illusion of 'wilderness' is maintained.

The niche of the sable antelope can thus be seen as, in a sense, 'successional', rather than being extraneous to disturbance by humans.

However, the sable antelope has not been recorded foraging on domestic species of plants.

This perspective raises the following question:

Where the long-standing pattern of slash-and-burn cultivation has been discontinued in conservation areas, has the result - other factors being equal - been an increase or a decrease in the population densities of the sable antelope?

Publicado el 08 de diciembre de 2023 a las 10:46 PM por milewski