The ostrich does not coexist with any monogastric or ruminant species sharing both its body size and its avoidance of a grass diet.
The diet of the ostrich is qualitatively and even quantitatively similar to those of ruminant concentrate-selectors or 'mixed feeder', particularly coexisting gazelles smaller than the ostrich.
The ostrich seems tolerant of silica-rich forbs, contributing to its ecological separation from ungulates.
The ostrich, in its most extreme habitat, coexists with
- a grazer larger-bodied than itself, viz. Oryx,
- a grazer/browser smaller-bodied than itself, viz. Gazella.
Both are adapted to reduced intakes (ruminants) and have advantages of foraging at night.
The grazer accepts up to 40% of the diet as browse, fruits, tubers, and forbs, largely for their water-content. The grazer/browser accepts up to 30% of the diet as the same, though smaller, items, and probably some insects too. Neither eats faeces, nor relies on forbs. Both avoid competition with the ostrich partly by foraging at times when atmospheric moisture condenses, and partly by resorting to landforms avoided by the ostrich.
The grazer is the arid-zone counterpart of semi-arid-adapted alcelaphins, which are more specialised grazers, partly because they can drink (and ultimately mesic hippltragin and large reduncins).
The grazer/browser is the arid-adapted counterpart of small-bodied reduncins and tragelaphins, because neither grass nor browse will support a specialist.
Where two spp. of gazelles coexist with the ostrich, the smaller-bodied one eats more grass (cannot reach much browse, and does not depend on forbs), and the larger-bodied eats more browse because it can reach it. They have about the same dietary quality, in terms of protein.
The more browsers extend into the arid zone, the ganglier they become (giraffes, gerenuk, dama gazelle). Nanger granti is the last outpost of a 'normal browser' towards dry country, after all the tragelaphins have expired.
Spatial separation and limited bulk demands/food quality are two sides of the same strategy. If a species can survive the shortage in the desert, then the quality is likely to be fair. If physical separation is hard, and coexistence is inevitable, then the animal must eat as little as possible in order to avoid competition and to exploit microspatial separation based on advantages in economy of movement. I.e. do what browsers do, but on the ground floor = go 'down and out'. If the animal can afford to pick and choose, then it can wait to find items others have found too awkward to eat.
The ostrich does not enhance mobility by reducing ingesta mass in body, but rather maximises this (compensating with e.g. reduction of toes) and draws indirect benefits from digestive power and hence reduced bulk demands, allowing it to move instead of having to eat so frequenty.
The ostrich differs from ungulates in the following:
- small head/lack of teeth/small brain
- gastric mill/hindgut fermentation/double caeca/cloaca
- feathers/uric acid/salt gland
- bipedality/air-sacs
- diurnality/high body temperature (1 degree Celsius or less higher than in ruminants)
- omnivory/carnivory/coprophagy
- large clutch/collective breeding/seasonal breeding.
Concentrate-selecting ungulates differ from roughage grazers in the following morphological features:
- small head and narrow muzzle
- smaller teeth and reduced dental occlusion
- long neck
- long legs
- small stomach (fermentation vat)
- large caecum
- short small intestine
