The ostrich (Struthio camelus) as a quasi-ungulate, part 6

The only ungulates rivalling the ostrich in length of leg and neck, and diminution of the head, are giraffes (Giraffa) and certain spp. of gazelles. However, all of these use this configuration to reach high rather than to walk far.

The ostrich resembles like-size ungulates in the size of the gastrointestinal tract and other organs, and presumably also digestive power and total metabolic costs.

Among ungulates, body size is associated with ecological separation among spp. of roughly similar shapes.

This is partly because

• large-bodied forms forage less selectively, and eat poorer foods, than is true for small-bodied forms, while potentially digesting their food more thoroughly, and
• there are associated differences in height of foraging (Owen-Smith 1985).

Large ungulates with long legs and necks, and small heads with fine muzzles, can potentially coexist with a wide variety of spp., by being more selective (and thus requiring lesser gastrointestinal volumes) than like-size forms, and foraging higher than small-bodied forms.

The ostrich, like e.g. giraffes, seems to maximise selectivity without compromising its mobility. It is potentially able to forage delicately over a wide vertical and horizontal range, thus rivalling ungulates a fraction of its size in the quality of food economically selected.

However, height effectively provides little separation between the ostrich and ungulates, particularly in the sparsest vegetation, where the few shrubs are apparently browsed by gazelles rather than the ostrich. This suggests that horizontal mobility is critical for a bird forced to compete with ungulates for the forbs on which it depends.

In well-vegetated areas, this presumably allows the ostrich to be extremely selective. In barren areas, it may allow the bird to find a sufficient quantity of any food available.

In addition, the ostrich potentially enjoys the flexibility of variable passage rate, since theoretically then food passage in monogastric spp. is independent of fermentation rate (in turn partly a function of food quality). At the same time, it may be able to digest its food more thoroughly than can a gazelle, which it matches in degree of comminution of food and exceeds in fermentation volume, retention time, and gastrointestinal surface area (while requiring scarcely more bulk of food).

If giraffes can be described as tree-specialists of open vegetation, then perhaps the ostrich will be found to be a forb-specialist of open vegetation.

The ostrich, like certain ruminants, is able to survive on a limited quantity of good quality.

The ostrich may use retarded rate of passage of finely-ground food to achieve particularly thorough digestion of limited quantities of food where necessary, rather than processing large quantities of food very superficially as equids potentially do.

This would suit the ostrich to extremely arid areas where the very sparse available vegetation is generally nutrient-rich.

The monogastric competitors of the ostrich are apparently limited by their dependence on

• drinking water,
• local, reliable concentrates (suids), and
• large quantities of food (equids).

Ruminant competitors are limited in ways that favour the ostrich particularly in arid climates. These limitations are in

• body size,
• mobility, and
• digestive power.

The ostrich and equids both coexist with ruminants, but forage with different ratios of quantity to quality. The ostrichnhas the grinding power of ruminants, the fermenting power of equids, and a particularly large area for absorption in the hindgut.

While the ruminant digestive system is specialised for a certain regime of intake rate and fermentation rate, the monogastric/hindgut fermentation system is relatively flexible to variation in quality and quantity of food. Caecal and colonic fermentation capitalise on microbial breakdown of cellulose without the costs of blocking the system (e.g. via an omasal filter) to variable throughput rate. Where grinding is performed by the stomach, it also avoids the costs to foraging time from which cud-chewers suffer - leaving the beak free to keep selecting.

Posted on May 24, 2024 01:54 AM by milewski