The ostrich (Struthio camelus) as a quasi-ungulate, part 4: avian advantages of mobility in arid climates

Which morphological differences permit the ostrich to compete with ecologically similar ungulates, particularly under arid climates?

Birds tend to be extremely mobile, exploiting resources too patchy in space and time for like-size, like-diet mammals. This hypothetically applies even to flightless birds, for subtle reasons.

The ostrich may exceed ungulates in certain aspects of mobility, partly by virtue of its concentration of mass near the centre of gravity (e.g. gizzard replaces Jaws, tongue, and teeth).

Greater mobility would help to explain the following:

  • why all like-size ungulates coexisting with the ostrich, whether dependent on drinking (Equus asinus) or not (Oryx), depend on grass rather than forbs; the foraging selectivity of the bird follows from aspects of mobility (including light, dexterous head); and
  • why - contrary to their respective body-heights - the ostrich apparently prefers forbs while gazelles prefer woody plants, since long legs and neck allow horizontal as well as vertical mobility.

Giraffa is not only the largest-bodied of extant ruminants, but also remarkably arid-adapted. The ostrich and Giraffa are similar in

  • diurnal foraging,
  • horizontal mobility, and
  • selectivity in foraging.

Morphologically, the ostrich resembles Giraffa in its

  • small head,
  • large eyeballs,
  • long neck, and
  • long feet.

Furthermore, I predict that, consistent with the relative mobility in the ostrich, its proportional allocation of body mass should differ from that of ungulates in being

  • greater for organs of vision, performance, and thermoregulation,
  • smaller for ingesta and urine,
  • smaller for the head (including the brain), and
  • smaller for offspring and maternal tissues.

In line with monogastric ungulates which share its limited digestion of food in the stomach, the ostrich has

  • a short small intestine, and
  • heavy caecum and large intestine,

The ostrich differs from most ungulates, and all monogastric ungulates, in having

  • a longer caecum and large intestine, and
  • additional partitioning inside these chambers.

This suggests greater assimilation efficiency of food and water than in hindgut-fermenter ungulates, particularly considering the finer grinding of food in the ostrich and the fluid nature of its caecum contents, relative to e.g. equids.

The observed ability of the ostrich to maintain itself on limited water fits adaptation to aridity, in which quantity of food is more limiting than its quality.

The ostrich seems to be capable of extraordinary water economy, judging from

  • the large size of its kidneys (loop of Henle; uric acid),
  • the presence of a bladder-like coprodaeum,
  • the mobility of feathers and wings, and
  • the likely large ratio of skin mass to surface area.

A small spleen possibly reflects high body temperatures, for immunological reasons.

The ostrich has a small brain. However, it produces far more offspring per breeding attempt, and particularly per lifetime, than is true for ungulates sharing the bird's predators.

In short, the ostrich has

  • the gastrointestinal tract (and presumably the flexibility of throughput rates) of an ungulate hindgut-fermenter, and
  • the gracile build, fine comminution of food, and thorough fecal water-retrieval of a ruminant concentrate-selector.

The ostrich has the potential for efficient digestion of starch, fat, and animal protein, and the rapid reproduction, of an equid, while the opposite of equids in its mobility (and precociality of offspring) and tolerance of thermal extremes on exposed plains.

In body composition and ecology, the ostrich has little in common with hippos (maybe contrast body composition of ostrich with that if pygmy hippo of similar body mass and dietary quality), except for foraging selectivity despite being inactive for half of each day.

The ostrich, in arid climates, fills the niches of

  • ruminant concentrate-selectors, and
  • pigs.

This is because the ostrich has

  • greater flexibility of food quality and quantity, and
  • greater mobility than either of these types of ungulates. (Relate back to bird themes...)

The quality of the diet of the ostrich resembles that of ruminant 'concentrate selectors', none of which penetrates flat, arid zones, except camel, which a is big enough to fall back on woody tissues, and b relaxes the inflexibility of the omasal filter of other cud-chewers.

The ostrich combines the digestive efficiency and water-retention of ruminants (heavy gut-walls and fine grinding by stones) with the flexibility of monogastric ungulates (no obligate limitation to throughput rates nor wasteful fermentation of animal protein), with a degree of selectivity greater than in either type of ungulate.

Long neck, small head, long limbs, combined with large body mass and efficient digestion, allow the ostrich to be a concentrate selector in extremely sparse vegetation, where quantity is even more limiting than quality.

This parallels

  • Giraffa in how coexistence with a wide variety of ungulates is achieved, and
  • Camelus (unique water economy and reduced bulk needs) in penetrating arid climates beyond most ungulate competitors.

However, the ostrich differs from both Giraffa and Camelus in potential fecundity, in keeping with the particularly ephemeral resource in arid climates.

So, mobility is a crucial element in the strategy of the ostrich, but mainly in terms of food selection, penetration of arid climates, and emancipation from parental ties.

Publicado el mayo 22, 2024 03:23 TARDE por milewski milewski

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A BASIC ASPECT OF ALLOMETRIC COMPARISONS OF BIRDS WITH MAMMALS

How birds differ from mammals depends on body mass. Small birds differ from small mammals differently from the ways in which large birds differ from like-size, large mammals.

The following organs scale more steeply, in allometric terms, in birds than in mammals (Calder, 1984, https://books.google.com.au/books/about/Size_Function_and_Life_History.html?id=-iBS6-2OO3wC&redir_esc=y): a) eyeballs, b) muscle mass, and c) (slightly) body surface area, kidneys, and gastrointestinal tract.

The following organs scale more flatly, in allometric terms, in birds than in mammals: a) brain, b) spleen, and c) (slightly) lungs, heart, skin, and feathers/pelage.

The intercept of the allometric regression is greater in birds than in mammals, in the case of a) lungs and heart, b) skin and feathers/pelage, and c) (possibly) brain and skeleton.

This suggests that only in small birds cf small mammals is organ size greater in the birds.

Publicado por milewski hace 26 días

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