How do the niches differ between the ostrich (Struthio camelus) and a coexisting ungulate, Grant's gazelle (Nanger granti)? part 1
INTRODUCTION
Ecological separation between the wild ostrich (Struthio camelus) and gazelles is unclear.
The ostrich is sympatric with gazelles, with a similar diet and fermentative digestion of fibre.
There is no obvious separation by foraging height, since the ostrich forages mainly near ground-level, and several gazelles reach to the height of the bird by bipedal standing.
The ostrich and gazelles are both tolerant of dry heat, with no obvious difference in their penetration of arid zones.
The ostrich is diurnal. Gazelles can potentially forage at night. However, this would not per se prevent competition for the same plants.
However, the ostrich exceeds gazelles in stride-length and body size. Furthermore, the bird breeds more synchronously than at least those gazelle spp. coexisting with it in mesic areas.
Offspring of the ostrich are, from the egg stage onwards, left to progressively fewer adult custodians as they develop towards adulthood.
Hence the movements of most adults are possibly not constrained by care of offspring as in gazelles - particularly in view of the potential masculine territoriality of gazelles.
Based on its body size alone, the ostrich should have an advantage of greater daily mobility and greater potential for nomadic movements than those of gazelles. The bird potentially walks faster and more efficiently, with a greater ability to forgo shade and to commute long distances to drink.
The economy of leg-length and bipedality would enhance this. However, such locomotory specialisation would also potentially bring costs of instability relative to quadrupeds.
Hence, the ostrich would be expected to avoid unstable substrates, such as deep, loose sand and rocky slopes.
Together, these considerations would suggest that the ostrich is better-suited than gazelles for exploiting the ephemeral appearance of food in remote areas on flat, firm ground.
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COMPARISON OF LOCOMOTORY ECONOMY, BASED ON STRIDE-LENGTHS, BETWEEN STRUTHIO AND STREPSICEROS
assuming similar speed
Struthio camelus
Body mass 130 kg (referring to source of data on height of back)
Basal metabolic rate 72% of that of S. strepsiceros
because = 38.5 kg at exponent 0.75
Locomotory costs 26.1 at exponent 0.67
28.9
100.9
Strepsiceros strepsiceros
Body mass 200 kg
= 53.2 kg at exponent 0.75
Locomotory costs 34.8 at exponent 0.67
38.5
138.5
Chris Magin told me that Gazella dorcas is a 'static' species, as opposed to being nomadic.
A Tuareg proverb invokes this gazelle as 'home-loving'; there is no corresponding saying for the ostrich.
This can be taken as anecdotal evidence that G. dorcas is not particularly mobile in the desert.
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