TheImprovement ofTropical and Subtropical Rangelands

222 IMPROVEMENT 0' TROPIOAL AND SUBTROPIOAL RANGBLANDS
depending upon complex balances among habit, tradition, the role
of agriculture in the tribe's economy, political power and prestige,
local variations in relief and climate, and other factors.
Camell are much more common in the arid north, where frequently
they are the only animal herded. Their frequency and
importance declines the nearer one comes to the more humid and
disease-ridden regions along the Senegal River. Cattle, being less
mobile, more demanding in their water requirements, and unable to
utilize dune formations for forage, are found in greatest numbers in
the south, where their ability to live on dry forage during the summer
months makes them especially valuable. Sheep and goats are found
everywhere, a function oftheir intermediate position between camels
and cattle, for they are more mobile and more omniverous in their
eating habits than cattle but less resistant to drought conditions than
are camels.
Thus, groups with a greater proportion of camels than other
animal species will cover longer distances in their annual migrations
than will those tribes whose herds are largely composed of cattle.
Also, tribes herding camels, sheep, goats, and cattle will, depending
upon the size and importance of the different herding components,
split their herds in an attempt to realize the optimal potential of
each species.
In addition to these regularities in the areal distribution of animals
and of the pastoral regimes that result from them, there is a
distinct regularity in the latitudinal zones occupied by the Mauritanian
nomads. During the hot, dry season, the herds and their masters
are always concentrated in the southern portion of the Sahel zone,
where numerous permanent wells supply water needs; in the rainy
summer season, the tribes move northward to exploit pastures made
green by the advancing tropical showers, and they usually remain
somewhere in the northern portion of their tribal territory during
the cool, dry season. us
The actual extent of northward penetration depends in part
upon the adequacy of the rains, while in especially dry years many
tribes, including the Rigaibat from the Saharan zone, can be found
in the extreme southern part of Mauritania, where grass and water
supplies are more assured. Also, most tribes have an association,
either of reciprocity or domination, with an oasis or group of oases
where they can obtain supplies of dates and grain, and this means
that most tribes have a gtJtftt& (date harvest) period when they are
back at the oasis for the harvest.l1