by means of the united senses of all. Every sportsman knows, as Dr.
Jaeger remarks (7. Die Darwin sche Theorie, s. 101.), how difficult
it is to approach animals in a herd or troop. Wild horses and cattle do
not, I believe, make any danger-signal; but the attitude of any one of
them who first discovers an enemy, warns the others. Rabbits stamp
loudly on the ground with their hind-feet as a signal: sheep and
chamois do the same with their forefeet, uttering likewise a whistle.
Many birds, and some mammals, post sentinels, which in the case of
seals are said (8. Mr. R. Brown in Proc. Zoolog. Soc. 1868, p. 409.)
generally to be the females. The leader of a troop of monkeys acts as
the sentinel, and utters cries expressive both of danger and of safety.
(9. Brehm, Thierleben, B. i. 1864, s. 52, 79. For the case of the
monkeys extracting thorns from each other, see s. 54. With respect to
the Hamadryas turning over stones, the fact is given (s. 76), on the
evidence of Alvarez, whose observations Brehm thinks quite trustworthy.
For the cases of the old male baboons attacking the dogs, see s. 79;
and with respect to the eagle, s. 56.) Social animals perform many
little services for each other: horses nibble, and cows lick each
other, on any spot which itches: monkeys search each other for external
parasites; and Brehm states that after a troop of the Cercopithecus
griseo-viridis has rushed through a thorny brake, each monkey stretches
itself on a branch, and another monkey sitting by, conscientiously
examines its fur, and extracts every thorn or burr.
Animals also render more important services to one another: thus wolves
and some other beasts of prey hunt in packs, and aid one another in
attacking their victims. Pelicans fish in concert. The Hamadryas
baboons turn over stones to find insects, etc.; and when they come to a
large one, as many as can stand round, turn it over together and share
the booty. Social animals mutually defend each other. Bull bisons in N.
America, when there is danger, drive the cows and calves into the
middle of the herd, whilst they defend the outside. I shall also in a
future chapter give an account of two young wild bulls at Chillingham
attacking an old one in concert, and of two stallions together trying
to drive away a third stallion from a troop of mares. In Abyssinia,
Brehm encountered a great troop of baboons who were crossing a valley:
some had already ascended the opposite mountain, and some were still in
the valley; the latter were attacked by the dogs, but the old males
immediately hurried down from the rocks, and with mouths widely opened,
roared so fearfully, that the dogs quickly drew back. They were again
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