are particularly abundant and frequently encountered.
Their diet, a mixture of mature and young leaves as well
as flowers, leaf petioles, and some fruit, affords sufficient
protein (mostly from mature leaves) and minimizes
the amount of indigestible fiber and potentially toxic
defense compounds ingested. Howlers are more often
heard than seen, as they tend to remain well up in the
canopy of mature rain forest and, as mentioned earlier,
their howling carries long distances. Nonetheless, there
are numerous areas where howler monkeys are easy to
observe.
Howlers and other monkeys on Barro Colorado
Island, Panama, are apt to suffer a high natural
mortality rate. Katherine Milton, in a now- classic
study focused on howler monkeys, found that seasonal
changes in food availability, plus periodic unpredictable
shortages of high- quality foods, placed major stresses
on the population. Years of severe El Niño events,
such as 2015– 16, may result in significant increases in
mortality (recall discussion in chapter 2). In addition,
monkeys are commonly parasitized by botflies (see the
Appendix), though most monkeys appear to survive
their botfly wounds. Still, such parasitic afflictions can’t
be pleasant for the simians.
Marmosets and tamarins (plate 16- 20), are diminutive
monkeys, the gnomes of the rain forest, scurrying
through the branches, peeking out from behind leaves
often larger than they are. They resemble hyperactive
squirrels as they scatter about in the low tangles of
branches, constantly scanning in all directions. They
share the habit of leaping from one branch to another,
landing vertically. Their tails, while used for balance, are
non- prehensile. These small monkeys may be found in
interior forest but also favor gaps and disturbed areas,
where insects and small fruits are abundant. Many feed
in the lower story of forest as well as forest edge.
Marmosets comprise 22 species, all of which are
confined to South America. Among them are the
Black- tufted Marmoset (Callithrix penicillata; plate
16- 21), found in cerrado forests from central and
southeastern Amazonia to coastal Brazil; and the
tiny Pygmy Marmoset (Cebuella pygmaea), which
ranges throughout much of western Amazonia. The
Pygmy Marmoset is yellowish- brown, with thick fur
surrounding its tiny face. Marmosets of the genus
Callithrix have somewhat shaggy fur, ranging in color
from nearly white to grizzled gray and black, and
long tails. Their faces are small but accentuated by
prominent tufts of fur about the ears.Plate 16- 20. The Emperor Tamarin (Saguinus imperator) sports
a prominent moustache. The species occurs widely in western
Amazonia. Photo by Sean Williams.
Plate 16- 21. The Black Tufted- ear Marmoset (Callithrix
penicillata) is one of six closely related and similar species that
were until recently considered subspecies. It inhabits dry and
upland forests in much of east- central Brazil. Photo by Kevin
Zimmer.
328 chapter 16 from monkeys to tarantulas: endless eccentricities