Monkeys with a bark far greater than their bite

Press, 4 May 1984, Page 15

Monkeys with a bark far greater than their bite

By

BARBARA MOFFET,

National

Geographic News Service

It’s the middle of the night on the plains of northern Venezuela; a full moon illuminates forest and field. Suddenly, a lion-like roar breaks the silence.

Dr Carolyn Crockett, asleep in a ranch house, is awakened but not alarmed. She recognises the familiar voice of the red howler monkey; a full moon often sets off its low, guttural roar. Red howlers also purr like kittens, cluck, grunt, wail, and squeal to communicate different messages. But they are at their loudest when they roar, usually in unison at dawn to announce location, or any time they feel threatened. Their throats — enlarged to enhance roaring — may be the most active part of the red howler monkeys, according to Dr Crockett. "These are really laid-back monkeys,” she says. “They move rather slowly and spend a great part of their day sleeping.” So subdued are they that the local people do not even consider them monkeys. “Mono,” the word for other monkey species in the area, is not used in reference to red howlers. Instead, they are called “flojio” — lazy. Lazy or not, the red howlers have kept Dr Crockett on the move for five years. The 38-year-old University of Washington professor has made a census of nearly 700 monkeys as part of her study of the population dynamics of red howler troops and the effects of two different habitats on them. Her work is partly supported by the National Geographic Society. Red-orange like an orangutan, the red hbwler (Alouatta seniculus) is one of six species of howler monkey and has the loudest, deepest voice. It lives in South America, north of the Amazon River.

Dr Crockett’s study subjects dwell on Hato Masaguaral, a private cattle ranch on the plains of Vanezuela’s state of Guarico. Giant anteaters, jaguars, ocelots, foxes, and other mammals, as well as 250 bird species, live there unmolested by people in an area protected for them.

The ranch makes an ideal field laboratory for comparing effects of different environments on a

species. The howlers live in two distinct habitats — a scrubby area known as the open woodland, and a denser section of trees, the gallery forest extending along the river.

The Smithsonian Institution began a study of the open woodland howlers in 1974, and now Dr Crockett has expanded it to the gallery forest, tracking 23 separate troops there. She trails monkeys on foot or sits for hours watching, in the style of the chimpanzee expert, Jane Goodall. Unlike chimps, red howler faces have little variety, so she has come to rely on scars and variations in pigment to tell the hundreds of individuals apart. Some monkeys also have been eartagged. Red howlers spend most of their time in trees, especially from May to November, when much of the ranch is flooded. When they do descend to travel, it is in a cautious, single-file style. About 60 per cent of the daytime is spent at rest, the remainder munching on figs, leaves, flowers, and other fruits. At night, they head as a troop for a “sleeping tree.” Generally, the troop’s dominant male

sleeps closer to the adult females, while subordinate males choose more far-flung branches.

The troop — an average of nine monkeys — may contain only one adult male, or two, or occasionally more. Fighting for dominance is rarely seen, the scientist says. Red howlers tend more towards barking than biting. One troop under study, however, was invaded by two males, providing a rare view of red howler aggression. All three males were injured in the battles for supremacy. Access to females is the main motivation for becoming top male; that male breeds with several females, perpetuating his genes.

Like most red howler activities, motherhood is a fairly relaxed experience. A mother does not pick up her infant; she waits for it to climb on her back, where it clings as she leaps from bough to bough. While adults nap, young red howlers frolic, springing among the trees, squealing and biting each other in fun. Some can be mischievous. One day, as Dr Crockett watched from below, two adolescent howlers were playfully wrest-

ling while hanging from branches by their prehensile tails, which can grasp like hands. A third monkey came up and unwound one of the tails, sending the monkey crashing to a branch below. It regained its grasp unharmed.

Some of the adults are not so winsome. The disappearance of several infants over the last few years has led the scientist to believe that infanticide by red howler males is widespread, at an average rate of one killing in each troop every five to six years. As is true of some other mammals, the males apparently kill dependent infants so the mother will be available for mating sooner. A few males, Dr Crockett believes, may have unwittingly killed their own offspring. In comparisons of 50 troops, she has found populations in the woodland to be denser, troop size larger, and troops to contain two adult males more often than the one found in gallery forest troops. Troop sizes in the two habitats have become increasingly more equal. Dr Crockett theorises that the forest study’s start coincided with a crash in population, possibly caused by the failure of a particular tree or fruit. This staple of red howler diet has recovered; now the forest troops are thriving.