Howler monkey census on Barro Colorado Island reveals steady population

Long before dawn on March 19 and 20, Katie Milton and a group of stalwart volunteers, each armed with flashlight and compass, spread out into the jungle to find 35 predetermined listening stations marked on their maps of the island.

Just before sunrise, howler monkeys launch into a chorus of howls, roars and barks. From 5:15 am until 6:30 am, each volunteer recorded the time and direction of these vocalizations and estimated the distance to each group that they could hear from their stations. As they walked back to the lab in the early morning light they noted locations of any monkey groups they saw.

"It amazes me that volunteers want to get up at 4 am to walk through the dark forest alone for an hour or more -- but it is a thrill when they hear that first howl and know the dawn chorus is beginning," said Katie Milton.

Lines drawn on a map from each station toward the compass angle of the observed sounds intersect, marking the estimated location of each group. Based on this technique called triangulation Milton estimated that there are 60-70 groups on the island. By multiplying the number of groups by the average number of individuals in a group -- between 17 and 18 monkeys -- she concluded that there are roughly 1200 individuals in fairly evenly spaced groups.

Census volunteers always include internationally known ecologists: Smithsonian staff scientists and visiting scientists as well as current students and fellows. Barro Colorado's game wardens and two Panamanian national forest police manned some of the most inaccessible listening stations. Eight highly-motivated students from the University of Panama arrived on the Island on March 18th to participate in a 2-day workshop on determining primate densities that included volunteering for the count.

Milton organized the first island-wide group count in March, 1977. That census also revealed approximately 65 groups and thus, around 1200 monkeys.

"It doesn't look like howlers on Barro Colorado have suffered from any devastating diseases or other catastrophic problems for the last three decades," said Milton, "groups are larger at some times of year and smaller at others, which seems to reflect births and the specific causes of death that affect different ages of monkeys at different times of the year."