Howler Monkeys, Home Range, and Human Encroachment

By David Love, Amy Milin, Lauren Peppers, and Sean Perkins

General Information

• Alouatta palliata have a highly variable home range size often listed as between 10 and 60 hectares depending on the group composition, resources, location, etc. (Estrada 1982, 1984b Ibid.; Heltner, Turner, and Scott 1976; Glander 1978; Clarke 1983.)

• Their basal metabolic rate (BMR) is relatively low (Strier 2000.)

• They are mainly folivorous, also eating flowers and fruits when available. They must eat constantly because leaves are such a low energy food. (Strier 2000.)

• Alouatta palliata are a species that uses conflict avoidance, mainly because of their low BMR’s and highly folivorous diet. Physically fighting would be an unnecessary expenditure of much needed energy.Because of this, they have developed throat sacs, which help them release their famous ‘howl’. These long calls are used to warn other howler troops of their presence and to stay away. They also ‘howl’ when disturbed.

• Question: Does human encroachment, such as crop fields, affect the size and shape of the home range of a troop of Alouatta palliata on Isle de Ometepe during the wet season?

• Hypothesis: Clearing for crop fields on Isle de Ometepe cuts off potential travel routes for Alouatta palliata. This will result in an irregularly shaped home range that forces the monkeys to travel around fields to get to certain resources. Because howler monkeys have a relatively low BMR, the extra energy required to travel a longer distance to reach certain resources will force the monkeys to stick to a smaller home range than they would normally use, one that is more manageable in terms of energy expulsion.

Question and Hypothesis

• On the southeast side of the volcano Maderas on Isle de Ometepe

• Group consists of 3 males (possibly 4), 5 females, 4 juveniles, and 1 infant.

The Selected Troop

Group identified by a specific male, “Frederick”, who had a large head and a black spot in between his testicles on his scrotum.

Also, one sub adult male was much smaller than the other two males along with one female who had particularly light markings on her mantle.

The group was tracked each day over a 3 ½ week period and each tree they were seen in was flagged. They were tracked for a total of 144 hours.

Tape measures were used to measure distances of fields and between fields.

Block count and least polygon methods were used to determinesize of the range from GPS coordinates.

Methods

Map Legend

Map

Facts About Map

Gridded Map

After gridding the map, it was determined that this particular howler troop’s home range was 8.75 hectares (compared with the average 10-60 hectares).

Data

• The map shows the howler troop’s range in between fields, disturbed areas, and other howler groups

• The monkeys were tracked around three different sides of one field

• Other howler groups were found in nearby areas and long calls between the troops were observed

• The home range based on our study was 8.75 hectares• No traditional statistical analysis was used, so our

conclusions were based on inferences made from the data.

• A longer term project is needed for more definitive conclusions

Analysis

It was concluded that the hypothesis presented was supported but would require more research to fully realize the extent to which human encroachment is affecting the howler groups.

The presence of an irregularly shaped home range that appears to be at least partly the result of human encroachment leads the researchers to believe that, if left with open forest, the howlers would create a more efficiently constructed home range in which it was easier to travel between resources without expending unnecessary amounts of energy and thus allowing them to create a larger home range.

Conclusion

Works Cited

Strier, Karen B. Primate Behavioral Ecology. Boston, 2000; Allyn and Bacon.

Smuts, Barbara B., Cheney, Dorothy L., Seyfarth, Robert M., Wrangham, Richard W., and Struhsaker, Thomas T. Primate Societies. Chicago, 1987; The University of Chicago Press.

Paterson, James D. Primate Behavior: An Exercise Workbook. Prospect Heights, 1992; Waveland Press.