Artwork by Jane Kim

What do howler monkeys make so much noise about? It takes a lot of effort and a large territory for a howler monkey- one of the largest of the New World primates- to feed itself, so they announce their claims on the land to the other monkeys around. The hyoid bone in the throat of howler monkeys is enlarged, which allows them to make a sound that can be heard up to 3 miles away! To make sure that other monkeys aren’t invading their territory, male howler monkeys check in with choruses of howling every morning before foraging, and every evening before they prepare to go to sleep. The greater the number of howler monkeys in the area, the more they howl.

Howler monkeys love to eat fruit, which is the base of their diet when it’s available. However, seasonal shifts in fruit availability mean they need to survive mostly on leaves for parts of the year. The fruit and leaves of the fig tree are staples in many howler monkeys’ diets, as they are a source of fruit during times of year when fruit is scarce. It’s more difficult for howler monkeys to subsist only on leaves, because they have to spend more time foraging, and eating lots of different leaves to make sure that they don’t consume too much of any one type of leaf. Leaves contain compounds that can be toxic to animals in large amounts, to deter foraging so that the plant can survive.

Howler monkeys have trichromatic color vision, just like humans!

Howler monkeys love to swing and hang from their tails, which can be 5 times as long as their bodies.

Their bark is worse than their bite: howler monkeys rarely fight, but their cries can be heard 3 miles away!

Photo by Sam Abell

Moving around in small groups of 15-20 individuals, howler monkeys spend most of their day high up in the trees. They have powerful tails, which they use to swing themselves around from branch to branch. Most howler monkey groups consist of 3-4 males, and the rest females. Both male and female juveniles leave the group they are born in to join another group, so many spend the majority of their lives in groups of individuals to whom they are not related.


Howler monkeys are an integral part of the rich fabric of the rainforest ecosystem. Scientists have noted that in areas where there are more howler monkeys, there are also more birds. Through careful research, they discovered that trees actually become more productive when there are more monkeys eating their leaves. This leads to an increase in insects eating young leaves, which means there is more food for birds in the area. Howler monkeys face predators too, though-- especially humans! Primates of many species are hunted as bushmeat, a practice which decimates their populations. Deforestation is also a threat to howler monkeys. Areas of the rainforest are burned and cleared every day, which reduces the area of land that howler monkeys can use to forage. These territorial animals need lots of land to find enough fruit and nutritious leaves to survive. Since they like to stay high in the canopy, areas without trees serve as barriers to howler monkeys and can limit their range to smaller patches of land.

 

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  1. Dunn et al. “Seasonal variations in the diet and feeding effort of two groups of howlers in different sized forest fragments.” Int. J. Primatol (2010) 31:887-903. http://www.prime.bioanth.cam.ac.uk/reprints/Dunn%20et%20al.%202010.%20Seasonal%20Variations%20in%20the%20Diet%20and%20Feeding%20Effort%20of%20Two%20Groups%20of%20Howlers%20in%20Different%20Sized%20Forest%20Fragments%20Int%20J%20Primatol%2031-%20887-903.pdf

  2. Cristobal-Azkarate, J. and Arroyo-Rodriguez, V. “Diet and activity patterns of howler monkeys (Alouatta palliata) in Los Tuxtlas, Mexico: effects of habitat fragmentation and implications for conservation.” Am J Primatol (2007) 69 (9): 1013-29.
    http://www.ncbi.nlm.nih.gov/pubmed/17330311

  3. Jacobs et. al. “Trichromatic vision in New World monkeys.” Nature (1996) 382 (6587): 156-8.
    http://www.ncbi.nlm.nih.gov/pubmed/8700203

  4. Duke University. ""Fowl-Howl" Ties Discovered Between Birds, Monkeys." ScienceDaily. ScienceDaily, 6 August 2002. https://www.sciencedaily.com/releases/2002/08/020806080028.htm

  5. Dunn, T. “The loudest animal in the New World.” Smithsonian National Zoological Park. n.d.
    https://nationalzoo.si.edu/animals/smallmammals/exhibits/howlermonkeys/loudestanimal/

  6. Milton, K. “Howler Monkeys.” Montclair State University.” 1998.
    https://www.montclair.edu/csam/prism/rainforest-connection/panama/mammal-directory/howler-monkeys/