High stress levels found in monkeys forced to spend more time foraging

Jan 22, 2013

(Phys.org)—New research shows that disturbed habitats are resulting in increasingly poor diets for monkeys, and that the additional time and energy required to find food is causing concerning levels of stress in already critically endangered primates.

Endangered Mexican howler monkeys are consuming more leaves and less fruit as a result of habitat disturbance by humans, which is forcing them to invest much more time foraging for sustenance and leading to increased 'stress' levels, as detected through hormone analysis.

The research, published today in the , took place in the of the Mexican state of Veracruz, which are being deforested and fragmented by human activity – primarily the clearing of forest for cattle raising.

It shows that increases in howler monkey 'travel time' – the amount of time needed to find requisite nourishment – are leading to increases in levels of stress hormones called glucocorticoids.

These hormones are not only indicators of stress, but are also known to relate to diminished and lower .

Researchers believe the study could serve as a model for behavioural change and resulting more generally in primates living in habitats disturbed by human activities, such as deforestation.

"Howlers are arboreal primates, that is to say they spend their wholes lives in the trees", said Dr Jacob Dunn from Cambridge's Department of , who carried out the research.

"As forests are fragmented, the howlers become cut off, isolated on forest 'islands' that increasingly lack the fruit which provide an important component of their natural diet. This has led to the monkeys expending ever more time and effort , often increasing leaf consumption when their search is, quite literally, fruitless."

Fruit occurs in , and the monkeys will naturally revert to 'fallback' foods, including leaves, when fruit is scarce. But as habitats shrink, and fruit is harder to find, leaves from second-choice plants, such as lianas, have increased in the Mexican howlers' diet.

While leaves may sound like a plentiful resource in a rainforest, many leaves are difficult to digest and can be filled with toxins – a natural defence mechanism in most trees and plants – so the monkeys are actually forced to spend more time seeking out the right foliage to eat, such as new shoots which are generally less toxic.

"The traditional view was that the leaves exploited by howler monkeys were an abundant food source – but this is not the case," said Dunn.

"The monkeys rely much more heavily on fruit than previously believed, and when turning to foliage for food – as they are increasingly forced to do – they have to be highly selective in the leaves they consume, visiting lots of different trees. This leads to the increased 'travel time' and consequent high levels of stress we are seeing in these primates as their habitats disintegrate."

As trying to catch the howlers to examine them would in itself be highly stressful for the animal, the best way of evaluating in wild primates is by analysing their faeces for glucocorticoid stress hormones, which are general to all vertebrates.

Through statistical modelling, the researchers were able to determine that it is the 'travel time' – rather than the increased foliage intake – causing high levels of stress.

"Monkeys in disturbed habitats suffering high is in itself unsurprising perhaps, but now we think we know why, the root cause from the primates perspective. Our results also highlight the importance of preserving and planting fruit trees – particularly those species such as figs that can produce fruit during periods of general fruit scarcity – for the conservation of howler monkeys¨ said Dr Jurgi Cristóbal-Azkarate, also from Cambridge, who led the research in collaboration with Dr Joaquim Vea from the University of Barcelona.

The authors say that further studies are required to fully understand the significance of increases in in howler monkeys living in disturbed habitats. "Determining the full relevance of our results for the conservation of primates living in forest fragments will require long-term studies of and survival", said Dunn.

Explore further: Male monkey filmed caring for dying mate (w/ Video)

add to favorites email to friend print save as pdf

Related Stories

Recommended for you

Male monkey filmed caring for dying mate (w/ Video)

Apr 18, 2014

(Phys.org) —The incident was captured by Dr Bruna Bezerra and colleagues in the Atlantic Forest in the Northeast of Brazil.  Dr Bezerra is a Research Associate at the University of Bristol and a Professor ...

Orchid named after UC Riverside researcher

Apr 17, 2014

One day about eight years ago, Katia Silvera, a postdoctoral scholar at the University of California, Riverside, and her father were on a field trip in a mountainous area in central Panama when they stumbled ...

In sex-reversed cave insects, females have the penises

Apr 17, 2014

Researchers reporting in the Cell Press journal Current Biology on April 17 have discovered little-known cave insects with rather novel sex lives. The Brazilian insects, which represent four distinct but re ...

Fear of the cuckoo mafia

Apr 17, 2014

If a restaurant owner fails to pay the protection money demanded of him, he can expect his premises to be trashed. Warnings like these are seldom required, however, as fear of the consequences is enough to ...

User comments : 0

More news stories

Biologists help solve fungi mysteries

(Phys.org) —A new genetic analysis revealing the previously unknown biodiversity and distribution of thousands of fungi in North America might also reveal a previously underappreciated contributor to climate ...

Making graphene in your kitchen

Graphene has been touted as a wonder material—the world's thinnest substance, but super-strong. Now scientists say it is so easy to make you could produce some in your kitchen.