Structural patterns and 24-hour rhythms of the loud calls of Alouatta seniculus were studied in a wild population living in primary rain forest in French Guyana. These loud calls are low-pitched noisy hoarse sounds with an upper frequency limit around 3,000 Hz. Their spectral energy is concentrated in two clusters, each of which includes two dominant frequency bands. Loud calls comprise one or several vocal units with alternating crescendos and decrescendos. Five different types of units can be distinguished. Depending on their duration, loud calls can be roughly divided into two classes: long and short calls. Long calls (mean duration 3.28 min) include three phases. After the end of the calls, one or two blowing noises of weak amplitude may be heard. Short calls (mean duration 11s) are frequently uttered in clusters of several calls given by the same troop at intervals of a few seconds to a few minutes. Observations carried out over the 24-hour cycle show that the calling rate is higher at night, with a peak around dawn and that long calls are more frequent at night, while short calls are mainly heard during the day. Most long calls are emitted from the sleeping sites and carry great distances. They probably play a role in inter-troop communication. Despite their intensity, short calls are used for short-distance communication. The occurrence of two different loud calls in Guyanan howlers and the amount of night calling in these monkeys are discussed with reference to studies of other howler populations.

We examined range use by great apes during logging activities and investigated associations between local variations in ape abundance and changes in the structure of the habitat or in the availability of fruits after disturbances. We carried out two annual censuses of western lowland gorilla (G. g. gorilla) and chimpanzee populations (Pan t. troglodytes) in an active logging concession in Southeast Cameroon. The results suggest that gorillas may adapt their range use to avoid most recently logged compartments, while chimpanzees appear to be more spatially resilient to logging. In our study site, selective logging affected 10% of the forest. After logging, gorillas nested in all types of vegetation, while chimpanzees nested exclusively in mixed mature forest. Fruit availability was not affected by logging and did not explain the distribution of ape nests in the study area.

The choice of a sleeping site is crucial for primates and may influence their survival. In this study, we investigated several tree characteristics influencing the sleeping site selection by proboscis monkeys (Nasalis larvatus) along Kinabatangan River, in Sabah, Malaysia. We identified 81 sleeping trees used by one-male and all-male social groups from November 2011 to January 2012. We recorded 15 variables for each tree. Within sleeping sites, sleeping trees were taller, had a larger trunk, with larger and higher first branches than surrounding trees. The crown contained more mature leaves, ripe and unripe fruits but had vines less often than surrounding trees. In addition, in this study, we also focused on a larger scale, considering sleeping and non-sleeping sites. Multivariate analyses highlighted a combination of 6 variables that revealed the significance of sleeping trees as well as surrounding trees in the selection process. During our boat surveys, we observed that adult females and young individuals stayed higher in the canopy than adult males. This pattern may be driven by their increased vulnerability to predation. Finally, we suggest that the selection of particular sleeping tree features (i.e. tall, high first branch) by proboscis monkeys is mostly influenced by antipredation strategies.

Reducing the size of food particles is crucial for herbivores. Seasonal dietary changes are known to influence animals’ chewing efficiency. Proboscis monkeys (Nasalis larvatus) are foregut fermenters, with a high chewing efficiency allowing them to achieve very fine faecal particles. In this study, we investigated how proboscis monkeys’ chewing efficiency varies between wet and dry seasons, hypothesising differences possibly related to diet change. Faecal particle size analysis is an established approach to estimate chewing efficiency in mammalian herbivores. We analysed 113 proboscis monkey faecal samples collected in the Lower Kinabatangan Wildlife Sanctuary, between 2015 and 2017. By following standard sieve analysis protocols, we measured a mean particle size MPS_0.025–8 of 0.45 ± 0.14 mm, and confirmed a previous result that proboscis monkeys have a very low faecal MPS. This study highlights a seasonal influence on proboscis monkeys’ chewing efficiency, with smaller MPS (better chewing efficiency) during the wet season. During that time of the year, individuals may potentially change their diet, as all faecal samples contained intact seeds. Whether the seasonal MPS difference in proboscis monkeys is smaller than in other colobines due to their “rumination” strategy remains to be investigated.

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