For arboreal primates, ground use may increase dispersal opportunities, tolerance to habitat change, access to ground-based resources, and resilience to human disturbances, and so has conservation implications. We collated published and unpublished data from 86 studies across 65 localities to assess titi monkey (Callicebinae) terrestriality. We examined whether the frequency of terrestrial activity correlated with study duration (a proxy for sampling effort), rainfall level (a proxy for food availability seasonality), and forest height (a proxy for vertical niche dimension). Terrestrial activity was recorded frequently for Callicebus and Plecturocebus spp., but rarely for Cheracebus spp. Terrestrial resting, anti-predator behavior, geophagy, and playing frequencies in Callicebus and Plecturocebus spp., but feeding and moving differed. Callicebus spp. often ate or searched for new leaves terrestrially. Plecturocebus spp. descended primarily to ingest terrestrial invertebrates and soil. Study duration correlated positively and rainfall level negatively with terrestrial activity. Though differences in sampling effort and methods limited comparisons and interpretation, overall, titi monkeys commonly engaged in a variety of terrestrial activities. Terrestrial behavior in Callicebus and Plecturocebus capacities may bolster resistance to habitat fragmentation. However, it is uncertain if the low frequency of terrestriality recorded for Cheracebus spp. is a genus-specific trait associated with a more basal phylogenetic position, or because studies of this genus occurred in pristine habitats. Observations of terrestrial behavior increased with increasing sampling effort and decreasing food availability. Overall, we found a high frequency of terrestrial behavior in titi monkeys, unlike that observed in other pitheciids.

There is wide variability in primate behavior and ecology. Understanding how frugivorous primates behave under different habitat fragmentation levels is key for effective conservation and management of species and their habitats. We evaluated the seasonality in activity budget, diet, and ranging behavior of two groups of Endangered Coimbra-Filho's titi monkeys (Callicebus coimbrai). One group inhabited a 14-ha forest fragment, whereas the other lived in a 522-ha fragment. We measured the monthly density of trees and lianas available as food sources over 8 months. We also collected behavioral and group location data every 5 min, from dawn to dusk, using the scan sampling method. The two forest fragments differed seasonally in the number of fruiting food-resource available. In the 14-ha fragment, we found that the time spent by titi monkeys feeding, foraging, resting, and traveling differed seasonally. In the 522-ha fragment, titi monkeys exhibited seasonal differences in time spent sleeping, socializing, foraging, and revisiting food sources. In both titi monkey groups, diets varied seasonally. Our findings indicate that Coimbra-Filho's titi monkeys can exhibit behavioral flexibility in their activity budgets, diets, and movement patterns. Such flexibility is important for this species to survive in fragmented habitats and may be linked to three key factors: speciesspecific resource availability, plant species diversity, and the vegetation structure of each forest fragment.

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