Camera Traps Uncover the Behavioral Ecology of an Endemic, Cryptic Monkey Species in the Congo Basin

The lesula monkey (Cercopithecus lomamiensis), described in 2012, is a semi-terrestrial rainforest guenon endemic to the central Congo Basin. It's a member of the Cercopithecini tribe, the most diverse and understudied group of Old World monkeys. The lesula's range overlaps with five other guenon species and the dryas monkey in the Tshuapa, Lomami, and Lualaba (TL2) Rivers Landscape. Due to bushmeat hunting, the lesula is listed as Vulnerable on the IUCN Red List. Genetic analysis shows lesula is a sister species to Cercopithecus hamlyni, with both species exhibiting a recent divergence and non-overlapping geographic ranges, suggesting allopatric speciation. However, there's a significant lack of ecological and behavioral data on this lineage, hindered by the lesula's cryptic behavior. This study uses intensive camera trapping to address this knowledge gap, focusing on lesula's terrestriality (contrary to the mostly arboreal nature of other Cercopithecus), activity pattern (potentially crepuscular or cathemeral despite being presumed diurnal), birth seasonality, and group size and composition. The study aims to provide species-specific data for effective conservation planning, bridging the gap between scientific knowledge and conservation management decisions. Preliminary results were incorporated into the recent IUCN Red List assessment of lesula.

The literature review highlights the limited understanding of the lesula monkey's behavioral ecology, especially concerning its terrestriality, activity patterns, birth seasonality, and group dynamics. While members of the Cercopithecus genus are generally arboreal, morphological analysis suggests a unique terrestrial adaptation in the hamlyni species-group, which includes the lesula. This contrasts with the arboreal nature of other Cercopithecus species and raises questions about the evolutionary drivers behind this terrestriality. Studies on other primates show a link between skull morphology and activity patterns, with larger orbits often associated with nocturnal species. Lesula skull morphology suggests potentially larger orbits than other guenon species, raising questions about their activity patterns. Existing literature on guenon species generally indicates reproductive and birth seasonality, often linked to rainfall and food availability. However, the secretive nature of females with newborns makes direct observation difficult. Previous studies have relied on methods such as camera traps and satellite telemetry to study activity patterns in mammals, revealing the influence of factors like food resources, temperature, and predation risks on nocturnal or crepuscular activity.

Ethical clearance was obtained from the Institut Congolais pour la Conservation de la Nature (ICCN) for research within Lomami National Park (LNP) and from the sector chief of the Yawende Lolo Sector for the buffer zone. Three systematic camera trap surveys were conducted over three years across three sites: two within LNP (Losekola and E15) and one in the buffer zone (Okulu). Camera traps were placed 500 m apart in Okulu and Losekola, and 1 km apart in E15, resulting in a total of 92 camera traps across 4.1 km², 3.8 km², and 12.0 km² grids respectively. Moultrie and Bushnell cameras were used, recording videos in high resolution. Cameras were positioned on the ground facing animal trails. Data analysis involved reviewing all videos (over 15,000 total) and defining events based on lesula activity clusters. Capture rates were calculated. To determine terrestriality, lesula capture rates were compared to those of sympatric arboreal monkey species. Activity patterns were determined by analyzing event times. Birth seasonality was assessed by classifying infant ages from video footage, comparing this with rainfall data (2008-2013). Group size and composition were determined by identifying unique individuals based on physical features and behavior, ensuring reliability through multiple observers. Statistical analyses were used to identify significant differences where applicable.

The study yielded 598 independent lesula events from 5960 camera trap days. Lesula capture rates were significantly higher than those of sympatric arboreal monkeys (67-500 times higher), and 98.8% of lesula events showed terrestrial locomotion. The activity pattern was exclusively diurnal, with few events at dawn and dusk. Analysis of 77 infant events suggested a birth peak from August to October, coinciding with the start of the second wet season. The mean group size, considering groups with three or more individuals, was 7.17 (±SD = 4.68). However, a substantial proportion of events (54%) showed only one or two individuals, potentially underestimating group size. The largest groups observed numbered 32, 31, and 25 individuals. Group composition analysis revealed a ratio of 1.67 mature to immature individuals and approximately a 1:1 ratio of adult males to females. A substantial number of mature individuals remained unsexed (75%). Approximately 45% of identified males were observed alone or in male-only groups.

The high lesula capture rates demonstrate the effectiveness of camera trapping for studying this cryptic species. The findings confirm lesula's strong terrestriality, possibly an adaptation to reduce competition for resources in the canopy. The diurnal activity pattern aligns with other Cercopithecus species. The observed birth seasonality, peaking during the second wet season, correlates with increased food availability. The lesula group size appears to be on the lower range for related terrestrial species, possibly due to limitations of camera trap detection. The observed group composition suggests a polygynous mating system with female philopatry and male dispersal. The presence of larger groups with two adult males could be due to several factors: immature males transitioning to adulthood, temporary male influxes during breeding season, limitations of group defense at large group sizes, or ephemeral associations between groups.

This study significantly advances our understanding of lesula behavioral ecology. The use of camera traps proved highly effective for collecting data on this elusive species, providing valuable insights into its terrestriality, activity patterns, birth seasonality, and social structure. These findings are crucial for effective conservation planning. Future research could incorporate arboreal camera traps to get a complete picture of substrate use and explore potential links between morning calls and arboreal activity. Further study is needed to validate the birth seasonality findings and improve group size estimations.

The study's limitations include the possibility of underestimating lesula group size due to the limited field of view of the ground-based camera traps. Also, difficulties in classifying infant ages from poor video quality influenced the accuracy of the birth seasonality analysis. Additionally, the inability to sex the majority of mature individuals limited the precision of the group composition analysis. The rainfall data used was from a nearby location and might not perfectly reflect the rainfall at all three sites.