The extension of distance sampling methods to accommodate observations from camera traps has recently enhanced the potential to remotely monitor multiple species without the need of additional data collection (sign production and decay rates) or individual identification. However, the method requires that the proportion of time is quantifiable when animals can be detected by the cameras. This can be problematic, for instance, when animals spend time above the ground, which is the case for most primates. In this study, we aimed to validate camera trap distance sampling (CTDS) for the semiarboreal western chimpanzee (Pan troglodytes verus) in Taï National Park, Côte d'Ivoire by estimating abundance of a population of known size and comparing estimates to those from other commonly applied methods. We estimated chimpanzee abundance using CTDS and accounted for limited availability for detection (semiarboreal). We evaluated bias and precision of estimates, as well as costs and efforts required to obtain them, and compared them to those from spatially explicit capture‐recapture (SECR) and line transect nest surveys. Abundance estimates obtained by CTDS and SECR produced a similar negligible bias, but CTDS yielded a larger coefficient of variation (CV = 39.70% for CTDS vs. 1%/19% for SECR). Line transects generated the most biased abundance estimates but yielded a better coefficient of variation (27.40–27.85%) than CTDS. Camera trap surveys were twice more costly than line transects because of the initial cost of cameras, while line transects surveys required more than twice as much time in the field. This study demonstrates the potential to obtain unbiased estimates of the abundance of semiarboreal species like chimpanzees by CTDS.HIGHLIGHTS Camera trap distance sampling produced accurate density estimates for semiarboreal chimpanzees. Availability for detection must be accounted for and can be derived from the activity pattern.

Effective monitoring methods are needed for assessing the state of biodiversity and detecting population trends. The popularity of camera trapping in wildlife surveys continues to increase as they are able to detect species in remote and difficult-to-access areas. As a result, several statistical estimators of the abundance of unmarked animal populations have been developed, but none have been widely tested. Even where the potential for accurate estimation has been demonstrated, whether these methods estimators can yield estimates of sufficient precision to detect trends and inform conservation action remains questionable. Here, we assess the effort-precision relationship of camera trap distance sampling (CTDS) in order to help researchers design efficient surveys. A total of 200 cameras were deployed for 10 months across 200 km 2 in the Taï National Park, Côte d'Ivoire. We estimated abundance of Maxwell's duikers, western chimpanzees, leopards, and forest elephants that are challenging to enumerate due to rarity or semi-arboreality. To test the effects of spatial and temporal survey effort on the precision of CTDS estimates, we calculated coefficient of variation (CV) of the encounter rate from subsets of our complete data sets. Estimated abundance of leopard and Maxwell's duiker density (20% < CV < 30% and CV = 11%, respectively) were similar to prior estimates from the same area. Abundances of chimpanzees (20% < CV < 30%) were underestimated, but the quality of inference was similar to that reported after labor-intensive line transect surveys to nests. Estimates for the rare forest elephants were potentially unreliable since they were too imprecise (60% < CV < 200%). Generalized linear models coefficients indicated that for relatively common, ground-dwelling species, CVs between 10% and 20% are achievable from a variety of survey designs, including long-term (6+ months) surveys at few locations (50), or short term (2-week to 2-month) surveys at 100-150 locations. We conclude that CTDS can efficiently provide estimates of abundance of multiple species of sufficient quality and precision to inform conservation decisions. However, estimates for the rarest species will be imprecise even from ambitious surveys and may be biased for species that exhibit strong reactions to cameras.

The use of camera traps (CTs) has become an increasingly popular method of studying wildlife, as CTs are able to detect rare, nocturnal, and elusive species in remote and difficult-to-access areas. It thus makes them suited to estimate animal density and abundance, identify activity patterns and new behaviours of animals. However, animals can react when they see the CTs and this can lead to bias in the animal population estimates. While CTs may provide many advantages, an improved understanding of their impacts on individual’s behaviour is necessary to avoid erroneous density estimates. Yet, the impact of CTs on detected individuals, such as human odour near the device and the environment, or the infrared illumination, has received relatively little attention. To date, there is no clear procedure to remove this potential bias. Here, we use camera trap distance sampling (CTDS) to (1) quantify the bias resulting from the different animal responses to the CTs when determining animal density and abundance, and (2) test if olfactory, visual and auditory signals have an influence on the animals’ reaction to CTs. Between March 2019 and March 2020, we deployed CTs at 267 locations distributed systematically over the entire Taï National Park. We obtained 58,947 videos from which we analysed four medium- to-large-bodied species (Maxwell’s duiker (Philantomba maxwellii), Jentink’s duiker (Cephalophus jentinki), pygmy hippopotamus (Choeropsis liberiensis) and Western chimpanzee (Pan troglodytes verus)) displaying different behaviours towards the CTs. We then established species-specific ethograms describing the behavioural responses to the CTs. Using these species-specific responses, we observed that the Maxwell’s duiker reacted weakly to CTs (about 0.11% of the distance data), contrary to Jentink’s duiker, pygmy hippopotamus and Western chimpanzee which reacted with relatively high frequencies, representing 32.82%, 52.96% and 16.14% of the distance data, respectively. Not taking into account the species-specific responses to the CTs can lead to an artificial doubling or tripling of the populations’ sizes. All species reacted more to the CTs at close distances. Besides, the Jentink’s duiker and the pygmy hippopotamus reacted significantly more to the CTs at night than during the day. Finally, as for olfactory signals, the probability of reaction to the CTs during the first days after CTs installation was weak in Maxwell’s duiker, but concerned 18% of the video captures in Western chimpanzees which decreasing with time, but they remained high in pygmy hippopotamus and Jentink’s duiker (65% and 70% of the video captures respectively). Careful consideration should be given to animal’s response to CTs during the analysis and in the field, by reducing human’s impact around the CTs installation.

Les périodes d’activité des animaux difficilement observables des forêts primaires sont peu connues. Des caméras pièges ont été utilisées pour établir les périodes d’activité de quelques mammifères au Parc national de Taï. Ces caméras non appâtées ont été installées d’octobre 2016 à août 2017. L’enregistrement de l’heure et la date de chaque observation a permis de déterminer des activités nocturnes, diurnes et cathémérales (actif le jour et la nuit) des espèces à travers le pourcentage des vidéos obtenues. Les heures d’activité pour chaque espèce étaient concentrées autour du vecteur moyen (heure moyenne autour de laquelle sont concentrées les heures d’activité de l’espèce) et réparties de manière non-uniforme pendant la période de 24h. Les mammifères de petite masse corporelle (< 10 kg) avaient des tendances nocturnes alors que ceux de grande masse corporelle (> 10 kg) avaient des tendances nocturnes, diurnes ou cathémérales. Certaines espèces cathémérales ont montré des périodes d’activité à prédominance diurne ou nocturne et doivent faire l’objet d’études supplémentaires enfin de connaître les facteurs influençant leur régime d’activité.Mots clés : caméra piège, taux de capture, mammifères, vecteur moyen.   English Title: Activity periods of some terrestrial mammals by using camera traps in Taï National ParkDetermination of cryptic animal’s activity periods is problematic in primary forests and reminds poorly known. Hence, in the Tai National Park, camera traps were used to establish the periods of activity of some mammals. These non-baited cameras were installed in the research area of the park from October 2016 to August 2017. Recording the time and date of each observation enabled to determine nocturnal, diurnal and cathemeral activities of each species through the percentage of videos obtained. Hours of activity for each species were concentrated around the mean vector and were not uniformly distributed over the 24-hour period. Furthermore, mammals with smaller body mass (<10 kg) had nocturnal tendencies whereas those with larger body mass (> 10 kg) had nocturnal, diurnal or cathemeral tendencies. Certain cathemeral species have predominantly shown day or night periods of activity and must be subject to further studies to find out more about the factors influencing their regimes of activity.Keywords: camera trap, detection rate, mammals, mean vector.