The animal is connected to the external environment (e.g., social environment, habitat environment) via their brain, and the hormone plays an important role as its transmitter substance. In other words, when the external environment changes, hormones are secreted to adjust the internal environment in response. Therefore, hormone analysis is a useful tool that enables researchers to know animal's physiological state under various environments.

Since the measurement of hormone concentration can be carried out at a relatively low cost and does not require advanced technique, it is applied to a wide research field. For example, many relations between behaviors and hormones were clarified, such as behaviors in estrus and rut, infant rearing, attacking behaviors, intra/inter-species communications, and response to the change of habitat environment. Recently noninvasive samples are often used for the hormone analysis in both captive and wild animals (e.g., urine, feces, and hair). However, when excrement is used as a sample, it should be noted that there is a species specificity in the excretion route of hormones and the time taken for excretion. Also, depending on hormones, it is necessary to thoroughly examine the sampling frequency according to the change of hormonal concentration. As Beach (1948) summarized, no behavior depends only on one type of hormone, and conversely, no hormone has only one kind of physiological function. Not one but multiple mechanisms are involved in the hormonal control of behavior. Therefore, focusing on multiple hormones and evaluating results from various aspects are also important keys to capturing their invisible physiological state accurately.

Stable isotope analyses are now widely used for estimation of diet, habitat use, and migration in ecology, and are especially useful for species that cannot be observed directly. Although most primate species can be habituated and observed directly, stable isotope analysis provides supporting and complementary evidence for ecological studies of such species. In this review, stable isotopic methodologies are introduced in Japanese for researchers in primate ecology. Elements and tissues that are typically used in stable isotope ecology are explained. Several previous works that applied stable isotope analyses to primate ecology are reviewed briefly, and possible future research avenues are also described. Some points that need consideration in the application of stable isotopic methodologies are also discussed. Because the quality and quantity of samples are important in stable isotope analysis, collaboration of researchers in primate ecology and stable isotope ecology is recommended prior to sample collection. The application of stable isotope analyses to primate ecology can expand with advances in analytical techniques.

Group-living primates often perform complex social behaviors. Traditionally, observational and experimental studies have provided important insights into the social behaviors of primates; however, these studies have limitations regarding unambiguous causality. The use of artificial stimuli can aid in understanding the mechanisms of animal behavior. A robot, which can perform some behavior sequences automatically or by remote control, serves as a new method to study the response of an animal to the stimulus of the same or other species. One of the advantages of using a robot is that researchers can change the appearance and behavior in line with their purpose. In addition, using a robot can help investigate the influence of more than one individual on another individual's behaviors. Although it is advantageous to use robots in the study of animal behaviors, it entails various challenges. This paper reviews the studies on animal behavior that used robots as stimuli and discusses the contribution of using robots in primate behavior study in the future.

The use of bio-logging, which involves collecting ecosystem data by attaching a small sensor and/or wearable camera to a part of the body of wild animals, is increasing in mammal ecology. Image information taken from an animal's own point of view can provide valuable insights into their behaviors, including their preferred habitats, diet, breeding, and competition. Moreover, wild animal studies using unmanned aircraft systems (UAS) also referred to as drones, are increasing especially in recent years. UAS is useful in places inaccessible or inhospitable to researchers, such as deep mountains or above the sea. The presence of species and the number of individuals can be confirmed using the UAS data. However, there are few reports using these methods in primate studies. This review explains what kind of useful information can be obtained in the future for primate researchers studying in the field with reference to previous research and explains the problems and future possibilities of these methods. In the future, technical development and progress is likely to increase the application of animal-borne camera systems and UAS in primate research.

Camera trapping is a new method widely used to assess animal distribution, density and behaviour. Although recent studies have reviewed general patterns in camera trap studies and provided recommendations in their usage, primate studies using camera traps have yet to be thoroughly reviewed. Here, I conducted a systematic search for studies using camera traps in primatology (camera trap primate studies [CTPS]). Finding 57 papers published between 2001 and 2017, I recorded their study objectives and methodologies. The number of CTPS started to increase from 2010, and more than half of CTPS (64.9 %) focused on behaviours. The majority of behavioural CTPS investigated foraging behaviours, including tool use, geophagy and predation, while we also found studies exploring activity rhythms, terrestrial behaviour, habitat use and social behaviours. Some studies used camera traps to complete mammal checklists in study areas and confirm the presence of focal primate species. Some ecological CTPS estimated population density using spatial capture-recapture models and capture rates, and I also found a study calculating occupancy probabilities of arboreal primates. I then point out several issues we have to consider when deploying cameras (sensor sensitivity, image type and camera placement) and analysing images obtained (definitions of independent events and potential biases in detection probability). Unfortunately, several CTPS were not designed to test their study questions sufficiently, and many articles failed to report essential information to facilitate repeatability. I argue that future researchers conducting CTPS should focus on nocturnal primates, explore novel methodologies to use the camera-trap images themselves for primate colour and morphology, develop methodologies for density estimation of arboreal primates, and use sophisticated study designs and reporting. Primatologists will be able to test their existing hypotheses using new technologies.

Genetic investigation of wild primates are crucial to understand kinship, population diversity, phylogeographic patterns, and heritable factors of phenotypes. Traditional DNA technology using polymerase chain reaction (PCR) and Sanger sequencing have restricted the genome-wide analysis of primates, particularly due to the low quality and low quantity of noninvasive DNA samples obtained from wild individuals. Following the post-genome era, next-generation sequencing (NGS) technologies have provided a new paradigm in primate studies. NGS has enabled the genome-wide analysis of primate DNA using noninvasive samples, such as feces. Metabarcoding and metagenomics analyses using fecal samples provide information on food items and commensal microorganisms of the host animal. Here, I review a history of DNA sequencing technologies and examples of NGS studies in wild primates. Further, I discuss the effectiveness of NGS application to noninvasive samples.

In October 2015, large quantities of the feces from Japanese macaques (Macaca fuscata) were found within the gallery of the abandoned Nokado Mine in Nikko City, Tochigi Prefecture, central Japan. More than 100 feces specimens were concentrated in two narrow areas of 1-2 m² less than 8 m inward from the mine entrance. The feces were quite similar in outline and plural-connectivity by woody fibers to typical macaque winter feces. Analysis showed that the feces were almost entirely composed of plant-derived fibers. The locations and morphological characteristics of the feces support that the macaques used the gallery during winter, particularly from December 2014 to March 2015, and most likely huddled together for warmth. The amount of snowfall during the winter of 2014 was greater than average, likely forcing the macaques to use the abandoned gallery as a shelter from the heavy snow/wind. Although the Nokado Mine constitutes the second case of cave use by macaques in Japan, it is noteworthy that the macaques used an artificial cave excavated through mining. This is in contrast to the first report of cave use where macaques used a natural limestone cave in the karst regions. By accumulating cases of cave use by macaques, our understanding of adaptive traits in cold areas will be advanced.

The origin of bonobos and the history of their dispersion in the southern Congo Basin were investigated from the perspectives of biogeography and the paleo-environment. Why bonobos are distributed in the southern Congo Basin was unclear because of limited geological records about the region and genetic information for bonobos. Recently, colleagues and I proposed some hypotheses concerning the history of bonobos based on recent reports about marine sediments and structural geology of the Congo basin, as well as new mitochondrial DNA haplotypes collected from seven wild bonobo populations. Here, I illustrate these hypotheses, namely: 1) the Congo River functioned as a biogeographical barrier for forest animals since 34 million years ago; 2) the ancestor of present bonobo populations crossed the upper parts of the Congo River from the right bank to the left bank during the early Quaternary period; and, 3) factors affecting the genetic structure of present bonobos. Lastly, I propose possible future studies to investigate bonobo evolution.