Emotion is understudied in nonhuman animals despite broad interests in the topic. This is partly due to the difficulty in measuring subtle emotional reactions, such as physiological changes, under ecologically-valid situations. It is particularly challenging because the majority of traditional physiological measurements require animal participants to wear electrodes and head/body restraints in a laboratory. Recent advances in infrared thermography (IRT), and its use in measuring changes in animals' skin-temperature, offer suitable solutions for these challenges. This article reviews a growing body of research employing IRT in the study of animal emotions and identify both merits and shortcomings of IRT which need to be considered when designing experiments and observations. Also, we introduce our recent efforts to facilitate the use of IRT for the study of large-body animals, such as chimpanzees. Finally, we illustrate some of the critical future directions of IRT for the study of nonhuman animals. In conclusion, the study of animal emotion is more possible than ever before with this novel technology.

The present study examined effects of pre- and post-reinforcer delays on pigeons' choice using concurrent-chains schedules. Subjects were exposed to concurrent-chains schedules in which a single variable-interval 30-s schedule was arranged for the initial link, followed by different fixed-time schedules for pre- and post-reinforcer delays as the terminal links. The pre- and post-reinforcer delays were fixed at 2 s for one alternative and they were manipulated across conditions (2 s, 8 s, 20 s, or 40 s) for the other alternative. Based on the sensitivities estimated from the overall reinforcement density model, it is suggested that pigeons have higher sensitivity to the pre-reinforcer delay relative to the post-reinforcer delay. These results are consistent with those reported in a previous study using rats. Comparisons of choice proportion data from different post-reinforcer delays reveal that pigeons have sensitivity to post-reinforcer delay, although it is not high.

Non-human animals commonly perform mutual communication in which two individuals simultaneously exchange information in an interaction that can comprise multiple components and multimodal signals. The forms of mutual communication vary widely, ranging from simple greeting ceremonies to complex paired dances including ultiple behavioural elements. Our understanding of mutual communication lags markedly because few systematic studies have examined this topic. In this review, we used the example of paired dances in red-crowned cranes (Grus japonensis) to examine two important aspects of mutual communication: structure and function. We reviewed the difficulty analysing the key characteristics of structure quantitatively (e.g. sequential pattern and complexity), and examined suitable analytic methods such as etermining the four levels of its characteristics. We proposed that a combination of current methods and new methods, such as the Shannon entropy index and temporal associations among behavioural elements, is necessary to quantify the complexity of mutual communication. Regarding the function of paired dances, we critically discuss the pair bond hypothesis, which suggests that a paired dance strengthens pair bonds in monogamous, long-living birds. Testing this classical hypothesis remains a challenge due to the difficulty of performing quantitative analyses of paired dances and the ambiguous concept of pair bonds. We hope that the questions and predictions raised in this paper will encourage future research on mutual communication in non-human animals.

Many studies have reported that wild and captive nonhuman primates have an advanced ability to understand causal relationships and use tools. However, the only studies investigating physical causal understanding in rodents under controlled experimental settings has been conducted in degus. The present studies conducted tool-use tasks in rats in an experimental setting, similar to tasks conducted in nonhuman primates and birds, and we used newly developed tasks to investigate their ability for physical causal understanding. In these tasks, rats were required to use tools to obtain food beyond their reach. Research 1 showed that rats chose appropriate tools to obtain food even when two novel tools were presented. In addition, Research 2 showed that rats were able to manipulate a tool according to the position of food. Our studies first suggest that rats have a primitive ability to understand physical causal relationships between pulling tools and approaching food. We propose the rat as an animal model to shed light on the evolution of physical causal understanding from rodents to humans.

Bio-logging, i.e., the use of animal-borne sensors such as acceleration, GPS, electroencephalogram, and video camera, allows researchers to measure the behavioral and physiological data of animals as well as the variables of the environments in which the animals move. The derived animal-borne data, coupled with statistical modeling, can be used to understand ecologically significant phenomena, such as the navigation strategies of animals, across multiple spatiotemporal scales. For example, streaked shearwaters (Calonectris leucomelas) have evolved the ability to locate their home far away from the colony and adjust the onset time of their homeward journeys according to the distance to the home. We discuss the current trends and limitations in bio-logging science and propose future directions for integrating bio-logging with engineering, robot technology, and data science.

The view that episodic memory and metacognition are human-unique abilities has been challenged over the recent decades. While much of the focus of nonhuman studies has been on primates, western scrub-jays contributed extensively towards the understanding of these abilities. Western scrub-jays demonstrate episodic-like memory by remembering what, where, and when they cached food items and by using this information flexibly during recovery to change their behaviour according to the situation. In addition to episodic-like memory, recent studies suggest that these birds are also capable of self-reflection in a form of metacognition. Further investigation of their self-reflective ability may be the critical approach in deepening our understanding of episodic memory in nonhuman animals.

Group living mammals have a distinct characteristic: when conspecific animals are together, they show a better recovery from experiences of distress. This phenomenon, termed 'social buffering', has been found in rodents, birds, non-human primates, and also in humans. This phenomenon is well-observed in bonded dyad; the mother-infant or pair-bonded dyads. Social contact, including allogrooming, appears to have a very positive influence on the psychological and physiological aspects of social animals, including human beings. These relationships depend on the neuroendocrine system, especially oxytocin. Oxytocin is released by affiliative social contact in dyads and can ameliorate stress and anxiety in both sides. This review overviews the classic finding of social buffering in animals and describe the recent findings of neuroendocrinological mechanisms for social buffering.

Nobody would dispute that cats and dogs are the two most popular species of companion animals for humans. It is no wonder that dogs are often considered to be the best friends and adopted family members of human beings: They developed the high social ability to communicate with humans during the two species' long history of co-habitation. However, it seems strange that cats are in the same position as dogs in human society, because they are more individual and less social than dogs. This review first presents the history of cat domestication. Next, it discusses the reasons why cats are now human companion animals despite such a history. This paper will discuss the cognitive and behavioral characteristics in cats that promote human-cat interaction and the similarities between cats and humans. Finally, it will discuss the significance of the study of cats' sociality.

The guppy (Poecilia reticulata), a polyandrous livebearing fish, is a model organism in the study of sexual selection. This species exhibits sexual dimorphism, such as male body coloration (orange, black, or iridescence color spots). Although there is evidence of a preference for colorful males in female mate choice, a wide variation in male color patterns is found even in a population. Recently, there has been an increase in studies examining the postcopulatory processes involved in sperm competition and cryptic female choice in this species. If male traits that favor sperm competition and/or cryptic female choice correspond with traits preferred in female mate choice, then postcopulatory processes will reinforce the selection to colorful males. In contrast, if males with traits preferred by females are not favored by sperm competition and/or cryptic female choice, then postcopulatory processes will weaken the selection to colorful males. In this paper, I review studies of the relationships between male guppy coloration and female mate choice, sperm competition, and cryptic female choice, and discuss the possible factors that maintain the variation in male coloration.