Traumatic lesions are often observed in ancient skeletal remains. Since ancient medical technology was immature, severely traumatized individuals may have frequently lost the physical ability for cultural skills that demand complex body movements. I develop a mathematical model to analyze the effect of trauma on cultural transmission and apply it to Neanderthal culture using Neanderthal fossil data. I estimate from the data that the proportion of adult individuals who suffered traumatic injuries before death was approximately 0.79-0.94, in which 0.37-0.52 were injured severely and 0.13-0.19 were injured before adulthood. Assuming that every severely traumatized individual and a quarter to a half of the other traumatized individuals lost the capacity for a cultural skill that demands complex control of the traumatized body part, I estimate that if an upper limb is associated with a cultural skill, each individual had to communicate closely with at least 1.5-2.6 individuals during adulthood to maintain the skill in Neanderthal society, and if a whole body is associated, at least 3.1-11.5 individuals were necessary. If cultural transmissions between experts and novices were inaccurate, or if low frequency skills easily disappeared from the population due to random drift, more communicable individuals were necessary. Since the community size of Neanderthals was very small, their high risk of injury may have inhibited the spread of technically difficult cultural skills in their society. It may be important to take this inhibition into consideration when we study Neanderthal culture and the replacement of Neanderthals by modern humans. (C) 2017 Elsevier GmbH. All rights reserved.

Empathy plays an important role in animal social behavior. Since emotional contagion forms one of the bases of empathy, here we study conditions for emotional contagion to be adaptive, compared with other behavioral rules such as behavioral mimicry. We consider the situation where the focal individual (=observer) reacts to a behavior of another individual (=demonstrator). By emotional contagion one spontaneously copies the emotional state of the demonstrator and takes a behavior driven by that emotion. By behavioral mimicry, in contrast, one copies the behavior of the demonstrator itself. Through mathematical models we show that emotional contagion is adaptive when the environmental similarity between the demonstrator and the observer is intermediate. The advantage of adopting emotional contagion over behavioral mimicry increases when observing others' behavior is difficult or cognitively demanding. We show that emotional contagion is often a more flexible strategy than behavioral mimicry in order to cope with the living environment. In other words, emotional contagion often works as a better social learning strategy. These results suggest some ecological conditions that would favor the evolution of emotional contagion, and give a part of the explanations of why emotional contagion is frequently observed in group-living animals. (C) 2015 Elsevier Ltd. All rights reserved.

The replacement of Neanderthals by modern humans may possibly have been influenced by the different cultural transmission mechanisms of the two hominins. Since teaching is widespread in modern human societies, but extremely rare in animals, it may have played an important role in human cultural evolution. In modern humans, how and whom to teach may, in part, be transmitted culturally. Therefore, in this paper, I develop a cultural transmission model of teaching. I show that even when costly, teaching can evolve provided that teachers transmit their cultural traits more actively than non-teachers. Teaching is more likely to evolve when the cost of social learning is low relative to individual learning, social learning is accurate, the environment is stable, and the effect of teaching is extensive. Under certain conditions, two states, existence and non-existence of teaching in the population, are evolutionarily stable (bistable).When this happens, social learning is sometimesmaintained by teaching under unstable environments where social learning cannot exist without teaching. Differences in subsistence strategy and group structure between Neanderthals and modern humans may have affected the evolution of the teaching behaviors of the two hominins.

Cultural transmission and cultural evolution are important for animals, especially for humans. I developed a new analytical model of cultural evolution, in which each newborn learns cultural traits from multiple individuals (exemplars) in parental generation, individually explores around learned cultural traits, judges the utility of known cultural traits, and adopts a mature cultural trait. Cultural evolutionary speed increases when individuals explore a wider range of cultural traits, accurately judge the skill level of cultural traits (strong direct bias), do not strongly conform to the population mean, increase the exploration range according to the variety of socially learned cultural traits (condition dependent exploration), and make smaller errors in social learning. Number of exemplars, population size, similarity of cultural traits between exemplars, and one-to-many transmission have little effect on cultural evolutionary speed. I also investigated how cultural interaction between two populations with different mean skill levels affects their cultural evolution. A population sometimes increases in skill level more if it encounters a less skilled population than if it does not encounter anyone. A less skilled population sometimes exceeds a more skilled population in skill level by cultural interaction between both populations. The appropriateness of this analytical method is confirmed by individual-based simulations. (C) 2014 Elsevier Ltd. All rights reserved.

The division of labor is an important component of the organization of human society. However, why this division evolved in hominids requires further investigation. Archeological evidence suggests that it appeared after the emergence of Homo sapiens and contributed to the great success of our species. We develop a mathematical model to investigate under what conditions division of labor should evolve. We assume two types of resources the acquisition of which demands different skills, and study the evolution of the strategy that an individual should use to divide its lifetime into learning and using each skill. We show that division of labor likely evolves when group size is large, skill learning is important for acquiring resources, and there is food sharing within a group. We also investigate division of labor by gender under the assumption that the genders have different efficiencies in acquiring each resource. We show that division of labor by gender likely evolves when skill learning is important and the difference in efficiencies between genders in acquiring resources is large. We discuss how the results of our analysis might apply to the evolution of division of labor in hominids. (C) 2014 Elsevier Ltd. All rights reserved.

Humans have created highly developed cultures, brought about by iterative improvements in technology. Using a mathematical model, I investigated the conditions under which cultural traits tend to be improved for a higher level of culture to evolve. In the model, I consider three ways of learning: individual learning, simple social learning, and improvements of socially learned cultural traits (social improvement). I obtain the evolutionarily stable number of cultural traits acquired through each way of learning. I show that organisms improve many socially learned cultural traits under the following conditions: (1) environmental stability is intermediate; (2) the environment is severe; (3) the success rate of individual learning is high; (4) the utility of cultural traits acquired by individual learning is large; (5) the accuracy of social learning is high; and (6) the increase in the utility of beneficial cultural traits attained by social improvement is large. I also show that when organisms have greater ability for social improvement, the average utility of the beneficial cultural trait increases, the proportion of beneficial cultural traits among all cultural traits decreases, and the total number of cultural traits acquired by the three ways of learning is constant. These results shed light on the origins of human cumulative culture. (C) 2012 Elsevier Inc. All rights reserved.

Long before the origins of agriculture human ancestors had expanded across the globe into an immense variety of environments, from Australian deserts to Siberian tundra. Survival in these environments did not principally depend on genetic adaptations, but instead on evolved learning strategies that permitted the assembly of locally adaptive behavioral repertoires. To develop hypotheses about these learning strategies, we have modeled the evolution of learning strategies to assess what conditions and constraints favor which kinds of strategies. To build on prior work, we focus on clarifying how spatial variability, temporal variability, and the number of cultural traits influence the evolution of four types of strategies: (1) individual learning, (2) unbiased social learning, (3) payoff-biased social learning, and (4) conformist transmission. Using a combination of analytic and simulation methods, we show that spatial-but not temporal-variation strongly favors the emergence of conformist transmission. This effect intensifies when migration rates are relatively high and individual learning is costly. We also show that increasing the number of cultural traits above two favors the evolution of conformist transmission, which suggests that the assumption of only two traits in many models has been conservative. We close by discussing how (1) spatial variability represents only one way of introducing the low-level, nonadaptive phenotypic trait variation that so favors conformist transmission, the other obvious way being learning errors, and (2) our findings apply to the evolution of conformist transmission in social interactions. Throughout we emphasize how our models generate empirical predictions suitable for laboratory testing.

Humans (Homo sapiens) generally form multiple-male-multiple-female groups that include multiple family units. This social structure is maintained because dominant males do not monopolize females and, thus, allow subordinate males to mate, and human females are not generally promiscuous. Although apes show great variation in mating systems, the human-type mating system is unique among primates. The mating systems of apes and humans have evolved in response to their adaptation to different ecological conditions. We created and analyzed a mathematical model to investigate the conditions for each type of mating system to evolve. We focused on the mating strategy of alpha males and the mating and grouping strategies of females. We defined the human-type mating system as one with multiple-male-multiple-female groups in which alpha males do not monopolize females and females are not promiscuous. We demonstrated that the human-type mating system could evolve when a large group is advantageous and the cost of female promiscuity is great. Moreover, the human- and Pan-type mating systems can be bistable. Our results shed light on the origin of the human family. (C) 2011 Elsevier Ltd. All rights reserved.

Humans strongly depend on individual and social learning, both of which are highly effective and accurate. I study the effects of environmental change on the evolution of the effectiveness and accuracy of individual and social learning (individual and social learning levels) and the number of pieces of information learned individually and socially (individual and social learning capacities) by analyzing a mathematical model. I show that individual learning capacity decreases and social learning capacity increases when the environment becomes more stable; both decrease when the environment becomes milder. I also show that individual learning capacity increases when individual learning level increases or social learning level decreases, while social learning capacity increases when individual or social learning level increases. The evolution of high learning levels can be triggered when the environment becomes severe, but a high social learning level can evolve only when a high individual learning level can simultaneously evolve with it. (C) 2010 Elsevier Inc. All rights reserved.

We study the effects of natural selection and migration on the numbers of individual learners and social learners in subdivided populations that occupy environmentally heterogeneous sites. The island model and the circular stepping model each have four classes of globally stable equilibria (fixation of individual learners, polymorphism of individual and social learners, fixation of social learners, and extinction). The linear stepping stone model has an additional class of equilibria, which are characterized by the complete absence of phenotypes adapted to the interior sites. Low and high rates of migration favor social and individual learners, respectively, in all three models. In addition, we use the stepping stone models to study the range expansion of a species, initially confined to one environmentally homogeneous site, into the spatially heterogeneous world. The successive peaks of the transient spatial distributions of the number of individual learners occur at initially empty sites. (c) 2008 Elsevier Inc. All rights reserved.

There are many examples of male mate choice for female traits that tend to be associated with high fertility. I develop quantitative genetic models of a female trait and a male preference to show when such a male preference can evolve. I find that a disagreement between the fertility maximum and the viability maximum of the female trait is necessary for directional male preference (preference for extreme female trait values) to evolve. Moreover, when there is a shortage of available male partners or variance in male nongenetic quality, strong male preference can evolve. Furthermore, I also show that males evolve to exhibit a stronger preference for females that are more feminine (less resemblance to males) than the average female when there is a sexual dimorphism caused by fertility selection which acts only on females. (C) 2008 Elsevier Inc. All rights reserved.

Conformity is often observed in human social learning. Social learners preferentially imitate the majority or most common behavior in many situations, though the strength of conformity varies with the situation. Why has such a psychological tendency evolved? I investigate this problem by extending a standard model of social learning evolution with infinite environmental states (Feldman, M.W., Aoki, K., Kumm, J., 1996. Individual versus social learning: evolutionary analysis in a fluctuating environment. Anthropol. Sci. 104, 209-231) to include conformity bias. I mainly focus on the relationship between the strength of conformity bias that evolves and environmental stability, which is one of the most important factors in the evolution of social learning. Using the evolutionarily stable strategy (ESS) approach, I show that conformity always evolves when environmental stability and the cost of adopting a wrong behavior are small, though environmental stability and the cost of individual learning both negatively affect the strength of conformity. (c) 2007 Elsevier Inc. All rights reserved.