Gillian R. Brown

Bateman's principles and human sex roles

In 1948, Angus J. Bateman reported a stronger relationship between mating and reproductive success in male fruit flies compared with females, and concluded that selection should universally favour ‘an undiscriminating eagerness in the males and a discriminating passivity in the females’ to obtain mates. The conventional view of promiscuous, undiscriminating males and coy, choosy females has also been applied to our own species. Here, we challenge the view that evolutionary theory prescribes stereotyped sex roles in human beings, firstly by reviewing Batemans principles and recent sexual selection theory and, secondly, by examining data on mating behaviour and reproductive success in current and historic human populations. We argue that human mating strategies are unlikely to conform to a single universal pattern.

Reconsidering the null hypothesis: Is maternal rank associated with birth sex ratios in primate groups?

Trivers and Willard hypothesized that vertebrates adaptively vary the sex ratio of their offspring in response to the mothers physical condition [Trivers, R. L. & Willard, D. (1973) Science 179, 90–92]. This hypothesis has produced considerable debate within evolutionary biology. Here we use meta-analysis techniques to evaluate claims that nonhuman primate females facultatively adjust the sex ratio of their progeny in relation to their own dominance rank in a uniform way. The magnitude of the difference in birth sex ratios of high- and low-ranking females declines as sample sizes increase, and the mean difference in birth sex ratios of high- and low-ranking females is zero. These results suggest that the observed effects could be the product of stochastic variation in small samples. These findings indicate that presently we cannot reject the null hypothesis that maternal dominance rank is unrelated to birth sex ratios.

Lessons from animal teaching

Many species are known to acquire valuable life skills and information from others, but until recently it was widely believed that animals did not actively facilitate learning in others. Teaching was regarded as a uniquely human faculty. However, recent studies suggest that teaching might be more common in animals than previously thought. Teaching is present in bees, ants, babblers, meerkats and other carnivores but is absent in chimpanzees, a bizarre taxonomic distribution that makes sense if teaching is treated as a form of altruism. Drawing on both mechanistic and functional arguments, we integrate teaching with the broader field of animal social learning, and show how this aids understanding of how and why teaching evolved, and the diversity of teaching mechanisms.

Sex-biased investment in nonhuman primates: can Trivers & Willard's theory be tested?

Abstract The study of how parents divide investment between sons and daughters has become one of the most popular areas of discussion in evolutionary biology. Trivers & Willard (1973, Science , 179 , 90–92) suggested that the physical condition of the mother may influence whether she maximizes her lifetime reproductive success by biasing her investment towards sons or towards daughters. In this review I discuss the evidence for sex-biased maternal investment in primates. No consistent pattern emerges from data sets on sex differences in infant birth weights and growth rates, measures of nipple contact, and lengths of interbirth intervals with respect to maternal condition. Studies on birth sex ratios have also proven highly inconsistent. To test the Trivers & Willard hypothesis, information on birth sex ratios, sex-biased mortality before and after birth, and patterns of maternal care during prenatal and postnatal life are required in a single population. Currently, no such information is available for any one population of primates, and will be difficult to obtain, owing to the long life span and complexity of social interactions of primates. An additional difficulty of testing the Trivers & Willard hypothesis in primates is that it is hard to conceive of any empirical evidence that would disprove it as long as alternative adaptive explanations continue to be suggested for particular species and populations. A lack of correlation between maternal condition, birth sex ratio and maternal care could be interpreted as suggesting that additional factors have been overlooked. Direct predictions from the theory are required.

Begging, stealing, and offering : food transfer in nonhuman primates

Publisher Summary This chapter discusses the food transfer in nonhuman primates and compares the data with predictions that stem from various functional explanations of food transfer. The transfers among adult primates are described and investigate whether patterns of food transfer may be explained in terms of trade or reciprocity, or whether alternative explanations provide a better fit to the data. The transfer of solid food to infant primates from parents and alloparents are described, the inter- and intraspecific evidence that food transfer reduces time to weaning, increases the infants growth rate, or increases the infants chance of survival are reviewed in the chapter. The transfer of food to infants may ensure that infants receive nutrients during the critical transition to independent foraging (nutritional hypotheses), and/or learn about diet breadth or food-processing techniques (informational hypotheses). It is suggested that interactions between mothers and infants may result in the transmission of tool-using skills. The chapter also investigates whether there is any evidence that adults actively use food transfer to direct offspring learning, and discuss potential directions for future research.

Darwin in mind: new opportunities for evolutionary psychology.

Evolutionary Psychology (EP) views the human mind as organized into many modules, each underpinned by psychological adaptations designed to solve problems faced by our Pleistocene ancestors. We argue that the key tenets of the established EP paradigm require modification in the light of recent findings from a number of disciplines, including human genetics, evolutionary biology, cognitive neuroscience, developmental psychology, and paleoecology. For instance, many human genes have been subject to recent selective sweeps; humans play an active, constructive role in co-directing their own development and evolution; and experimental evidence often favours a general process, rather than a modular account, of cognition. A redefined EP could use the theoretical insights of modern evolutionary biology as a rich source of hypotheses concerning the human mind, and could exploit novel methods from a variety of adjacent research fields.

The Ontogeny of Exploratory Behavior in Male and Female Adolescent Rats (Rattus norvegicus)

During adolescence, rats gain independence from their mothers and disperse from the natal burrow, with males typically dispersing further than females. We predicted that, if dispersal patterns are associated with responsiveness to novelty, exploratory behavior in novel environments would increase across adolescence, and males would explore more than females. Alternatively, females might explore more than males, if females are more motivated than males to learn about the immediate environment or if females have poorer spatial abilities than males. Twenty-five male and 21 female rats were exposed to two novel environments (open field and elevated plus-maze) during early, mid-, or late adolescence. Total locomotion and amount of exploration directed towards aversive areas increased across adolescence, even when body weight was included as a covariate. Female adolescents locomoted more and spent more time exploring aversive areas than males. Developmental changes in neural function potentially underlie age and sex differences in exploratory behavior.

The exploratory behaviour of rats in the hole-board apparatus: Is head-dipping a valid measure of neophilia?

The exploratory behaviour of laboratory rodents is of interest within a number of areas of behavioural pharmacology. However, how best to measure exploratory behaviour in rodents remains a contentious issue. Many unconditioned tests, such as the open field, potentially confound general locomotor activity with exploration. The hole-board apparatus appears to avoid this confound, as head-dipping into holes in the floor is assumed to be a valid measure of the subjects attraction towards novelty (neophilia). This study aimed to investigate whether head-dipping should be considered a valid measure of neophilia by comparing performance of adult male and female Lister hooded rats on the hole-board task (a) over repeated sessions and (b) when novel objects were absent or present underneath the holes. The results show that head-dipping initially decreased across repeated exposures, while time spent in the aversive central area increased. No change in head-dipping was seen in response to objects being placed underneath the holes. Rather than being a measure of neophilia, these results support the hypothesis that head-dipping represents an escape response, which declines as the subject becomes less fearful. These results are compared with previous studies of repeated exposure to other novel environments.

Sex differences in sensation-seeking: a meta-analysis

Men score higher than women on measures of sensation-seeking, defined as a willingness to engage in novel or intense activities. This sex difference has been explained in terms of evolved psychological mechanisms or culturally transmitted social norms. We investigated whether sex differences in sensation-seeking have changed over recent years by conducting a meta-analysis of studies using Zuckermans Sensation Seeking Scale, version V (SSS-V). We found that sex differences in total SSS-V scores have remained stable across years, as have sex differences in Disinhibition and Boredom Susceptibility. In contrast, the sex difference in Thrill and Adventure Seeking has declined, possibly due to changes in social norms or out-dated questions on this sub-scale. Our results support the view that men and women differ in their propensity to report sensation-seeking characteristics, while behavioural manifestations of sensation-seeking vary over time. Sex differences in sensation-seeking could reflect genetically influenced predispositions interacting with socially transmitted information.

Evolutionary accounts of human behavioural diversity

Human beings persist in an extraordinary range of ecological settings, in the process exhibiting enormous behavioural diversity, both within and between populations. People vary in their social, mating and parental behaviour and have diverse and elaborate beliefs, traditions, norms and institutions. The aim of this theme issue is to ask whether, and how, evolutionary theory can help us to understand this diversity. In this introductory article, we provide a background to the debate surrounding how best to understand behavioural diversity using evolutionary models of human behaviour. In particular, we examine how diversity has been viewed by the main subdisciplines within the human evolutionary behavioural sciences, focusing in particular on the human behavioural ecology, evolutionary psychology and cultural evolution approaches. In addition to differences in focus and methodology, these subdisciplines have traditionally varied in the emphasis placed on human universals, ecological factors and socially learned behaviour, and on how they have addressed the issue of genetic variation. We reaffirm that evolutionary theory provides an essential framework for understanding behavioural diversity within and between human populations, but argue that greater integration between the subfields is critical to developing a satisfactory understanding of diversity.