Why men are men...

This book is the proceedings of a conference on comparative physiology with the stated aim of fostering a wider awareness of the diversity of research in sex differences. The various chapters convey this extremely well. Instead of references in the text, each author gives a short list of suggested further readings and cross-references to studies contained in other chapters. The result is an extremely readable set of introductions to current work over a full range of studies of sexual dimorphisms. While useful for honours undergraduates (mine found the chapters on comparisons across species and mating systems excellent), it is also an important reference for researchers.

R. V. Short begins with an historical overview of why different sexes exist and speculates about the origin of anisogamy. Is the large female vertebrate egg to ensure the transmission of mutation-free mitochondria? The chapters range from the constraints on parthenogenesis, the costs of producing sons or daughters, and the surprising extent of vocal communication in the African elephant, to the production of pheromones in drosophila and the influence of the local environment on the differentiation of the gonads in some vertebrates.

Comparisons are central to many explanations of sex differences and cross-taxa comparisons are used here to describe the ways sexual selection can produce differences between the sexes, the ways sex change in fishes is condition dependent, and the size of weapons in mammals. R. D. Martin and his colleagues examined selection acting on both sexes in primates rather than just using females as a baseline for comparison with males. They found evidence for selection not only for the usual increased body size in males, but also for decreased body size in females compatible with selection for increased reproductive output. The translation of genotypic sex into phenotypic sex and sexual dimorphisms in various organs is an area of vigorous current research. Due to their relatively undifferentiated gonads at birth, the effects of hormonal administration are better understood in marsupials than in eutherian mammals, but the rule that sex differences are solely the result of gonadal secretions does not apply in marsupials. To my surprise, sexually dimorphic metabolic processes in the liver appear to be masculine unless feminised by secretions of the pituitary, whereas the default sex for the gonads is female.

Sexual dimorphism in areas of the brain has been described for years in terms of the overall size or neuron number of a brain area, but these are poor parameters to use when trying to link structures with behavioural functions. M. Gahr argues that there will be little progress in making direct links between brain sex differences and behaviour until we know how sexually dimorphic cells actually function within their neural circuits.

J. A. Marshall Graves presents a very readable and fascinating detective story of the search to pinpoint and sequence the SRY gene, the gene that controls the development of the testis. While some progress has been made towards understanding sex determination for most mammals, there is a set of delightful rodents that break all the rules for normal chromosomal sex determination. Some wood lemmings have the male XY karyotype, yet are female and only give birth to daughters. Both males and female mole voles are XO and after 40 years of research we still do not know how sex is determined.

Many studies have produced conflicting results for testosterone implants and aggression and reproductive success in birds, sometimes within the same species. J. C. Wingfield used both laboratory and field studies to show that the stability of the social group and the breeding system have to be taken into account. He found a relationship between degrees of dimorphism and male and female testosterone levels. As dimorphism in size, plumage and behaviour increases, so does the ratio of male to female testosterone levels for monogamous birds, but not for polygamous species. The relationship for monogamous species would be even stronger but for the inclusion of the mallard as a monogamous species. Since mallard drakes frequently force copulations on other females and give no parental care, this is a curious use of the term monogamous. DNA fingerprinting studies on birds over the past few years have made it clear that monogamy is better defined as a system where parental care is shared by one male and one female than by the number of sexual partners.

Jeff Graves is senior scientific officer, school of biological and medical sciences, University of St Andrews.