Comparative reproductive biology

Comparative reproductive biology looks across species, from fish and birds to rodents, ruminants and primates, to understand which features of reproduction are ancient and widely shared and which have evolved separately in particular groups. The approach serves two purposes. It reveals general principles, such as the central role of the brain hormone GnRH, and it warns against assuming that results from one laboratory species apply automatically to others, including humans.

Many ways to make males and females

Because humans and many familiar animals have X and Y chromosomes, it is tempting to think that sex is determined the same way everywhere. In fact the mechanisms are remarkably diverse and can change quickly over evolutionary time. Almost all mammals and many flies have XY systems, in which males carry two different sex chromosomes; birds, snakes, butterflies and some fish have ZW systems, in which females do. In all crocodiles, most turtles and some fish, the temperature at which eggs develop decides sex. In ants, bees and wasps, males develop from unfertilised eggs, a system found in about 12 percent of animal species. Hermaphrodites, individuals with both male and female organs, are the rule in most flowering plants but make up only around 5 percent of animal species.

Despite this variety at the top of the cascade, some of the genes acting further down, which direct the developing gonad towards testis or ovary, are shared across distant groups. Why sex determination systems turn over so often is still debated; conflicts between the interests of different parts of the genome, or of the two sexes, are among the leading explanations, but few systems have been studied in enough depth to test them.

Two ways to trigger ovulation

GnRH controls reproduction in all vertebrates, but mammals use it in two distinct ways to trigger ovulation. In spontaneous ovulators, including rats, mice, guinea pigs, sheep, monkeys and women, rising oestrogen from maturing follicles provokes the surge of luteinising hormone that releases the egg. In induced ovulators, such as rabbits, ferrets, cats and camels, the surge is triggered by nerve signals generated during mating, which activate noradrenaline-producing neurons in the brainstem that in turn stimulate GnRH release. In these species, oestrogen acts in the brain to promote sexual behaviour, while its ability to trigger the surge on its own appears reduced or absent. The boundary is not absolute: mating-induced surges occasionally occur in spontaneous ovulators. Because most research on GnRH has used spontaneous ovulators, induced ovulators remain comparatively under-studied, and comparisons between the two groups are still refining models of how ovulation is controlled.

Model organisms and their limits

Comparative work also depends on choosing the right model. The zebrafish, a small tropical fish with almost transparent embryos, has become a popular species for studying vertebrate gene function. When its genome was sequenced, roughly 70 percent of human genes were found to have at least one obvious zebrafish counterpart, and the analysis also identified chromosome regions that influence sex determination in the fish. Such overlap makes fish useful for studying gonad development and fertility, but the remaining differences are a reminder that findings must be tested in other species before being generalised. Rodents, rabbits, ruminants and birds, all well represented in the archive, fill similar roles for other questions.

Stress and reproduction across species

Stress responses offer another example of a shared system with species-specific details. When an animal faces a challenge, its adrenal glands release glucocorticoid hormones such as cortisol or corticosterone. Elevated glucocorticoids can inhibit reproduction at several levels: they can reduce GnRH release from the hypothalamus, suppress the release of the pituitary hormones that drive the gonads, and act directly on the testis or ovary. Balanced glucocorticoid levels are needed for normal gonadal function, and exposure to stress or high glucocorticoid levels before birth can have lasting effects on the offspring’s own stress system. How these effects differ between species and between males and females remains an active comparative question.

Can mothers influence the sex of their offspring?

A long-running debate in evolutionary biology concerns whether mammalian mothers can bias the sex ratio of their offspring. A 1973 theory, known as the Trivers–Willard hypothesis, proposed that when mothers are in good condition, natural selection should favour producing sons, who can gain more from extra investment, and that mothers in poor condition should favour daughters. The original paper cited mammal data consistent with this idea.

Testing it has proved difficult. A 2023 analysis of five decades of records from wild baboons in Kenya, covering 1,372 offspring, found no support for adaptive biasing of offspring sex, and its authors noted that evidence for such hypotheses in mammals, especially primates, has been mixed. Whether and how mothers could adjust offspring sex around the time of conception is still an open question.

What is established and what is still debated

• Established: sex determination mechanisms are diverse but share some downstream genes; mammals ovulate either spontaneously or in response to mating; stress hormones can suppress the reproductive axis; zebrafish share many genes with humans.
• Still debated: why sex determination systems change so often; how induced ovulation is controlled at the molecular level; how far stress effects differ between species and sexes; and whether mammals adaptively adjust offspring sex ratios.

The archive collected under this topic is unusually broad. Alongside research on cell adhesion molecules, signalling pathways and the effects of plant extracts on fertility in rats, it includes society proceedings and obituaries of pioneers of reproductive endocrinology published in the 1960s, a reminder that the comparative approach has been part of the discipline since its early days. For a conservation perspective on the same diversity, see the wildlife reproduction and conservation page.