Parental care in humans is remarkably extensive, with many individuals living only a fraction of their lives outside of the sphere of parental influence, and parental care is exceptionally plastic, with the extent and form of care varying across contexts and individuals. Behavioral ecological studies of parenting fall into three main foci. First, parental care is studied in terms of life history tradeoffs, including between mating and parenting effort, and between fertility and investment per offspring. Second, parental care is studied as a conflict trait, including sibling conflict, parent-offspring conflict, and sexual conflict between mothers and fathers. Finally, parental investment theory addresses how individual characteristics, such as birth order, sex and putative relatedness, influence parental strategies. For each focus, predictions are context-specific with socioecology dictating the local costs and payoffs to parental care. Over 50 years of behavioral ecological studies of parenting have transformed understandings of the human family, exposing the fragile mix of cooperation and competition underlying family relationships. Questions remain, however, particularly with regard to parental strategies in nonnuclear family structures, the role of physiological, psychological and cultural mechanisms in regulating adaptive strategies, and the coevolution of parental care with culturally variable beliefs about kinship, gender and reproduction.

For most people, the most obvious thought or image that sexual reproduction brings to mind is that of sexual intercourse, a mating between two individuals of opposite sexes, which will result in the birth of their common offspring. While biparental reproduction is certainly the most common mode of sexual reproduction among all eukaryotes, it is not the only one, and the way it is carried out can depart substantially, in many different ways, from the ‘canonical’ description above. What is common to all these modes is that two distinct sexually compatible individuals (parents) undertake a sexual exchange that leads to the generation of new individuals with a genetic constitution obtained from the association and/or the reassortment of those parents’ genomes. The key event in this mode of reproduction, technically called amphigony, is the fusion of two gametes or two nuclei functioning as gametes (syngamy), each produced by one parent, to form a zygote. While in species with anisogamy (i.e. with distinct male and female gametes; Chapter 4), only gametes of opposite sex are compatible, the two individuals that produce them are not necessarily a male and a female.

We are all familiar with the changes in an organism during development, followed by its reproduction, which are repeated generation after generation. Biologists describe this development–reproduction sequence as the life cycle: the series of transformations and reproductive events that, from a given stage of life of an organism, leads to the corresponding stage in a subsequent generation. We can describe a biological cycle as going from zygote to zygote, but also from adult to adult, or from embryo to embryo: in a cyclical process, the choice of the ‘initial phase’ is arbitrary or conventional, as the notorious ‘the chicken or the egg’ dilemma beautifully illustrates.

In the course of their lives, organisms spend time and energy on a number of activities and functions, of which reproduction is only one – think of growth, defence against predators and pests, and others. How many resources are used for reproduction, how much time is devoted to it and how this time is distributed over the course of life are all elements that characterize the different reproductive strategies. From an even wider perspective, in those organisms that at certain times in their lives can opt for one or another reproductive mode (e.g. sexual or asexual reproduction, as in many plants and many marine invertebrates), a reproductive strategy includes also this reproductive policy.

On February 1997 the birth was announced of a sheep named Dolly, the first mammal to be cloned from an adult cell of a mother individual. The event attracted enormous media attention. Dolly, born on 5 July 1996, actually had three ‘mothers’: one provided the egg (whose nucleus was removed), another the nucleus with the DNA picked out from a somatic cell (i.e. a cell of the body not specialized for reproduction), while the third mother carried the cloned embryo in her womb until parturition.

Ever since living beings arose from non-living organic compounds on a primordial planet, more than 3.5 billion years ago, a multitude of organisms has unceasingly flourished by means of the reproduction of pre-existing organisms. Through reproduction, living beings generate other material systems that to some extent are of the same kind as themselves. The succession of generations through reproduction is an essential element of the continuity of life. Not surprisingly, the ability to reproduce is acknowledged as one of the most important properties to characterize living systems. But let’s step back and put reproduction in a wider context, the endurance of material systems.

Acquiring the traits specific to a given sex, during early development or at another point during the life of an organism, is usually a complex process. Although the sex condition of an individual is conventionally defined based on the type of gametes it is able to produce (Chapter 4), the sex-specific phenotype is generally not limited to the organs of reproduction. Each of these characters can maintain a certain degree of independence from other sexual traits in the same organism, be subject to different developmental control, and show different degrees of sensitivity to the environment. Therefore, sexual differentiation extends to the development of the secondary sexual characters, which can be morphological, physiological, behavioural, or combinations of these. An exploration of this fascinating subject requires some preliminary clarification about systems and mechanisms of sex determination and sex differentiation.

In Chapter 1 we defined sexual reproduction as a form of reproduction that generates new individuals carrying a genome obtained by the association and/or the reassortment of genetic material from more than one source. In the most familiar form of sexual reproduction, the new genome is formed by the union of (partial) copies of the genomes of two parents through the fusion of two special cells produced for that purpose, the gametes, into a single cell, the zygote. This is the way most multicellular eukaryotes, ourselves included, reproduce sexually.

A zygote does not necessarily derive from the fusion of gametes or gametic nuclei produced by different individuals. Both egg and sperm may instead be produced by the same individual, a sufficient simultaneous hermaphrodite (Chapter 4). In this case, the offspring has only one parent. However, the gametes that merge are the products of independent processes of meiosis undergone by different germ cells, although in the same individual: this distinguishes self-fertilization (or selfing) from some forms of parthenogenesis where there is the fusion of two of the four nuclei deriving from the same meiosis, as we will see in the next sections (Figure 6.1).

Sexual conflict arises from differences in the fitness interests of males and females. A trait that is beneficial for the reproductive success of one sex reduces the fitness of the other sex, resulting in opposing selection pressures on the two sexes. The two sexes need each other for reproduction, but their dependence is asymmetric. Males benefit from a higher mating rate than females, as their reproductive success is usually constrained by the number of receptive mates, while female reproductive success is limited by egg production. Sexual conflict can occur at any stage of reproductive interactions – before or during copulation, or after insemination – and over almost any aspect of reproduction, from the decision to mate to the investment into parental care. The conflict results in a perpetual tug of war between the sexes. Each sex attempts to maximize its fitness at a cost to the other sex, which results in sexually antagonistic selection. This can cause the rapid evolution of sexual traits, and ultimately results in the diversification of traits, and possibly even in speciation. Sexual conflict can manifest in two ways, intra- and interlocus sexual conflict. Intralocus sexual conflict occurs when a trait expressed in both sexes (determined by alleles in the same locus in the two sexes) has opposite effects on male and female fitness. Interlocus sexual conflict occurs when the conflicting traits are determined by alleles in different loci in the two sexes and the optimal outcome of male–female interactions differs between the two sexes. Intralocus sexual conflict generates a genetic tug of war between the sexes over the optimal trait expression, while interlocus sexual conflict can lead to open-ended cycles of sexually antagonistic coevolution. Sexual conflict before mating has resulted in a diversity of tactics and strategies that males use to overcome female unwillingness to mate, from forced copulations and sneaky fertilization to the emission of love darts. Sexual conflict after mating has favored the evolution of male traits that increase success in sperm competition, such as postcopulatory mate guarding, toxic seminal substances that destroy the sperm of other males, mating plugs that prevent other males from mating with the female, and morphological and physiological traits that harm females, such as spines on the intromittent organ that pierce the reproductive tract of the female. Females have evolved cryptic choice of sperm to influence which males fertilize her ova. Whether the sexual conflict between males and females can be resolved depends on the type of conflict. Several mechanisms may reduce the strength of intralocus conflict, such as sex-limited expression of traits, but the interests of males and females are unlikely to become aligned when it comes to interlocus conflict. The relative influence of sexual conflict on the fitness of organisms, and the degree to which it can be resolved, are open questions.

This chapter discusses interference in minority affairs when they engage in physical harm to others. Relevant considerations are the extent of harm, consent (or lack of) of those who are subjected to harm, parental care and responsibility, significance of religious and culture norms and values, and the extent to which a liberal society should intervene in group and individual affairs. It first analyses the practices of suttee, self-starvation, scarring, murder for family honour, female circumcision and female genital mutilation. It is argued that liberal intervention is justified in the case of gross and systematic violation of human rights, such as murder, slavery, expulsion or inflicting severe bodily harm on certain individuals or groups. Such norms are considered by liberal standards to be intrinsically wrong, wrong by their very nature. Physical harm includes cases of widow burning, female infanticide, murder for family honour, and harsh forms of female circumcision, deformation or alteration which are rightly termed female genital mutilation.

This chapter describes the breeding cycle of kestrels from egg laying to the nestling-rearing phase. It illustrates the different reproductive strategies of males and females, the endogenous mechanisms and environmental conditions that affect the laying date and the clutch size, the adaptive meanings of egg volume and of hatching asynchrony, and the factors that affect the probability of nestlings surviving until fledging.

Chapter 4 deals with investment in reproduction by organisms, first considering the destiny and care of the products of reproduction and the alternative between oviparity and viviparity, followed by a brief mention of the forms of parental care given to the offspring by one or other parent, or both. Some energetic and metabolic aspects of reproduction, in particular vitellogenesis in animals and the formation of endosperm in flowering plants, are also considered, concluding with a short section on the various strategies of parental investment, including different fecundity/fertility levels, considered against different environmental contexts.