Mice

Mice serve as evidence in two separate arguments in The Selfish Gene, and the examples reveal different levels at which reproductive outcomes can be explained. In an enclosure experiment, overcrowding was followed by lower fertility among females, a result that could fit either group-selection reasoning or an account based on individual reproductive advantage. In a separate discussion of mate desertion and replacement, a male mouse’s chemical signal is proposed as a cause of pregnancy loss when a female encounters the scent of a male other than her former mate. The first case concerns population density and birth rates; the second concerns the reproductive interests of particular males and females.

Population Density and Birth Rates

In the experiment described in the book, mice were placed in an outdoor enclosure with abundant food and allowed to breed freely. The population increased for a time, then levelled off. The observed explanation was not a shortage of food: females became less fertile as a consequence of overcrowding and had fewer babies. The example establishes a connection between density and reproductive output, but by itself does not settle why such a response evolved.

The distinction between an observed effect and its evolutionary explanation is central to the argument. The immediate cause of reduced fertility was often labelled stress, but the author notes that naming a proximate cause does not explain its evolutionary significance. The further question is why natural selection would favour females whose birth rates decline under crowded conditions. Thus the enclosure result is treated as a starting point for comparing competing explanations, not as proof of either one.

Competing Accounts of Regulation

Wynne-Edwards’s group-selection account treats overcrowding as a signal that animals use to regulate reproduction for the welfare of their group. On this view, females reduce births so that the population does not overuse food supplies. The enclosure experiment had abundant food, but the proposed explanation allows that mice may be adapted to conditions in the wild, where crowding could reliably foreshadow famine. They need not assess the food supply in the enclosure itself for the response to have been shaped by environments in which density predicted scarcity.

The gene-centred alternative accepts that overcrowding can lower birth rates but interprets the adjustment through the reproductive success of individual females. In the account associated with David Lack’s theory, a female tends to have the number of offspring that lets her rear the greatest number successfully. If crowding signals a likely future famine, the number she can rear may be lower than it would be when the population is sparse. A female that responds to this warning by bearing fewer offspring may ultimately rear more than a rival that bears more but cannot raise them successfully.

The two explanations can therefore arrive at nearly the same observed outcome—reduced reproduction in crowded conditions—while assigning it different evolutionary causes. Under the group-selection account, restraint benefits the population as a whole. Under the gene-centred account, it can result from competition among individuals whose reproductive success depends on how many young they can rear. The example shows why a behavioural or demographic pattern alone cannot establish that animals restrain reproduction for the good of the group.

The Bruce Effect

Mice also enter a discussion of what may happen when a female has been deserted by a mate. R. L. Trivers considers the possibility that a female might induce a new male to adopt a developing offspring as his own. Such adoption could benefit her, but the offspring would carry no genes from that male. The book argues that natural selection would therefore penalize male gullibility and could favour responses that prevent investment in another male’s offspring.

The proposed mouse example is the Bruce effect. A male mouse secretes a chemical that, when smelled by a pregnant female, can cause her to abort. According to the account, this response occurs when the scent differs from that of her former mate. The author presents the effect as a possible way for a male to eliminate potential step-offspring and make the female receptive to his own advances. This is an evolutionary interpretation of the reported response, rather than evidence that the male consciously plans or understands its consequences.

The same phenomenon is also used to contrast explanations of animal population control. Wynne-Edwards interprets the Bruce effect as a mechanism of population regulation. The gene-centred reading instead places it within conflict over reproductive interests: a male’s response may prevent investment in offspring that are not his. The example thus shifts the discussion from density-dependent fertility across a population to competition surrounding a particular female’s reproduction.

Significance in the Book’s Argument

Together, the two mouse examples show how similar reproductive outcomes can be placed within sharply different explanatory frameworks. Reduced fertility in a crowded enclosure may look like restraint that protects a population, yet it can also be interpreted as an individual response to conditions that affect the survival of offspring. The Bruce effect can likewise be described as population control or as a consequence of male reproductive interests. In both cases, the book distinguishes what is observed from the evolutionary account proposed to explain it.

The examples also fit the broader account of genes and behaviour developed in The Selfish Gene. Genes do not make moment-to-moment decisions; nervous systems mediate behaviour, and genetic effects operate through development and interactions among genes and environment. Accordingly, a gene associated with a behavioural tendency is not a solitary cause that constructs the whole behaviour. It is a shorthand for a genetic influence that can alter the likelihood of an outcome amid many other developmental conditions. The mouse cases make the gene-centred argument concrete while showing that an explanation of reproductive behaviour must separate immediate mechanisms, environmental circumstances, and the longer-term consequences of differential reproduction.