How is it possible then to sustain groups in which some individuals are prevented from breeding? They would have no lifetime reproductive success, none of their characteristics could be passed on to offspring.
Name two of the species in LoM that had females in the group that were unable to breed or were prevented from breeding.
Marmot [p. 67]. Naked mole-rat [p. 78].
What function is ascribed to the behaviour of the young female marmots?
They assist in keeping the burrow warm in the winter, enabling their mother's current litter to survive the sub-zero temperatures [p. 67].
Such a behaviour pattern, that benefits another individual at a cost to the performer's own individual fitness, is said to be altruistic. This is another word with a specific biological meaning that differs from lay usage and certainly does not have the same moral connotations.
Is parental behaviour altruistic by this definition?
No, because the parenting increases the offspring's chance of surviving long enough to breed itself which, by definition, increases the parent's fitness.
In fact, careful thought shows that the daughter's non-breeding behaviour is not altruistic either because they are increasing the fitness of close relatives. A number of characteristics that have passed to them from their mother, and in particular the tendency to remain with her (which they can potentially pass on to their own offspring), are also passed to siblings and indeed are found in other close relatives. In such an instance, particular genes of an individual - genes being the factors that give rise to the characteristics in question - have been selected. But in this instance, an individual's behaviour has increased the chances of her relative's genes being carried into future generations - genes that the relatives are likely to have in common. Selection of this type is termed kin selection, and working to increase the fitness of your relatives benefits your own inclusive fitness. Inclusive fitness is your own lifetime reproductive success plus the lifetime reproductive success of related individuals that share, by inheritance, the altruistic characteristic.
How long do the young marmots remain with their parents?
LoM suggests that the daughters only remain with their mother for a year [pp. 66-67]. DA says 'Marmots live in families of up to twenty individuals - a female and her mate, together with one or two young that were born at the beginning of the summer, and sometimes one or two of the female's sisters'. In the TV programme you saw that the young males leave home during the summer.
What would happen to a young female that left home with her male siblings?
Any young female that left the group in the first summer along with the young males might mate and produce young, but without the warmth of the group of female bodies her young are more likely to die.
That is a strong selection pressure operating against the characteristic of 'running away from home in your first year of life'. However, the following year a group of young females, perhaps with one of their mother's sisters leave and form a new colony. For marmots, living in an extended family group in this way is not without its risks and costs - the amount of inbreeding (i.e. mating between closely related males and females) is known to increase but although in other species such a habit would lead an increased incidence of deformity and weakness, marmots for some reason seem to suffer no obvious deleterious effect. So the advantages of this form of group living must outweigh the disadvantages. Increased survival during hibernation apart, the origins of such 'staying at home' behaviour may reflect a severe limit on available new habitat - when suitable accommodation is at a premium, gaining social experience by remaining on home territory makes particular sense.
DA also describes the life of another rodent that lives in groups, Belding's ground squirrels. These animals live in much larger colonies than marmots for they are 'several hundred strong' [p. 74]. When foraging above ground they are vulnerable to predators but there are always some individuals on guard. 'But sentry duty is not shared by all. It is a dangerous job. … Matrons in this society, it seems, are more prepared than anyone else in the family to give their lives on behalf of the next generation' [p. 74].
Why is it the females with young who are most prepared to take on sentry duty?
I am sure that you spotted that the chance of improving the survival odds for her offspring would be likely to benefit her inclusive fitness more than saving her own skin. Her offspring are younger than her, so they have greater potential to produce more offspring than she has.
You may wonder why the same argument doesn't show that the males should also take turns at sentry duty. To understand this paradox you will have to look a little more closely at the sexual habits of rodents.