How long does a calf stay with its mother?

In mammals, the length of time a young animal stays with its mother is a valuable indicator of its way of life: simple or complex societies, solitary or gregarious species, the ways in which skills are passed on. In cetaceans these durations run from the strict biological minimum, a few months are enough for a harbour porpoise to become independent, to astonishing extremes: among the resident killer whales of the north-eastern Pacific, sons and daughters stay with their mother for their whole lives. This spread reflects the extraordinary diversity of cetacean societies, and reveals how evolution fits reproductive strategy to the ecological constraints of each species.

The spectrum of durations

Lactation and cohabitation durations vary enormously from one species to another. The harbour porpoise is at the short end: lactation of about 8 months, rapid weaning, independence at around a year. This is a “fast” strategy, that of a short-lived mammal that breeds frequently and invests relatively modestly in each offspring.

The common bottlenose dolphin illustrates an intermediate strategy: lactation of 3 to 6 years, sometimes up to 8 years. Weaning is gradual, the calf starts eating fish in its first year while still suckling, and cohabitation with the mother continues well beyond that. It is also during this long period that complex hunting techniques are passed on, such as the use of marine sponges as tools among the dolphins of Shark Bay.

The mysticetes, despite their gigantic size, have relatively short lactation periods: 7 to 12 months for most rorquals and for the humpback whale. This is consistent with their migratory way of life: the mother has to rebuild her reserves quickly, and the calf learns filter feeding within a few months.

The sperm whale represents the other extreme of the spectrum. “Main” lactation lasts about 2 years, but some young go on suckling sporadically up to 13 years under conditions of stress or food scarcity (Whitehead, 2003). Young males stay with their maternal unit for 6 to 15 years before leaving it to join the groups of young males and then the high latitudes. Females, for their part, remain for life in their mother’s unit.

The exceptional case of resident killer whales

Among the resident killer whales of the north-eastern Pacific, the situation is unique in the mammalian world. Neither males nor females leave their maternal pod in the course of their lives. A 50-year-old adult killer whale may swim side by side with its 80-year-old mother, as it has done all its life. This total absence of dispersal is exceptional. In other socially complex mammals (elephants, chimpanzees) there is at least one sex that disperses, generally the males. In resident killer whales, neither does.

This structure produces particularly strong and lasting mother-son bonds. Darren Croft and his team published a disquieting study in Current Biology in 2017: the survival of adult males depends on the presence of their mother. More precisely, in the year following their mother’s death, adult sons carry a risk of death multiplied by 14. It is one of the most critical transgenerational bonds ever documented in a wild mammal. The maternal presence may provide better access to resources (the matriarch knows the productive areas and periods), social protection, or simply emotional and physiological support.

Menopause, an extremely rare phenomenon

The killer whale, with two other species, shows true menopause. The term denotes a cessation of reproduction long before death, unlike ordinary mammals, which remain fertile until the end of their lives. Beyond humans, only three cetacean species are concerned: the killer whale, the short-finned pilot whale and the beluga. That is all. Evolution has not selected this trait anywhere else in the mammalian kingdom.

The hypothesis known as the grandmother effect proposes that these post-menopausal females go on contributing significantly to the reproductive fitness of their family by sharing their knowledge, minding the young, locating resources and maintaining social cohesion. Natural selection would thus have favoured a long post-reproductive life wherever intergenerational investment is essential. A study by Emma Foster et al. in 2012 in Science measured this effect directly in resident killer whales: the presence of a post-menopausal mother in the group significantly increases the survival rate of her grandchildren.

The male sperm whale, solitary wanderer

The sperm whale illustrates a radically different strategy. Females remain for life in their maternal unit. Males, by contrast, follow a trajectory resembling that of several primates: between the ages of 6 and 15 they leave their natal unit and join “bachelor groups” of unrelated young males. These groups break up gradually with age, and the older males become solitary, migrating towards the high latitudes where they hunt squid in cold water. They return to temperate and tropical waters only sporadically, to breed.

This segregation by sex and age produces a particularly complex geography for the species: tropical waters are occupied essentially by females and young; polar waters are occupied by solitary adult males. See our page on sperm whale clans.

Sources

• Croft, D.P. et al. (2017). “Reproductive conflict and the evolution of menopause in killer whales”. Current Biology, 27.
• Whitehead, H. (2003). Sperm Whales: Social Evolution in the Ocean. University of Chicago Press.
• Mann, J., Connor, R.C., Tyack, P.L. & Whitehead, H. (eds.) (2000). Cetacean Societies. University of Chicago Press.
• Foster, E.A. et al. (2012). “Adaptive prolonged postreproductive life span in killer whales”. Science, 337.
• Wells, R.S. (2003). “Dolphin social complexity: Lessons from long-term study and life history”. In de Waal & Tyack (eds.), Animal Social Complexity.

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Further reading