Life history of cetaceans

Life history concerns the timing and patterns of growth, survival, and reproduction of an organism. Animals that take a long time to reach reproductive age also live long lives and are said to have “slow” life histories. Life history parameters include gestation duration, physical size, brain size, growth rates, age of weaning, age of sexual maturity, age of last reproduction, maximum and expected lifespan. Cetaceans exhibit some of the most fascinating life history traits among mammals and these traits set the stage for every facet of social living.

A group of Indo-Pacific bottlenose dolphins off the island of Reunion. Indo-Pacific bottlenose dolphins are slightly smaller than common bottlenose dolphins and speckle ventrally with age. They can live for up to six decades.

LIFE EXPECTANCY

Cetaceans are at some of the extremes in terms of life history traits because many species live for decades. The bowhead whale is the longest-living mammal at more than 100 or even 200 years. Killer whales have also been documented as living for over 100 years. Wild bottlenose dolphins in Sarasota Bay, Florida, are living into their sixties. Sexual maturity (onset of reproduction) can be as late as the teens for many dolphin and whale species. Mysticetes have a faster life history strategy than odontocetes in that they grow faster, reach sexual maturity within a few years, and wean their offspring earlier. A prominent exception is the bowhead whale, where females reach sexual maturity at about 20 years of age and nurse their offspring for longer than most mysticetes.

Estimated lifespan and gestation period of cetaceans.

DEVELOPMENT

Prolonged development, specifically a long infancy and juvenile period, is associated with social complexity and relatively large brain size. Life history theory does help explain this pattern, as it is a theory about trade-offs, namely: trade-offs between growth and reproduction, trade-offs between growth and survival, trade-offs between brain and body size, and trade-offs between survival and reproduction. Large animals, such as whales, tend have good survival and reproductive prospects, but it takes time to grow big. Animals that invest in growth must “wait” to reproduce because reproducing too early detracts energy from growth. Reproduction, and especially early reproduction, also increases mortality risk for both sexes because both are shifting effort and resources away from self-maintenance and toward whatever it takes to reproduce. And, finally, growing and maintaining a large brain is energetically expensive, more so than any other organ. Dolphins, compared to the larger mysticetes, exemplify the pattern whereby species with larger brains have slower growth and reach adult size later. Put simply, it is difficult to grow a big brain and a big body at the same time. Presumably natural selection has favoured brain-size expansion in dolphins and body-size expansion in whales. There is good evidence for this in the anatomical record of ancient cetacean skulls and bones—which show the divergence of odontocetes and mysticetes.

Sperm whales, the largest of the odontocetes, produce clicks for sonar and communication. They can live for up to eight decades.

With a prolonged developmental period, there is also a longer period of learning before facing the challenges of adulthood, namely reproduction. Given that it takes dolphins a decade or more to reach maturity, how they spend their time is critical. The most common hypothesis of a prolonged juvenile period is the “learning hypothesis,” whereby delaying reproduction allows young to develop social, cultural, and ecological skills that favour later survival and reproductive success. This is effective only if adult and juvenile mortality is low—otherwise speedy reproduction would be the way to succeed. In species with high social complexity, larger brains would be favoured, which would in turn favour slower growth. Those that spent their youth well would be better prepared to take on the vagaries and intricacies of social life. Primates, dolphins, elephants, and a few other species fit this pattern—even though the extent of social complexity for many delphinids is not yet known.

FAMILY BONDS & GRANDMOTHERS

In odontocetes such as killer whales, bottlenose dolphins, and pilot whales, a female might have several generations of offspring that associate with her. One adult female in our Shark Bay dolphin population is a great-grandmother, with five surviving offspring ranging in age from 30 to 6. Her 30-year old son checks in every once in a while, spending time with his siblings, nieces, and nephews. Persistence of family bonds across the lifespan could have important benefits. Kin have a stronger tendency to protect each other and share resources than non-kin. In other words, the benefits of group living are enhanced among kin because they tend to cooperate and share more than unrelated individuals. This is sometimes referred to as kin-altruism.

Long-finned pilot whales are highly sociable and remain in family groups, making them an easy target for hunters in the Faroe Islands and Japan.

For killer whales, short-finned pilot whales, and possibly false killer whales, this is taken one step further, showing evidence of menopause, a trait once thought to be uniquely human. Menopause has consistently puzzled evolutionary biologists because why would natural selection favour cessation of reproduction? Some have posited that menopause is just a by-product of modern living where humans outlive their normal reproductive lifespan. Theories about menopause are varied and complex, but there is growing consensus that menopause is not really about ceasing reproduction, but about prolonging the lifespan. In all the species mentioned, females stop reproduction in their forties, but live until their sixties or later. Granny, the infamous killer whale in the waters off British Columbia, was born circa 1911 and was still swimming around with her elderly offspring until her death in 2016. As mentioned earlier, there is a trade-off between reproduction and survival. If survival is favoured, it is difficult to favour reproduction simultaneously. All whales with menopause have their adult offspring still living with them in strong matrilineal units—where neither sex disperses from the group. The preferred hypothesis is that post-reproductive females provide benefits such as information and resources to adult offspring. If they reproduced until death, the last few offspring would be unlikely to survive, and perhaps the adult offspring wouldn’t do very well either.

Evidence in support of this comes mostly from over four decades of research into fish-eating killer whales. The eldest matriarchs are the leaders of their groups, and this becomes more apparent when salmon supplies are low. Killer whale females share fish with male kin, who, with their larger size might be less able to manoeuvre to catch prey. In addition, killer whale adult sons experienced higher than expected mortality following the death of their mothers. Finally, short-finned pilot whales live into their sixties, unlike the closely related long-finned pilot whales, which do not have a post-reproductive lifespan, and die in their forties. It is curious as to why, with similar social structures, short-finned pilot whales found the secret to long life while long-finned pilot whales did not. Such mysterious patterns cry out for scientific investigation.

Extract taken from Deep Thinkers: An Exploration of Intelligence in Whales, Dolphins, and Porpoises by Janet Mann, Published by Ivy Press, RRP £20.