Deborah, Ari and I. Photo by L. Drake
(Click on the following link to listen to an audio version of this blog … More on The active grandparent hypothesis
Deborah and I moved to Greeley, Colorado two and a half years ago primarily to help take care of our grandson, Ari. The pandemic had made daycare very difficult and both our daughter and her husband had full-time jobs. It was hard to leave all of our friends in Pennsylvania but the choice was emotionally (and, maybe, even evolutionarily) satisfying.
Zofia. Photo by L. Drake
This past summer a granddaughter, Zofia, joined our Greeley family-pod. It is an incredible experience to watch Ari’s growth into a sentient human being (his “I have an idea” stratagems that inevitably result in his getting an Oreo or some extra screen time are amazingly sophisticated if not positively Machiavellian!), and watching Zofia grow neurons and establish better and better synaptic pathways to control her body is like observing the miracle of creation itself! She is transform to functioning little human! Also, all she has to do is smile at me, and I am set for days! Both Deborah and I feel that being a day-to-day part of our grandchildren’s lives is one of the most important things we have ever done! Plus, I get to play with Hot Wheels, plastic dinosaurs, Legos and KNEX’s all over again (not to mention getting to be a “monster” in all sorts of chase games!)! Life is good!
This is the essence of the “active grandparent hypothesis” (a set of ideas that are also referred to as the “grandmother hypothesis”): if adults that have aged past their reproductive years can stay active and healthy, they can be very useful in helping to sustain and rear their children’s children or maybe even their children’s-children’s children! (see Signs of Summer 1, May 19, 2022). It is possible that the long span of post-reproductive life seen in human beings was, at least in part, evolutionarily selected for by the survival benefits that these extra life years convey on our offspring’s offspring!
Public Domain.
A few other species also demonstrate this “active grandparent phenomenon.” Young orcas in pods (especially young females) have an increased rate of survival if the mother of their mother is still living in the pod. These “grandmother” orcas provide food for the young, female whales, teach them how to hunt and maybe even protect them from other predators. The ecological cost of providing for a non-reproductive individual in pod is more than compensated for by the overall benefit to the young females (see Signs of Summer 1, June 4, 2020). Some new research just published in Current Biology (February 27, 2023) entitled “Costly lifetime maternal investment in killer whales,” examines these maternal and grand-maternal care-behaviors in orcas more closely.
Orcas are found all around the world. They tend to live, though, in geographically discrete populations. The orca population that was the focus of both this maternal behavioral study and also the grandmother behavior study mentioned above is the one that occupies the coastal waters off of the British Columbia, Washington and Oregon coast. This orca group is referred to as the “southern residents.”
Chinook salmon. USFWS. Public Domain.
The southern residents, like most orca groups, are very specialized predators. The southern resident orcas primarily rely on Chinook salmon for most of their food. Chinook salmon populations, though, have been, for a variety of reasons, declining over the past few decades and this decline has resulted in consequential declines in the southern resident numbers. Their current population is estimated to be only 73 individuals (there were 97 individuals in this group in 1996) . While most orca populations around the Earth are thriving, this southern resident population is considered to be on the verge of extinction.
A key aspect of the basic biology/ecology of a Chinook-salmon-hunting population of orcas is the recognition that these salmon are very fast, very agile swimmers. Adult male orcas, because of their great body sizes (they are 20 to 26 feet long and weigh around 12,000 pounds) have difficulty catching these salmon. Adult female orcas are smaller and much less bulky (they are 16 to 23 feet long but only weigh 6,000 to 8,000 pounds) and with experience can become quite efficient salmon catchers. This basic feature of the gender influenced, predator/prey relationship between orcas and the Chinook salmon is the foundation for the maternal and grand-maternal behaviors observed in the southern resident orcas.
Male orca (foreground) female orca (background). Public Domain.
Female orcas with male offspring tend to extensively share their salmon catch with their sons even after the sons have reached maturity. This long-term energy commitment to a son has a logical, evolutionary basis: male orcas can sire 20 or more calves in their lifetimes while a female during her lifetime is only capable of having 4 or 5 calves. Ensuring the health and survival of her son, then, is an excellent way to pass on more of her DNA to the next orca generation. The importance of this long-term maternal commitment has been shown in previous research which discovered that male orcas over 30 years of age were eight time more likely of die during the year following their mother’s death. The cause of death, apparently, was not grief but starvation!
Having a maternal grandmother to primarily care for and feed the female calves evens out the feeding benefit for the population. These female calves, presumably, learn the hunting techniques of their grandmothers through their interactions and, eventually, are able to feed themselves and then feed their own offspring in an efficient manner.
But, all of this is dependent on an abundance of Chinook salmon for food!
Orcas in Alaska. Photo by C. Michel. Wikimedia Commons.
When the salmon became scarce, the balance and benefit of these behaviors began to fall apart especially with regard to the maternal/son interaction. The female orca giving her son so much of her increasingly scarce salmon results in her becoming so deficient in caloric intake that she is half as likely to reproduce in a given year as a lone female or a female with a female calf. This removal of a significant percentage of the reproductive potential of these already slowly reproducing animals may be writing the final chapter on the viability and existence the southern resident population. There are just not enough calves being born each year to generate a sustainable population!.
So a behavior that made great evolutionary sense for many thousands of generations, may stop being a positive factor for survival under changing environmental conditions. It may even be transformed into an eroding force that leads to decline and extinction of the population. Inflexibility and resistance to change, as we have seen in other animals and also in many human societies, may be a significant hidden flaw in many grand schemes of existence!