Whispering Past, the Graveyard
When I was a boy, one of our favourite summer outings was to the camp of our relatives on Grand Lake, New Brunswick (a “camp” in the Maritimes is what other Canadians call a “cottage,” or what some of the rest of you might call a “cabin”). Once there, we would swim in the lake or maybe pick strawberries or raspberries in the old fields, evidence of the farms that once occupied the sandy soil of this area. And we would take long walks down the gravel Scotchtown road to the Thoroughfare, where the rippling Grand Lake Meadows grade gently into the blue of the lake. The area was the ghost of a late eighteenth century settlement, with a few dispersed farmhouses, summer vacation places, forest, and a well-tended little cemetery.
My family were all interested in the past (my father was an historian), and we would always stop at the cemetery to read the gravestones. If we travelled elsewhere, we would do the same thing: walk in cemeteries and read the inscriptions. We were taught to be respectful, not yelling and running, and never stepping on the graves. Unlike my own children, I never found graveyards at all creepy, probably because we spent so much time visiting them. At 10, I did find it boring to spend more than three minutes reading gravestones, but 40 years later the experience has stuck with me.
I realized this a few weeks back, as I sat in a graduate seminar discussing the lives and interactions of ancient animals. Jeff, the student, was describing a book chapter on evolutionary strategies of species, and he made a statement that really caught my attention. He said, quoting the textbook, that humans belong to the category of organisms that have the strategy of producing relatively few offspring, because the great majority of us live to be mature adults. My immense experience on the graveyard shift told me that, while humans do produce relatively few offspring, through most of the history of our species it has not been an expectation that the great majority of babies will live to a ripe old age.
But perhaps I should first explain how paleontologists know about the life expectancies of long-dead creatures. We tend to think of fossils as living animals, but of course they are not; they are the preserved remnants of dead ones. Unlike population biologists who can capture live animals and measure for various features to determine growth and age, we are stuck with trying to understand life cycles from the dead bodies. Bodies may tell us a lot about how “our” animals died, but nevertheless, some of the bodies can also give information about the creatures’ lives.
Paleontologists have to use indirect methods, which range from very good to extremely difficult, depending on the organisms being studied. Some things have growth rings: we can examine yearly or even finer increments in trees, corals, clams, and many vertebrates. Some animals moult and we can collect their moult stages; this is true of arthropods, in particular. These creatures are like cemeteries that are full of distinct tombstones; given enough time and energy, we can walk around and gather the inscribed information.We can use all of this to reconstruct life histories and develop graphs of death and survival. Survivorship curves are known for modern creatures and can be developed for extinct ones. For example, considerable work has been done on tyrannosaurs.
But many other fossils are not particularly willing to share how old they were at the time of death, and we can only work from the size distributions of the fossils themselves. Just like a graveyard, the fossil collection can be a sample showing how old the creatures lived to be. It is not necessarily a perfect sample; young animals may be absent because they were too small and fragile to be preserved (in the same way that babies were quite often buried in unmarked graves). Others might have been lost because they died where they could not be preserved, like people who were buried outside the graveyard, or lost at sea.
If we have not collected the fossils ourselves, we also have to be wary of “trophy hunting.” Some fossil collectors do not collect a representative sample, but will try to get the best fossils, which means that they will tend to have far more large specimens than small ones. This is, perhaps, like a cemetery in which the most successful people will leave the biggest, best kept, and most legible gravestones. To get a good sample, the cemetery-reader has to go looking for the stones that are half-buried and covered in moss, while the paleontologist must examine the fragmentary and unattractive specimens.
If data can be gathered, then survivorship curves and death curves can give important information about a species and how its reproduction, growth, and death rate are all closely interconnected. Some species (which the ecologists call r-selected) may produce many offspring, take little care of them, and reach reproductive age relatively quickly. r-selected creatures, such as houseflies and mice, are often small and are adapted to a high death rate. K-selected organisms, on the other hand, produce relatively few offspring, invest a lot of energy in caring for them, and reach maturity more slowly. Animals such as elephants, whales, and humans clearly fit into this category. What modern civilization has done for humans is to accentuate this quality: we are the ultimate K-creatures, if you like.
The old graveyards suggest that were three life intervals in which people died at an elevated rate in horse and buggy-era Canada (there is, of course, also a lot of non-graveyard data out there on death rates). My parents always used to point out how many small children died in rural New Brunswick in the nineteenth and early twentieth centuries. Prior to the development of modern vaccines and antibiotics, childhood diseases took an immense toll. In my father’s own family, two of his older siblings died at the age of about six. Women commonly died from complications related to childbirth and childbearing. But though there was a steady rate of mortality from farming and lumbering accidents, many who survived childhood and childbearing lived to a relatively healthy old age.
The graves, and the fossils, whisper fascinating stories to those who will listen.
© Graham Young, 2011