The reality of Life: Longevity versus fertility

Posted on 19 June, 2019 at 4:00 Comments comments (186)

Throughout all living organisms there is an inverse relationship between the number of offspring produced and the chance that those offspring will survive to reproduce. We know that this is true of bacteria and viruses but this becomes very obvious when we can really see this 'in action' in animals and plants. 

One of my first very beautiful experiences of this relationship was on a biology excursion to Herron Island, an island in Australia's magnificent Barrier Reef, one Christmas over 50 years ago. Here, our task was to creep out at night in groups, by dim torch-light, and wait for the female green seas turtles to arrive on the high tide and make their way up above the high tide mark to lay their eggs. Each group's task was to count the number of eggs laid by our turtle. The ecologists would then eventually discover how many eggs hatched, how many baby tutles survived the treacherous journey back to the ocean after hatching and then in the longer term how many survived life in the ocean to the age of maturity? 

Our female turtle started at quite a disadvantage because like many turtles, she had lost half one of her back flippers in a shark attack. Nevertheless, she made her way up the sand, dragging her her heavy body until she was above the tide line. We had to be absolutely quiet at this stage while she first dug a shallow nest like resting place. Then she went into a fairly frenzied digging where she dug a deep tunnel using her back flippers as bores and scoops to remove the sand. The only problem was that her amputated limb was pushing some of the sand back into the hole so we were surruptitiously removing the sand for her. Once she was finished her digging, her insides emitted an extremely loud gurgling sound and out popped about six to ten eggs. Our job was to count the eggs and I am proud to say that our mother turtle laid 120 eggs over the space of a couple of hours. Once done, she covered up the top of the hole with sand and then made her way slowly back to the water. The eggs and potential next generation were now on their own! Only very few of those 120 would be expected to survive to adulthood.

As humans living in a modern world, we tend to forget that we are just part of the bigger biological picture and that the urge and ability to reproduce is being governed by the much bigger picture of survival of the species. Because we are part of this huge dynamic of life, we should expect that some of the very things that enable us to live longer will be the very things that reduce our fertility! I suspect that instead of worrying about the drops in sperm count and couples not being able to reproduce, we should be celebrating this as a reflection of our ability to live much longer and healthier lives. I don't think we should be spending vast amounts of money on infertility treatments when lowering fertility is just part of the bigger picture of survival. The following graph has been created from the world health data. As in the whole of the plant and animal kingdoms, the ratio between life expectancy and fertility is an inverse one. If your life expectancy is only about 55, you are likley to be very fertile but 'congratulations', if your life expectancy is 80 or longer, you might have very low fertility!