The ‘slings and arrows of outrageous Fortune’ wear us down eventually but, when it comes to longevity, they’re not the only players. Our body cells have their own agendas; they decide when it’s time “to shuffle off this mortal coil”, writes Richard Collins
Little strings, known as ‘telomeres’, are found at the ends of cells. They protect our DNA, ensuring that it’s not corrupted or damaged. Kate Bebbington of the University of East Anglia likens them to ‘the hard plastic ends of a boot lace’. Our cells divide repeatedly as we age. The telomeres ‘get broken down and become shorter because they absorb all the damage experienced during life’ she says.
“The rate at which this happens reflects how much stress the body is under and, how long it can continue to function. In humans, things like smoking, eating foods that are bad for you and putting your body through extreme physical or mental stress, all have a shortening effect on telomeres. The healthier you are, or have been, the better telomeres you have and the less quickly you age. When its telomeres are gone, a cell can’t divide any more and dies. These little biological clocks place a limit on how long we last,” she says.
A little songbird, living in a favourite haunt of honeymooners and tax dodgers, gave Bebbington an ideal opportunity to study telomeres. The Seychelles warbler is closely related to the sedge and reed warblers which come to Ireland from Africa each spring. Unlike them, however, it’s a stay-at-home bird, holding territory all year round. In 1968, the Seychelles warbler was on the brink of extinction with only 26 individuals left. Conservation measures saved it from oblivion; there are around 2,500 alive today.
In 1994, researchers began monitoring and colour ringing warblers on Cousin Island in the Seychelles. This bird community of about 320 adults is closed and when juveniles are trapped on their natal territories, their parentage is known. Inbreeding occurs frequently; 5% of chicks have parents that are first-order relatives.
Over 1,000 DNA samples were taken from 592 individuals. Bebbington used them to examine telomere strings. When she compared their lengths, some curious patterns emerged. Inbred individuals had shorter telomeres than out-bred ones. This had been found in other studies, but there was also a cross-generational effect not previously known; individuals whose mothers were inbred also had shorter telomeres.
Their mothers were “less well able to provide for their offspring, either in terms of investment in the egg or during early life”, Bebbington thinks. “This in turn would make an offspring’s life experiences more stressful or make it less able to deal with the stress, leading to a more rapid shortening of its telomeres,” she says.
The warblers feed on leaf-insects they catch within their territories. Conditions on Cousin Island vary with the seasons and from year to year. The number of insects fluctuates and the resulting food shortages stress the birds.
“Inbred animals are more susceptible to disease,” says Bebbington. They are “poorly developed because they don’t have much variation in the genes they carry, plus whenever life gets difficult, they can’t cope as well as out-bred animals”.
These results have important implications for endangered species. With reduced numbers, individuals have fewer potential mating partners and may have to breed closer to their kin. If such in-breeding shortens life, as Bebbington claims, an individual’s prospects of successfully producing young are reduced. Zoos nowadays are the modern equivalent of Noah’s Ark. They maintain stocks of species which are declining in the wild. Keepers and stud-book holders, therefore, must be vigilant to ensure that in-breeding is minimised and gene-pools are kept as rich as possible.
Thanks to our immigrants, the human population of Ireland has never been as diverse as it is today. This augurs well for our longevity. Vive la difference!
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