As animals age, they may not move so well, generally look less good and the offspring they produce later in life tend not to live as long. In addition, as creatures grow older, the tiny capping structures that protect chromosomes from fraying, known as telomeres, become shorter. Recently, Valeria Marasco at the University of Veterinary Medicine Vienna, Austria, together with Pat Monaghan and colleagues at the University of Glasgow, UK, discovered that the telomeres of zebra finch offspring produced by older mothers are shorter than those of the offspring produced when they were younger. But why? The researchers speculated that there could be two possible reasons. Either older mums provided poorer care for their later broods, shortening their telomeres, or simply passed on shorter telomeres. But it wasn't clear which. To find out, the team decided to look at the grand-chicks of the original mums to find out whether the length of the grand-chicks’ telomeres were affected by the age that their grandmother was when she produced the eggs that went on to become their mums.
The team allowed the daughters of the original mothers – some of which had hatched from their mother's first clutch when she was young and others from their mother's fourth, and final, clutch – to breed when they were young (∼7.5 months old). After waiting for the grand-chicks to fledge, the team collected tiny droplets of blood from the youngsters to find out how long their telomeres were.
They found that telomeres of the young grandmas’ grand-chicks were 43% longer than the telomeres of the older grandmas’ grand-chicks. The grand-chicks from older grandmas inherit shorter telomeres from their grandmothers, which likely causes them to have a shorter lifespan and to have issues with their own offspring; so being born to younger mothers is an advantage for zebra finch descendants. The reason why older grandmas have shorter telomeres has to do with the natural aging process in which all tissues, including the eggs that the grand-chicks will develop from, deteriorate over time. This deterioration occurs when the mitochondria, tiny parts of the eggs that generate energy, get damaged and stop being properly maintained. Therefore, grand-chicks that are born with shorter telomeres are effectively ‘older’ and so may die between 1–4 years of age while grand-chicks of young grandmas can live to 7 years and have telomeres that are, on average, 33% longer. While this may mean that shorter-telomered chicks may need to grow up and move out faster to compensate for their shorter lifespan, mammals and birds continue to reproduce throughout their lives, which suggests there is some benefit in reproducing later in life; life experience is truly an invaluable asset. Interestingly, having a younger father did not protect offspring from inheriting shorter telomeres from an older grandma.
In short, Marasco and colleagues discovered that zebra finch lifespan is a legacy inherited from mothers and previous female ancestors, which naturally shortens the life expectancy of chicks. Future investigations of the lives of zebra finch grand-chicks that were effectively born older because of their inherited shorter telomeres could also teach us more about aging and how it has evolved.
