Female warblers live longer when they have help raising offspring
Our long-term study of the Seychelles warbler – a small, tropical songbird – on the island of Cousin in the Indian Ocean is a useful case study for understanding the ageing process. Since the 1990s, all the warblers on this tiny island – just 40 football fields in size – have been fitted with coloured leg rings so they can be tracked and identified. Birds don't disperse on or off this isolated island so we were able to follow them from birth to death. We also monitored their health, reproduction and survival, which all decline rapidly in elderly individuals.
We also measured the warbler's telomeres – repetitive DNA sequences which protect the ends of chromosomes but shorten in response to physiological stress. Telomere shortening has been shown to be a useful marker of biological condition and ageing in various animals, including humans. In the Seychelles warbler, telomere length predicts future survival. By measuring the telomere shortening that occurs in response to any given experience we can determine the impact that specific factors have on ageing.
Our previous studies on the Seychelles warbler have already found that certain factors influence the rate at which individuals age. For example, having a territory surrounded by unrelated and unfamiliar neighbours leads to more territorial fights, and hence more rapid telomere shortening. Growing up in a territory with limited food availability also has a detrimental impact on later ageing.
Our recent paper in Nature Communications has focused on how raising offspring is stressful and may lead to premature ageing – something that may not surprise many parents. Due to a lack of space on Cousin, many adults can't find a territory in which to pair up and breed. Instead, these individuals may join up as subordinates to a dominant breeding pair within an already established territory – often the one in which they were born. They then sometimes help the dominant breeding pair raise their next batch of offspring—a process known as "cooperative breeding".
Our analyses showed that the dominant birds that receive help have less telomere shortening than those that are left to do all of the parenting work themselves. This help also results in better survival of the dominant females. Therefore, we can see that the help that the dominant breeding birds received reduced the stress of breeding and delayed ageing, at least in females. The dominant males don't appear to benefit from receiving help as much, probably because in the Seychelles warbler males invest much less energy in raising chicks than females do.
Our study confirms a long-held hypothesis that cooperative breeding – which is the norm in humans – can reduce the health costs to parents of raising young and may, therefore, slow down ageing. This could explain why more social species tend to have longer lifespans.
