Asian elephants born during times of the year when their mothers are experiencing stress have significantly fewer babies themselves throughout their lifetimes—despite having higher rates of reproduction at an early age.
They also decline much more quickly decades later when they are in old age.
“Poor early life conditions have been linked to many disease outcomes in humans, but is unknown whether stress in early life also speeds up aging rates in long-lived species,” says Hannah Mumby of the University of Sheffield.
“We found that the decline in reproduction with age is much steeper in the elephants born at the poorer time of year. Even though they reproduce slightly more when they’re young, this still doesn’t compensate for the steep decline and they end up with fewer offspring.”
The scientists made the discovery after getting access to a unique record of the lives and deaths of more than 10,000 elephants from Myanmar spanning three generations and almost a century. The elephants in the study are semi-captive animals working in the timber industry by pushing and dragging logs.
The researchers used measures of a hormone associated with stress (glucocorticoid metabolites) to determine which months represented stressful condition for the elephants.
Published in the journal Scientific Reports, the study shows that the months from June to August, which is monsoon season and typically when elephants work hard dragging logs to rivers, are the most challenging for the animals. The number of calves born at this time is low and their survival prospects are the poorest.
“Fertility and reproductive rate decline with age for all of us, but for some faster than others—and this variation was how we measured differences in aging,” says senior author Virpi Lummaa.
The findings highlight the potential for maternal stress to be associated with offspring aging and could also have important implications for Asian elephant populations both in western zoos, where they may experience stressful conditions associated with captivity, and in range countries where both captive and wild elephants may experience seasonal exposure to stress.
Improving developmental conditions could therefore delay reproductive aging in species as long-lived as humans.
The Natural Environment Research Council, Leverhulme Trust, and European Research Council funded the work, which was carried out at the University of Sheffield, Myanma Timber Enterprise, Chiang Mai University, and University of Edinburgh.
Source: University of Sheffield