Scientists have long observed that the offspring of younger parents tend to live longer than the offspring of older parents in many animal species, including humans. But this phenomenon, dubbed the “Lansing effect” for the first scientist who described it, has proven difficult to document experimentally.

Now, researchers at the University of Glasgow in Scotland have demonstrated the effect in birds. Using zebra finches and a controlled breeding design, they showed that older parents—especially older fathers—produce young with shorter lifespans. Their findings, published in the Proceedings of the Royal Society B, also point to one possible mechanism of the effect.

The team began by forming 44 pairs of male and female zebra finch mates. They then tracked the survival of a chick from each pair. The percent of surviving offspring from older fathers, the researchers discovered, dropped dramatically to less than ten percent within the first 16 months. Meanwhile, the percent of offspring surviving from younger fathers slowly but steadily declined over the course of about five years.

In search of a possible cause for this disparity, the team turned to telomeres—the repeating DNA sequences that cap the ends of chromosomes and play an important role in preventing chromosome degradation. Some studies have suggested that older fathers produce offspring with shorter telomeres, while other studies, including in humans, suggest just the opposite. But in most studies, researchers couldn’t tease out the effect of older fathers from other variables such as the mother’s age.

Using an animal model like zebra finches, says lead author José Noguera, an affiliate researcher at Glasgow, allowed the researchers to experimentally manipulate the age of the fathers. This allowed the experimenters to avoid “a number of confounding factors,” he adds.

In another phase of the study, the team worked with a separate group of zebra finches, pairing each of 32 females with either a young male around 4 months old or an older male around 4 years old. In a second breeding round, they paired the same females with males from the alternative age group. The scientists extracted genetic material from the resulting embryos and measured telomere length.

Unlike in humans, the team discovered that zebra finch embryos from older fathers had shorter telomeres. According to senior author Pat Monaghan, a professor of zoology at Glasgow, the contrast with humans could be the result of a species difference or could stem from the researchers having successfully eliminated some potentially confounding factors that could cause the effects. Since Monaghan’s previous work has shown that zebra finches with longer telomeres in early life live longer, a shortened telomere may be one explanation for the negative effect of a father’s age in this species.

The study is an important contribution, says evolutionary biologist Nicholas Priest of the Milner Centre for Evolution at the University of Bath in the United Kingdom, whose own research similarly suggests that older fruit flies produce offspring with shorter lifespans. “A lot of papers have described age effects,” he says, “but haven’t been done with careful manipulated work.”

As couples in the US and other countries take on child rearing increasingly later in life, some researchers have investigated what a Lansing effect might mean for the health of human populations. In one 2017 study, researchers scrutinized groups of siblings in four different human populations spanning four centuries. In three of the populations, all of them rearing children prior to 1850, the scientists found that siblings born to older fathers had a lower chance of surviving infancy and produced fewer offspring. The infant mortality effect, however, all but disappeared in the twentieth-century population, presumably because infant mortality in general has declined dramatically.

The lead author of this human study, Ruben Arslan, now a postdoctoral fellow at the Max Planck Institute for Human Development in Germany, appreciates the zebra finch researchers’ experimental contribution but notes that the research did not directly test whether telomere length drives the Lansing effect within one experiment. Citing also their modest sample size, he remains unconvinced that telomere length is the main driver of the Lansing effect. “For now, the best understood pathway for the Lansing effect,” he says, “is that the father’s age increases the number of genetic mutations and genetic mutations are generally bad for health.”

“Extrapolating results from one species to another is always complicated,” says Noguera. “We still lack a lot of basic information about telomere biology in non-model organisms to really know whether the patterns we observe are species-specific or not.”