New research suggests prenatal thyroid hormones influence the 'biological age' of birds at birth.

The findings, published Wednesday in the journal Biology Letters, could have implications for aging in humans.

Numerous studies have shown environmental conditions, like exposure to maternal stress hormones, can influence embryo development -- triggering genetic changes that influence health, aging and lifespan.

One of the ways scientists study cellular aging is by measuring the length of telomeres, the protective ends of chromosomes that protect strands of DNA and promote cell replication.

As living organisms age, telomeres get shorter -- but shorter than average telomeres at a given age can signify an increased risk of disease and mortality.

Studies have previously shown that exposure to maternal stress hormones and other environmental instabilities during embryonic development are associated with shorter telomeres.

For the new study, researchers used egg injections to expose collared flycatcher embryos to maternal thyroid hormones.

Scientists were surprised to find that baby birds that experienced prenatal exposure to thyroid hormones were born with longer telomeres than birds from the control group.

"The telomere biology of humans is closer to the telomere biology of birds than those of traditional laboratory models," lead study author Antoine Stier said in a news release.

"In both humans and birds, telomere length is measured in a minimally-invasive way from small blood samples," said Stier, a researcher at the University of Turku in Finland.

Because scientists had previously found prenatal exposure to thyroid hormones was associated with accelerated early-life growth, researchers assumed exposed embryos from the more recent study would feature shortened telomeres.

"Based on the natural decline of telomere length observed with age in the same collared flycatcher population, we estimated that chicks hatching from thyroid hormones injected eggs were approximately four years younger at birth than chicks hatched from control eggs," said Turku researcher Suvi Ruuskanen.

The findings suggest thyroid hormones influence the establishment of a newborn bird's biological age. However, scientists suggest there could be evolutionary costs to this phenomenon.

The researchers also suggest the findings could have implications for the study of aging in humans.

"Considering the interest and controversies surrounding gene therapy trials in humans to elongate telomeres as an anti-aging therapy, this discovery opens potential avenues to better understand the influence of telomere elongation in animal models," Stier said.

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