Aging is an evolutionary balancing act with reproductive effort.

A Japanese quail (Coturnix japonica). (Credit: Dennis Hansen, image provided)

Dennis Hansen, image provided

I ran across an elegant little study that investigates the relationship between aging and reproductive effort. Basically, living beings are confronted by an evolutionary trade-off between investing their limited resources into reproduction versus maintaining their bodies so that investing more into reproduction is counter-balanced by accelerated aging and a shorter life span. However, the significance of these trade-offs remains debated because previous studies have produced inconsistent results.

“All living things have limited energy and resources, and face trade-offs between competing priorities,” observed the study’s lead author, evolutionary ecologist Barbara Tschirren, an associate professor at the University of Exeter. One of Professor Tschirren’s main research interests is studying the evolution of life histories, with a special focus on cross-generational effects.

“Evolutionary theory suggests there’s an intrinsic link between aging and reproductive effort — but this is quite difficult to test.”

To investigate this relationship between aging and reproduction, Professor Tschirren and an international team of collaborators randomly split a captive flock of Japanese quail, Coturnix japonica, kept at the University of Zurich, into two flocks to create two independent biological replicates. They selectively bred the two captive populations so the birds either produced large eggs or small eggs. Those that produced large eggs represented an increased (high, H-line) maternal egg investment whereas those that produced small eggs represented a reduced (low, L-line) maternal egg investment, which correlates with reproductive effort.

Japanese quail chicks are precocial, meaning that the young take care of themselves quite well from the moment of hatching, so adult quails’ main investment into their offspring is limited to the amount of resources they transfer to their eggs (chicks from larger eggs are more likely to survive).

“There is a very strong relationship between egg size and hatchling size,” Professor Tschirren observed. “All the resources the embryo has available for growth are provided through the egg.”

“Birds are a great model for research into reproduction-self-maintenance trade-offs because most of the offspring development takes place outside of the mother’s body,” Professor Tschirren told me in email. “This allows us to standardise rearing conditions after the eggs are laid, and to disentangle pre- and post-natal parental effects.”

Japanese quail have a rapid growth rate and a short life cycle, becoming sexually mature only 8 weeks after hatching. They are an increasingly important model species for developmental biology and biomedical research, including the fields of neuroscience, reproductive biology and immunology.

“Quail in particular are a great model because they have the shortest generation time of all birds (comparable with the small flowering plant, Arabidopsis!) — making it possible to perform artificial selection experiments,” Professor Tschirren added.

After four generations of directional selection, the H- and L-lines were laying eggs that were significantly different in size but the rate of egg laying across both groups was the same.

After five to six generations of selective breeding, Professor Tschirren and collaborators found that females who laid large eggs lived just 595 days on average, compared to 770 days for females bred to lay small eggs. (Male Japanese quails live much longer than females, so the study was too short to reach firm conclusions about the impact of selective breeding on male lifespan.)

The findings of this study support a fundamental evolutionary theory: that high “investment” in offspring unavoidably leads to faster aging and a shorter life.

“There clearly is genetic variation for longer lifespan present in quail (and likely other vertebrate populations) — why is there no selection acting on this variation and why does lifespan thus not evolve?” Professor Tschirren pondered. “Our findings show that a longer life comes at the expense of lower reproduction, and this is ultimately not favoured by natural selection.”

The evolutionary trade-off is whether to put energy into reproduction or “self-maintenance” — so for example, quails selected for high egg investment also have lower rates of cell repair and immune function, as previous research has found (ref).

Is this just an effect seen in quails, or might it also apply to people?

“There is no reason to assume that similar trade-offs do not exist in other taxa, including humans,” Professor Tschirren replied in email. “Indeed, a recent study demonstrated a similar trade-off in a historical human dataset during a famine (ref).”

“Our study … shows there is substantial genetic variation in reproductive effort and aging, that this genetic variation is linked, and that it can evolve quickly.”

The study is the first to use artificial selection to test this link between aging and reproductive effort in a vertebrate.

Already, Professor Tschirren and collaborators are looking into the molecular mechanisms that underlie this observed trade-off.

“Why does it occur? What does antagonistic pleiotropy mean at the molecular level?”

Source:

Barbara Tschirren, Joel L. Pick, Dave W. Hudson, Oscar Vedder, Erik Postma, and Ana Angela Romero Haro(2026). Artificial selection for increased reproductive effort accelerates actuarial senescence and reduces lifespan in a precocial bird, Proceedings of the Royal Society B: Biological Sciences 293(2069):20252908 | doi:10.1098/rspb.2025.2908

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