Line Sass Kierkegaard’s PhD research reveals a biological trade-off between reproduction and lifespan in ewes, where environment and nutrition shape survival, genetics, and future resilience in Norwegian sheep production.
How many lambs can a ewe produce without compromising her health and lifespan? This question has been at the heart of the doctoral research conducted by PhD candidate Line Sass Kierkegaard at the Norwegian University of Life Sciences (NMBU).
Through analyses of Norwegian data from the Norwegian Sheep Recording System (Sauekontrollen) and the use of advanced simulation models, she has uncovered a clear biological trade-off between reproduction and survival in ewes. Her findings also demonstrate why understanding these trade-offs is becoming increasingly important as sheep farming faces the challenges of a more demanding climate.
– Vi ønsket å forstå hvilke kostnader reproduksjon har for dyrenes levetid, og hvordan disse kostnadene påvirkes av miljøet, sier Kierkegaard.
The study focused on the Norwegian White Sheep under Norwegian production conditions and examined how ewes balance resource allocation between reproduction and long-term survival. Using data from the Norwegian Sheep Recording System (Sauekontrollen), it was possible to track individual animals throughout their entire lives.
Early lambing, shorter lifespan
One of the clearest findings was that age at first lambing affects lifespan. Ewes that produced their first litter as yearlings lived, on average, shorter lives than those that first lambed at two years of age.
The relationship between litter size and lifespan was also not straightforward. Neither very small nor very large litters resulted in the longest lifespan. What is considered “optimal” depends, among other factors, on the ewe’s age at first lambing and on how many lambs she successfully raises relative to the number she gives birth to.
Difficult to separate environmental and management effects
Kierkegaard also sought to link these patterns to large-scale environmental conditions, such as climate and natural variation. This proved challenging.
“In practice, it was difficult to separate the effects of nature from the effects of human management and husbandry, making it impossible to identify unambiguous environmental effects from the registry data alone,” she explains.
A virtual sheep barn for the future
To investigate the effects of future environmental conditions, the researchers developed a digital “laboratory” by calibrating an existing simulation model for cattle so that it could be used to simulate Norwegian ewes. The model combines information on plant growth in outfield grazing areas with various scenarios for future feed availability. An entire sheep flock was then simulated over a period of one hundred years.
The aim was to create a virtual laboratory where biological mechanisms could be studied in detail, including those that are difficult or impossible to measure in real-life conditions.
“With the model, we can study biological mechanisms that would otherwise be inaccessible to us,” says Kierkegaard.
The results clearly showed that the environment influences the trade-off between reproduction and lifespan. When feed availability was limited, the ewes produced fewer lambs. Even more interesting, however, were the long-term effects. The flock developed different genetic characteristics depending on the environment in which it lived. This suggests that environmental conditions can shape the genetic composition of a flock and that future sheep may require different abilities for acquiring and allocating energy than they possess today.
Robustness is becoming increasingly important
In a harsher and more variable climate, there may be a need for animals with characteristics different from those considered optimal today, particularly regarding their ability to maintain stable production over time.
These findings have clear implications for breeding, food production, and our understanding of the biological mechanisms that regulate energy allocation to growth, reproduction, and survival.
“If we are to safeguard food production in the future, we need animals that can withstand more,” says Kierkegaard.
Understanding how biological trade-offs operate in practice may therefore be crucial for defining and selecting for robustness, and this doctoral work contributes new knowledge about the interaction between biology, environment, and production.
The doctoral candidate has also established a methodological foundation for future research. The calibrated simulation model for ewes can be further developed and linked to breeding programs. In the long term, this could provide researchers and breeding organizations with a tool for testing strategies and evaluating their consequences before implementing them in practice.
Thus, the PhD project not only provides insights into the lifelong balancing act of sheep but also into how future livestock can be adapted to a changing climate, benefiting both animals and humans.
Line Sass Kierkegaard is 36 years old and comes from Denmark. She will defend her doctoral thesis, entitled "Weathering the storm: The trade-off between reproduction and longevity in ewes" at the Norwegian University of Life Sciences (NMBU) on 30 April 2026.
Professor Bente Aspeholen Åby (NMBU) served as Kierkegaard’s principal supervisor. Co-supervisors included Nicolas C. Friggens (INRAE), Laurence Puillet (AgroParisTech, INRAE), Geir Steinheim (NMBU), Gunnar Klemetsdal (NMBU), and Øystein Holand (NMBU).