Frigid winters, food scarcity, intense competition for territories and the threat of becoming prey to large predators like the Canada lynx are just some of the trials they face.

Early-life struggles and trauma can literally get under their skin, affecting long-term survival, said Lauren Petrullo, a University of Arizona assistant professor in the Department of Ecology and Evolutionary Biology. Scientists want to know what factors, if any, can buffer young squirrels against these threats.

Petrullo is part of the Kluane Red Squirrel Project, a multi-university long-term field project involving the University of Alberta, University of Michigan, University of Colorado Boulder and University of Saskatchewan. The project has tracked and studied thousands of wild North American red squirrels in the southwestern part of Canada’s Yukon territory for over 30 years.

A new study — which Petrullo led with David Delaney, a postdoctoral fellow at the University of Colorado Boulder — finds that the more challenges young squirrels face in the year they are born, the shorter their adult lifespan.

Red squirrels who make it past their first year of life live about 3 1/2 years, on average, but early life adversity can cut life expectancy by at least 14%.

But there’s a big caveat.

“The ecosystem red squirrels inhabit in this region is unique,” Petrullo said. “Every three to seven years, their favorite food — seed from cones of white spruce trees — is produced in superabundance during what we call a food boom. We found that these booms, even though rare, can interrupt the biological embedding of early-life adversity. If a squirrel had a harsh first year of life, if they were lucky enough to experience a food boom in their second year of life, they lived just as long — if not longer — in spite of early-life adversity.”

The team replicated a food boom by offering wild squirrels in the Yukon peanut butter as a supplemental food source. The peanut butter didn’t have the same effect as the naturally occurring food boom did.

“This suggests that the buffering effect we see is not really just about an increase in available calories,” Petrullo said. “It’s probably about shifts in larger population-level dynamics, like competition.”

What squirrels can teach us about humans

Petrullo and her colleagues are eager to tease out the mechanisms that link squirrels’ early developmental conditions with later-life survival. What they learn could inform scientific understanding of human resilience, too.

“Our findings in red squirrels echo what we know about how early-life adversity can shorten adult lifespan in humans and other primates,” Petrullo said. “Humans vary widely in how vulnerable or resilient they are to challenges faced during early development. Our study demonstrates that future environmental quality might be an important factor that can explain why some individuals appear to be more, or less, susceptible to the consequences of early-life adversity.”

While it might be surprising that scientists can glean insights about human resilience from wild red squirrels, Petrullo pointed out that squirrels are rodents, and rodents are commonly used as models for humans in laboratory settings.

“Many lab experiments have limited relevance for broader dynamics between ecology and evolution, because it can be hard to really replicate the ecological challenges that animals have evolved to cope with in a lab setting,” she said.

Wild red squirrels, on the other hand, allow for such investigations and are an especially useful study group for questions regarding the early-life environment, Petrullo said. Although growing up as a young squirrel in the Yukon can be difficult, with lots of things making early development challenging, there are also things that can go right.

“Some red squirrels have the luck of being born into gentler early environments, akin to being born with a silver spoon,” Petrullo said. “Because of this, we’ve got this really nice individual variation in early-life environmental quality across a natural ecological environment.”

This environment, however, is expected to experience a great deal of change as global temperatures continue to rise.

“As food boom patterns begin to change,” Petrullo said, “the pathways that connect early-life experiences and lifespan may change as well, potentially offering important insight into how animals may adapt to increasingly challenging environments.”

The analysis was led by the German Centre for Integrative Biodiversity Research (iDiv) and the Martin Luther University Halle-Wittenberg (MLU) and is the largest modelling study of its kind to date. The researchers compared thirteen models for assessing the impact of land-use change and climate change on four distinct biodiversity metrics, as well as on nine ecosystem services.

GLOBAL BIODIVERSITY MAY HAVE DECLINED BY 2% TO 11% DUE TO LAND-USE CHANGE ALONE

Land-use change is considered the largest driver of biodiversity change, according to the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES). However, scientists are divided over how much biodiversity has changed in past decades. To better answer this question, the researchers modelled the impacts of land-use change on biodiversity over the 20th century. They found global biodiversity may have declined by 2% to 11% due to land-use change alone. This span covers a range of four biodiversity metrics¹ calculated by seven different models.

“By including all world regions in our model, we were able to fill many blind spots and address criticism of other approaches working with fragmented and potentially biased data,” says first author Prof Henrique Pereira, research group head at iDiv and MLU. “Every approach has its ups and downsides. We believe our modelling approach provides the most comprehensive estimate of biodiversity trends worldwide.”

MIXED TRENDS FOR ECOSYSTEM SERVICES

Using another set of five models, the researchers also calculated the simultaneous impact of land-use change on so-called ecosystem services, i.e., the benefits nature provides to humans. In the past century, they found a massive increase in provisioning ecosystem services, like food and timber production. By contrast, regulating ecosystem services, like pollination, nitrogen retention, or carbon sequestration, moderately declined.

CLIMATE AND LAND-USE CHANGE COMBINED MIGHT LEAD TO BIODIVERSITY LOSS IN ALL WORLD REGIONS

The researchers also examined how biodiversity and ecosystem services might evolve in the future. For these projections, they added climate change as a growing driver of biodiversity change to their calculations.

Climate change stands to put additional strain on biodiversity and ecosystem services, according to the findings. While land-use change remains relevant, climate change could become the most important driver of biodiversity loss by mid-century. The researchers assessed three widely-used scenarios — from a sustainable development to a high emissions scenario. For all scenarios, the impacts of land-use change and climate change combined result in biodiversity loss in all world regions.

While the overall downward trend is consistent, there are considerable variations across world regions, models, and scenarios.

PROJECTIONS ARE NOT PREDICTIONS

“The purpose of long-term scenarios is not to predict what will happen,” says co-author Dr Inês Martins from the University of York. “Rather, it is to understand alternatives, and therefore avoid these trajectories, which might be least desirable, and select those that have positive outcomes. Trajectories depend on the policies we choose, and these decisions are made day by day.” Martins co-led the model analyses and is an alumna of iDiv and MLU.

The authors also note that even the most sustainable scenario assessed does not deploy all the policies that could be put in place to protect biodiversity in the coming decades. For instance, bioenergy deployment, one key component of the sustainability scenario, can contribute to mitigating climate change, but can simultaneously reduce species habitats. In contrast, measures to increase the effectiveness and coverage of protected areas or large-scale rewilding were not explored in any of the scenarios

MODELS HELP IDENTIFY EFFECTIVE POLICIES

Assessing the impacts of concrete policies on biodiversity helps identify those policies most effective for safeguarding and promoting biodiversity and ecosystem services, according to the researchers. “There are modelling uncertainties, for sure,” Pereira adds. “Still, our findings clearly show that current policies are insufficient to meet international biodiversity goals. We need renewed efforts to make progress against one of the world’s largest problems, which is human-caused biodiversity change.”

¹global species richness, local species richness, mean species habitat extent, biodiversity intactness

The researchers fed hungry mice that could only see and smell the food without eating it. After just a few minutes, the researchers analysed the mitochondria in the liver and found that processes normally stimulated by food intake were activated.

Mitochondria in the liver get ready

The studies show that it is sufficient for the mice to see and smell food for a few minutes to influence the mitochondria in the liver cells. This is mediated by a previously uncharacterised phosphorylation in a mitochondrial protein. Phosphorylation is an important modification for the regulation of protein activity. The researchers also show that this phosphorylation affects the sensitivity of the liver to insulin. The researchers have thus discovered a new signalling pathway that regulates insulin sensitivity in the body.

Nerve cells in the hypothalamus

The effect on the liver is mediated by a group of nerve cells called POMC neurons. These neurons are activated within seconds by the sight and smell of food, signalling the liver to prepare for the incoming nutrients. The researchers also showed that the activation of POMC neurons alone is sufficient to adapt the mitochondria in the liver, even in the absence of food.

“When our senses detect food, our body prepares for food intake by producing saliva and digestive acid. We knew from previous studies that the liver also prepares for food intake. Now we have taken a closer look at the mitochondria in liver cells, because they are essential cell organelles for metabolism and energy production, and realised how surprisingly fast this adaptation takes place,” explains Sinika Henschke, first author of the study. Jens Brüning, head of the study and director at the Max Planck Institute for Metabolism Research: “Our study shows how closely the sensory perception of food, adaptive processes in the mitochondria and insulin sensitivity are linked. Understanding these mechanisms is also important because insulin sensitivity is impaired in type 2 diabetes mellitus.”

Jens Brüning is also a research group leader at the CECAD Cluster of Excellence in Ageing Research at the University of Cologne and Director of the Department of Endocrinology, Diabetology and Preventive Medicine at Cologne University Hospital.