In nature, growing fast and dying young is the most common life history strategy. Many animals reproduce early to maximize their chances of passing on their genes before something kills them. But not all animals follow this norm. Humpback whales, for instance, do not mature until around age 10 to 15, and typically raise just one calf every few years. So, what makes these animals to adopt such a slow life history strategy?

Although we now have a fairly good understanding of why some organisms live fast and die young, the reasons behind slower life histories are less clear. We do know that larger animals have low metabolism, which can slow down aging. We also know that relaxing the risk of mortality favours longer lives because it allows selection to eliminate harmful genes and to enhance investment in body maintenance. But none of these explanations fully account for the extreme slow life history of some birds and mammals.

The new study, which is coauthored by researchers of the Institute of Evolutionary Biology (IBE), a joint center of the Spanish National Research Council (CSIC) and Pompeu Fabra University (UPF), the CREAF, with the participation of reserarchers of the CEAB-CSIC and IRBio of the University of Barcelona, highlights something important: to justify reproducing late and infrequently, animals must be able to survive for a long time. That means evolving protective traits—larger bodies, better defenses, smarter behaviors—that increase adult survival. These kinds of adaptations are more likely to develop when animals enter new environments that are less dangerous but also more challenging in other ways.

And, according to the new study, that is exactly what happened when some mammals and birds returned to the ocean.

The challenges of living in the sea

Life in the sea is not easy, especially for animals whose ancestors lived on land. Finding food is tough—prey is scattered and often unpredictable. Marine species have had to invent new ways to hunt and survive in often cold and harsh waters. But these adaptive innovations—like powerful traveling abilities, advanced navigation, and complex social hunting—take a long time to develop. Once they do, though, they give animals a survival advantage that can stretch across decades.

Take the albatross, for example. It can glide effortlessly over thousands of kilometers by riding wind currents, barely using any energy. Or consider dolphins and orcas: they have evolved large, flexible brains that allow them to cooperate and use clever strategies to catch prey.

Adaptive adjustments for a life in the sea

According to the study, species that transitioned from land to sea have tended to evolve slower life histories strategies. These changes went hand in hand with new adaptations for life in the ocean—such as more efficient locomotor morphology and higher encephalized brains —adaptations that are costly to develop but offer a big payoff in terms of survival and long-term reproduction.

“For albatrosses, for example, the fact that prey is sparse and spread over vast areas is not a problem”, argues Daniel Sol, the leading author of the paper, “because they have evolved a morphology that allows them to use wind currents to glide effortlessly, enabling long-distance travel with minimal energy cost”.

As for toothed whales like dolphins and orcas, their large and highly plastic brains allow them to learn complex hunting techniques and often collaborate socially, for example by herding prey in cooperative groups to then use their tails to kill the fish.

But there is a catch.

Modern Dangers for Ancient Survivors

The same traits that helped marine mammals and birds thrive for millions of years now make them vulnerable. The slow-paced life of marine endotherms makes their populations highly vulnerable to threats that impact adult survival—such as ship strikes, acoustic pollution, fishing net entanglements, oil spills, and commercial hunting—threats to which they have had little opportunity to adapt. To make matters worse, their long generation times mean that any evolutionary adjustment—if possible—will take a long time to unfold. “The great paradox is that the very adaptations that enabled the extraordinary evolutionary success of marine birds and mammals may now be putting that diversity at risk,” concludes Sol.

Referenced article:

Daniel Sol, Antón Prego, Laura Olivé, Meritxell Genovart, Daniel Oro & Antonio Hernández-Matías (2025) Adaptations to marine environments and the evolution of slow-paced life histories in endotherms. Nature Communications 16, 4265