A study published in Nature Communications has analysed the evolution of marine birds and mammals, which transitioned from land to sea tens of millions of years ago, and found that these transitions have been associated with a slowing down of life, linked to the need to develop adaptations to survive in the sea. Paradoxically, this slow pace of life and their low reproductive rate make these animals particularly vulnerable to human impacts such as pollution or collisions with boats.
In nature, growing fast and reproducing early is the most common life strategy because it reduces the risk of dying before passing on one’s genes. But not all animals follow this strategy. Humpback whales, for instance, do not mature until they are 10 to 15 years old and only produce a single calf every few years. Understanding the evolution of such slow life strategies remains one of the great enigmas of evolutionary biology.
A new study recently published in Nature Communications and led by the Institute of Evolutionary Biology (IBE) — a joint centre of the Spanish National Research Council (CSIC) and Pompeu Fabra University (UPF) — and CREAF, with contributions from researchers at CEAB-CSIC and the University of Barcelona, concludes that when terrestrial mammals and birds made the transition from land to sea, they evolved towards increasingly slower life strategies. This life slowdown is explained by the fact that living in the sea requires a series of adaptations that improve individual survival, thereby promoting longevity. The possibility of sustaining a long reproductive lifespan would make it feasible to support low fertility and slower development. “Our study suggests that the evolution of slow lives is not simply the result of increased body size or reduced mortality risk, but also relates to the colonisation of new environments that, like the sea, pose major challenges — such as the difficulty of finding food in such a vast space. These challenges favour adaptations that protect individuals from external mortality risks,” says Daniel Sol, lead author of the paper and researcher at both IBE and CREAF.
Examining the present to understand the past
Since life strategies do not leave a trace in the fossil record, the study’s authors had to investigate past evolutionary changes using phylogenetic data and evolutionary models. The first step was to reconstruct transitions from terrestrial and freshwater habitats to marine environments, using phylogenetic data from over 90% of bird and mammal species currently alive on Earth. By combining this data with evolutionary models, the scientists were able to determine that these transitions were accompanied by adaptive changes in longevity, fertility and developmental time — all associated with increased body size, improved long-distance movement efficiency, and greater encephalisation — a measure of a species’ cognitive capacity.
“While we have a fairly good understanding of why some organisms live fast and die young, the reasons behind slower life strategies are less clear. We know that larger animals have slower metabolisms, which may slow down ageing. We also know that reducing mortality risk favours longer lifespans, as it allows natural selection to eliminate harmful genes and promote investment in body maintenance. In contrast, the role of protective adaptations in response to new challenges has received less scientific attention,” Sol explains.
The challenges of living at sea
Life in the ocean is not easy, especially for animals whose ancestors lived on land. Finding food is particularly difficult: prey are spread across vast and unpredictable areas, and some live at inaccessible depths. Marine species have had to “invent” new feeding methods, often in the inhospitable and extreme environments of the planet’s coldest oceans. “Take the albatross, for example. Its body shape allows it to glide effortlessly for thousands of kilometres using wind currents, almost without expending energy,” says Daniel Sol, the paper’s lead author. “Or consider dolphins and orcas: they have evolved large, flexible brains that enable them to cooperate and use clever strategies to catch prey,” adds Sol. These adaptive innovations take a long time to develop. Once they do, however, they give animals a survival advantage that allows for long lifespans.
Modern threats to ancient survivors
The same traits that helped marine mammals and birds thrive for millions of years may now make them more vulnerable. The slow pace of life in these animals means their populations are highly sensitive to threats that affect adult survival — such as ship strikes, noise pollution, bycatch (accidental capture in nets or hooks), oil spills and commercial hunting — threats for which they have had little time to adapt. To make matters worse, since they also reproduce slowly and have long generation times, this leads to slow population recovery and any evolutionary adjustment — if even possible — would take a long time to occur. “The great paradox is that the very adaptations that enabled the extraordinary evolutionary success of marine birds and mammals may now place this diversity at risk,” concludes Sol.
