In the spring of 2023, we returned to PenÃnsula Valdés, a rugged coastal region in Argentine Patagonia, expecting to witness the familiar sights and sounds of southern elephant seals during their breeding season. These massive marine mammals, with males weighing up to 4,000 kilograms, gather in large colonies on the beaches to give birth, nurse their young, and mate. The air usually resonates with the cries of thousands of pups calling out to their mothers, the grunts and bellows of males competing for dominance, and the buzz of life thriving on the rocky shores.
Instead, we were met with an eerie silence and a devastating sight: beaches once bustling with thousands of seals were littered with hundreds of dead pups and adults. The usual cacophony had been replaced by the stench of decay, and the empty spaces where seals should have gathered were painfully obvious. This mass mortality event had unfolded over just a few weeks—a stark and sudden collapse that no one could have predicted with such speed and severity.
Southern elephant seals lead challenging lives. Adult males arrive early in the breeding season and fast for months while defending harems of females. Females give birth to a single pup, nurse it for about a month, and then mate again before returning to the sea, pregnant once more. The pups are entirely dependent on their mothers; without constant nursing, they quickly perish. In 2022, our aerial surveys recorded ca. 18,000 females that gave birth to a pup. In the most crowded areas of the colony, we recorded 4,145 pups alive. But in 2023, in the same crowded areas, the numbers had decreased to 135 pups alive, most of which had died a few weeks later. Many of the mothers were gone. A year later, in 2024, some females returned, but, once again, the numbers were low compared to 2022, a 67 percent decrease in the most important sampled areas of the colony. Many adult seals displayed abnormal behaviors, such as reduced aggression in males and scattered female groups without male attendance.
This tragedy of the 2023 season was not just a population decline; it was a profound disruption of the social fabric that governs elephant seal life. Dominant males, whose fierce competition had long been a defining feature of the breeding grounds, were largely absent. Females were seen isolated or grouped without protection, which likely affected their ability to mate and successfully rear pups. The entire colony was struggling under a shadow of illness and loss.
The Virus and Its Unprecedented Spread
The culprit behind this catastrophe was a highly pathogenic strain of avian influenza virus known as H5N1. First identified in China in 1996, H5N1 emerged in an ecological cycle where viruses move through wild birds, spill into domestic poultry, and then re-enter wild populations, allowing the disease to evolve and spread across continents. The virus has caused massive die-offs in both wild and domestic birds worldwide, and its ability to infect mammals, including humans, has raised significant public health concerns.
What made the outbreak at PenÃnsula Valdés particularly alarming was the virus’s jump from birds to marine mammals—specifically to southern elephant seals and South American sea lions—and its subsequent spread between seals. This type of mammal-to-mammal transmission, spanning thousands of kilometers along the coast, came as a surprise. Transmission has occurred among elephant seals, but what makes this event especially alarming is the massive die-off—a level of mortality that would not have occurred in this population for at least a century.
This kind of spillover event is rare but increasingly concerning. Viruses like H5N1 continue to evolve, and the interface between wildlife, domestic animals, and humans is expanding due to habitat destruction, climate change, and globalization. These changes facilitate opportunities for viruses to cross species barriers—sometimes with devastating consequences. This growing overlap between humans and animals has already fueled the emergence of several major diseases, including SARS, avian influenza strains such as H5N1 and H7N9, MERS, Nipah virus, Ebola, the 2009 H1N1 pandemic, and, most recently, COVID-19.
For the seals, the timing of the outbreak could not have been worse. The virus struck during their critical breeding season, when the animals congregate densely on beaches, and their immune systems are likely compromised by fasting and reproductive stress. Seal pups are born with surprisingly weak immune defenses—lacking both potent maternal antibodies and the usual innate immune factors that fight infection. Yet they still manage to survive, raising questions about how they cope with disease risk and whether this unusual vulnerability is unique to seals or shared by other marine mammals, such as sea lions, fur seals, and walruses.
Meanwhile, adult females and males weakened by the virus were mainly absent from the coast, making them unable to maintain the social structures necessary for breeding success.
Our Long-Term Research and What It Revealed
We have dedicated much of our scientific careers to studying the southern elephant seals of PenÃnsula Valdés. Beginning with aerial surveys in the early 1980s and continuing through decades of on-the-ground counts and behavioral observations, our work has documented the growth, social behavior, and ecological dynamics of this unique continental colony. This longitudinal dataset was crucial in helping us understand the scale of the crisis in 2023 and assess the likely trajectory for recovery.
Before the outbreak, the colony had experienced steady growth, increasing at about 3.4 percent per year until the early 2000s, then slowing to around 1 percent per year more recently. These trends reflected a population approaching what ecologists call its ‘carrying capacity’—not only the maximum number of individuals the ecological environment can sustain, but also the limits imposed by social factors, such as overcrowding and the structure of the colony, which may affect survival and reproduction in ways we do not yet fully understand. Such slowdowns in growth are typical and not a cause for alarm when balanced by a healthy ecosystem.
However, the mass mortality event triggered by avian influenza abruptly reversed this trend. If the virus had affected only pups—which commonly experience high mortality anyway—the colony might not reach pre-outbreak levels until around 2035, based on a 1 percent recovery rate, with a possible range from 2029 to 2051, reflecting the resilience of surviving adults and new births. But the 2024 breeding season counts revealed a grim reality: reproductive females had declined by approximately two-thirds at some of the most densely populated beaches, suggesting significant adult mortality. Therefore, a fast return supported by an increase in population size of much more than 1 percent seems unlikely. Future counts will allow for improving the estimate.
If indeed half or more of the adult female population perished, and survival of juvenile females does not increase significantly, recovery could take many decades—possibly until the end of the century. This lengthy recovery timeline is especially worrisome because the colony’s social structure, essential for reproductive success, has been disrupted. Dominant males, who defend harems and ensure selective mating, are fewer in number. Females without male protection or access to mates may fail to reproduce successfully. These complex social disruptions add layers of uncertainty to the population’s future.
Unfortunately, once such an epidemic spreads among wild marine mammals, direct interventions, such as vaccination or treatment, are nearly impossible. The logistics and ethics of vaccinating thousands of wild seals scattered across remote beaches are daunting at best. Moreover, culling sick animals in the wild is not just controversial—it is unacceptable. Humans have already driven much of the mortality, and deliberately killing more animals to try to prevent further deaths is ethically indefensible.
Instead, our best hope lies in prevention, monitoring, and mitigating human disturbances that can exacerbate pup mortality. As the colony grew, seals began colonizing new beaches that were previously unused for breeding, some of which are now subject to human activities such as off-road vehicles and sport fishing. These disturbances cause mother-pup separations, almost certainly leading to pup starvation. Protecting every possible breeding site from such disruptions is a vital conservation step.
A Global Wake-Up Call
The catastrophic impact of avian influenza on the PenÃnsula Valdés seal colony is a stark reminder of how interconnected life on Earth truly is. The virus responsible originated on poultry farms thousands of kilometers away, spread globally through birds, humans, and other species that can carry it, and ultimately adapted to infect marine mammals at the far end of the world. This pathway exemplifies the “One Health” concept—the idea that the health of wildlife, domestic animals, humans, and ecosystems is deeply intertwined.
Climate change, habitat loss, and increased global connectivity accelerate the risks of such spillover events. Warmer temperatures may alter bird migration patterns or stress animal populations, making them more susceptible to disease. Habitat encroachment brings wildlife into closer contact with humans and livestock, creating new pathways for pathogen transmission.
Conservation categories such as the International Union for Conservation of Nature’s “Least Concern” can provide a false sense of security when faced with rapidly changing environmental conditions and emerging diseases. The PenÃnsula Valdés event teaches us that no population is invulnerable and that continuous monitoring is essential.
As researchers, we remain committed to conducting annual counts and behavioral observations, aiming not only to document the recovery but also to inform global strategies to prevent wildlife diseases. The seals’ story is a cautionary tale—a call to action for greater investment in integrated health approaches that treat wildlife, domestic animals, humans, and the environment as a single system.
Ultimately, protecting biodiversity is about preserving species for their own sake, which requires safeguarding the delicate balances that sustain life on our planet. The 2023 epidemic made this painfully clear: a virus that originated in birds spread globally and adapted to infect elephant seals in Patagonia. Conservation today is profoundly complex—some battles may already be lost, and environmental movements must take epidemics seriously. Humanity must adopt and invest in the One Health concept, recognizing that our welfare is inseparable from the health of all life. This may seem like common knowledge, but it bears constant reminding. The epidemic was a shock, yet we will work relentlessly to help this population recover. We believe the seals are resilient. It will take time, but they must come back.
[Authors’ Note: We are thankful to Dr. Burney Le Boeuf and Marcela Uhart, DVM, for their comments on several drafts of the original article, and to Reynard Loki, for his editorial guidance on this updated version.]
This article was produced by Earth | Food \ Life, a project of the Independent Media Institute.
