Changing environmental conditions could make it difficult for marine mammals to find prey, according to a new study published in the journal Ecology Letters. The research shows how gradual reductions in prey could have profound implications for animal populations.
For female elephant seals, reproductive success depends on the ability to find prey and gain weight. Researchers at UC Santa Cruz studied the relationships between elephant seal behavioral strategies in the open ocean, weight gain, and lifetime success at producing pups.
The findings reveal a relationship between mass gain and pup production, suggesting a tipping point at which insufficient mass gain leads to reproductive failure.
“We found that diving deeper during foraging allowed the females to gain more mass, and gaining more mass led to a marginal increase in their chances of survival and a massive increase in the number of pups they produced in a lifetime,” said Professor Roxanne Beltran.
The team used 25 years of data to investigate the influence of behavioral strategies and foraging success on survival and reproduction in female northern elephant seals. Migration activities were recorded, including where the seals went and the depth of their dives. Data collected on the beach at Año Nuevo told researchers which animals survived, how much weight they gained, and how many pups they had.
“Conceptually it makes sense that an individual would have to gain enough energy to survive and reproduce, but we were able to demonstrate how entwined these are and suggest when survival may be prioritized over reproduction,” said study co-first author Keith Hernandez.
Female elephant seals give birth to a single pup in winter. They spend four weeks on the beach nursing their pup, relying on stored energy until they can return to the ocean to feed. After the winter breeding season, they head out to sea for two months before returning to the colony to molt. Then they leave on a long migration across the North Pacific Ocean over seven to eight months.
During foraging trips, seals pursue their own strategies such as traveling different distances from the coast, diving to different depths, and targeting different prey. The researchers found that deeper diving seals migrated farther and had more energy-rich diets than shallower diving seals.
Mass gain was found to directly affect female seal reproduction. The threshold is around 205 kilograms (450 pounds): animals that gained less rarely had a pup, while those gaining more than 260 kilograms (573 pounds) almost always reproduced.
Previous research has shown that a relatively small number of long-lived female elephant seals produce most pups in the colony. These “supermoms” may live as long as 23 years and produce more than 15 pups in their lifetime. The strategies that allow these moms to be so successful, however, have long been a mystery.
“We discovered that an additional 5 percent of foraging success led to a 300 percent increase in lifetime pup production due to the effects of mass gain on both survival and giving birth and raising a pup each year,” said Professor Beltran. “These findings tell us which strategies allow these long-lived mammals to succeed at their most important job, which is to stay alive and contribute to the next generation.”
After being hunted to near extinction during the 19th century, the northern elephant seal population has recently increased by about four percent each year. They do, however, face new threats. Climate change is impacting ocean environments and the fishing industry is exploring areas where elephant seals find most of their prey.
“Elephant seals are incredibly successful now, but that could change as their environment shifts in the coming years.” said Professor Beltran.
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