Elephant seals take more risks when their body fat is lowest, and strategically change their behavior as their body condition improves, according to a new study from UC Santa Cruz. The research shows that when they have more fat stored up, the seals prioritize safety over feeding.
The investigation is the first of its kind to continuously measure changes in behavior relative to body fat, and provides new insight into how wild animals balance risks and rewards.
“Theory predicts that fat animals take fewer risks, but it’s very difficult to measure body condition continuously in wild animals,” said study first author Professor Roxanne Beltran. “Our results show that fat seals prioritize safety, and skinny seals prioritize feeding.”
Each year in May, female elephant seals leave the beaches in California where they gove birth and nurse their young to set out on a seven-month foraging trip across the Pacific Ocean. As they hunt for fish and squid, the seals must avoid becoming prey themselves.
The researchers used satellite tags and time-depth recorders to track the movements and diving patterns of 71 adult female northern elephant seals. Their intense migration covers about 6,000 miles, and invloves an uninterrupted series of dives. The seals average 23 minutes per dive with just two minutes at the surface between dives.
The elephant seals minimize their time at the surface to avoid attacks by sharks and killer whales. Because these are visual predators, the seals are safer in water depths that light does not penetrate. The researchers refer to this phenomenon as “lightscapes of fear” that shape animal behavior in the open ocean.
“We don’t know a lot about predator-prey dynamics in the open ocean because there are so few sightings of attacks, but we regularly observe elephant seals on the beach with shark bites,” said Professor Beltran.
The tracking data revealed several different types of dives, including foraging dives, when the seals pursue prey; transit dives with no feeding activity; and rest dives, when the seals drift passively at the bottom of the dive and are presumably sleeping.
During the drift dives, the researchers can calculate how much body fat the seals have based on their buoyancy in the water.
“It gives us a nearly continuous record of how fat they are, which is unparalleled for any wild animal other than elephant seals,” said Professor Beltran. “At the beginning of the migration, they are really skinny and they sink through the water column. As the feeding migration progresses, they slowly gain weight and their buoyancy becomes less negative until, at some point, they switch to positive buoyancy and stay positive for the rest of the trip.”
The timing of the resting dives were found to change as the seals’ body condition improves. Seals in poor condition started their drift dives just after sunrise, while seals in good condition started their drift dives just before sunrise. As a result, the amount of time the seals rested during the dangerous daylight hours decreased from 80 to 30 percent.
The drift dives occurred progressively earlier with respect to sunrise as the seals’ body condition improved throughout the migration. This indicates a shift in their priorities from feeding to safety.
“Light is a fundamental environmental constraint that these animals are using strategically to maximize rewards and minimize risks,” said Professor Beltran. “In the ocean, light is an interesting way to look at risk-reward tradeoffs because it integrates time and space, especially for a migratory species that travels across time zones and latitudes with constantly changing day lengths.”
The study is published in the journal Science Advances.