By Kathleen Wong, UC Nature
When northern elephant seals first come into this world, males and females are nearly indistinguishable. Pups of both sexes resemble sable-brown torpedoes with oversized eyes and glamorously curled whiskers. But by the time these marine mammals reach adulthood, the sexes have diverged so much they could be mistaken for different species, with bulls sporting a bulbous proboscis and weighing up to two tons more than cows.
Now researchers from UC Santa Cruz have described for the first time how these differences emerge in juvenile northern elephant seals. Madeleine Stewart initially conducted the study for her senior thesis as a UC Santa Cruz undergraduate and UC Nature Field Science Fellow. Stewart examined the majority of her seal subjects at UC Nature’s Año Nuevo Reserve along the San Mateo County coast.
“Sexual dimorphism is such a foundation for every single part of elephant seal life history, from breeding to migration, throughout the year,” Stewart says.
Bulls need mammoth proportions to defeat other males for breeding rights, and ward off rivals with hearty bellows amplified by that trumpetlike schnozz. The sexes also forage in different parts of the Pacific, males exploring the richer hunting grounds of the shallower continental shelf, and females striking out midocean where prey is more scarce but predators are less concentrated.
As a student in UC Santa Cruz Associate Professor Roxanne Beltran’s lab, Stewart studied animals already being handled for a longstanding elephant seal research program.
“We have a huge six-decade database of known-age animals that includes every animal in my study,” Stewart says. “We know when they were born, we know their mother, they’ve been tracked over their lifetime. There’s not many places where you’re able to handle animals this readily, or have this much data on an animal like this.”
Stewart measured features such as the dimensions of the snout and teeth, the shape of flippers, and body mass using calipers, a tape measure, and photographs she later analyzed by computer.
The 60 seals analyzed in the study were up to four years old. Stewart could deduce the age of each because they were flipper tagged as young-of-the-year seals at Año Nuevo. This aspect of elephant seal biology offered Stewart an advantage over previous studies of seal development, which were based on museum specimens of unknown age.
“It was cool to fill that gap and connect the adult sexual dimorphism studies done on wild adults, and then the museum specimen studies,” Stewart says.
Stewart’s measurements revealed that the sexes diverged in dimensions, though not mass, within the first four years of their lives. Males grew longer faster, so that by age three they were on average five centimeters longer than females. The growth of males’ snouts, and the length of their canine teeth also significantly outpaced that of females. Likewise, by ages two and three, both the front and rear flippers in males were clearly larger than in females.
“Males are already prioritizing becoming bigger and becoming longer early in life because of their highly polygynous mating system” where males mate with many females, Stewart says.
Females may invest more in body mass because they start breeding earlier in life, at around age three. By comparison, males aren’t likely to be successful at securing a harem until they are 8–10 years old.
The early morphological differences Stewart observed between the sexes continue to magnify with age. When males do finally start siring pups, they may be 4 feet longer and 2.5 times heavier than females, and sport a nose with a startling resemblance to an abbreviated elephant trunk.
“Maddi’s findings provide the anatomical and morphological information underpinning the differences in physiology, diving and movement patterns, and demographic processes that other graduate students and postdocs are discovering in the same seals,” Beltran says. “Putting together all these puzzle pieces will help us understand the mechanisms underlying niche divergence between males and females, in elephant seals and beyond.”
Stewart’s research was funded by a 2024 Field Science Fellowship from UC Nature, which supports UC undergraduate-faculty teams conducting field research at UC Nature reserves. The fellowship’s $5,000 paid her for her field work time, while the $1,000 provided for project costs bought the camera and digital calipers Stewart used to measure seal features.
The opportunity to work on the project with her postdoctoral mentor Stephanie Adamczak and program leader Beltran on this research, Stewart says, “has informed my whole career and passions, and definitely has launched my career as a marine mammal scientist.”
Stewart has already made crucial strides toward her career goal. After graduating from UCSC in 2025, she interned at The Whale Museum in Friday Harbor, Washington, studying orcas in the Salish Sea. She is currently a sea otter care intern at the Monterey Bay Aquarium.
“My team had the pleasure of helping Maddi develop her confidence and leadership skills in the field and lab for three years. We can’t wait to see what she does next!” Beltran says.
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