Understanding the ontogeny of diving behavior in marine megafauna is crucial due to its influence on foraging success, energy budgets, and mortality. We compared the ontogeny of diving behavior in two closely related species – northern elephant seals (Mirounga angustirostris, n = 4) and southern elephant seals (Mirounga leonina, n = 9) – to shed light on the ecological and evolutionary processes underlying migration. Although both species have similar sizes and behaviors as adults, we discovered that juvenile northern elephant seals have superior diving development, reaching 260 meters diving depth in just 30 days, while southern elephant seals require 160 days. Similarly, northern elephant seals achieve dive durations of ~11 minutes on their first day of migration, while southern elephant seals take 125 days. The faster physiological maturation of northern elephant seals could be related to longer offspring dependency and post-weaning fast durations, allowing them to develop their endogenous oxygen stores. Comparison across both species suggests that weaned seal pups face a trade-off between leaving early with higher energy stores but poorer physiological abilities or leaving later with improved physiology but reduced fat stores. This trade-off might be influenced by their evolutionary history, which shapes their migration behaviors in changing environments over time.

Study System and Animal Handling

In 2018, 24 juvenile northern elephant seals (15 females and 9 males) from the population at Año Nuevo Reserve, CA, USA (Figure 1A; 37°5’ N, 122°16’ W) were equipped with an archival time-depth recorder (MK9, Wildlife Computers, measures time, depth, light) to record their very first trip to sea. The tags of only four individuals (4 females; mean weaning mass ± SD = 133 ± 10 kg; mean straight length from nose to tail tip ± SD = 139 ± 7 cm) were recovered when they returned to land after 229 ± 16 days at sea. Animals were sedated with an initial injection of tiletamine hydrochloride and zolazepam hydrochloride (Telazol), administered intramuscularly. Immobilization was maintained with intravenous injections of Ketamine when needed. Using quick-setting epoxy (Loctite®, Epoxy General Purpose), the MK9 tag was affixed to the fur on the center of the back.

In 2014, 20 juvenile southern elephant seals (10 females and 10 males) from Kerguelen Islands, sub-Antarctic French territories (49°20′S, 70°20′E) were equipped with a custom-designed Argos relay satellite tag (SPLASH10-F-2961-DSA tag, Wildlife Computers, hereafter DSA tag) and a smart position transmitting tag (SPOT 293A, Wildlife Computers) that measured diving depth to record their very first trip to sea. The satellite tags of nine individuals (7 females and 2 males; mean weaning mass ± SD = 108 ± 16 kg; mean straight length from nose to tail tip ± SD = 134 ± 9 cm) were recovered when they returned to land after 175 ± 25 days at sea. Animals were captured with a canvas head-bag and anesthetized using a 1:1 combination of tiletamine hydrochloride and zolazepam hydrochloride (Zoletil 100) injected intravenously. Using quick-setting epoxy (Araldite® AW 2101), the DSA tag was attached to the fur on top of the head and the SPOT tag to the center of the back.

Data Collection

The MK9 tags attached to northern elephant seals sampled pressure every 4 s during the entire trip to sea. The DSA tags sampled pressure every 1 s, but only by recording one complete dive (max depth > 15 m and duration > 60 s) every ~2.25 h. 

Data Preprocessing

Time-depth records of southern elephant seals were downsampled to 4 s to match the sampling frequency of the northern elephant seals and facilitate comparison. Dive identification and zero offset correction of depth were performed using the IKNOS Toolbox with Matlab 9.1 (The MathWorks, Natick, MA, USA). Dives were identified as excursions from the surface reaching a maximum depth of at least 10 meters and lasting a minimum duration of 30 seconds.

For northern elephant seals, location data were obtained from the light-level data using the Wildlife Computers GPE3 algorithm, with the exception of seal ID 2018074 in which the light levels were corrupted. Using the aniMotum R package, we fitted a correlated random walk with a maximum travel rate of 3 m.s-1 on our location data to re-estimate the animal's position every hour in order to get a regular time step and handle gaps. The time of day was calculated using the function sunriset() from the R package maptools, which estimates night-time and day-time based on date-time and location from the NOAA Solar Calculator.