DOI: 10.5061/dryad.tht76hf3t
title: Diving behaviors of juvenile northern and southern elephant seals

description: 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.
description: 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 &gt; 15 m and duration &gt; 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. 
description: # Diving behavior of juvenile northern and southern elephant seals ---
 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. ## Description of the data
 and file structure The dataset `data_ontodive.csv` includes a single table
 that contains all information for both northern and southern elephant
 seals, with a single row for every dive. * .id: individual ID *
 divenumber: # of dive * date: date and time of the dive * maxdepth:
 maximum depth reached (m) * dduration: dive duration (s) * botttime:
 bottom duration (s) * desctime: descent duration (s) * descrate: descent
 rate (m/s) * asctime: ascent time (s) * ascrate: ascent rate (m/s) * pdi:
 post dive interval (s) * dwigglesdesc: # of wiggles in descent phase *
 dwigglesbott: # of wiggles in bottom phase * dwigglesasc: # of wiggles in
 ascent phase * totvertdistbot: vertical distance at the bottom (m) *
 driftrate: drift rate (m/s) * divetype: Dive type (transit, foraging,
 drift or benthic) * day_departure: # of days since departure *
 lightatsurf: light level at the surface * lat: latitude * lon: longitude *
 dist_dep: distance from the first data location point (m) * sp: species
 (nes or ses) * ssh: sea surface height from Copernicus * psu: Practical
 Salinity Unit from Copernicus * vel: current velocity in m/s derived from
 Copernicus * temp: sea surface temperature (C) from Copernicus * bathy:
 bathymetry from marmap package * phase: phase of the day (day or night) ##
 Code/Software An R package called `ontodive` has been developed to analyse
 this dataset, available on github:
 .