Weddell seals have been observed to dive as deep as 600 m for up to an hour[14] Such deep dives involve foraging sessions, as well as searching for cracks in the ice sheets that can lead to new breathing holes.[15] Weddell seals exhibit a diel dive pattern, diving deeper and longer during the day than at night.[23] After dropping away from a breathing hole in the ice, the seals become negatively buoyant in the first 30 to 50 m, allowing them to dive with little effort as they make a “meandering descent".[24] The seals can remain submerged for such long periods of time because of high concentrations of myoglobin in their muscles.[25]
Weddell seals' metabolism is relatively constant during deep-water dives, so another way to compensate for functioning with a lack of oxygen over an extended period of time must exist. Seals, unlike other terrestrial mammals such as humans, can undergo anaerobic metabolism for these extended dives, which causes a build-up of lactic acid in the muscles.[26] The seals can also release oxygenated blood from their spleens into the rest of their bodies, acting as an oxygen reserve.[27]