Weddell seals truly need that more highly developed brain. They must be able to orient themselves under water right away, and at less than three weeks old they start to dive under the ice. Such a dive is dangerous: if they don’t find their way back out in time, they drown. These seals must develop this capability quickly, as they must live independently very soon — Weddell seal mothers leave them to their own devices from the time they are 40 to 50 days old.
The seal pups’ large and well-developed brain comes at a price: brains cost a good deal of energy in the form of glucose. While nursing the pups, the seals’ mothers have to supply this energy. The mothers rapidly lose body mass, sacrificing their own reserves for the sake of their young’s survival and growth.