Humans detect drowning by elevated CO2 in the blood. This means people walking into downed weather balloons have no warning that they're running low on oxygen before passing out and dying.
IIRC most aquatic mammals detect low oxygen directly instead, allowing them to dive for over an hour without panicking. CO2 builds up in the blood, but they can feel their oxygen levels directly.
IIRC most aquatic mammals detect low oxygen directly instead, allowing them to dive for over an hour without panicking. CO2 builds up in the blood, but they can feel their oxygen levels directly.