Study

Stereotypical resting behavior of the sperm whale

Miller, Patrick J. O.; Aoki, Kagari; Rendell, Luke E.; Amano, Masao

Current Biology, 2008

Peer-reviewedfield observationwild

What they did

Movement and orientation records from tagged sperm whales were examined for periods of inactivity, and the posture and depth of those periods characterised.

Studied in: Physeter macrocephalus
Sample size: tag records from dozens of whales, plus opportunistic observation

Sperm whales rest in a near-vertical posture, drifting motionless in the upper water column with the head either up or down, in bouts of around ten to fifteen minutes. Resting occupied roughly 7% of the record. Resting animals were unresponsive to approaching vessels until physically contacted, indicating a genuinely reduced state of awareness.

What it means

The authors' reading, and ours. Where they differ, that difference is the point.

How the authors put it

Sperm whales adopt a stereotyped vertical drift posture for rest, and the unresponsiveness during these bouts suggests a sleep-like state in which both hemispheres may be involved.

How NatureHQ reads it

One of the very few animal facts where the strange popular version is simply correct: they do sleep upright, in silent vertical groups. The detail worth adding is the unresponsiveness, because it is what makes it sleep rather than loafing — the whales did not react to a boat until touched. That is also notable against the dolphin comparison, where sleep is famously half-brained; the evidence here is at least consistent with sperm whales shutting down more completely, which would be unusual for a cetacean and is not yet settled.

Limitations

Sleep is inferred from posture and unresponsiveness; no brain activity was recorded.
Resting bouts may be under-sampled if tagging disturbs the behaviour.
Whether both hemispheres sleep simultaneously remains untested in this species.