Sperm whales have a problem. They find most of their food (squids!) in the mesopelagic zone, typically at depths more than 600m deep, but their calves are unable to dive this deep for very long, with the youngest being even unable to reach these depths.
As a consequence, whilst the adults and sub-adults from a social group are foraging, the calves have to remain on or near the surface where they are vulnerable to predators or could simply get lost.
Two Sperm whale calves being baby sat by a sub-adult bull - photo by Justin Hart
To help protect the babies, but also probably to help prevent the calves from getting lost, older members of the social group take turns to baby sit. Adult females in the group will also give milk to any calf seeking it, not just their own offspring, a behaviour known as allosuckling.
This means that a calf does not have to struggle to keep up with its mother whilst she is diving for food, as they can get all the milk they need by simply moving between any of the groups females as they surface between dives.
During our Whale Watching trips we often witness baby sitting in Sperm whales and also, perhaps, how they sometimes choose to communicate at the surface by breaching. In one particular tour, after we had already seen a variety of different females and sub-adult males from one group surfacing between dives, we also noticed two very young calves sometimes travelling together at the surface.
The forehead and blow-hole of a baby Sperm whale seen as it passes close by our boat - photo by Justin Hart
After an interlude when none of the whales were at the surface, we suddenly saw one sub-adult bull breach twice in the distance. We immediately moved the boat to where this whale laid at the surface and there, almost certainly in response to his signal, we witnessed the two calves speedily swimming to his side (first photo in this post).
An amazing sight and a noteworthy observation too at least good anecdotal evidence of why Sperm whales sometimes breach and that in their social groups not only mothers take responsibility for the care of their young.