We had to do some field work in Monterey County yesterday, but we arrived a little early, so had a few minutes to observe a few sea otters while eating lunch in Pacific Grove.
The first sea otters we saw were actively munching on sea urchins -- above, and next two photos:
But then our attention was drawn to a hard slapping sound, and we turned to see an otter trying to crack open a large mussel against a rock that was balanced on the otter's chest:
Several times we watched this otter dive down, bring up a rock and a mussel (or clump of mussels)...and then use the rock like an anvil -- hammering the mussel against the rock.
Sea otters are known to do this, but I hadn't photographed it before, so here are a few more images. (Tool-use by mammals aside from humans is rare.) In each picture, look for a rock resting on the otter's chest and a mussel between the otter's paws.
We were reading up on this behavior at the end of the day and came across an interesting research paper about tool use by sea otters just published in May 2024.
It turns out that using rocks as tools to access hard-shelled prey might be especially important for female otters in areas where their preferred and easier-to-process prey (e.g., abalone and urchins) aren't as common. And using the rocks to open hard-shelled prey can prevent tooth damage and give the otters access to an alternative food source that otherwise wouldn't be available by biting alone.
Law et al. 2024. Tool use increases mechanical foraging success and tooth health in southern sea otters (Enhydra lutris nereis). Science 384: 798-802.
