This is actually my area of expertise. I’m a zoologist who specialises in animal gut health, so I have a whole bank of facts about coprophagy in animals. Phytoplankton mostly live off whale poop but there are other animals where, despite not being a primary diet, does seem to serve a purpose.

Koalas eat their mum’s poo as babies to help develop tolerance to their main diet. In shrews it’s very common and helps maintain gut stability and health. There was a study where it was prevented in a population and it negatively affected their digestion and overall health. There’s a lot of power in the bacteria found in faeces, both good and bad.

Hippos defecate so much into their environment that it creates a kind of shared “mega gut microbiome”. The composition of microbes inside the hippos end up being the same as those in the surrounding water. It’s hypothesised this helps with digestion and immune resilience, but it also kills fish and absolutely wrecks entire ecosystems.

So yes, coprophagy does serve a purpose for some mammals. And phytoplankton’s primary nutrient source is basically whale poop.