Sentient and capable of suffering are two different things.

Technically true, but in the opposite way to what you're thinking. All those capable of suffering are by definition sentient, but sentience doesn't necessitate suffering.

Whether they can feel suffering like ants can is an unsolved scientific question

No it isn't, unless you subscribe to a worldview in which sentience could exist everywhere all at once instead of under special circumstances, which would demand you grant ethical consideration to every rock on the ground in case it's somehow sentient.