The level 4 spell Polymorph requires concentration to maintain. The subject retains their personality and alignment, but they have the mental statistics of the new form, which must be a beast and cannot speak or cast spells.
If a caster targets themselves with this spell, can they concentrate to maintain it, or does this depend on the mental statistics of the beast form they choose?
This situation certainly occurs in practice, since Wizards reaching level 10 in the School of Transmutation gain the Shapechanger feature. This allows them to cast Polymorph, but only upon themselves and only to change into a beast of CR 1 or lower. Beasts with CR 1 or less range in Intelligence from the Giant Eagle (Int 8) to the Frog (Int 1).
Best Answer
You maintain concentration, regardless of the form. Your ability to concentrate on a spell does not depend on any mental statistic. You can also continue to concentrate on a spell, even if you cannot cast spells. For example, Silence, Druid Wildshape, and the Antimagic Field spell all restrict spellcasting, but not do not restrict your ability to concentrate. If, hypothetically, your concentration were tied to your ability to cast spells, then a cleric who casts Antimagic Field would immediately lose concentration, and the spell would be incapable of having an effect. From this we can safely conclude that these two mechanics are not intrinsically tied to one another.
Page 203 lists the requirements for concentrating on a spell. You lose concentration if you:
Being polymorphed does not fall under any of these categories. A character can therefore maintain concentration while polymorphed.