The sentence you provided is grammatically correct!
Here's why:
- 'The big ears' is a clear subject.
- 'help' is the verb, which agrees with the plural subject 'ears'.
- 'fennec foxes' is what the ears belong to.
- 'keep cool' and 'catch the tiniest noises' are two actions connected by 'and', which the ears help the foxes to do.
So your sentence, "The big ears help fennec foxes keep cool and catch the tiniest noises," is well-written and easy to understand!