There are many types of animals. An earthworm is an animal, and it is undoubtedly not sentient. It has no brain, just a nerve ganglion, with negligible intelligence. A chimpanzee is also an animal, and they are estimated to have an intelligence comparable to that of a five year old human child, which clearly does fall within the range of sentience. So it depends which species we are talking about.
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It depends on how you use the term "sentient".
- If it broadly defined to simply "able to experience sensations", then any organism with sensory organs is sentient.
- If it is limited a bit to he concept of "able to feel pleasure and pain" then it gets more murky since we have no way to determine for certain if the response of an animal translates to the same things we call pleasure and pain but we believe most "higher" animals are able to be sentient by this second definition.
- If we further limit it to the ability to reason and think abstractly, this is little evidence that animals are sentient to that degree.
