My interpretation of that is that we are part of nature, part of it all, even though we don't act it. We acknowledge that in these intuitive feelings we have, and so do other animals. Still, Marshall Thomas is not one to over sentimentalize animal nature. And if you do report on everything, including the things that are less than totally happy to us, then that gives much more credibility to the things that are totally happy to us. Realism is a very good vehicle.
Animals can express themselves.
You'll never get into trouble if you do that. In my book, when the Magi make this wonderful journey to see the Christ child, they block off the view of two dogs. One dog assumes, because he no longer sees the eyes of the other dog, that the first dog has had a failure of courage. To the dogs the entrance of the Magi into the stable has dog meaning. Dog life is very important to dogs, and human life is very important to humans, and sometimes the two conflict in some seemingly trivial way.