Hi Vika， It's really a great question and you're really observing. 脖子 and 颈 means the same.It has none differences in people or animals. 脖子 is more casual，daily conversation. 颈 is more formal，and widely used in medical science documents. Hope that‘s helpful. :-D

脖子 is more widely used than 颈. People rarely say 颈 in daily conversation.

这两个词的意思是相同的，只是“脖子”更多用在口语中，人们的日常口语表达会使用；“颈”则更书面语。

The latter one is more formal,the former is more spoken. Yes,same.