There are multiple synonyms for the phrase "there backs". These can include "their rear end," "their backside," "their buttocks," and "their hindquarters." Depending on the context, other synonyms could include "their posterior," "their tail end," or simply "their behind." These synonyms are all used to refer to the area of the body that includes the buttocks, hips, and lower back. While some may be considered more formal or appropriate in certain situations, they all convey the same basic meaning and can be used interchangeably depending on the speaker's preference and the context of the conversation.