The joke is in the use of the word tail. A tale is another word for a story, but since they are talking about lizards, it is literally about the tail body part of a lizard. 'Come listen to this tail' sounds like the speaker is beginning to tell a long story, but they literally mean that you can hear the heartbeat of the lizard through its tail.
Lizards have a very strong heartbeat which can be faintly heard anywhere on their body , said Mr . Johnson , beginning a story . ' ' Come listen to this tail . ' '