Meursault is a Reptile

Reptiles and being 'cold-blooded' are often associated with a lack of empathy and feeling-- Meursault also shares these traits. When his mother dies, he lacks any kind of recognizable emotional response other than his frustration with having to go to her funeral. This doesn't necessarily mean that he's behaving abnormally, he could just be in shock. However, as the story continues no evidence of any kind of normal human emotion surfaces. Meursault continues to show a lack of empathy and emotion through his response to Raymond's mistreatment of his "mistress" (basically ignoring it), and how little he feels after having killed the Arab. He also doesn't seem to be able to relate with anyone else in the novel, observing their emotional responses in a detached way that would seem to indicate that (while does sometimes say he can understand people's point of view) he doesn't empathize with them, like when he was unhappy with the crying woman at his mother's funeral. Meursault's detachment from the rest of the population coupled with his lack of emotions relate strongly to the traits associated with being reptilian. 

Meursault seems to also share a reptile's passiveness and responsiveness to physical stimuli rather than mental or emotional stimuli (that he generally just doesn't respond to). His behavior on the balcony represents this idea well as Meursault is content to just sit on his balcony for hours, not reflecting on anything, simply watching-- he only responds to physical things such as the soccer team yelling, or his own hunger. Similarly, a lizard basking on a rock is content to stay there for long stretches of time, detachedly observing the things happening around it without passing judgement-- things happen around it but unless it's hungry or their is a loud noise it won't. 

His response to temperature, in particular heat, is also something that he shares with reptiles. From the funeral scene right at the beginning of the book, to the scene on the beech where he murders the Arab, Meursault is very conscious of the temperature, and examines it in a way that he doesn't seem to examine anything else around him. His behavior is also largely controlled by the temperature in a similar way to a reptiles. If we return to our friend the lizard, if the temperature were to suddenly drop, he would become lethargic, similar to Meursault's ability to calm down and reflect on his execution when he is in his relatively cool temperature cell. The lizards would also be much more energetic if it was warm in a similar way to how much more active Meursault was on the beech. 

While Meursault obviously isn't actually a lizard, these similarities combined with the fact that he killed the Arab in 'cold-blood' (a metaphor which is hard to ignore in this context), show a strong relationship between Meursault and our scaly friends. Also, standard disclaimer: I'm not sure if this is important or if Camus intended it but I thought it was interesting.