system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are two headed reptiles more common than two headed mammals?
gpt
Reptiles on average have more offspring than mammals and reproduce by laying eggs. Animals in eggs are more susceptible to environmental conditions which may affect the embryo inside. 

Being two-head, which is called bicephaly, can also result from an embryo splitting in two (this is how twins form) but sometimes they don't split completely and this is how conjoined twins form.