They're even different sexually. Last year I learned from the mainstream media that echidna penises have four glans (heads). Researchers from the Universities

They're even different sexually. Last year I learned from the mainstream media that echidna penises have four glans (heads). Researchers from the Universities of Melbourne and Queensland and Monash University studied the structure. An article [0] in Melbourne Uni's magazine gives a technical overview and refers to the peer-reviewed paper [1]. They only use two at a time. The researchers speculate that two can rest while the other two are in use, outcompeting proto-echidnas with fewer glans. On a sad note, the research studied echidnas injured in road accidents, handed in to the Currumbin Wildlife Sanctuary on the Gold Coast, that had been euthanised. I imagine the numbers available are the result of the rapid urbanisation in the area. [0] https://pursuit.unimelb.edu.au/articles/solving-the-mystery-of-the-four-headed-echidna-penis https://pursuit.unimelb.edu.au/articles/solving-the-mystery-... [1] https://pubmed.ncbi.nlm.nih.gov/33915542/ https://pubmed.ncbi.nlm.nih.gov/33915542/