Science writer Liam Drew thinks of himself as a mammal first and a human being second. Members of the mammal club – which include tiny shrews and giant blue whales – share lactation, warm blood and sometimes body hair, backbones and scrotums.
Liam was a neurobiologist for 15 years and had always thought of himself as a 'stream of consciousness' until his first child was born.
Fatherhood set him on an explorative journey into evolutionary biology which became the new book I Mammal: The Story of What Make Us Mammals.
While researching the book, Liam fell in love with the platypus – a highly unusual mammal that had 19th-century scientists baffled because it lactates despite not having nipples and, along with its cousin the echidna, lays eggs.
The tuatara is almost like the platypus of the reptile world, he says, as adult male tuataras don't have penises,
A couple of years ago the New Zealand native became "the unexpected star" of an investigation into the origins of the penis.
(Tuatara embryos digitally reconstructed at Harvard University were found to have a nascent penis that stops growing as the tuatara develops, confirming mammalian penises are an inherited version of the first land animals' penis.)
One definitive trait of mammals is lactation – the word 'mammal' comes from word 'mammary' – but why don't men lactate?
Although human males have developed some of the tissue required and there seems to be a fundamental barrier to it, we're not sure, Liam says.
One reason male mammals haven't developed the ability may be that only 5 percent of mammal fathers participate in parenting.
Of all of the species on Earth, the really spectacular ones are mammals, he says.
"The fact that a polar bear came roam across the Arctic or a camel can cross a desert is an amazing accomplishment ... It would be nice if the species of mammal of which there are 7 billion could take care of its mammalian cousins."