"Ninjas are mammals" is the first rule of ninjas, as enumerated by Robert Hamburger on realultimatepower. The line, like most of the web page, is perhaps just a piece of random absurdism. It is also perhaps a reference to the fact that one of the most famous carriers of the ninjitsu banner were not, in fact, mammals. But let us for a moment take this statement from a nine year old webpage that somehow maintains its cachet as a serious statement: what is the connection between being a ninja and being a mammal?
Mammals come in all shapes and sizes, but there is actually a shape and size that represents the symplesiomorphy (which is a big compound word being "together early shape" -- the shape that the common ancestor of a clade shared) of mammals. This shape was pretty similar to a modern rodent, at least on the outside. Primates, too, are fairly close to the early form and lifestyle of our mammalian ancestors. Although mammals have branched out into large herd animals, pack predators, flying insectivores, armored forms, gigantic sea-based filter feeders, and large, bipedal hoppers, the core form of the mammal is small, quadrupedal, lives in a burrow or undergrowth, has prehensile hands, and is a crepescular or nocturnal omnivore. Mammals were also, for the longest part of their history, small and near the bottom of the food chain, so had to learn to use their delicate senses and ability to escape to survive.
So while there are many exceptions to the rules, large and fierce mammals that can fight face-to-face during the day time, the prototypical mammal survives through alertness, stealth, the cover of darkness, the use of acute senses, being able to climb and escape, as well as heavy doses of opportunism. In other words, the survival strategy of the ninja. And of course, this is the survival strategy of the ACTUAL ninja, as opposed to the more confrontational ninja celebrated in Hollywood and by Mr. Hamburger. Another relevant fact about mammals is that compared to reptiles and some birds (and perhaps most dinosaurs), they spent more time educating their offspring in the ways of survival. In other words, the system of the ninja clan hearkens directly back to their Jurassic ancestors. The only thing that perhaps does not fit in our ninja/mammal connection is the defining characteristic of mammals: the presence of mammaries. Hot ninja babes to the contrary, the presence of breasts is not an important cornerstone of the ninja's strategy. However, in so much else, we find that ninjas are indeed prototypical mammals.
As Auduster forced me to acknowledge, mammals do not usually use poison, while ninjas often use poison. This is two strikes against my theory, and I will cry tears of regret if someone brings a third disjunction between mammals and ninjas to my attention