I wish I were a dog

As I've mentioned recently, I'm now in a Meetup for people in the Des Moines, IA area where we're seriously doing the problems and paying close attention to the text, so I noticed this function intended to map your dog's age in years to a corresponding age in people years, while also introducing the beginning Haskell programmer to guards:

dogYrs :: Integer -> Integer
dogYrs x
   | x <= 0    = 0
   | x <= 1    = x * 15
   | x <= 2    = x * 12
   | x <= 4    = x * 8
   | otherwise = x * 6

Looking at it, you can see that yes, dogs mature faster than people but the rate tapers off with increasing age (but still faster than people). OK...but then something looked wrong. A function like this ought to be monotonically increasing, right?

map dogYrs [0..5]
[0, 15, 24, 24, 32, 30]

So dogs don't age in human years between 2 and 3, and get younger in human years between 4 and 5? (If myDog took and returned types in Fractional, things would get even weirder, because then dogs would celebrate their second birthday plus epsilon by getting almost three years younger in human years, and their fourth birthday plus epsilon by getting almost eight years younger in human years.)

As a human, I feel cheated. As a person looking to get better at Haskell, I think I'd better write a quickCheck test that a function is monotonically increasing.