The distinctive feature, if anyone remembers the mention of it yesterday, aside from the hollow crest and height of Hypacrosaurus which sets it apart from other hadrosaurs is the tall neural spines. The crest on the head is a lot like that of Lambeosaurus and Corythosaurus and, being a Lambeosaurine dinosaur, this makes sense along with making it slightly more difficult to differentiate between the two animals when no height reference is given;the neural spines of Hypacrosaurus are between five and seven times taller than the vertebrae they are found on whereas the other two animals have much more similar heights of spines and vertebrae. These neural spines, therefore, would more than likely be sticking at least a little ways out of the back of the animal, as they do in this illustration with one exception as far as theories go. This exception would be if a theory that proposes a lump of fat along the spine existed for protection or camel like purposes of storing fat and hydration. As yet I have neither heard nor seen this type of theory brought to light, but that does not mean it does not exist.
This illustration, shared yesterday, not only shows the height of the animal, and that distinct Lambeosaur/Corythosaur crest, but also what the back of the animal would look like with more of a fat hump as opposed to the neural spines protruding from the back. In this case it is not simply the curvature of a hadrosaur back but that typical back meeting where the spines support the lump, in this case it looks like it is just skin. The lump of fat idea, in addition to being a survival necessity in Cretaceous droughts or during migration, may have also served as a defensive barrier. remember that this dinosaur was almost as tall as Tyrannosaurus Rex, meaning that the T. Rex would not have to bend down a whole lot to attempt to take a chunk of back fat off a running Hypacrosaurus. A lump supported by the neural spines could have offered a buffer zone between precious spinal material and giant carnivorous death utensils, though it may have also led to broken neural spines, it could have been the difference between being a snack and running away when the predator went slightly off balance after tearing out a small chunk of fat.
The evidence for the neural spines comes not only from the adult specimens found but also from the hatchlings. Clearly the hatchlings are lacking in the crest department, which certainly grows as the infant becomes a juvenile and then an adult, but we can see here that those tall neural spines begin sprouting out of their vertebrae very early on, perhaps even while still in the egg. This actually tells us that the high neural spines were important to the species in one way or form from birth to adulthood whereas the crest, not evident in hatchlings, obviously was not of the greatest importance at birth. If the crest were that important to the animal than even the hatchlings would possess at least a small crest so that they could begin using it from birth. This, to me, means that the crest did not begin to sprout until either the juvenile could begin to sound warnings, because it must have been able to make some sound without a crest just maybe not a warning tone, or began to mature and practice mating calls and perhaps both reasons. A hatchling has no need for either as it would be kept in the center of a herd and would certainly not be mating. These are just ideas though.