Stop-and-go mammals and “warm-blooded” dinosaurs

Ever since Thomas Huxley opined that dinosaurs and birds might be related, a debate has waxed and waned over whether dinosaurs were simply inflated, “cold-blooded” reptiles or active, “warm-blooded” creatures.  Tied into these metabolic “states” have been judgement calls as well: we mammals tend to have warmer feelings for endotherms than for ectotherms (pun very much intended).

Our best data for inferring dinosaur physiology comes to from studies of the microscopic structure of long bones in thin cross-sections, a science called histology.  Bone is a dynamic, living tissue that provides a record of the relative speed at which its tissues were laid down during growth and development.  Bone growth and development rely on a good blood supply, and in a typical long bone cross-section, one can see the conduits that once carried life-sustaining blood vessels.  These circular rings of bone are called Haversian canals.

The rate or speed at which bone tissues are laid down effects how they look in cross-section, an observation first made by the famous Italian researcher Amprino and known as Amprino’s Law.  When bone grows slowly, fewer blood vessels are necessary to sustain the growth, and consequently there are few Haversian canals.  In contrast, when bones grows quickly, many blood vessels are needed to supply fuel for the growth, and therefore Haversian canals abound.  Additionally, slow-growing bone has time to lay itself down in nice, concentric layers like the rings of a tree.  This type of bone is called lamellar because of its distinctive layering.  In contrast, fibrolamellar bone occurs when bone is being laid down quickly — somewhat like a mason throwing together a wall of bricks as fast as possible and in no particular order.  In other words, fibrolamellar bone can look a mess compared with lamellar bone.

Ectothermic vertebrates like an alligator tend to have lamellar bone with few Haversian canals, whereas endotherms such as a bird will show fibrolamellar bone pocked with many Haversian canals.  But there’s a wrinkle: in addition to these features, we will also see dark, concentric rings called Lines of Arrested Growth (LAGs) representing times when long bone growth temporarily ceased.  Traditionally, it was thought that slow growth and seasonal cessations of that growth produce LAGs, such as the seasonal warming and cooling of “reptiles” from Spring and Summer to Fall and Winter.  However, there have been sporadic reports of LAGs in otherwise fast-growing mammals and birds, and it has been suggested that changes in diet might play a role here.

And this is where a lot of controversy has come in: dinosaur long bones often show LAGs in section, yet are often fibrolamellar with many Haversian canals.  What gives?  If there is any consensus currently, it is apparent that all dinosaurs had some fibrolamellar bone, and most researchers recognize that this indicates a qualitatively rapid growth rate elevated at least somewhat above “reptiles.”  But what about those pesky LAGs?

Enter a new study by Kohler and colleagues published in Nature which shows that lines of arrested growth occur in numerous wild ruminants over a large range of climates.  That’s right — these are endothermic mammals, your cows and antelopes and other hoofed herbivores that have large, multichambered stomachs for processing grasses.  By examining the histological cross-sections of 115 femora from ruminants across Africa and Europe, Kohler and colleagues have shown many examples of LAGs in otherwise “good” endotherms.

What is going on here?  Seasons.  Kohler and colleagues matched the LAGs in their ruminant sample to average rainfall across the various biomes from which the ruminants lived.  What they discovered was that LAGs and slowed growth occurred during times when precipitation was low.  As you might expect, low rates of precipitation will effect plant biomass: fewer plants = fewer calories = less energy available for growth during the lean season.

LAGs in dinosaurs have often been pointed to as either definitive evidence of ectothermy or at least have raised flags concerning how well bone histology conserves information about growth and development.  This study by Kohler and colleagues pretty much ends that argument: LAGs are not associated with the underlying metabolic rate of the animal.  Instead, they reflect changes in bone growth due to the seasonal availability of food and water.

The authors actually say it best at the end of their paper:

The consistently seasonal formation of rest lines in homeothermic endotherms debunks the key argument from bone histology in support of dinosaur ectothermy. Our study instead suggests that the extensive vascularization of the fibrolamellar bone in most dinosaurs and other extinct vertebrates is tightly correlated with seasonal maxima of endogenous heat production, an association that should be explored in future studies. — Kohler et al. (2012), doi:10.1038/nature11264, p. 4

Is this “proof” that dinosaurs were endothermic?  Not exactly, but what is exciting about this new study is that it does indicate that what we are seeing in dinosaur long bone thin-sections is a realistic approximation of their growth rates and an indirect measure of their environments.