Alligators are pretty tough. I’m not just referring to the fact that they faced down extinction in the 20th century and thrived. They’re also physically hardened, a set of ridged bones beneath their skin called osteoderms giving them a built-in coat of armor. But those skin bones aren’t just for protection. They’re also a handy source of calcium for egg-laying female gators.
Alligators, like all their other crocodylian relatives, lay eggs. In any given season a female alligator can lay up to 40 eggs in a clutch adding up to a total of 90 to 200 grams of calcium. That puts a pretty big demand on mother alligators to come up with enough material to create all the shelly capsules for their embryos. In a new paper, University of Portsmouth biologist Chris Dacke and colleagues considered the options.
Frogs and some lizards can ossify their skeletons thanks to the calcium-rich contents of a special sac next to the skull. Perhaps pregnant alligators drew on a similar pathway for their eggs. Yet when Dacke and coauthors looked at these sacs in female alligators before and after laying eggs, they didn’t spot any significant differences. And given that alligators can’t lay down and then utilize an ephemeral tissue called medullary bone like birds and non-avian dinosaurs, the calcium has to come from somewhere else. X-rays and tissue samples suggest that the answer is in the armor.
The osteoderms of nesting females used in the study weren’t as dense as in other alligators. They were lighter, and the degree to which the bone had been “mobilized” led Dacke and coauthors to estimate that a nesting mother alligator may use at least 10% of the calcium in her own armor to form her eggs. How this happens isn’t yet known. But should you ever see a nesting alligator (hopefully at a respectful distance!), have a look at the bumps on her back. Some of that armor was sacrificed to cradle the next generation.
Dacke, C., Elsey, R., Trosclair, P., Sugiyama, T., Nevarex, J., Schweitzer, M. 2015. Alligator osteoderms as a source of labile calcium for eggshell formation. Journal of Zoology. doi: 10.1111/jzo.12272