Developmental plasticity highlighted in Crocodylus niloticus through in vivo fluorochrome labelling

In vivo fluorescent labelling has been widely used in osteohistological studies for age estimation, and to measure the daily/annual bone depositional rates. Here, we conducted a detailed microstructural study of limb bones of captive Crocodylus niloticus. We studied two male and two female specimens which were in vivo labelled three times over a period of approximately 14 months to evaluate the nature of their bone tissues, daily bone depositional rates and the periodicity of growth marks in their bones. We identified different types of embryonic bone tissues, and the widespread occurrence of woven parallel complex bone in several of the limb bones. Notably, we determined substantial intraskeletal and intraspecific variability in bone microstructure and bone depositional rates among the four individuals. The bone depositional rates calculated are within the range expected of reptiles and lower than those of birds and mammals. The labelling administered permitted the identification of stochastic and seasonal growth marks. Unexpectedly, we documented temporal differences in periosteal bone deposition and growth in length, which raises pertinent questions concerning the regulation of these growth processes. We propose that additional carefully constrained osteohistological in vivo labelling studies are warranted to expand our understanding of the factors affecting crocodilian growth.