Abstract
The annual cyclicality of cortical bone growth marks (BGMs) allows reconstruction of some important life history traits, such as longevity, growth rate or age at maturity. Little attention has been paid, however, to non-cyclical BGMs, though some record key life history events such as hatching (egg-laying vertebrates), metamorphosis (amphibians), or weaning (suggested for Microcebus and the hedgehog). Here, we investigate the relationship between non-cyclical BGMs and a stressful biological event in mammals: the moment of birth. In the present study, we histologically examine ontogenetic series of femora, tibiae and metapodia in several extant representatives of the genus Equus (E. hemionus, E. quagga and E. grevyi). Our analysis reveals the presence of a non-cyclical growth mark that is deposited around the moment of birth, analogous to the neonatal line described for teeth. We therefore refer to it as neonatal line. The presence of this feature within the bone cross-section agrees with a period of growth arrest in newborn foals regulated by the endocrine system. The neonatal line is accompanied by modifications in bone tissue type and vascularization, and has been identified in all bones studied and at different ontogenetic ages. Our discovery of a non-cyclical BGM related to the moment of birth in mammals is an important step towards the histological reconstruction of life histories in extant and fossil equids.
Highlights
The study of bone microstructure provides key insights into the life history strategy of extant and extinct vertebrates
We describe for the first time a non-cyclical bone growth marks (BGMs) in the limb bones of mammals that records the moment of birth
During this life history event, physiological levels of cortisol, thyroid hormones, and growth hormones agree with a period of growth arrest in the newborn foal that leads to the deposition of the neonatal line (NL)
Summary
The study of bone microstructure provides key insights into the life history strategy of extant and extinct vertebrates. The arrangement of collagen fibers within the bone matrix, its vascularization and the density and number of bone cells are directly related to the rate of bone deposition [1,3,14,15,16,17]. Analysis of these histological characteristics, allows estimation of growth rate in extant and extinct taxa [2,4,6,10,18,19]. This sort of studies relies on the annual periodicity of cyclical growth marks (CGMs), which record hormonal and physiological cycles [22] that are synchronized with seasonal photoperiod and resource availability [15,23]
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