Mature hematology in neonatal walruses supports diving alongside their mothers

An animal's physiology limits the environmental conditions where it can persist; quantifying the physiology of the walrus is timely since they are being impacted by alterations in sea ice. We examined postnatal changes in hematology, an important attribute that supports diving, by analyzing a longitudinal data set from aquaria walruses (five males and nine females) sampled from 0.04 to 12.0 years of age (n = 795 samples). Red blood cell count (RBC), hemoglobin (Hb), hematocrit (Hct), mean corpuscular volume (MCV), mean cell hemoglobin (MCH), and mean corpuscular hemoglobin content (MCHC) did not change markedly after birth and appears to have not been influenced by sex. Estimated values at birth were RBC: 3.78 ± 0.12 × 10⁶ mm^?3, Hb: 17.62 ± 0.82 g dl^?1, Hct: 45.21 ± 2.01%, MCV: 118.99 ± 3.99 fl, MCH: 47.10 ± 1.77 pg, and MCHC: 39.60 ± 0.70 g dl^?1. Compared to newborns, there were only subtle decreases in RBC, Hb, and MCHC, and a slight increase in MCV in the years following birth; Hct and MCH appear not to have changed. Unlike other pinnipeds, walruses swim within days of birth and have a prolonged 2-year nursing interval. Mature hematology early in life supports breath-holding, as young walruses must transit under sea ice with patchily distributed breathing holes.