Timing hypothesis explains the mystery of the missing blue whale calves

Few mother-calf pairs are sighted in blue whale Balaenoptera musculus populations worldwide, averaging only 3.1% (95% CI 2.7-3.4%) of sighted individuals, despite 33 to 50% annual pregnancy rates among mature females. Multiple hypotheses were examined to explain the low rates of sighted calves. Observed rates are too low to be explained by low fetal survival, low calf survival, low birth rates, or calf separation from mothers, although mother-calf pairs might avoid higher-density regions where field studies are concentrated, and accounting for males and immature individuals reduced the expected proportion of mother-calf pairs to 7 to 12%. However, the timing hypothesis best explained observed patterns, by proposing that most blue whales produce calves shortly after departing their summer feeding grounds and wean their calves 7 mo later, on their return. A conceptual model of this hypothesis, with some variability around calving dates and a few year-round births, predicted mother-calf proportions peaking in winter at 8 to 9% and reaching a low of 1 to 4% in summer. These predictions matched the low proportions observed in 7 of 8 summer feeding regions (1.5-3.5%), but not off New Zealand (9.8%), while in winter, observed proportions were high in the Gulf of California (12.3%) and Timor Trough (9.3%) but lower in the Galapagos (3.8%). The lowest mother-calf proportion was off Sri Lanka (0.7%), where blue whales reside year-round. These results suggest that the mystery of the missing blue whale calves can largely be explained by mothers calving immediately after leaving summer feeding grounds and weaning on their return.