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Sex ratios in blue whales from conception onward: the effects of space, time, and size
Trevor A. Branch and Cole C. Monnahan
International Whaling Commission
Mammalian sex ratios should be close to equality between males and females, or the rarer sex will on average produce more offspring. Deviations in this prediction across time, space, age and length may reveal insights into sex-specific growth, survival, movements, and behavior. For blue whales, extensive sex records are available for fetal (n = 21,542) and postnatal (n = 311,901) individuals from whaling records, and are examined here with a special focus on the Antarctic. Male blue whales are significantly more common than females in both fetal (51.3% male) and postnatal (52.1% male) data. Historically, Antarctic catches shifted from slight male-dominance before 1951 (52.4% male) to slight female-dominance thereafter (48.0% male), even though females are larger and were hence preferentially selected by whalers. Subtle daily shifts in sex ratios were observed at the three land stations with the greatest catches, with statistical evidence for linear or sine wave patterns. Among land stations, temperate land stations in southern Africa caught fewer males (46–47%), South Georgia was evenly split among the sexes (50% male), and the southernmost stations in the South Orkneys (60% male) and South Shetlands (56% male), caught more males. Among pelagic catches, there were no clear spatial patterns in sex ratios, except that more males were caught in the Ross Sea. Fetal data yield evidence that the smallest females were misidentified as males, and that the proportion of males declines with increasing length, probably because of higher prenatal male mortality. After birth, sex ratio differs by length, with more males at intermediate lengths and more females among the longest individuals; this pattern is mimicked by a conceptual model in which females grow faster and to a longer asymptotic size. Overall, sex ratios in blue whales are remarkably close to equality across time, space, and length; with any deviations from equality explained best the larger sizes attained by females, together with size selectivity in whaling due to economics and regulations.