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SC/69A/SH/06
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Resource ID
20052
Access
Open
Document Number
SC/69A/SH/06
Full Title
Fetal sex misidentification and adaptive sex ratio behavior in large whales
Author
Zoe R Rand, Trevor A Branch
Authors Summary
The authors analyzed fetal sex data from catches of Antarctic blue, pygmy blue, fin, sperm, humpback, sei, Antarctic minke, and common minke whales. Using Bayesian models, the authors estimated the extent to which the sexes of small fetuses were misidentified, and found evidence of misidentification in fetuses between 30-120 cm (1-4 ft) in length. In addition, the authors used Bayesian generalized linear mixed models to test whether females in rorqual whale species adapt the sex ratio of their offspring in relation to their size. They found that longer mothers produced higher proportions of female offspring than male, with the posterior probability of this ranging from 0.87-0.99 depending on model formulation.The authors conclude that these results provide evidence that differences in adult female body size may confer a greater reproductive advantage than in males. These findings suggest that calculations using fetal sex data from the catch database should account for sex misidentification at small lengths, and provide possible evidence of adaptive sex ratio behavior in rorqual whales.
Publisher
IWC
Publication Year
2023
Abstract
There are many theories about how mammals divide their reproductive effort between males and females, including how individuals may adapt fetal sex ratios to ensure their lifetime reproductive success. These theories are difficult to test without large sample sizes because differences from equal sex ratios are often small. Whaling data provides one of the largest fetal sex ratio datasets for non-domesticated mammals and could be used to test some of these theories. One issue with whaling data, however, is the potential misidentification of the sex of small fetuses. To estimate the extent of this issue, we fit Bayesian models to estimate the lengths at which >99% of fetuses are correctly sexed for seven great whale species, finding evidence for frequent sex misidentification at lengths shorter than 30–120 cm (1–4 ft), with females recorded as males in Antarctic blue, fin, sperm, humpback, and sei whales, and males recorded as females in Antarctic minke whales. Using fetal sex data for fetuses longer than lengths where misidentification occurred, we tested whether female rorqual whales adapt the sex ratio of their offspring in relationship to their size with Bayesian generalized linear mixed models (GLMM). Longer females should be able to devote proportionately more resources to lactation and growth, and hence should have calves that grow faster and reach sexual maturity earlier. If longer females produce more male offspring, this implies that size differences between adults provide a greater advantage to male-male competition than female fecundity; while if longer females produce more female offspring, this implies that faster-growing and larger females enjoy much greater lifetime reproductive success than smaller females. We found some evidence in rorqual whales that longer mothers produced a lower proportion of male offspring, with overall posterior probability ranging from 0.87 to 0.99 depending on the model formulation, although this relationship varied among species. This evidence favors the hypothesis that differences in female body size have a greater impact on reproductive success than differences in male body size and that individual female rorqual whales adapt the sex ratio of their offspring to facilitate lifetime reproductive success.