Sex ratios of migrating southern hemisphere humpback whales: A new sentinel parameter of ecosystem health

Abstract

Southern hemisphere humpback whales have evolved energetically demanding capital breeding and migratory life-history behaviours. It has been hypothesised that not all individuals of a population participate in the seasonal migration each year, or only undertake partial migrations.Given the cost of migration and reproduction, we explored the possibility that specifically, not all mature females participate in the seasonal migration every year, or significantly delay or shorten their migration, in response to poor feeding conditions. That is, females must attain a minimum threshold of accumulated energy reserves to commit to a reproductive event that likely occurs as a product of mating during migration. With a 1:1 male to female birth ratio, yet a male bias observed along the main migratory corridor; this study utilised inter-annual migratory cohort sex ratios to explore their potential to serve as measures of population fecundity, as a function of ecosystem health. The sex ratios of randomly biopsied adult humpback whales, sampled at a defined location and set time-points along the main migratory corridor from 2008 to 2016 were investigated. Northward migration sex ratios in 2009, 2014 and 2016 revealed a lower male bias suggesting good female participation in the migration and therefore apparent optimal provisioning during the two preceding summers. By contrast, the 2011 southward migration, revealed the highest male bias recorded of 5.75:1. Southward migration sex ratios were found to oscillate closely with measures of population adiposity, a sentinel parameter employed for longterm surveillance of the Antarctic sea-ice ecosystems under the Southern Ocean Observing System-endorsed Humpback Whale Sentinel Program. Anomalously poor humpback whale body condition recorded in 2011 was attributed to poor Antarctic feeding conditions during the extreme La Niña event of 2010/11. These findings lend support for the application of migratory cohort sex ratios, standardised by time and location, as a measure of relative inter-annual

Interannual variability in the lipid and fatty acid profiles of east Australia-migrating humpback whales (Megaptera novaeangliae) across a 10-year timeline

“…All samples were stored on ice in the field, and then transferred to – 20 °C freezers until lipid and fatty acid analysis. Sub-samples of skin were similarly stored at – 20 °C for genetic sex determination 64 . Observational notes were recorded for pod composition.…”

Sex ratios in blue whales from conception onward: effects of space, time, and body size

“…Antarctic minke whale catches also display strong latitudinal segregation by sex, shifting from 80% male north of 60°S to less than 20% male south of 70°S (Kasamatsu & Ohsumi, 1981); and minke whales off West Greenland also show a shift towards females nearer the poles, from 80% male at 59°N to <10% male female at 72°N (Laidre et al, 2009). Humpback whales are also well‐known to have sex‐segregated migrations (Chittleborough, 1965): for example, in Hervey Bay, Australia, 75% are female, and this may be a strategy to minimize male harassment of mature females (Franklin et al, 2018); although in general humpback migrations to Australian breeding areas are male‐dominated, ranging from 57% to 85% male depending on the year (Druskat et al, 2019).…”

“…Sex ratios at a given location should change over the course of a year if the sexes have segregated migrations, as seen in humpback whales (Chittleborough, 1965; Druskat et al, 2019; Franklin, Franklin, Brooks, & Harrison, 2018). Daily patterns were examined at land stations with more than 5,000 catches, for all days where 25 or more whales were sexed, with data for each day of the year summed across all years.…”

“…While sex ratios at birth reveal selective pressures, postnatal sex ratios can reveal far more. From birth to death, sex ratios will differ: by age if one sex has higher survival than the other; by body length if growth rates vary by sex; seasonally if migration timing differs by sex; by area if there are sex‐specific migration pathways or tendencies (Druskat, Ghosh, Castrillon, & Bengtson Nash, 2019; Kasamatsu & Ohsumi, 1981; Laidre, Heagerty, Heide‐Jørgensen, Witting, & Simon, 2009); and across years due to human selection if catches preferentially target one sex (e.g., Beddington & May, 1980). Thus, a careful examination of sex ratios across age, length, day, area, and year can reveal much about the growth patterns, movements, and behavior of a given species.…”

Interpretation of southern hemisphere humpback whale diet via stable isotopes; implications of tissue-specific analysis

“…Southern hemisphere humpback whales ( Megaptera novaeangliae ; SHHWs) have been implemented as a sentinel species for the circumpolar surveillance of pollution and climate change in the Southern Ocean (1,2). As capital breeders, these populations rely on intensive summer feeding on Antarctic krill ( Euphausia superba; hereafter ‘krill’; Groß et al, 2021; Waugh et al, 2012), to sustain their annual winter migrations to lower latitudes for breeding and calving.…”