Humpback Whales, Megaptera novaeangliae, were nearly exterminated around the Western Antarctic Peninsula by commercial Whaling in the first part of the twentieth century. The species has been protected since 1946, and in recent years their population has appeared to be recovering in many areas, including the Western Antarctic, where populations have risen rapidly in recent years. However, these Whales are living in a very different environment to their ancestors, with average annual sea temperatures in the region having risen by 7°C since 1950, and a much shorter seasonal sea ice coverage. The Whales migrate annually between winter feeding grounds on the Atlantic coast of South America, where mating occurs, and the Western Antarctic, where calves are born and spend the first months of their lives, though while this basic pattern is understood and the population can be observed to be recovering, little is understood about their breeding behaviour, due to the difficulty of studying the species.
In a paper published in the journal Royal Society Open Science on 2 May 2018, Logan Pallin of the Fisheries and Wildlife Department at Oregon State University, and the Department of Ecology and Evolutionary Biology at the University of California Santa Cruz, Scott Baker and Debbie Steel, also of the Fisheries and Wildlife Department at Oregon State University, Nicholas Kellar, of the Marine Mammal and Turtle Division of the National Oceanic and Atmospheric Administration, Jooke Robbins of the Center for Coastal Studies, David Johnston of the Division of Marine Science and Conservation at Duke University, Doug Nowacek, also of the Division of Marine Science and Conservation, and of the Pratt School of Engineering at Duke University, Andrew Read, again of the Division of Marine Science and Conservation at Duke University, and Ari Friedlaender, again of the Fisheries and Wildlife Department at Oregon State University, and the Institute for Marine Science and Department of Ecology and Evolutionary Biology at the University of California Santa Cruz, publish the results of a study into pregnancy rates among Humpback Whales around the Western Antarctic Peninsula between 2010 and 2016.
A range of methods have been developed to detect pregnancy in Marine Mammals in recent years, including testing for hormones and steroids in the milk, urine, blood and other bodily fluids of the animals, and even ultrasound evaluations. Such techniques typically involve close sampling or capture and release of the study subjects, suitable for use on Seals, Dolphins and even small Whales, but difficult to apply to large Baleen Whales such as Humpbacks.
Instead Pallin et al. looked for pregnancy (and sex) in Humpback Whales by testing for progesterone in skin and blubber samples obtained from biopsies of Whales, which can be obtained using sampling devices launched from crossbows or similar devices, a method not thought to cause any harm to the Whales, which seldom notice the sample being taken.
Using this method Pallin et al. obtained biopsy samples from 583 Whales between 2010 and 2016, of which it was possible to identify the sex of 507 individuals (239 males and 268 females). During the study 54 individuals were resampled two or more times during the same year, and 11 were resampled on more than one year. Excluding samples from females who were resampled the same year, a total of 265 samples were available for pregnancy testing, from 244 females. Of these 166 were determined to be from pregnant females, 99 from females that were not pregnant, and one was inconclusive.
These results indicate that on average 63.5% of the females sampled were pregnant; this figure was the same for females accompanied by yearling calves as it was for females without calves, though it did vary significantly from year-to-year, with a high pregnancy rate of 86.27 in 2014 and a low pregnancy rate of 36.36 in 2010.
A rising population requires two factors, a high birthrate and a low infant/juvenile mortality rate. The Humpback Whales of the Western Antarctic Peninsula. clearly have a high birthrate, and Pallin et al. do not believe they are being hampered by a high infant/juvenile mortality rate either. The two reasons why the mortality rate could be high are a shortage of food and a high level of attacks by predators. Humpback Whales are among the largest Whale species, reaching 12-16 m and weighing 20-30 tonnes. As such they have very few natural predators, with few hunters willing to risk attacking even a calf guarded by such a large mother, a factor which helped make the Whales vulnerable to Human predation, but which is now counting in their favour again. Nor do they appear to be suffering from food shortages, as their favoured food, Krill, is highly abundant in these region, and other Baleen Whales, which might compete for food, have not recovered and recolonised Antarctic waters as quickly.
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Instead Pallin et al. looked for pregnancy (and sex) in Humpback Whales by testing for progesterone in skin and blubber samples obtained from biopsies of Whales, which can be obtained using sampling devices launched from crossbows or similar devices, a method not thought to cause any harm to the Whales, which seldom notice the sample being taken.
A Humpback Whale off the Antarctic Peninsula. Robert Brears/Oceanwide Expeditions.
Using this method Pallin et al. obtained biopsy samples from 583 Whales between 2010 and 2016, of which it was possible to identify the sex of 507 individuals (239 males and 268 females). During the study 54 individuals were resampled two or more times during the same year, and 11 were resampled on more than one year. Excluding samples from females who were resampled the same year, a total of 265 samples were available for pregnancy testing, from 244 females. Of these 166 were determined to be from pregnant females, 99 from females that were not pregnant, and one was inconclusive.
These results indicate that on average 63.5% of the females sampled were pregnant; this figure was the same for females accompanied by yearling calves as it was for females without calves, though it did vary significantly from year-to-year, with a high pregnancy rate of 86.27 in 2014 and a low pregnancy rate of 36.36 in 2010.
A Humpback Whale breaches in Wilhelmina Bay, on the Western Antarctic Peninsula. Ari Friedlander.
A rising population requires two factors, a high birthrate and a low infant/juvenile mortality rate. The Humpback Whales of the Western Antarctic Peninsula. clearly have a high birthrate, and Pallin et al. do not believe they are being hampered by a high infant/juvenile mortality rate either. The two reasons why the mortality rate could be high are a shortage of food and a high level of attacks by predators. Humpback Whales are among the largest Whale species, reaching 12-16 m and weighing 20-30 tonnes. As such they have very few natural predators, with few hunters willing to risk attacking even a calf guarded by such a large mother, a factor which helped make the Whales vulnerable to Human predation, but which is now counting in their favour again. Nor do they appear to be suffering from food shortages, as their favoured food, Krill, is highly abundant in these region, and other Baleen Whales, which might compete for food, have not recovered and recolonised Antarctic waters as quickly.
See also...
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