Insights Into Nursing and Weaning From Whale Teeth
New estimate of nursing duration cuts species’ reproductive potential in half.
Posted July 9, 2020
A century of commercial whaling across the North Atlantic cut a swath through the population of northern bottlenose whales (Hyperoodon ampullatus). Now, samples collected from whaling expeditions are actually helping scientists learn more about these elusive animals and work towards their conservation.
The northern bottlenose whale is a member of the beaked whale family, one of the least known groups of mammals.
“Much of what we know about beaked whales has come from deceased specimens that have washed ashore,” says Laura Joan Feyrer, a doctoral student at Dalhousie University in Canada. “Not many people have seen beaked whales alive in the wild.”
One reason that northern bottlenose whales are so little known is their habitat. They spend their time in the deepest, remotest areas of the North Atlantic.
Commercial whaling in these waters ended in the early 1970s. Up to then, northern bottlenose whales were popular targets of Scottish and Norwegian whalers — both for the valuable oil filling the males’ large, square foreheads and for their ease of capture. Unlike many beaked whales, which are shy and boat-averse, northern bottlenose whales are considered boat-curious. When they encounter a vessel, these whales will often approach it in small groups of three or four animals.
Feyrer’s studies are focused on understanding the population of northern bottlenose whales and how they are recovering from the whaling era. One crucial piece of this puzzle is figuring out how long their babies nurse. The duration of nursing has implications for infant survival, the period of time between births, and the number of babies a female can raise over her lifetime — all of which could affect this species’ recovery from whaling.
It’s in the Teeth
In a recent study, Feyrer and her colleagues analyzed northern bottlenose whale teeth for the presence of certain isotopes indicative of milk or other items in a whale’s diet. Although they are considered a toothed whale, northern bottlenose whales have only two tusk-like teeth, which erupt in the males but remain embedded in the jawbones of females.
These teeth are pretty difficult to obtain, says Feyrer. For this study, the team used teeth from two whales that had stranded as well as a collection of northern bottlenose whale teeth from the Norwegian Marine Research Institute that were taken from animals during the last decades of commercial whaling in the North Atlantic.
Whale teeth contain a record of the animal’s diet over time. Within teeth, dentin is deposited in layers annually. These chronicle the molecular signature of the food eaten during that year. Feyrer and her colleagues extracted samples from the teeth and analyzed them for the signatures of nitrogen and carbon stable isotopes that would indicate the transition from feeding exclusively on milk to independent foraging.
Feyrer and her colleagues determined that nursing is prolonged in northern bottlenose whales, lasting until calves are three to four years old. This is substantially later than the previous estimate for this species, which was one year (and based on the stomach contents of one individual calf). Compared to other whale species, it’s on par with sperm whales and orca.
What do northern bottlenose whale calves get out of three to four years with mom? Feyrer says that nursing in mammals serves multiple functions beyond just providing necessary nutrition.
“Whale calves depend on this nutrition for energy and thermoregulation,” she says. “Whales convert that high-fat milk into blubber, which is critical in the first few months of life as a whale living in cold North Atlantic waters.”
During nursing, babies are protected by association. Potential defenders include their mothers, which are much larger than they are, and potentially the social group to which the mother belongs. Nursing also provides opportunities for socialization and other important lessons, like foraging strategies or migratory routes.
In this species, prolonged maternal care may also be related to the whales’ complex foraging strategy. Beaked whales regularly dive to extreme depths (~1,000m) to feed. Calves may not be physically capable of deep diving until they are larger. Plus, more time with mom gives them more time to master the technical, socially learned aspects of foraging at depth, such as prey identification and capture.
Conservation and Questions
Prolonged maternal care and a longer interval between calves has implications for population growth in northern bottlenose whales. For most whales, pregnancy and lactation rarely overlap, so extended nursing decreases the lifetime reproductive potential of this species.
“Our findings suggest that the reproductive rate of northern bottlenose whales is about half of what was previously estimated,” says Feyrer. “Since they have a smaller reproductive capacity due to the extended time spent caring for their calves, it means they cannot replace themselves as easily. That affects estimates of the time it would take to recover from whaling.”
In addition to prolonging their recovery from commercial whaling, the decreased reproductive rate in these whales could also impact their ability to withstand contemporary risks, such as disease outbreaks or mass strandings.
Feyrer hopes her research will help scientists better understand the lives of northern bottlenose whales. The whales are listed as endangered in Canada, but their population size elsewhere is poorly understood.
“There are still so many questions about beaked whales because they are so difficult to study,” she says. “In general, there remains a lot we don’t know about their social structure, communication, and other aspects of their fundamental biology — information which could help us protect and conserve these animals.”
Feyrer LJ, Zhao St, Whitehead H, Matthews CJD (2020) Prolonged maternal investment in northern bottlenose whales alters our understanding of beaked whale reproductive life history. PLoS ONE 15(6): e0235114. https://doi.org/10.1371/journal.pone.0235114.