Long baleen whale mothers are more likely to have female calves than males, according to a new study led by the 天美影视传媒. The findings contradict a popular evolutionary theory postulating that strong mammals benefit more from birthing males.

In 1973, that fit female mammals can improve their odds for grandchildren by having males. Large strong mothers will raise large strong offspring that, if male, can outcompete other males for mates.聽But, according to the theory, female fitness is less consequential. The studies backing this argument focused on land mammals, such as deer and elk, and often included just tens or hundreds of animals.

UW researchers tested the theory in marine mammals by comparing maternal length and fetal sex in more than 100,000 baleen whales. They found that the fetal sex ratio skews female for longer 鈥� and thus more fit 鈥� rorqual whales, the predominant baleen whale family that includes humpbacks and blue whales. The findings, on Sept. 24, suggest that female calves benefit more from heritable fitness than males do.

Carrying and caring for young is exhausting, and whales often breed far from food sources. They must rely on stored fat to sustain themselves and their young during and after pregnancy.

鈥淭he question we wanted to answer was: if you are in good condition, if you鈥檙e big and fat and you鈥檙e going to have a big fat calf that will survive and reproduce 鈥� do you want that calf to be a male or a female?鈥� said , a UW doctoral student of quantitative ecology and resource management.

To answer this question, the researchers turned to historical whaling data.

Back in the early 1900s, when people hunted whales, a group from Norway began collecting data on their catch. The practice was codified into a law that required all Norwegian hunters to record the whale鈥檚 length, sex and pregnancy status, as well as the sex and size of a fetus. In the 1930s, the Norwegian regulation became international law.

鈥淲hen they hunted whales, there were often biologists around who were knee-deep in the carcasses, measuring and collecting samples,鈥� Rand said. in 1986 to protect dwindling populations from further decimation. The IWC data, however, is a treasure trove for researchers.

鈥淲e have this enormous data set with hundreds of thousands of data points that doesn鈥檛 exist for almost any other wild population,鈥� said , a UW professor in the School of Aquatic and Fishery Sciences. In 2023, Branch and Rand helped create an interactive map depicting whale distribution from the data.

The data also gave Rand an opportunity to investigate fetal sex ratios in marine mammals. Experts argue that some animals just after conception. No one knows exactly how this works for mammals, but adapting sex ratios based on physical or environmental conditions is considered advantageous.

鈥淚 think for our mammal brains, it is a little bit confusing,鈥� Rand said, 鈥淏ut insects, and ants, have a lot of control over the sex of their offspring, so it鈥檚 not entirely surprising that mammals might have a little bit of control.鈥�

In this study, the researchers modeled maternal length against sex for fetuses measuring three feet and longer 鈥� the size at which sex becomes evident. They included seven whale species in the rorqual family, totalling more than 100,000 whales.

If the Trivers-Willard hypothesis were correct, researchers would have seen a slight increase in the number of male fetuses as maternal length increased. Instead, they observed a downward trend, indicating that fewer males were born to larger mothers. The results varied some by species: There was a 77% chance that longer female humpbacks have more female calves, and that probability increased to 99% for sei whales.

There are several possible explanations for why these findings flip the Trivers-Willard hypothesis, and the trends observed in land mammals. Some male whales compete for mates, but competition might not be as significant a pressure as female size because small female whales will likely struggle to reproduce and raise healthy young. Big whales, on the other hand, will have big female calves that will grow into long mothers with strong reproductive potential.

For baleen whale mothers, investing energy in female calves is the best way to ensure generations of grandchildren.

Research also suggests that some whale species are , which could spell trouble for future generations if females are unable to support offspring. The findings could have implications for conservation, but Rand said that this will require further research to confirm.

鈥淧reviously it was assumed that if you have male-male competition for mates, bigger mothers will have males,鈥� Rand said. 鈥淥ur paper shows that you can鈥檛 make that assumption because there鈥檚 also an advantage to being big as a female.鈥�

Other authors include , the Leader of the USGS Washington Cooperative Fish and Wildlife Research Unit and a Professor in the UW School of Aquatic and Fishery Sciences.

This research was funded by the National Oceanic and Atmospheric Administration.

Contact Rand at zrand@uw.edu for more information.

The Columbia River is home to one of the West Coast鈥檚 most important Chinook salmon runs. Through late spring and early summer, mature fish return from the sea and begin their arduous journey upriver to spawn. In recent years, these fish have faced an additional challenge: hungry California sea lions.

A new 天美影视传媒 and NOAA Fisheries study found that sea lions have the largest negative effect on early-arriving endangered Chinook salmon in the lower Columbia River. The 聽were published Oct. 19 in the Journal of Applied Ecology.

Opportunistic sea lions have learned that by swimming as far as 145 miles upriver, they can easily feast on migrating salmon, including those hindered by the Bonneville Dam.

鈥淲e investigated whether mortality rates varied depending on the specific threatened Chinook salmon population, determined by when they arrive in the river,鈥� said lead author , a doctoral student at the UW School of Aquatic and Fishery Sciences. 鈥淲e found that, based on their individual return timing and the abundance of sea lions in the river when they return, individual populations experience different levels of sea lion-associated mortality.鈥�

Researchers learned that the earliest arriving populations of Chinook salmon experienced an additional 20% mortality over previous years, and the later arriving populations experienced an additional 10%. This increase in mortality was associated with increased sea lion abundance at those times of year in the period of聽2013 to 2015 compared to the period of 2010 to 2012.

The numbers of California sea lions are highest at the mouth of the Columbia in early spring, before they depart for their breeding grounds in southern California. The researchers also discovered that the earliest arriving salmon migrate through the lower Columbia River more slowly than those arriving later in the season, thereby increasing their exposure to predation.

鈥淭his information on how different populations are affected by sea-lion associated mortality is key because recovery of endangered Chinook salmon requires multiple of the individual populations to be healthy,鈥� said Sorel.

California sea lions have seen their numbers rebound along much of the U.S. West Coast since the passage of the Marine Mammal Protection Act of 1972, which protects them from being killed, captured and harassed. The increased presence of sea lions is now at odds with the endangered salmon populations on which they feed, putting managers in a difficult position.

Researchers are concerned that something must be done quickly as these hunting behaviors are learned, and the problem could continue to grow exponentially. In August, the National Marine Fisheries Service for Washington, Idaho, Oregon and several Pacific Northwest tribes to capture and euthanize both problematic California and Steller sea lions within a larger area of the lower Columbia and Willamette Rivers. Previously, only California sea lions could be killed in these rivers if managers deemed them a threat to salmon.

This complicated decision was enacted after non-lethal methods, such relocation and hazing, to limit the impact sea lions have on salmon 鈥� plus some targeted lethal removal 鈥� were met with limited success.

鈥淭his is often a challenging management problem as both sea lions and salmon are of strong interest to the public, and both are protected under federal statutes,鈥� said Sorel. 鈥淢anagement must consider multiple social values and operate within existing legal frameworks.鈥�

Continued monitoring will help to reduce the remaining uncertainty about the effects of sea lions on salmon and the expected outcomes of alternative management actions.

Other co-authors are and A. Michelle Wargo Rub of NOAA Fisheries Northwest Fisheries Science Center; of NOAA Fisheries Alaska Fisheries Science Center; and , leader of the U.S. Geological Survey Washington Cooperative Fish and Wildlife Research Unit and UW associate professor. This research was funded by the National Marine Fisheries Service West Coast Protected Resource Division.

For more information, contact Sorel at marks6@uw.edu and Converse at sconver@uw.edu.