Rising temperatures are causing declines in Antarctic fur seal populations, but according to a study published in Nature this week, their genetic diversity seems to be increasing. By itself, this diversity isn’t an evolutionary response -- since it can’t be passed down -- but maintaining higher levels of genetic variation offers a better chance of evolutionary adaptation later on, which will hopefully buffer the impacts of long-term climate change.
Antarctic fur seals (Arctocephalus gazella) were almost hunted to extinction in the 19th century, but their numbers began to recover when these sealing operations ceased. However, in the past three decades, climate warming has been cutting down the availability of krill, an important part of the fur seal diet. Climate change is altering environmental pressures, and the effect is intensifying.
Jaume Forcada of the British Antarctic Survey and Joseph Hoffman from the University of Bielefeld analyzed a plethora of data on a fur seal population on South Georgia in the South Atlantic Ocean dating back to 1981: diet, annual pup counts, biometric measures (age, body length, weight), and life histories such as survivorship and breeding success. They also obtained genetic data on 1,728 seals, as well as a decade-long record of climate data from satellites. Watch this delightful 2-minute video.
The reduced krill availability has caused a decline in the birth weight of female pups, compared with 20 years ago. At the same time, the average age and size of females entering the breeding population are increasing. “Those that survive and return to breed tend to be the bigger ones and they have their first pup later in life than they used to,” Forcada explains in a news release. “Such changes are typically associated with food stress.”
Females who made it to motherhood were more heterozygous -- when an individual has a higher level of genetic variation between chromosome pairs. Heterozygosity has increased 8.5 percent per generation over the last two decades. Relatively homozygous (genetically homogenous) females are increasingly excluded from the breeding population, with an estimated decline of 17 percent.
But here’s the thing: Daughters can’t inherit heterozygosity from their mothers. It also depends on the male, so it arises mostly through chance. Since the advantage isn’t passed on from one generation to the next, the population continues to decline with the decreasing viability of homozygous seals.
“Strong selection by the environment can drive rapid evolution,” Hoffman says. But these seals aren’t evolving if surviving females don’t pass on their high heterozygosity. “With each new generation, the process of selection has to start all over again, with only those individuals that happen to be born more heterozygous having a good chance of survival.”
Why does heterozygosity confer an advantage? The likeliest explanation is that it's a sign of reduced inbreeding, The Scientist explains. Genetic variety helps animals respond to unexpected or variable conditions. The duo plans to sequence many more markers throughout the seal genomes to pinpoint where heterozygosity is most important.
Here's a crèche of pups waiting for their mothers to return from a foraging trip at sea:
Images: Antarctic fur seals at Bird Island, South Georgia / Jaume Forcada