Fighting the good fight: survival in male Weddell seals

Earth is now in its 6th mass extinction. As a result, we must race against time to conserve species. A useful conservation tool is modeling population size. In this post, we cover Weddell seals‘ survival in Antarctica and show that fighting decreases the long-term survival rate of adult males.

Population parameters

Changes in population size occur from 4 processes:

1. births: how many animals reproduce.
2. deaths: how many animals do not survive
3. immigration: how many animals move into the population from another location
4. emigration: how many animals move out of the population

Information about these four processes can be useful to know how to conserve animals best. However, these kinds of data can be time-consuming to collect and often include how humans have impacted populations. Knowing how populations function using species that are not at risk of extinction can provide more detailed information about wildlife. Then, we can apply this information to endangered species.

Weddell seals

Weddell seals are a great species to learn from because they can be easily spotted and form large populations. Additionally, they only live on the ice and in the waters around Antarctica. Because they live in remote areas, Weddell seals are among the animals that humans have impacted the least. Finally, Weddell seals provide a unique opportunity for study because they are easy to see: they spend several weeks on the ice, unlike many marine mammal species that live underwater year-round.

Survival and reproduction

One useful metric for learning about populations is the survival rate. The first few years of life can be the most challenging years to survive because young animals need to learn how to escape predators and find food. They also are still growing. For example, young Weddell seals cannot dive as deep or for as long as adults, making hunting more difficult. As a result, young Weddell seals often do not obtain foods as high in quality as adults. Like many other mammals, survival for Weddell seals for the first few years is relatively low. However, if Weddell seals can survive to their 3rd year of life, their survival chances are much higher.

Weddell seals, like all living things, have finite fuel (energy from food) to give to growth, survival, or reproduction. Yet, investing in one of these three can come at the cost of not investing in the other two. For example, spending more energy to reproduce can come at the expense of lower survival chances. In some mammal species, males and females might invest their fuel differently in growth, survival, and reproduction. These differences might occur because males and females have unique roles. In Weddell seals, females care for pups by nursing them and teaching them to swim. Caring for seal pups requires a large fuel investment. Although males leave pup care to females, they spend fuel fighting each other for chances to mate with females.  

Male Weddell seals set up underwater territories, where they can show off their quality as a mate to females. Also, they can produce unique sounds to attract females to their territories. However, gaining and keeping territory can prove difficult; males must fight other male competitors. Fighting can make males more susceptible to harsh winters or other environmental challenges.  

Survival rates in male Weddell seals

Interestingly, male survival rates peak during early adulthood and decrease as males get older. The oldest males have the lowest survival rates. It seems that engaging in fights with other males for a chance to mate comes at a direct survival cost to male Weddell seals.

In conclusion, Weddell seal population studies show that sex and age can play an important role in survival rates. More knowledge of the complex processes that affect population size changes, such as what we learned from Weddell seals, can help people to make conservation decisions about threatened or endangered species when little data for those species are available.  

Further reading

Information about survival rates in male seals:

• Brusa, J. L., J. J. Rotella, R. A. Garrott, J. T. Paterson, and W. A. Link. 2020. Variation of annual apparent survival and detection rates with age, year, and individual identity in male Weddell seals (Leptonychotes weddellii) from long-term mark-recapture data. Population Ecology 62:134-150.

For more general information about survival rates and reproduction:

• Beauplet, G., C. Barbraud, W. Dabin, C. Küssener, and C. Guinet. 2006. Age-specific survival and reproductive performances in fur seals: evidence of senescence and individual quality. Oikos 112:430-441.
• Hastings, K., K., L. A. Jemison, and G. W. Pendleton. 2018. Survival of Steller sea lions in Alaska: senescence, annual variation and covariation with male reproductive success. Royal Society Open Science 5:170665.

Check out our other posts on seals survival:

How do Arctic seal pups survive in the cold?