The Life of a Female Northern Elephant Seal

NPS / Ellen Murphy

Burney Le Boeuf, Richard Condit, and Joanne Reiter spent over five decades—from 1963 to 2005—tracking almost 8,000 female northern elephant seals. With a team of researchers, they marked the females with tags and monitored their lives at the Año Nuevo rookery every winter breeding season and during the summer molting period. They focused on how lifetime reproductive success of females varied within the study group.

Through the act of sustained observation for this group of females, our understanding of female northern elephant seals has expanded.

[Information for this article is derived from:
Le Boeuf, B., R. Condit, and J. Reiter. 2019. Lifetime Reproductive Success of Northern Elephant Seals (Mirounga angustirostris). Canadian Journal of Zoology. 97(12): 1203–1217. Available at https://doi.org/10.1139/cjz-2019-0104. (accessed 12 March 2022).]

Yearly Cycle

A female normally gives birth annually to a single pup in the winter. She then nurses her pup for about four weeks, copulates during the last few days of nursing, and then weans and abandons her pup to go to sea on a foraging trip lasting approximately 2.5 months. The egg is fertilized but does not implant to the uterine wall until three months later during the one-month-long molting period on land. After molting, the female returns to sea again to forage for the entire eight-month-long gestation period. The pup mass at birth or weaning reflects the success of maternal foraging during the eight months prior to giving birth.

Figure 1. This graph provides an example of what a single adult female yearly cycle can look like. The timing for these events can vary amongst females. They are at sea on foraging trips for most of the year, but they return to land for: a) birthing and nursing from the end of December to mid-February; b) mating from the end of January to the beginning of March; and c) molting from mid-March to the end of May.

NPS / Ellen Murphy

Lifetime Reproductive Success (LRS)

Females lifetime reproductive success (LRS) greatly varies because:

1. 75% of the females died before reaching breeding age and produced no pups;
2. nearly half of the survivors bred for only a few years before dying and young females had low weaning success; and
3. less than 1% of the females in the sample were exceptionally successful producing up to 20 pups in life, otherwise known as "Super Moms."

Figure 2. This pie graph shows the mortality timing of the females in the study group.

NPS / Ellen Murphy

Super Moms

The top 1% of females go above and beyond earning this title of super mom. Super moms are long-lived (up to 23 years) and can successfully nurse as many as 20 pups in their lifetime. Of the 8,000 females that were studied, these super moms almost single handedly created the next generation of northern elephant seals at the Año Nuevo rookery.

How did these females achieve super mom status? Well, in the rookery, mature females express size-related dominance over younger, smaller females. This gives Super Moms an advantage while nursing because they can choose the safest location to protect their pups, which is next to the alpha male. This keeps younger females in less desirable areas where high surf and mating attempts of peripheral males disturb their nursing and can lead to mother–pup separation and pup mortality.

A northern elephant seal rookery.

NPS / Ellen Murphy

Pups & Weanlings

Pups have it tough. At four weeks old, the pup is weaned and abandoned forever by its mother. Weighing an average of 300 pounds, 59% of which is blubber, the weanling fasts for 2.5 months while learning to swim and dive in the shallows of the rookery before going to sea for its maiden foraging trip. The weight of the pup after nursing predicts their survival rate. The lifespan of the heaviest weanlings was more than double that of the lightest weanlings.

At departure from the rookery, weanlings have lost 25% to 30% of their weaning mass. The weanling is naïve and knows not what to eat, where to find its prey or how to catch it, and what predators to avoid. Surviving foraging trips at sea during the first three years of life poses many dangers and is the first and greatest barrier to reproducing. High mortality at sea during these early years is mainly caused by predation and poor foraging.

Figure 3. This graph shows an example of an individual pup/weanling yearly cycle. The timeline for pups and weanlings can vary depending on when they are born.

NPS / Ellen Murphy

Yearlings

As a yearling, the seals are losing and regaining weight as they begin to successfully forage, but they will not reach their post weaning weight in their first year or two. The lean muscle to fat ratio increases and they are about 10 cm longer but weigh no more than when they departed the rookery on their maiden foraging trip at 3.5 months of age. Nevertheless, the diving pattern and migratory routes of young seals approach that of adults even on the first trip to sea.

Figure 4: This cartoon shows the difference in size and coloration between a weanling pup and a yearling.

NPS / Ellen Murphy

Immatures

An immature female is around two to four years old. There are a lot of immatures in a population but, collectively, they do not produce as many pups compared to females that are 12 years or older. If an immature female starts reproducing while she is still growing, she may die young. In addition to this, because immature mothers are less experienced and do not have enough fat reserves to nurse a pup, her pup may not survive. An immature female needs to learn to forage lanternfish, hake, ragfish, and cocked-eyed squid, to gain the fat reserves necessary to nurse a pup. Most young females breed only once before they die; the rest may breed two to four times before they die. If a female successfully gives birth for the first time, she may take a long recovery period before attempting pregnancy again.

The diet of a female northern elephant seal yearling.

NPS / Ellen Murphy

The Unknowns

Most elephant seal deaths occur at sea. To understand lifetime reproductive success in females, we need to explore what happens during the two annual foraging trips at sea, where an adult female spends 10 months of the year. Researchers hypothesize that foraging for sufficient food and avoiding predators may be the major obstacles to survival. We have little information on how well females avoid predators, ship strikes, or being caught in fishing gear, or marine debris.

Obstacles to female northern elephant seal survival.

NPS / Ellen Murphy