As June comes to a close, the anxiety of finals, chaos of family visits and parties, and bustling excitement of graduation comes to an end and young adults begin to open up new life chapters. High schoolers may be preparing for a big college move or applying for their first “adult” job. Undergraduates have now entered the “real world” armed with a degree that will hopefully be the key to a successful career path or allow them to take the next step into graduate school. While we send our best wishes to the class of 2019, there is another graduating class of young, spry creatures heading off to forge a successful path of their own: Eastern Pacific harbor seals (Phoca vitulina richardii).

Despite the fact that Eastern Pacific harbor seals are non-migratory (individuals stay within the same home range), they can be found all along the coast of North America— from Baja California all the way up to the Pribilof Islands in Alaska. They are solitary animals, only seen in proximity of each other on the beach, yet females have the ability to coordinate their pupping season each year. Not only do they coordinate pupping, but their wide range in latitude means that where moms are dictates when pups are born; for harbor seals down in Baja, pupping season starts in February while up in Alaska pups are born in July (see figure below). This reproductive phenomenon is due to delayed implantation; once a female’s egg is fertilized, it pauses development for 1.5 to 3 months and then resumes gestation [1]. Delayed implantation is beneficial for females who have an eleven month gestation, as it gives them a physiological break between nursing a pup and developing a new fetus, as well as allows her enough energy to molt. Delayed implantation is also beneficial for pups to be born during optimal times of year when the upwelling of nutrients is common. It is also an efficient way for adult males to coordinate mating with females so they can resume their solitary lifestyle.

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Harbor seal moms are one of the smallest phocids (Phocidae is the taxonomic family that includes all “true” or earless seals), and unlike other phocids that are able forego foraging and rely on body stores during nursing, harbor seal females are unable to take on the energetic cost of nursing without making foraging trips [2]. Thus, they are the only phocids to care for their pups both in the water and on land [1]. For the harbor seal class of 2019 on the central California coast,  pups were born in April, and moms didn’t waste any time in starting their pup’s education. Unlike altricial (read: helpless at birth) human babies, harbor seal pups are highly precocial (read: alert and mobile at birth) and are able to swim within hours of being born. Females take their pups on dives soon after they are born and by two to three days old, pups can hold their breath for up to two minutes [1] [3]. By the end of the four to six-week nursing period, pups are diving up to 30 meters deep (as a frame of reference, the average community pool is usually around 25 meters in length) and matching the average dive duration of their moms [3].

Harbor seal moms are quite involved in their pup’s education, teaching their pups important life lessons such as how to maneuver through currents and waves, how to anticipate when and where to begin diving, and where to go foraging for food. By going on swimming and diving bouts with their moms early on, harbor seal pups are also challenged physiologically. Longer and deeper dives assist in the development of aerobic capacity and ability to store oxygen in their blood, which translates to better foraging capabilities after pups are weaned.

Past research has indicated that many phocids have abrupt weaning periods. This is a seal mom’s way of saying, “Bye kid, I am starving after X days of not eating and giving you energetically costly milk, so you and your chubby blubber buffer will have to sustain you while you figure out how to adult!” Since we landlubber researchers don’t have the aquatic grace and hydrodynamic efficiency of harbor seals, understanding their repertoire of underwater behaviors is often difficult, and we presumed that all harbor seal mother-pup dyads follow the same pattern. However, thanks to technology and some creative thinking, we now are learning more about what is going on between mom and pup under the sea. Using a pill that collects stomach temperature data and transmits to a data recorder placed on the pup’s back (called stomach temperature telemetry), researchers were able to update this abrupt weaning theory for harbor seals. Stomach temperature pill data illustrate changes in the temperature of the pup’s stomach that can be correlated to cooler temperature profiles of seawater, milk, or solid food ingestion [4]. This innovative data show that 63.7% of milk ingestion occurred underwater and what’s more, pups are ingesting solid prey during the nursing period [4]! Thus, not only are harbor seal pups taking diving classes from their moms, they are also taking nutrition classes while still having the buffer of having a cafeteria meal (a.k.a. milk) when they’ve made a foraging folly .

Dorm life (a.k.a beach time) for harbor seal pups also teaches important lessons. In the first 6 days after birth, harbor seal pups spend about 40% of their time in the sea [3]. The 60% of time they spend on land, their moms may or may not be with them (remember, mom has to go eat something to maintain energy stores for both her and her pup). Thus, pups must learn to understand the tide patterns so that they can move up the beach to prevent getting swept away while their mom is out grabbing a snack. An issue with this latch-key kid strategy is that many human beachgoers see a “lonely, helpless” pup on the beach and are tempted to rescue it. What they don’t notice is that mom is usually never far off the coast, and even the sight of humans can spook her and keep her away from her pup. Thus, it is important to always observe the cuteness of any seal or sea lion from a distance of one football field so that moms and pups can be reunited. If you a California beach goer and are concerned that a pup is truly alone, call The Marine Mammal Center, The Pacific Marine Mammal Center, or California Wildlife Center. These are trained rescue and rehabilitation teams that can assess, monitor, rescue, and rehabilitate pups, should they need intervention.

While there are no robes, graduation caps, or commencement ceremonies for harbor seal pups— if you hang out at the common pupping spots (e.g. Pacific Grove, CA) from April through June, you can watch the pups grow chubbier, the moms become leaner, and the mother-offspring pair gradually spending more time apart. This is the anticlimactic graduation ceremony for the harbor seal class of 2019. Harbor seal graduates (a.k.a. weaners) have hopefully learned enough life lessons of diving, foraging, and avoiding predators to get them to their next successful life chapter.

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Karli Chudeau is a graduate student in the Animal Behavior Graduate Group and a part of the UC Davis Center for Animal Welfare. She is interested in conservation management and assessing animal welfare in wildlife rehabilitation settings. Her current research examines how we can use behavioral management interventions, such as environmental enrichment, to improve reintroduction success with pinnipeds. She is also an avid ocean nerd.

References:

[1] Riedman, M. (1990). The Pinnipeds: Seals, Sea Lions, and Walruses. Berkeley: University of California Press.

[2] Bowen, W.D., Oftedal, O.T., & Boness, D.J. (1992). Mass and energy transfer during lactation in a small phocid, the harbor seal (Phoca vitulina). Physiological Zoology, 65, 844-866.

[3] Bowen, W.D., Boness, D.J. & Iverson, S.J. (1999). Diving behavior of lactating harbour seals and their pups during maternal foraging trips. Canadian Journal of Zoology, 77, 978-988.

[4] Sauve, C.C., Van de Walle, J., Hammill, M.O., Arnould, J.P.Y., & Beauplet, G. (2014) Stomach temperature records reveal nursing behaviour and transition to solid food consumption in an unweaned mammal, the harbour seal pup (Phoca vitulina). PLoS ONE 9(2): e90329.