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The Case of Stones in Sea Lions’ Stomachs

Did you know that stones are commonly found in the stomachs of Steller sea lions?

These stomach stones or “gastroliths” are as big as 12 cm!

Share your theories about why you think this might be after viewing the video below. It provides you with information to help with this Marine Detective case.

Happy sleuthing to you!


Note that yes, gastroliths have been found in the stomachs of other seal and sea lions species including the California Sea Lion, Zalophus californianus (Source: Drehmer and Oliverira).

Click here for SeaDoc footage of Steller Sea Lions playing with California Sea Cucumbers.

Research into gastroliths in Steller Sea Lions

C. R. Shuert and J. E. Mellish “Size, mass, and occurrence of gastroliths in juvenile Steller sea lions (Eumetopias jubatus),” Journal of Mammalogy 97(2), 639-643, (11 January 2016).


Abstract: “In summary, our opportunistic assessment of gastroliths in temporarily captive Steller sea lions showed that a large proportion of juvenile animals (e.g., one-third or more) may haveone or more stones at any given time. The regurgitation of gastroliths correlated best with leaner, but not poorer body conditions, and during the summer months, possibly indicating a weak association as a digestion aid. We found little evidence to suggest that they assist in buoyancy and satiation; it is even more unclear as to what drives an individual to regurgitate them. With a lack of strong conclusions relating to a particular use, we can only conclude that they appear to serve a function in sea lions and are not ingested accidentally. A combined assessment of regurgitated and in vivo gastrolith measurements may shed more light on the subject and allow for direct evaluation and conclusions as to their functionality in sea lions.”

“Gastroliths, defined as stones or concretions in the digestive tract, occur in many extant vertebrate taxa and throughout the fossil record of marine tetrapods (Wings 2007). Most information is anecdotal, with limited data on occurrence and size(e.g., Labansen et al. 2007). Adjusting buoyancy, alleviating hunger, and aiding in digestion are the most popular theories for the presence of gastroliths in marine mammals (Taylor 1993). Gastroliths have been described in all 3 families of  pinnipeds including otariids (e.g., South American sea lions Otaria byroniaDrehmer and Oliveira 2003), phocids (e.g.,  harp seals Phoca groenlandicaNordøy 1995), and odobenids (Gjertz and Wigg 1992)” 

A note about the Marine Mammal Regulations in Canada

Amended Regulations (since July 2018) include that it is illegal to target marine mammals for the purposes of swimming with them (unless permitted by a research license). See the Marine Mammal Regulations at

Further sources

Drehmer, C J, and L R. Oliveira. “Occurrence of Gastroliths in South American Sea Lions (Otaria byronia) from Southern Brazil.” Latin American Journal of Aquatic Mammals. 2.2 (2003).

Scheffer, V.B. and Neff, J.A. (1948) Food of California sea lions. Journal of Mammalogy 29(1): 67-68

Taylor, Michael A. “Stomach Stones for Feeding or Buoyancy?: The Occurrence and Function of Gastroliths in Marine Tetrapods.” Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. (1993)

Wings, Oliver. “A Review of Gastrolith Function with Implications for Fossil Vertebrates and a Revised Classification.” Acta Palaeontologica Polonica. 52 (2007): 1-16. Print.




8 Responses to “The Case of Stones in Sea Lions’ Stomachs”

  1. Stacey

    Such a great puzzle – one I’ve spent some time scratching my head over in WIC days past! I think more information about the size and sex of sea lions with gastroliths could help to narrow down the list of possible explanations for this phenomenon. The buoyancy regulation and stomach fullness hypotheses would work better for large males than females. Perhaps water movement could explain why the smaller stones have little or no growth in comparison to the bedrock though? xo

    • The Marine Detective

      Thanks so for this insight Stacey. It would be so valuable to have age and gender information re. gastroliths. Regarding the stones not having growth, it is pretty rare to have a bed of rocks like this with no current while everything around them does.

  2. Gillian Butler

    I’m wondering if similar behaviour has been observed in captive animals? If access to stones is available (if it isn’t perhaps it should be provided). Thanks Jackie xoxoxo

    • The Marine Detective

      Thanks so much Gillian. I know of no instances where this has been observed in captive animals but, as you identify, this is likely because stones of ingestible size are not available.

  3. Amber

    I think this behaviour is very interesting! My best guess would be that the sea lions are manipulating their mouth and jaw in order to prepare for the real thing – forage fish! I am curious to know if this behaviour is seen more in young Steller sea lions.

    • The Marine Detective

      In my opinion, this is the best hypothesis to date! That the behaviour is to learn how to manipulate and eat prey – even bigger fish like salmon. It would thereby be very valuable to know if the behaviour was more common in juveniles. Thank you so much Amber!

  4. Anonymous

    I have witnessed captive animals swallowing a lot of foreign objects including stones that fallen in the pool. One study I was part of had them eat a heavy concentration of squid, they would regurgitate the pens daily.

  5. Tre Willison

    I was diving in the channel islands 5 years ago and had a female sea lion spot out an oblong, baseball sized smooth pure quarts stone in front of me. It was beautiful and like nothing else around so I kept it as a gift. It also had no growths on it.


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