Credit: CC0 Public Domain
Pinnipedsa group that includes seals, sea-lions, and walruses are relatively recent marine mammals that evolved from terrestrial carnivorans. Baylor University assistant professor of biology Sarah Kienle is particularly interested in their evolution to an amphibious lifestyle.
Kienle stated, "I am fascinated by the way animals feed underwater." "Mammals like dolphins, pinnipeds and sea otters have all reintroduced the marine environment. They have all adopted the same feeding strategies for entering the water environment. The repeated evolution of filter feeding and suction feeding is also evident. It is amazing to see all these distinct lineages converging on the same feeding strategies: suction feeding, biting, and filter feeding.
Kienle led a study that was published in the Journal of Anatomy and compared pinniped craniofacial muscle to determine its role in aquatic feeding.
"I was curious to see how the different facial muscles could play a part in different aquatic feeding strategies. I was studying the skull morphology to determine if there are morphological specializations in feeding, and that naturally led me to ask the question "What do the muscles look like?" Kienle agreed.
Rare data exist on the function of aquatic feeding and the pinniped craniofacial muscles in pinnipeds. Kienle's research has provided foundational data that will allow for further research on pinnipeds, and their evolution in functional morphology.
"The first section of the study was quite surprising, as there wasn't any reference material to do these dissections. Kienle stated that there aren't any detailed drawings of how the muscles should look. I knew that they should have a massager (a facial muscle that opens and closes the jaw to chew), but it wasn't obvious what this looked like in pinnipeds. After the first dissections, I was unsure what muscles to expect. We would then write a tag that said "mystery muscle" with a question mark, until we found out what it was.
Researchers performed anatomic dissections on 35 specimens from six species of pinnipeds: California sea lions harbor seals northern elephant seals, Weddell seals, California seals, California sea-lions, California seals, and California sea lions. The researchers described 32 pinniped craniofacial muscle types, including facial expressions, tongue, hyoid, and soft palate muscles, through the dissections.
It was fascinating to see what was missing in different species. It was my expectation that the study would be fairly continuous among pinnipeds, and that all species would have the exact same muscles. Kienle stated that this was not always true.
Pinnipeds generally conform to mammalian patterns in craniofacial muscle musculature. However, they have unique musculoskeletal structures. You can see differences in the size, position, attachments, and muscle positions. These may be adaptations for aquatic feeding strategies.
Northern elephant seals and bearded seals are suction feeding experts and have a larger masseter muscle that biters. Northern elephant seals also have a unique and distinctive tongue and hyoid morphology, which is different from other pinniped species. Comparatively, biting species like California sea lions harbor, ringed, and Weddell sealings don't have consistent craniofacial muscle skeletal adaptations that distinguish them from suction feeders.
Kienle stated that pinnipeds are generalists and opportunistic. This is good for their ability to adapt to changing environments, as they have done in the past. From an evolutionary perspective, closely related species should be more alike than distantly related ones. Perhaps there are ecological forces that drive the muscular anatomy more than evolutionary history.
Researchers note that pinnipeds' strong facial morphology allows them to change their feeding strategies depending upon the environment. This ability is crucial for survival in rapidly changing environments underwater.
To help distinguish craniofacial muscles from their use, the researchers used data from other mammals. Kienle stated that it was difficult to find data about mammalian craniofacial muscles beyond humans.
"It is amazing how little we know about these large marine carnivores' internal anatomy. This is the part that most surprises me. She said that she was surprised at how little information there is about the internal structures of mammals in general. "This is a foundational comparative research. Previous research has been very isolated and limited in scope. This is the first time that multiple species have been compared to see what is common and different across each group. This opens up the possibility of more research.
Continue reading
Further information: Sarah S. Kienle and colleagues, Comparative examination of pinniped creniofacial musculature in aquatic feeding, Journal of Anatomy (2021). Sarah S. Kienle and colleagues, Comparative examinations of pinniped craniofacial muscles and their role in aquatic feeding. (2021). DOI: 10.111/joa.13557
