This blog post is provided by Rocio Iliana Ruiz-Cooley, Tim Gerrodette and Susan Chivers and tells the #StoryBehindThePaper for the paper “Cooperative feeding in common dolphins as suggested by ontogenetic patterns in δ15N bulk and amino acids“, which was recently published in Journal of Animal Ecology. Featured image: Four short-beaked common dolphins surface to breathe, part of a group of about 250 dolphin foraging over the continental drop-off west of Santa Rosa Island, southern California.
Short beaked common dolphins (Delphinus delphis delphis) are bycaught by the gillnet fisheries set to catch swordfish in the Southern California Bight, USA. In an effort to characterize the selectivity of the fishery (i.e., species, sex, stage class) and learn about the life history and ecology of this dolphin species, tissue samples and data have been routinely collected by California Gillnet fisheries observers in collaboration with scientists from Southwest Fisheries Science Center, National Oceanic and Atmospheric Administration (NOAA). Dr. Iliana Ruiz-Cooley, the lead author of the recent study published in the Journal of Animal Ecology, used biochemical tracers, stable isotope analysis of bulk tissue and individual amino acids (AAs), to learn about the main feeding strategy of this small pelagic odontocete using time-series skin samples. By measuring atoms (the heavy (15N) and light (14N) nitrogen (N) stable isotopes) from skin and individual amino acids, Dr. Ruiz-Cooley and colleagues reconstructed variation in dolphin diet and habitat from nursing calves to adulthood. The proteins of dolphin skin are composed of smaller units known as amino acids; each amino acid has a specific function and structure. The N isotope value from each of these amino-acid protein building blocks can reveal information related to the animal habitat biochemistry and diet even for species difficult to study, such as pelagic cetaceans like dolphins and whales.
Dr. Ruiz-Cooley and colleagues evaluated the effect of life-stage (nursing calves, weaning calves, juveniles, and adults) and sex (males and females) on the feeding habits of the common dolphin using isotope values from various amino acids. They developed a hierarchical Bayesian Model (HBM) to analyze the complex and highly variable data obtained from amino acid stable isotope analysis. In previous studies, researchers had considered each source of variability separately (implicitly assuming independence), and carried variability from one step to the next with approximations such as “propagation-of-error” methods. However, modern statistical analysis recognizes that there are important benefits to considering all the data together in a single likelihood framework. The hierarchical nature of stable isotope data (variability among replicates within AAs, among AAs within individuals, and among individuals within the study) can be easily and naturally represented using Bayesian methods. The HBM presented by the authors represents an efficient and realistic way to analyze these complex data and investigate foraging behavior of long-lived marine species.
Previous observations in the field suggest that common dolphins travel together in groups of a hundred to a thousand individuals (see photo below) and may work together for aggregating and catching prey. It was unknown how this important and frequent strategy was conducted, since humans can mainly observe dolphins at the sea surface and have limited access to long-term observations underwater. The recent publication in the Journal of Animal Ecology explains how researchers found that adults and juveniles, as well as males and females, share similar diet and habitat, indicating that common dophins may frequently cooperate to obtain similar diet composition (therefore similar isotope values in their proteins) and succesfully feed in the highly dynamic marine environment.
Ruiz-Cooley, R. I., Gerrodette, T., Chivers, S. J., & K. Danil. Cooperative feeding in common dolphins as suggested by ontogenetic patterns in δ15N bulk and amino acids. Journal of animal Ecology. In press.
Ruiz-Cooley, R. I., Gerrodette, T., Fiedler, P. C., Chivers, S. J., Danil, K., & Ballance, L. T. (2017). Temporal variation in pelagic food chain length in response to environmental change. Science advances, 3(10), e1701140.