cetacean feeding ecology
If we know what cetaceans eat, we can better understand how they affect their ecosystems, and how environmental changes may impact them. For instance, dietary information is relevant to assess how co-occurring species interact (sharing or competing for food), to estimate their energetic requirements, and to anticipate how human-related disturbances might affect their feeding success, distribution and survival.
There are different methods that we can apply to study the feeding habits of cetaceans, and these include visual observation, the analysis of gut contents of animals that are washed ashore after they die, and the analysis of natural chemical tracers (e.g., stable isotopes) in their tissues, such as skin, teeth or baleen.
Stable isotopes are chemical elements of the same type that have different atomic masses (hence, different weights!). They make up the different macromolecules that build our tissues, and we obtain them through our food (we are what we eat!).
Because it is easier to process the lighter elements, there are some natural discriminations against the heavy stable isotopes during metabolic processes, and organisms end up excreting the lighter isotopes, and accumulating the heavy ones in their tissues. This accumulation propagates through the food chain in a predictable way. Therefore, by looking at the stable isotope values in the tissues of cetaceans (or in any other consumer, for that matter), we can learn about their food sources and estimate their trophic positions within food webs.
I have been applying this method since my masters' studies to investigate the feeding habits of coastal and oceanic cetacean populations.
Trophic ecology and interactions of oceanic cetaceans from the south atlantic
Several cetacean species inhabit the oceanic waters off Brazil. Because of the distance from the coast, relatively little is still known about what they eat and how they interact with one another. During my PhD, I analysed skin samples of different toothed-whales that were collected during marine-mammal dedicated research expeditions along the oceanic waters off southern Brazil (Projeto Talude).
We found that the nitrogen stable isotopes in cetacean skin followed the same patterns as those in zooplankton, which had been previously described in a paper mapping the isotopic signature of organisms at the base of the food webs in these oceanic waters. By considering these spatial differences in the stable isotopes throughout this region, we could better interpret the isotopic values in cetacean skin, and make more accurate inferences about the feeding habits of these odontocete species. These analyses helped refine our knowledge about the trophic ecology and interspecific trophic interactions among several odontocete species that occur in the oceanic waters of the western South Atlantic. Click here to read the full paper.
Nevertheless, the patterns in the nitrogen isotope values of some cetacean species remained unresolved. For instance, δ15N values indicated that the rough-toothed dolphin (Steno bredanensis) could be occupying a higher trophic position than species such as the sperm whale (Physeter macrocephalus) or the killer whale (Orcinus orca), which is unlikely.
A schematic of the difference in δ15N values between the source and trophic amino acids (AA) in organisms of different trophic positions.
Trying to understand these patterns that remained unexplained in those previous analyses, I decided to analyse nitrogen isotopes in the individual amino acids of cetacean skin.
Source amino acids: because some amino acids change very little during metabolic processes, they preserve the isotopic signature of organisms at the base of the food webs. Hence, they can be used to trace the baseline sources where the consumer is foraging.
Trophic amino acids, on the other hand, undergo great changes in the nitrogen values, and can, therefore, be used to estimate a consumer's trophic position within food webs.
These analyses allowed the characterization of the trophic structure of the cetacean community that inhabit these offshore waters, as well as how they use this habitat. We found that the nitrogen stable isotopes in cetacean skin source amino acids were correlated with those in zooplankton, following the same latitudinal patterns. This revealed some level of fidelity to these regions, and allowed us to assess their different foraging areas.
Additionally, with the analysis of nitrogen isotopes in amino acids we could see that the trophic position of the rough-toothed dolphin was in fact lower than bulk-tissue isotope data had previously suggested.
These new analyses also resolved the high isotopic overlap that had been observed between sperm whales and the other smaller odontocetes. We could see that this overlap is much smaller, as species feed at different depths. For example, the sperm whales forage in the oceanic waters of the continental slope, and occupy the highest trophic position among the cetacean species studied.
The common dolphin (Delphinus delphis) feeds in neritic waters, while the Atlantic spotted dolphin (Stenella frontalis) and the bottlenose dolphin (Tursiops truncatus) search for food along the outer continental shelf. These last two species were often observed together, and the high overlap in their isotopic values indicated that they forage together, but consume different prey and occupy different trophic positions.
These analyses revealed that these cetacean species rely on spatial or trophic segregation as a means to minimize or avoid competition. The complete article can be downloaded here.
feeding habits of franciscana dolphins
Franciscanas (Pontoporia blainvillei) are small cetaceans that are endemic to coastal waters off Brazil, Uruguay and Argentina. During my Masters, I analysed carbon and nitrogen stable isotopes in teeth, to assess whether there were sexual or age-related patterns in the feeding habits of franciscanas from southern Brazil. I also used isotopic mixing models to estimate the importance of different prey types to the diet of franciscanas from distinct age groups (juveniles vs. adults). This paper is published in the journal Marine Mammal Science, and can be accessed here.