Dolphins rub against coral to ‘self-medicate’

Author: Jill Ashey

Editor: Sara Gagliardi

Coral reefs provide important services to many communities around the world – they foster biological diversity, support a booming tourism industry and cultivate vital fisheries. But a recent study found that coral reefs may also be helping dolphin communities take care of their skin! Researchers observed the Indo-Pacific bottlenose dolphin (Tursiops aduncus) in the Northern Red Sea preferentially rubbing against certain invertebrates on coral reefs. While it is unclear what the intention behind this rubbing behavior is, Morlock et al. (2022) hypothesized that Indo-Pacific dolphins were selectively rubbing against certain invertebrates as a means for targeted self-medication. These invertebrates contain active metabolites that may be sought out by the dolphins for targeted self-medication. 

Dolphins target specific invertebrates on coral reefs In their study, Morlock et al. used photo and video identification to observe group behavior and composition, location and features of targeted invertebrates. The dolphins specifically targeted three different marine invertebrates: a gorgonian coral (Rumphella aggregata), the leather coral (Sarcophyton sp.), and a sponge (Ircinia sp.) (Figure 1).

Figure 1: Dolphins rubbing on (A) gorgonian coral R. aggregata, (B) leather coral Sarcophyton sp., and (C) sponge Ircinia sp.

The dolphins preferentially rubbed different parts of their body on the marine invertebrate species. For instance, the dolphins slid into the branches of the gorgonian coral and often repeated this behavior, rubbing various parts of their body on the coral (Figure 2). 

Figure 2: T. aduncus rubbing on gorgonian coral. Click here to watch the video!

On the other hand, the dolphins typically rubbed only one isolated body part into the leather coral and sponges, as they are harder and more compact (Figure 3). The dolphins were also seen queuing up behind one another to take turns rubbing on these invertebrates!

Figure 3: T. aduncus rubbing on sponge (Ircinia sp.). Click here to watch the video!

Invertebrates contain beneficial metabolites

Researchers were interested in whether these invertebrates had any active metabolites that the dolphins were potentially using to care for their skin. Small fragments of the gorgonian coral, leather coral and sponge were collected for high-performance thin-layer chromatography (or HPTLC). A complex lab technique, thin-layer chromatography separates components of a mixture from one another; this technique can be used to monitor reactions or identify small amounts of unknown compounds. High-performance thin-layer chromatography is an enhanced form of thin-layer chromatography that allows for multi-detection assays, increased resolution and more accurate measurements (Figure 4). In this study, HPTLC was utilized to identify the metabolites present in the marine invertebrates thought to provide dolphins with some skin-care benefits. Ten different assays were performed using HPTLC techniques to identify any metabolites with bioactive properties. 

Figure 4: Visualization of high-performance thin-layer chromatography (HPTLC). 10 different biological assays were performed.

Overall, HPTLC detected 17 bioactive metabolite compounds across the three marine invertebrates selectively accessed by the dolphins. These included antibacterial (acting against Gram-positive and Gram-negative bacteria), antioxidative, acetyl-/butyrylcholinesterase inhibiting, estrogenic, antiestrogenic, antiandrogenic, and genotoxic compounds, indicating these compounds may have self-medication properties that the dolphins are actively exploiting. Furthermore, some of the compounds found could not be readily identified based on the existing literature, so future work is needed to fully understand the breadth of substances that these dolphins are being exposed to by rubbing on these invertebrates. 

Conclusions

This innovative study integrates behavioral biology and biochemistry to suggest that dolphins rub on specific marine invertebrates on coral reefs that contain bioactive metabolites for skin self-medication. The relationship between the selective rubbing behavior of the Indo-Pacific bottlenose dolphin and the presence of bioactive substances provides evidence for self-medication. Given the fascinating results of this study, more research is needed on vertebrate-invertebrate interactions on coral reefs and how climate change may impact these important relationships. 

Paper: Morlock et al. (2022). Evidence that Indo-Pacific bottlenose dolphins self-medicate with invertebrates in coral reefs. iScience. http://doi.org/10.1016/j.isci.2022.104271

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