Living on the Edge: Settlement Patterns by the Symbiotic Barnacle Xenobalanus globicipitis on Small Cetaceans


By Rachel Skubel, RJD Intern

If you were a barnacle, how would you choose your home? For X. globicipitis barnacles residing on striped dolphins, this question was ‘put under the microscope’ by Juan Carillo and colleagues at the University of Southern Mississippi and Cavanilles Institute of Biodiversity and Evolutionary Biology (Valencia, Spain).

Of all obligate barnacles studied, X. globicipitis has been found on animals that experience the most intense currents (Bearzi and Patonai, 2010). These organisms will settle on dolphins to optimize for (a) availability of passing current, to provide food, and (b) low drag from said current, to reduce physical degradation of the animal. Here, the investigators asked the following questions:

  1. Where do these barnacles choose to settle?
  2. How does this choice affect the barnacles’ recruitment (define), survival, and growth?

The researchers examined stranded striped dolphins (Stenella coerleoalba) along 556 km of the spanish mediterranean coastline (map), from 1979 to 2009. In 1990 and 2007, many of the dolphins examined had been killed by the morbillivirus (link to – infected animals would have swam slower and had weaker immune systems than otherwise, making them more likely to be colonized by the barnacles. For each animal, the researchers looked at the abundance (i.e. amount), location, and size of the barnacles. Then, they used a model to investigate why barnacles were colonizing certain locations of the dolphins.

Figure 1: Barnacles were mainly found on the trailing edges of dorsal fins, flippers, and flukes, over an area ~2cm wide (Dr. Mariano Domingo, Autonomous University of Barcelona, Spain)

Figure 1: Barnacles were mainly found on the trailing edges of dorsal fins, flippers, and flukes, over an area ~2cm wide (Dr. Mariano Domingo, Autonomous University of Barcelona, Spain)


Out of 242 dolphins examined, 104 had the X. globicipitis barnacles – on either their dorsal fins, flippers, and tail flukes. Of these locations, the tail flukes were by far the most common. Even if the dolphins had barnacles in multiple locations, linear density (barnacles/cm) was significantly higher on the tail. Also, the shell size of barnacles on the flukes was higher than on the flippers and dorsal fins. For these dolphins with barnacles on their tail flukes, it was more common to find them on the dorsal (top) than ventral (bottom) size of the tail.

Figure 2: When dolphins were found with X. globicipitis barnacles, they were most likely on the caudal fin.

Figure 2: When dolphins were found with X. globicipitis barnacles, they were most likely on the caudal fin.

Dolphins thought to have died from the morbillivirus did not have any significant differences in where the barnacles were located, or their size, compared to the unaffected animals.

Explaining the trends

When interpreting these results, it was important to consider that these were all pre-deceased study subjects, and the barnacles might have even settled on the carcasses. However, the finding of tail flukes being a popular settlement area for these barnacles matches with observations in the wild (see video below).

 Beginning around 0:13, you can see barnacles are common on the tails of wild dolphins, supporting the findings of the present study by Carillo et al.

How do the barnacles choose where to dig in? The researchers propose that once they’ve used chemical cues to recognize the dolphins as proper hosts, a two-pronged mechanism follows.

  • First, attachment success: those that choose the tail to latch onto will be less likely to fall off in the process because there is some shelter from strong currents. And once one barnacle settles, it actually becomes easier for more to do the same because they will be ‘sheltered’ by this first individual.
  • Second, there is less early cyprid mortality, which means that once fully attached, it is easier to stay attached.

Lastly, the authors considered why there were more barnacles on the dorsal sides of the tails. This could be due to an asymmetrical swimming style by the dolphins, which means that their ‘downstroke’ is stronger than their ‘upstroke’, so there is less force on the settled barnacles if they settle on the top of the tail. However, whether the swimming style of these dolphins is symmetrical or assymetrical is not conclusively known.



Bearzi M, Patonai K (2010). Occurrence of the barnacle (Xenobalanus globicipitis) on coastal and offshore common bottlenose dolphins (Tursiops truncatus) in Santa Monica Bay and adjacent areas, California. Bull South Calif Acad Sci. 109: 37–44. DOI: 10.3160/0038-3872-109.2.37

Carrillo JM, Overstreet RM, Raga JA, Aznar FJ (2015) Living on the Edge: Settlement Patterns by the Symbiotic Barnacle Xenobalanus globicipitis on Small Cetaceans. PLoS ONE 10(6): e0127367. DOI: 10.1371/journal.pone.0127367



Investigating the Intellectual and Emotional Lives of Cetaceans

By Heather Alberro, RJD Intern

The question of intelligence in animals other than human beings and perhaps some species of primates is a provocative and widely contested one. However, there is a growing body of evidence suggesting that cetaceans, the mammalian order that includes whales and dolphins, may possess many of the “intelligence markers” we typically ascribe to intelligent beings such as primates, including language, a sense of self, culture, and displays of emotional complexities. Despite having evolved along quite different evolutionary paths that were shaped by vastly different physical environments, both cetaceans and primates evolved the two largest brains in the animal kingdom. Consequently, as a large body of literature suggests, cetaceans display many of the signs of intelligence often exclusively attributed to the order of primates while even surpassing them in areas such as brain-to-body-size ratio. From living in tight-nit and highly structured social groups to their displays of emotional complexity and self-awareness, cetaceans are indeed evolutionary marvels that appear to be close to primates, particularly humans, in terms of the cognitive and behavioral complexities they exhibit.

Having originated from a hoofed land mammal turned aquatic inhabitant from the Paleocene nearly 50 to 60 million years ago, and despite the radically different physical environment that gave way to a different neuroanatomical structure, cetaceans have nonetheless undergone a similar brain size evolution, known as encephalization, to that of its terrestrial counterpart, the primate brain (Marino, 25). In fact, primates and cetaceans possess the highest encephalization levels in the animal kingdom. The common dolphin, a member of the cetacean sub-order odontoceti that also includes toothed whales, is known to have even higher encephalization levels than non-human primates such as chimpanzees, coming in second only to humans (Marino, 25).  In terms of EQ or “emotional intelligence value”, many modern odontoceti species have a value of 4.5, the highest in the animal kingdom apart from the average 7.0 for humans. Despite variations in neuroanatomical organization and the stark differences in the physical environments that shaped the evolutionary trajectories of primates and cetaceans, it is remarkable that encephalization levels between the two mammalian orders are in fact so similar in terms of size and complexity.


Comparison of the brains of a wild pig, bottle nose dolphin, and modern human.

When assessing the relative intelligence and cognitive capacities of cetaceans, particularly those of the odontoceti sub-order that include highly social species such as the common dolphin and the orca, various lines of enquiry have been pursued, such as whether or not these animals are self-aware. One test typically employed by researchers to test for advanced cognitive developments such as self-awareness is the mirror test. In her article, Convergence of Complex Cognitive Abilities in Cetaceans and Primates, Lori Marino describes a mirror test that she and a fellow researcher conducted with two bottlenose dolphins, whereby they placed marks on their bodies and allowed them to observe themselves in a mirror. Lori notes that, “both dolphins in our study used a mirror to investigate parts of their bodies that were marked [Reiss and Marino, 2001]” and that the findings of the study “open up the possibility that the emergence of self-recognition, and perhaps other forms of self-awareness, are not byproducts of factors unique to humans and great apes (29).” Indeed, the possibility that cetaceans may possess a sense of self, an attribute originally thought to be exclusively human, suggests that there is some level of cognitive complexity that warrants further research.


Dolphin mirror test (Reiss and Marino, 2001)

Another marker of intelligence originally believed to be exclusive to humans and some non-human primates such as macaques and chimpanzees is the presence of “culture”, which is defined as the information or behavior that is shared by a population or subpopulation, and which is acquired from conspecifics through some form of social learning (Rendall and Whitehead, 2001).  As Lori Marino elucidates, “Recently, enough data has been amassed on wild cetaceans to show that many species possess cultural traditions with regard to dialects, tool use among some wild dolphin populations, methods of prey capture in killer whales, and other related social behaviors (28).” Similarly, populations of wild orcas off the west coast of Canada have been known to display various hierarchical divisions, much of which seems cultural as the primary division is between resident and transient orcas (Baird, 2000). Such displays of complex social behavior and organization bear a striking resemblance to those of primates, suggesting continuities in their intellectual lives, despite disparities in the outward physical appearance of the two orders.

The idea that cetaceans experience emotional states such as grief, joy, fear, and the like, while difficult to corroborate for the simple reason that cetaceans cannot express any feelings they may have vocally, is nonetheless frequently maintained by many researchers who have spent a number of years working with these animals. In Into the Brains of Whales, Mark Peter Simmonds cites examples such as the “prolonged grief” displayed by Orcas upon losing an infant or other family member. One case involves two male orcas that, after encountering the body of an older female they had grown up with in mind November, 1990, spent the rest of their lives isolated from other orcas and visiting old places that the female had visited when she was alive (Rose 2000a)(Simmonds 108). Simmonds also notes the prominent field biologist Denise L. Herzing’s remarks on the “joy” often expressed by her long-studied Atlantic bottlenose dolphins. Such examples, while undoubtedly inconclusive, still warrant further examination, as they suggest that cetaceans may be as emotionally complex as humans and non-human primates.

Cetaceans have been known to display remarkable behaviors such as rudimentary forms of “culture” for the transfer of information and outward displays of emotionally complex behavior such as grief and excitement. Indeed, they appear to be rather close to humans and above many non-human primates in terms of cognitive, social, and emotional complexity. In terms of the size and anatomical complexity of their brains, many members of the odontoceti sub-order come in second only to modern humans. Further research should aim at gaining a closer look at the lives of these fascinating and intelligent animals, as there is much we have yet to learn, such as whether they indeed experience emotion, whether they can develop significant emotional attachments to members of their group like humans and non-primates do, and just what exactly they are capable of, cognitively.  Such questions lead to the issue of conservation: if these animals are indeed as intelligent and self-aware as they appear to be, should they therefore be granted increased protection from pollution, habitat destruction, hunting, and other man-made dangers? As fellow sentient beings with advanced emotional and intellectual lives, do we owe them the sort of consideration often awarded to members of our own species?



  1. Marino, Lori. “Convergence of complex cognitive abilities in cetaceans and primates.” Brain, Behavior and Evolution 59.1-2 (2002): 21-32.
  2. Rose, N.A., 2000a. A death in the family. In: Berkoff, M. (Ed.), The Smile of the Dolphin. Discovery Books, London.
  3. Simmonds, Mark Peter. “Into the brains of whales.” Applied Animal Behaviour Science 100.1 (2006): 103-116.
  4. Rendell, Luke, and Hal Whitehead. “Culture in whales and dolphins.” Behavioral and Brain Sciences 24.02 (2001): 309-324.

Cetacean Species Affected by Warming Arctic

By Hannah Armstrong, RJD Intern

Global climate change, among other anthropogenic issues, is becoming an increasingly significant threat to the Arctic region of the world.  Specifically, higher average temperatures and rapidly disappearing sea ice are of conservation concern for ice-dependent species.  Arctic marine mammals are specifically adapted to take advantage of the climatic conditions that have prevailed in the Arctic for millions of years, and have been a target of conservation based on their role in the functioning of Arctic ecosystems and surrounding communities.  Despite these conservation concerns, with impacts of climate change likely to worsen in coming decades, there is increased industrial interest in Arctic areas previously covered by ice.

Armstrong 1

A graph showing the evident decline in average monthly arctic sea ice extent from September 1979 through 2012 (Reeves et al).

In a recent study, scientists observed and mapped the distribution and movement patterns of three ice-associated cetacean (marine mammal) species that reside year-round in the Arctic: the Narwhal (Monodon monceros), Beluga (white whale, Delphinapterus leucas), and Bowhead Whale (Balaena mysticetus) (Reeves et al. 2013).  Then they used these ranges and compared them to current and future activity sites related to oil and gas deposits, exploration, development and commercial shipping routes, to assess areas of overlap, as a means of highlighting areas in the Arctic that might be of conservation concern.  Some of the results indicated the sensitivity of Bowhead whales to industrial activity; the sensitivity of Narwhals to climate change and noise, as well as a shift in distribution due to ice conditions; and the sensitivity of Beluga whales to noise, as well as a wider distribution extending into the sub arctic (Reeves et al. 2013).  These observations ultimately triggered the need for a better understanding of the implications of environmental changes in the Arctic for cetacean species, in order to develop effective conservation and management policies (Reeves et al. 2013).

Poorly documented shipping routes and operations, in addition to accelerating Artic pressures in Arctic Norway, Arctic Russia, the Alaskan Arctic, Arctic Canada and Arctic Greenland, indicate that immediate measures need to be taken to mitigate the impacts of human activities on these Arctic whales, as well as the people who depend on them (Reeves et al. 2013).  As indicated by researchers, some of these measures include: careful planning of ship traffic lanes (re-routing if necessary) and ship speed restrictions; temporal or spatial closures of specified areas (e.g. where critical processes for whales such as calving, calf rearing, resting, or intense feeding take place) to specific types of industrial activity; strict regulation of seismic surveys and other sources of loud underwater noise; and close and sustained monitoring of whale populations in order to track their responses to environmental disturbance (Reeves et al. 2013).

After comparing maps of Arctic whale ranges with maps of recent and anticipated oil and gas activity and shipping traffic in the Arctic, researchers noticed the unquestionable overlap between Arctic whales and harmful human activities.  Based on unparalleled current and predicted rates of climate change, the futures of these three Arctic whale species are uncertain.  Based on the significance of these species, both culturally and for proper functioning of the Arctic ecosystem, well-informed management decisions related to human activities will be imperative going forward.



Reeves et al.  Distribution of endemic cetaceans in relation to hydrocarbon development and commercial shipping in a warming Arctic.  Marine Policy 44 (2014).

“Narwhals Breach.” WikiMedia Commons. WikiMedia, 1 Oct. 2012. Web. 29 Jan. 2014.