Zoo Biology 16:173–177 (1997)

ZOO 492

© 1997 Wiley-Liss, Inc.

BRIEF REPORT

Lateralized Behavior in Two CaptiveBottlenose Dolphins ( Tursiops truncatus )Lori Marino* and Jennifer Stowe

Department of Biology, Emory University, Atlanta, Georgia

The study of behavioral laterality in humans and nonhumans can contribute toour understanding of brain evolution and functional similarities across species.Few studies have focused on cetaceans. This report exams lateralized behaviorsin two captive bottIenose dolphins (Tursiops truncatus). Observations were madeby videotaping through a 90 × 150 cm underwater one-way Plexiglass mirror.Directional bias in swimming, “barrel-roll” maneuvers, and circular head move-ments was assessed for each subject. There was a strong clockwise bias in swim-ming direction and direction of “barrel-rolls,” but not circular headmovements. The clockwise bias in swimming direction and “barrel-roll” ma-neuvers may be consistent with a rightward turning bias. Zoo Biol 16:173–177,1997. © 1997 Wiley-Liss, Inc.

Key words: laterality; cetaceans; asymmetries

INTRODUCTION

Lateralized behavior (i.e., a consistent preference or bias for one side of thebody) exists in a number of nonhuman species [Bradshaw and Rogers, 1993; Hopkinsand Morris, 1993; Springer and Deutsch, 1989, for reviews]. In humans, popula-tion-level lateralized behavior (e.g. handedness) is associated with functional asym-metries in such complex processes as language [Springer and Deutsch, 1989]. Suchfindings are important for understanding the evolution of the brain in general andbecause they may reveal underlying functional asymmetries in other species that aresimilar to those in humans.

Despite these important implications, there have been few systematic stud-ies of behavioral laterality in the large-brained and behaviorally complex ceta-ceans. Those findings that do exist point compellingly to the possibility that thelevel of behavioral asymmetry in some cetaceans is on a par with that in humans

Received for publication 7 June 1996; revision accepted 29 August 1996.

*Correspondence to: Lori Marino, Department of Biology, Emory University, Atlanta, GA 30322.

174 Marino and Stowe

and other primates. Studies of gray (Eschrichtius robustus) and humpback whales(Megaptera novaeangliae) strongly suggest a population-level, right-side bias inbottom-feeding [Clapham et al., 1995; Kasuya and Rice, 1970] and flipper slaps[Clapham et al. 1995]. Also numerous reports suggest that wild coastal bottle-nose dolphins (Tursiops truncatus) exhibit a right-side-down bias when intention-ally beaching themselves in order to feed on fish herded ashore [Caldwell andDolphin Project, 1993; Hoese, 1971; Norris and Dohl, 1980; Petricig, 1993; Rigley,1983; Rigley et al., 1981; Silber and Fertl, 1995].

Assessments of behavioral laterality in captive odontocetes (i.e., T. truncatus,Lagenorhynchus sp., Pseudorca crassidens, Orcinus orca, and Cephalorhynchus sp.),although limited, are consistent with a right eye viewing preference under certainconditions [Ridgway, 1986] and a bias toward a counterclockwise swimming direction[Caldwell et al., 1965; Ridgway, 1972, 1986], which would place the right eye in aposition to access important visual information in the direction of the outer wall of thepool [Ridgway, 1986]. The present study is a preliminary assessment of consistency inbehavioral lateralities in two captive bottlenose dolphins.

MATERIALS AND METHODSSubjects and Facility

Subjects were two half-sibling, 7-year-old captive-born male bottlenose dol-phins (T. truncatus), Pan and Delphi, that have resided together since birth. Theywere housed together at Marine World Africa USA (Vallejo CA) in a circular 18 ×5.4 m deep pool in which no other animals resided. The subjects were not trainedfor a show regime at any time and did not acquire any of the behaviors assessed inthis study through direct training, nor were any of these behaviors explicitly rein-forced. All data in the present study were collected while the subjects participatedin an unrelated study of mirror responses [Marino et al., 1994]. Behavior was vid-eotaped by a Sony Video 8 Handycam CCD-M8u camera through a 90 × 150 cmunderwater one-way Plexiglass mirror at the mouth of a channel on one end of thepool. Video-taped observations consisted of body-length views of the subjects wheneither stationed at the mirror or while swimming directly up to the mirror. Thesubjects’ behavior under these conditions was recorded for an average of 4 hoursper day for 10 days for a total of 36 hours.

Behavioral Analysis

The following behaviors were scored separately for each subject as clockwiseor counterclockwise (with respect to the subject): swimming direction (i.e., direc-tion of their forays around the pool, where a foray was defined as any type ofswimming around the edge of the pool that covered at least one lap around thepool), direction of turn axis during a “barrel-roll” (i.e., corkscrew) approach per-pendicular to the mirror, and direction of circular (i.e., around body axis) headmovements in front of the mirror. There was no circular current in the pool, only aweak current toward the center. Also, in order to eliminate the possible influence ofswimming direction on the direction of “barrel-rolls,” only those barrel-rolls that wereproduced when the subject approached the mirror in a straight line from the oppositeside of the pool were included in the analysis. “Barrel-rolls” that were produced whenthe subjects stopped at the mirror during a circular swim were not.

Lateralized Behavior in Dolphins 175

The first author reviewed all 36 hours of videotape and noted the tapesequences that contained clear instances of “barrel-rolls” and circular head mo-tions and the identity of the subject producing the behavior. Although both sub-jects were observed simultaneously, possible mimetic behavior was eliminated byexcluding sequences in which both subjects simultaneously exhibited the samebehavior type (swimming direction was excepted because both subjects swamtogether most of the time). The direction of each instance of these behaviors foreach subject was then analyzed by the second author, who was naive to anydirectional aspects of the behaviors noted by the first author. Thus frequency ofthe direction of “barrel-rolls” and circular head motions was obtained for eachsubject independently. Swimming direction was noted throughout the period.

RESULTSSwimming Direction

Both subjects swam in a clockwise direction during all 36 hours of recordedbehavior across the 10 day period. Additional observations confirmed that swimmingdirection was clockwise throughout the entire 10-day period regardless of whether vid-eotaping during mirror exposure was occurring or not. Furthermore, facility personnelwho were highly familiar with these two subjects for several years confirmed that theirswimming direction prior to the test period was almost always clockwise. Also, swim-ming direction did not differ when either subject swam with the other or alone (e.g.,when one stationed at the mirror while the other swam around). In general, the swim-ming behavior of the two subjects was characterized primarily by almost continuousswimming around the edge of the pool, punctuated by interruptions in this pattern whenstationing for feeding, playing with each other or a toy, or investigating the mirror orsome other feature of their environment.

Several months after the present study, these two subjects were transferred toanother captive facility, The Gulfarium (Ft. Walton Beach, FL) where personnelconfirm that they continue to swim consistently in a clockwise direction (there is awater current in the new facility that runs in a counter clockwise direction).

Barrel-rolls

During the 36-hour, 10-day videotaped observation periods all of Pan’s 11“barrel-rolls” (that met the study criterion) were in a clockwise direction. Eleven ofDelphi’s 12 “barrel rolls” were in a clockwise direction. It has also been confirmedthat both Pan and Delphi continue to “barrel-roll” almost exclusively in a clockwisedirection at The Gulfarium.

Circular Head MovementsDuring the 36-hour, 10-day videotaped observation period, 21 bouts of circu-

lar head movements were displayed by Pan, of which 10 were clockwise. Ninebouts of circular head movements were displayed by Delphi, of which five wereclockwise. Clockwise and counterclockwise circular head movements wereunsystematically disbursed across days for both subjects.

DISCUSSION

The results of the present study provide evidence that two captive bottlenosedolphins displayed a consistent clockwise bias in swimming and “barrel-roll” direc-

176 Marino and Stowe

tion. There was no evidence for a directional bias in circular head movements.Furthermore reports from facility personnel provide preliminary evidence that thesedirectional biases have been stable across several years despite changes in facility andwater currents. Tentatively, it is noteworthy that the two directional biases found (i.e.,in swimming and “barrel-rolling”) were both clockwise.

Biases in “barrel-roll” direction were found despite exclusion of “barrel-rolls”observed simultaneously. Thus the same clockwise bias in both subjects was notlikely due to mimicry of one subject by another. Additionally, although swimmingdirection was observed simultaneously, it did not differ when each subject swamalone or with the other. Nevertheless, as in the case with any subjects housed to-gether, nonindependence, especially during the development of behavioral biases,cannot be entirely ruled out. The negative findings regarding bias in circular headmovements may be due to it being a measure of laterality that is not analogous tobehaviors studied in other species. “Barrel-rolls,” although obviously not found innonaquatic animals, may be analogous to turning in nonaquatic animals.

The clockwise bias of the “barrel-rolls” involves a rightward movement of thebody, which is consistent with previous findings suggesting a strong right-side biasin viewing [Ridgway, 1986], feeding [Caldwell et al., 1993; Clapham et al., 1995;Hoese, 1971; Kasuya and Rice, 1970; Norris and Dohl, 1980; Petricig, 1993; Rigley,1983; Rigley et al., 1981; Silber and Fertl, 1995], and flipper use [Clapham et al.,1995] in several cetacean species. However, the present finding of a clockwise biasin swimming direction is not consistent with previous reports of an overall counter-clockwise swimming bias in captive cetaceans [Caldwell et al., 1965; Ridgway,1972, 1986], but may be consistent with an overall right-side body bias because itinvolves a leaning toward the right side of the body, much in the same way clock-wise “barrel-rolls” are produced. Nevertheless, the establishment of population levellateralized behavior in cetaceans goes well beyond the scope of the present study,and it is not presently possible to speculate meaningfully about the inconsistency inswimming direction bias shown by our subjects and that of those in previous studies[Caldwell et al., 1965; Ridgway, 1972, 1986]. Finally, in both the present study andin that of Ridgway (1972), water current was unrelated to swimming direction.

Overall, the present findings add to the growing evidence for behavioral later-ality in cetaceans and, although not directly applicable, are tentatively consistentwith evidence for neuroanatomical asymmetry in the bottlenose dolphin [Morganeand Jacobs, 1972; Morgane et al., 1980; Ridgway and Brownson, 1979, 1984].Future studies with larger sample sizes and longitudinal assessment of individualsubjects are important to compare the consistency, extent, and strength of thesebehavioral biases with those found in other species.

CONCLUSIONS

1. A consistent clockwise bias in swimming direction and “barrel-rolling,” butnot circular head movements, was found for two captive bottlenose dolphins.

2. Reports from facility personnel imply these biases have existed overseveral years.

ACKNOWLEDGMENTS

The authors thank Greg Siebenaler for his helpfulness in reporting Pan andDelphi’s behavior at The Gulfarium and William Hopkins for his critical commen-

Lateralized Behavior in Dolphins 177

tary. Note: Since the completion of this report, Delphi, one of our research sub-jects, has died.

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