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Open Access Publications from the University of California


The International Journal of Comparative Psychology is sponsored by the International Society for Comparative Psychology. It is a peer-reviewed open-access digital journal that publishes studies on the evolution and development of behavior in all animal species. It accepts research articles and reviews, letters and audiovisual submissions.

Volume 26, Issue 2, 2013


Are Conversations Between Dolphins and Humans Possible?

Scientific speculations concerning the sophistication of dolphin communication systems have contributed to the notion that meaningful two-way communication between dolphins and humans is possible. This notion has garnered considerable support in the media and popular literature, resulting in an enduring myth that dolphins and humans can communicate in ways that rival, and perhaps even surpass, human-human communication. The truth, however, is quite different from the myth. Although humans and dolphins can certainly communicate with one another, communication between dolphins and humans has been quite limited to date. In fact, there is no compelling scientific evidence that humans and dolphins have engaged in meaningful conversations that involved mutual exchanges of information. In this paper, I consider the reasons why communication between humans and dolphins has been much more limited than many media reports suggest.

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Directionality of Sexual Activities During Mixed-Species Encounters between Atlantic Spotted Dolphins (Stenella frontalis) and Bottlenose Dolphins (Tursiops truncatus)

In the Bahamas, interspecific groups of Atlantic spotted dolphins, Stenella frontalis, and bottlenose dolphins, Tursiops truncatus, have been observed underwater since 1985 on Little Bahama Bank. Mixed-species groups engage in associative behaviors and aggression on a regular basis. Because of their complex cognitive behaviors and large brain encephalization, dolphins are likely capable of complex social interactions, even between species.Between 1993-2003, 177 Mixed-Species Encounters (MSE) were categorized by the age class of male spotted dolphins, the ratio of spotted dolphins to bottlenose dolphins, behavior as Associative (traveling, babysitting, play) or Aggressive (chases, mounting, head to heads) and by directionality of sexual behavior. The majority (68%) of MSE involved adult spotted dolphin. Associative behaviors were observed more than aggressive behaviors in groups where no adult male spotted dolphin, only male calves, or male juvenile spotted dolphins were present. Aggressive behaviors were observed more frequently than associative behaviors in adult male spotted dolphin groups. When groups were unbalanced in favor of one species or the other, differences in social interactions occurred. Male spotted dolphins were never observed attempting to mount male bottlenose dolphin although they chased them. Despite the larger ratio of male spotted dolphins to bottlenose dolphins during MSE, directionality of male-to-male sexual contact was primarily one-way. Male bottlenose dolphin mounted and copulated with male spotted dolphins but not the reverse. Opportunities for cross-species mating and hybridization clearly occurred. Male bottlenose dolphins copulated with female spotted dolphins and male spotted dolphins copulated with female bottlenose dolphins. These sympatric dolphins in the Bahamas have a complex and dynamic relationship that varies with sex and age and revolves around potential reproductive isolation issues.

Social Competence of Adult Chimpanzees (Pan troglodytes) with Severe Deprivation History: A Relational Approach

The development of social competence depends on feedback from partners. We evaluated the social competence of 18 adult re-socialized chimpanzees with respect to (1) social group membership and (2) deprivation history combination. The groups comprised either a majority of early (EDs; mean age at onset of deprivation: 1.2 years) or late deprived chimpanzees (LDs; mean age at onset of deprivation: 3.6 years). We reapplied our model of social competence with five grades of social stimulation and found a diminished toleration of social stimulation (1) in ED-majority groups compared to the group where LDs predominate and (2) in homogeneous ED-majority dyads compared to homogeneous LD-majority dyads. LDs but not EDs representing the minority within their group were able to adjust their stimulation seeking to the majority of partners. Only the LD-dominated group and the homogeneous LD-majority dyads, respectively, showed improvements of social competence from the first to the second year following re-socialization.

Underwater Mirror Exposure to Free-Ranging Naïve Atlantic Spotted Dolphins (Stenella frontalis) in the Bahamas

The “mirror state,” described for human self-recognition, has been found in captive or human-raised species. In marine mammals, bottlenose dolphins and killer whales have shown evidence of body examination, self-directed and contingency checking behaviors whereas false killer whales appeared ambiguous and California sea lions did not recognize themselves in a mirror. Self-recognition processes in wild cetaceans remain unknown. Since 1985, a resident community of Atlantic spotted dolphin (Stenella frontalis) has been studied underwater in the Bahamas. We describe the reaction of free-ranging dolphins during 14 exposures to the presence of a mirror from 1994/1995 and 2004/2005. Responses to the mirror were mixed. Initial reactions of mother/calf groups were to swim around mirror and stay in close physical proximity. Others ignored the mirror entirely, or swam around or underneath. A single male became stationary and postured in an aggressive stance in front of the mirror. The wild spotted dolphins showed a significant preference to exposing and/or orienting their right side to the mirror versustheir left side. We suggest that the animals assign different meanings to a mirror in the wild versus the same object in captivity.

Visual Processing Speed in Capuchin Monkeys (Cebus apella) and Rhesus Macaques (Macaca mulatta)

Visual acuity is a defining feature of the primates. Humans can process visual stimuli at extremely rapid presentation durations, as short as 14 ms. Evidence suggests that other primates, including chimpanzees and rhesus macaques, can process visual information at similarly rapid rates. What is lacking is information on the abilities of New World monkeys, which is necessary to determine whether rapid processing is present across the primates or is specific to Old World primates. We tested capuchin (Cebus apella) and rhesus (Macaca mulatta) monkeys on a computerized matching-to-sample paradigm to determine the shortest presentation duration at which stimuli could be correctly identified. In Study 1, using clip art images, both species achieved presentation durations as short as 25 ms while maintaining high accuracy rates. In Study 2, we used logographic Asian language characters to see if stimuli that were more similar in appearance would reveal species differences. Neither species was as accurate, or achieved as short of presentation durations, as with clip-art images. In particular, capuchins were initially less accurate than rhesus in Study 2, but with experience, achieved similar accuracy rates and presentation durations. These data indicate that rapid visual processing abilities are widespread in the primate lineage, and that the form of the stimuli presented can have an effect on processing across species.