International Journal of Comparative Psychology

These guidelines follow the recommendation of a number of external bodies to regulate the use of animals in research. They can be used both for transparency in publication, and in this sense they extend what is being requested by journals, or for regulatory or funding institutions, to request information prior, during, or after funding, and to ensure adherence to regulations.

This checklist focuses on the use of rodents in research. Other species (such as marine mammals, primates, or invertebrates) will be covered in future separated checklists.

This checklist is based on and extends the following guidelines: Animals in Research Ethical Guidelines, Guidance for the Description of Animal Research in Scientific Publications, Animals Welfare Act, and ARRIVE guidelines.

On 14 April 2016, the scientific community lost Dr. Stan Kuczaj, professor at the University of Southern Mississippi and Director of the Marine Mammal Behavior and Cognition Laboratory. He was a beloved teacher, researcher, friend, mentor, and colleague. By age 65, this well-liked, respected professor had achieved world-renowned status in multiple disciplines—comparative psychology, behavioral sciences, and developmental psychology. His tremendous success in these areas resulted in a legacy of more than 50 master’s- and doctoral-level students working in a variety of fields; he also had hundreds of collaborators from around the world. Stan significantly contributed to and influenced the current direction of these fields and had many plans and research projects still to accomplish.

Several nonhuman animal species have been claimed to successfully pass tests indicative of relational matching and to therefore engage in analogical reasoning. Here, we address these claims by focusing on one recent case study. We illustrate several potential methodological limitations that make it uncertain as to whether the subjects in this particular study were indeed showing relational matching. To the extent that similar or analogous limitations apply in other studies, this undermines the claim of relational matching. Apart from this, however, even if relational matching was to be conclusively demonstrated in non-humans, this behavior alone is profoundly different from analogical reasoning as performed by humans. Substantial converging evidence now suggests a critically important difference between humans and nonhumans at the level of behavioral process that explains why nonhumans do not engage in complex language and therefore do not engage in processes that require complex language, including analogy. In accordance with both these arguments, we suggest that caution is needed in the comparative cognition literature when extrapolating from nonhuman to human cognitive capacity.

This paper is an introduction to the first part of a double special issue inspired by the work of and dedicated to Dr. Stan Kuczaj, who passed away in April 2016. The introduction reflects the contents of the first part of the special issue, which included a number of different species, research designs, and questions. Comments regarding Stan's influence on each contributer are also shared.

Play and tool use are controversial in part because both have been challenging to define. Play behavior continues to elude specific definition but is currently recognized as a legitimate behavioral classification, especially when an activity involves handling objects (toys), although play does not require object handling. In contrast, animal tool use behavior requires object handling that also meets criteria of purposeful and conditional handling in a specific context to achieve a goal. This report describes a form of object handling, grass-wearing behavior, exhibited by free-ranging bottlenose dolphins in St. Petersburg, FL, USA, to see if play or tool-use-like behavior explains it. During 9,551 sightings of 311 dolphins across 8 yrs of study (Jan 2006 – Dec 2013), N = 79 dolphins were observed with one or more blades of grass splayed across the dorsal fin 190 times. Grass-wearing was unrelated to activities conducted in seagrass meadows, age-sex class, or adult female reproductive phase. Grass-wearing was primarily related to changes in group composition (fusion events). It occurred in larger groups that were significantly more likely to be socializing in affiliative, explicitly sexual and playful contexts with only one observation during conflict, although grass-wearing occurred during travel, forage/feeding, and resting. The behavior was partly explained by play and tool-use-like behavior but is more consistent with dolphins self-decorating with grass as a stimulus enhancement in greeting or bids for attention.

Object use by cetaceans is associated with complex cognitive processes, social relations, play and tool use. A comparative approach of how cetacean species use objects will increase our understanding of how this behavior evolved. This study reports on observations of object use by a small group of wild, provisioned Australian humpback dolphins ( Sousa sahulensis ) in Tin Can Bay, Australia. Data were collated from attendance records, interviews and photographs revealing 23 separate occasions of object use over seven years. A variety of objects, biological and artificial were used by male dolphins during social and play interactions often directed at people. Comparable interactions have occurred in another provisioning program suggesting the behavior may be unique to these situations. The behavior observed in the current study also indicates variations of object use within the species, as objects were not associated with foraging as has been reported in the literature.

We examined side preferences in the Florida manatee (Trichechus manatus latirostris) through observations of limb use (right and left flipper) in 123 wild and 16 captive individuals.  We also analyzed archival data on wild manatees to develop an index of boat-caused body scars to determine lateralization of evasive action.  Wild and captive manatees displayed flipper lateralization at the individual, but not the population level for several behaviors including substrate touches, sculling, and feeding.  In contrast, manatees were lateralized at the population level for boat-scar biases with more manatees showing a left scar bias (45%) versus right (34%) or dorsal (21%).

Behavior is lateralized when it is performed preferentially by one side of the body, and this phenomenon is seen across a wide range of vertebrate taxa. Furthermore the brain and body are contralateral in many animals, meaning that the left brain hemisphere most dominantly controls the right side of the body and vice versa. Lateralized behavior in humans and nonhuman primates reveals a population right-hand bias. Recent studies in primates have also begun to link differences in lateralized behavior to task complexity, and responses to novel versus familiar stimuli. Parallel research on cetaceans is sparse although evidence accrued over the last decade suggests captive dolphins have a preference for swimming counter-clockwise, a right-eye advantage in spatio-cognitive tasks and a right-eye preference for viewing novel objects, although this is the reverse of the general vertebrate pattern. Lateralized behavior was examined in a group of six male bottlenose dolphins ( Tursiops truncatus ) in response to a novel underwater maze, and compared to behavior during a baseline condition (no maze present). Dolphins were significantly more likely to swim counter-clockwise round their pool during both the baseline and maze condition, interpreted as a right eye bias. Swimming rotation was also weaker in dolphins during the maze condition, suggesting that the maze may have disrupted routine circular swimming behavior. There was no clear preference for using the left or right side of the maze, except in two high- using subjects with a strong right preference. Modifications and extensions to the methods are discussed.

The population of the Pacific walrus ( Odobenus rosmarus divergens ) is currently a topic of conservation efforts. Understanding the mating behaviors of a species can be utilized in conservation efforts to preserve the species. Little is known about the behavioral repertoire of Pacific walruses, due to their isolated Arctic habitats, with limited studies previously describing observations of walrus mating behaviors. The aim of the present case study was to observe the mating behaviors of a single captive male Pacific walrus to examine overall frequency of specific mating behaviors in both social and solitary contexts. The subjects, one male and two females, were recorded at Six Flags Discovery Kingdom from November 2013 through January 2014. Only behaviors exhibited by the male walrus directly associated with mating were noted. Grabs were the most frequently observed behavior, and holds were not significantly observed which could contribute to the infrequent successful copulation attempts. Pharyngeal sac inflation, a vocal and visual behavior, was not frequently observed in a sexual context but has been observed in mating contexts in the wild. The male walrus used other sexual outlets such as self-gratification and toy use; however, these behaviors occurred significantly less than sexual encounters with females. There appeared to be a mate preference for the female with tusks, as the male interacted significantly more with the tusked female compared to the non-tusked female who was in estrus. Studying mating behavior in controlled settings such as this can be revealing of the capabilities of the species as a whole. Understanding more about how walruses interact in their environment can be used for future management and breeding strategies.

This study aimed to expand on previous efforts to evaluate the ontogeny of echolocation in Atlantic bottlenose dolphins ( Tursiops truncatus ). Data consisted of echolocation recordings and concurrent behavioral observations taken from one calf in 2000 and from five additional dolphin calves and their mothers in 2002 housed at the U.S. Naval facility in San Diego, CA. A total of 361 echolocation click train samples from calves were recorded weekly over the first 6 months of the calves’ lives. The earliest calf echolocation train was recorded at 22 days postpartum and the number of echolocation attempts from calves increased steadily with age. Calf echolocation trains increased in duration and the number of clicks per train with age while train density (clicks/sec) and interclick interval values remained more consistent. Calves swimming independent of their mothers produced more click trains, especially when multiple calves were present in the social grouping. When considering these results in the context of possible maturation of a calf’s melon physiology, it seems very likely that the first two months of life are critical for the development of echolocation and related behaviors. While the first click train recorded in this sample was approximately 3 weeks of age by two different calves, it is possible that dolphin calves may innately produce functional sonar clicks immediately after birth, which were not captured in the current study.  Future research will need to investigate this possibility using more controlled conditions and a better understanding of the anatomy and physiology of the sonar system of neonates as well as the possible role of the mother in echolocation development.

Chickadees produce many vocalizations, including chick-a-dee calls which they use as a mobbing call in the presence of predators. Previous research has shown that chickadees produce more D notes in their mobbing calls in response to high-threat predators compared to low-threat predators, and may perceive predator and corresponding mobbing vocalizations as similar. We presented black-capped chickadees with playback of high- and low-threat predator calls and conspecific mobbing calls, and non-threat heterospecific and reversed mobbing calls, to examine vocal and movement behavioural responses. Chickadees produced more chick-a-dee calls in response to playback of calls produced by a high-threat predator compared to calls produced by a low-threat predator, and to reversed high-threat mobbing calls compared to normal (i.e., non-reversed) high-threat mobbing calls. Chickadees also vocalized more in response to all playback conditions consisting of conspecific mobbing calls compared to a silent baseline period. The number of D notes that the subjects produced was similar to previous findings; chickadees produced approximately one to three D notes per call in response to low-threat mobbing calls, and produced more calls containing four to five D notes in response to high-threat mobbing calls, although this difference in the number of D notes per call was not significant. The difference in chickadees’ production of tseet calls across playback conditions approached significance as chickadees called more in response to conspecific mobbing calls, but not in response to heterospecific calls. General movement activity decreased in response to playback of conspecific-produced vocalizations, but increased in response to heterospecific-produced vocalizations, suggesting that chickadees may mobilize more in response to predator playback in preparation for a “fight or flight” situation. These results also suggest that chickadees may produce more mobbing calls in response to high-threat predator vocalizations as an attempt to initiate mobbing with conspecifics, while they produce fewer mobbing calls in response to a low-threat predator that a chickadee could outmaneuver.

In odontocetes, synchronous visible displays accompany a small proportion of vocalizations but the function of these multimodal signals is still unclear.  Bouts of stereotyped pulsed calls were collected from two adult female killer whales ( Orcinus orca ) concurrently with behavioral observations and the incidence of two synchronous behaviors, bubble streams and nodding, were measured.  Thirty-four hours of focal individual data were collected in the presence of dependent calves in 1993 and 1994.  Overall, 471 pulsed calls were attributed to the two subjects using synchronous behaviors or independent cues (proximity, localization by ear in air).  Both subjects used the same stereotyped pulsed call repertoire and they ordered calls within bouts similarly, despite dissimilar previous histories.  Both nodding and bubble streams were disproportionately associated with a subset of stereotyped pulsed calls, but the subset was different for the two behaviors.  General Linear Model analysis was used to predict the relative odds that the subjects would be swimming with calves given call class and attribution cue.  Bubbling was not associated with significant odds that a subject would be swimming with one or more calves, but nodding was associated with significantly higher odds and resting calls with lower odds.  Given these observations, synchronous behaviors in the presence of calves could function in one or more of the following ways: altering the signal value of calls, emphasizing an aspect of the social context, and facilitating learning.  All are possibilities at the interface between cognition and communication that would have interested Stan Kuczaj.

We have long observed dolphins producing recognizable sounds—bursts of pulses with sweeping peak frequencies—at prey capture.  We call this the victory squeal.  When dolphins hunt fish, there are three sequential sounds:  sonar clicks, terminal buzz, and the victory squeal. When dolphins find a fish with sonar clicks, but reject the fish during the terminal buzz phase, they omit or truncate the victory squeal.  We also observe dolphins producing the victory squeal after a trainer’s bridge, which serves as secondary reinforcement that bridges the time gap between the dolphin’s performance and delivery of food reinforcement.   It signals the dolphins that they responded correctly and that reward is forthcoming.  Before training, the victory squeal came after fish capture, but with successive trials, there was a forward shift in the victory squeal to come about 200 ms after the bridge.  The victory squeal immediately following the bridge suggests the dolphin expects reward. Although there are no direct studies of dopamine release in cetaceans, early brain stimulation studies demonstrated consistent timing that may link the victory squeal with brain dopamine release. In the current study, we asked if dolphins might produce the victory squeal after task completion, but without the trainer’s bridge.  Dolphins carried cameras, recording video and sound, while performing tasks in the open ocean, away from trainers, during swimmer/mine marking and retrieving.  In each task, we observed the victory squeal immediately upon completion of task components.  We suggest that the victory squeal signals that these experienced dolphins recognized their success.

Asian small-clawed otters ( Aonyx cinerea ) demonstrate remarkable hand dexterity when gathering and consuming prey, but little is known about their ability to use objects as tools. The present study used a tool choice paradigm in which six Asian small-clawed otters were tested individually and presented with two identical hook-shaped tools. For each trial, only one tool was positioned such that pulling it allowed an otter to obtain food. Pulling the other hook resulted in the correct hook being moved out of reach, necessitating selection of the correct tool as its first choice The two males performed above chance levels, but the four females did not. The females’ poor overall performance may have reflected their initial inability to understand the tool choice task. Two of the females’ performances improved by 20% over the course of the trials, and another female showed 5% improvement over time. In addition, some incorrect responses appeared to be due to the development of a side preference, rather than to the configuration of the apparatus. Four of the otters exhibited a significant side bias toward the left, but there were individual differences in how these preferences presented in each otter. For all otters, latency to approach and make a choice on the tool-use task decreased over time, regardless of success. Although otters do appear capable of learning which tool should be used in a forced choice comparison such as the one used here, other factors appear to influence the choices individual otters make.

The current study tested six Asian elephants ( Elephas maximus ) on a means-end behavioral task of pulling a support to retrieve a distant object; a systematic replication of the Irie-Sugimoto et al. (2008) study. The paradigm was somewhat modified from the original research to accommodate a protected contact setting, reduce the total number of trials, and one condition was excluded. Each elephant was tested on three conditions of increasing difficulty. Specifically, subjects were asked to select from a choice of two trays where one intact tray was baited with a highly-valued produce item and the other was A) empty; B) baited adjacent to the tray; and C) baited on the far side of a break in the tray. Results indicated that the elephants met or exceeded the criteria established for conditions A and B, but performed at chance levels on condition C. These data are contrasted with those of the original study where one elephant met criteria for all three conditions. We discuss potentially relevant variables affecting performance including differences in visual access to the trays, motivation levels, and training style.

When analyzing animal behavior, it is important to consider the influence of learning principles. The create response of bottlenose dolphins, elicited by a discriminative stimulus, or an SD (visual cue presented to an animal by a trainer), has been described as an elective, often novel response based on arbitrary preferences of individual animals. The goal of this study was to identify the potential influence of reinforcement theory, response class, and primacy and recency on the create responses of bottlenose dolphins. Three, male subjects with an established mastery of the create paradigm, identified in this study as a non-specific, non-repeat contingency, were assessed over the course of two months while under stimulus control ( pre-assessment ), followed by evaluations of the create response ( create assessment ) using a double-blind sampling model. During the pre- and create assessments, each response was quantified regarding response class, frequency of request, and reinforcement type, frequency, and magnitude. When presented with the create SD, the dolphins elected to produce behaviors predominantly associated with the more recent training context (create assessment) versus behaviors associated with training that occurred months prior (pre-assessment), which may demonstrate the effects of primacy versus recency. Additionally, the create trials were associated with reinforcement on a high frequency and magnitude, fixed, low ratio schedule, and the subjects most often performed the behaviors associated with the greatest magnitude of primary reinforcement, which highlights the influence of reinforcement and the law of effects. Lastly, two subjects never responded with high energy behaviors in the create contingency, and one subject performed significantly more low and medium energy responses when compared to high energy behaviors, capturing the effects of a response class characterized by intensity under a fixed ratio reinforcement schedule. Thus, the create response was not represented by arbitrary elective preferences but rather, partially driven by the learning theories examined.

I measured the amount of time mice spent in a compartment with either a mirror or an opaque screen and found that mice stayed longer in the compartment with the mirror. This finding suggests that mice prefer mirrors. They also showed a preference for the mirror over unfamiliar live mice but did not show a differential preference for the mirror over a familiar live mouse (cage mate). Restraint stress caused hyperthermia (known as stress-induced hyperthermia) in the mice. When cage mates received the restraint stress together, the hyperthermia was reduced. Placement of mirrors instead of the cage mates also showed stress-reducing effects, while restraint with unfamiliar mice did not reduce the hyperthermia. These results suggest that mirrors have familiar cage mate-like social effects in mice.

Previous research has documented that cetaceans can discriminate between humans, but the process used to categorize humans still remains unclear.  The goal of the present study was to replicate and extend previous work on the discrimination between familiar and unfamiliar humans by three species of cetaceans.  The current study manipulated the familiarity and activity level of humans presented to 12 belugas ( Delphinapterus leucas ) housed between two facilities, five bottlenose dolphins ( Tursiops truncatus ), and six Pacific white-sided dolphins ( Lagenorhynchus obliquidens ) during free-swim conditions.  Two measures of discrimination were coded from video recordings of each trial: lateralized visual processing and gaze duration.  No clear lateralization effects emerged at the species level, primarily due to extensive individual variability.  The results also indicated that activity level influenced gaze durations across species, and for some individuals, the interaction between human familiarity and activity level influenced gaze durations and eye preferences.  Unexpectedly, bottlenose dolphins had longer gaze durations for familiar humans whereas belugas and Pacific white-sided dolphins had longer gaze durations for unfamiliar humans.  All three groups displayed longer gaze durations for active humans as compared to neutral humans, and belugas and bottlenose dolphins had significantly longer gaze durations than Pacific white-sided dolphins.  These results indicate that the cetaceans can discriminate between unfamiliar and familiar humans and preferred active humans.  However, discrimination of humans via lateralized visual processing did not appear at the group level, but rather at the individual level which countered previous research.  This study is discussed within the contexts of attention and individual differences across animals of different species.

We investigated theoretical accounts of spatial overshadowing using a landmark-based spatial-search task in a touchscreen preparation with pigeons. Pigeons first learned to find a hidden target on a screen using a compound of two visual cues as landmarks. Landmark A was proximal to the target while landmark X was distal to the target. Experiment 1 replicated our prior spatial overshadowing effect whereby landmark A overshadowed the development of spatial control by X. Spatial control by X was also poorer than by landmark Y which had been paired with the target alone but with the same absolute distance to the target as X had. Thus, the poor spatial control by X was not merely due to the greater X-target distance (relative to the A-target distance). Experiments 2a and 2b failed to find recovery from spatial overshadowing of X through either post-training extinction or counterconditioning of overshadowing landmark A, respectively. We interpret our results as being consistent with acquisition-focused models of elementary associative learning, but not with performance-focused models.

Comparative research has proven to be a fruitful field of study on the ontogenetic and phylogenetic evolution of language, and on the cognitive capacities unique to humans or shared with other animals. The degree of continuity between components of human language and non-human animal communication systems, as well as the existence of a core factor of language, are polemic subjects at present. In this article, we offer an overview of the research on animal communication, comparing the resulting data with the current knowledge on human language development. We try to summarize what is currently known about “language abilities” in multiple animals, and compare those facts to what is known about human language. The aim of the article is to provide an introduction to this particular topic, presenting the different sides of the arguments when possible. A special reference is made to the question of syntactic recursion as the main component of language, allegedly absent among non-human animals. We conclude that the current state of knowledge supports the existence of a certain degree of continuity between different aspects of animal communication and human language, including the syntactic domain.

During play fighting, rats attack and defend the nape and during the course of this competitive interaction, they may adopt a configuration in which one animal stands over its supine partner (i.e., pin). Because the pin configuration is typically frequent and relatively easy to identify, it has been widely used as a marker to detect the effects of experimental treatments. In the present study, the frequency of pinning during standardized, 10min trials in three strains of rats, Long Evans hooded (LE), Sprague-Dawley (SD) and wild (WWCPS), was compared. LE and SD had higher rates than WWCPS rats (#/min: 6.5, 5.5, 1.5, respectively). When adjusted for strain differences in the frequency of attacks, SD as well as WWCPS rats had lower rates of pinning compared to LE rats. Both SD and WWCPS rats were less likely to use tactics of defense that promote pinning. Moreover, while the majority of the pins achieved in LE rats arose from the defender actively rolling over onto its back, the majority of pins in WWCPS rats arose because one partner pushed the other onto its back. SD rats were intermediate in this regard. Finally, once they do adopt the pin configuration, SD rats are less likely to remain supine than LE and WWCPS rats. That is, both SD and WWCPS rats have significantly fewer pins than LE rats, but a different combination of factors account for this. These data highlight the need to use a battery of measures for ascertaining the effects of experimental manipulations on play. Some suggested guidelines are provided.

When following a free-living ring-tailed coati Nasua nasua group behind a tourist complex on Ilha do Campeche (an island in the State of Santa Catarina, Brazil), we observed them rubbing laundry and cleaning substances onto their bodies. In order to describe this anointing behavior, spontaneous and induced anointing sessions were studied over two visits to the island. The induced events were prompted by offering bar soap in five experimental sessions. In all experimental sessions, one to three animals of both sexes performed soap-anointing behavior. It was most commonly self-directed (self-anointing), but also sometimes applied onto others (allo-anointing), or sometimes performed collectively and in close proximity to other group members. The genital area was the most often rubbed location, followed by the tail. We suggest that ring-tailed coatis may be deterring ectoparasites when applying soap to their integument. Ring-tailed coatis are known for anointing their fur with resin or arthropods, but this is the first description of the use of soap. Close contact with humans and easy access to soap inadvertently left outside may have been responsible for this arbitrary innovation. Because this behavior has persisted for more than 10 years and is practiced by different age groups, we suggest that this behavior is being socially transmitted across generations within the group from older to younger individuals.

This study compared three captive orangutans and a group of 5-10 year-old children in their ability to use stick tools to solve a series of mazes in a puzzle box, including three puzzles that required semantic prospection. The puzzle box had seven levels and moveable plastic inserts that created three easy, three intermediate, and three difficult maze configurations. Three wood and three plastic stick tools were presented with each maze. All 26 children immediately solved the easy and intermediate mazes. Seventy-nine percent of the children solved the difficult mazes on their first attempt, and nearly all the children solved the difficult mazes on the second attempt, which suggested a majority of children engaged in effective planning. Girls took significantly longer to solve the intermediate mazes while boys took significantly longer to solve the difficult mazes. Two of three orangutans also successfully avoided the dead ends in the difficult mazes and consistently used stick tools to move peanuts to the goal slots, and took longer to solve the intermediate or difficult mazes. Both the children and orangutans preferred to use plastic tools, although both tool types were functional. These results suggest many similarities between orangutans and children’s abilities to use tools in a puzzle box task that requires planning to avoid dead ends.

Social species need conflict-resolution mechanisms to maintain group cohesion and diminish aggression. Reconciliation (affiliative contact between opponents) and consolation (affiliative contact between the victim and an uninvolved third party) have been postulated for this function in various species. The purpose of this work is to study post-conflict affiliative behaviors toward humans in domestic dogs. This study has looked into post-conflict affiliative behaviors in domestic dogs toward their owners. To this end, a conflict situation was created where the animal was scolded by one of the owners for “stealing” human food. Behaviors were recorded along a period of 3 min and 30 s before and after the scolding. Results show that dogs exhibit affiliative behaviors (significant increase in closeness, gazing, and tail wagging) as well as appeasement behaviors (averting eyes, low tail carriage, lowered ears, lip licking, and crouching) toward the owner that scolded them (reconciliation). In other words, this is the first work that presents reconciliation in dogs in a conflict situation with humans. It discusses the importance of this phenomenon in the dog-human bond.

The aim of this study was to determine whether California sea lions ( Zalophus californianus ) are capable of using subtle human gestural cues in a series of object choice tests. Four sea lions, housed at  Parc Astérix Dolphinarium (Plailly, France), were tested using three gestural cues: hip-based finger points, chest-based finger points and eye glances (no head movement involved). Above chance performance was found in response to these cues in 4/4, 2/4, and 1/4 sea lions, respectively, suggesting that the sea lions were able to generalize their response from conspicuous pointing gestures to subtle finger pointing, as well as to eye glance cue for one subject. Discrepancies in accuracy rates between the cues confirmed however that conspicuousness of the pointing gesture is determinant for the ability of the sea lions to exploit it efficiently. These findings reinforce the hypothesis that human-socialization of undomesticated species can lead some individuals to develop an affinity for interpreting very subtle human gestural cues.

The sperm whale Physeter macrocephalus (Linnaeus, 1758) is the largest toothed whales and possesses the highest absolute values for brain weight on the planet (together with the killer whale Orcinus orca ). Former calculations of the encephalization quotient (EQ), which is used to compare brain size of different mammalian species, showed that the sperm whale brain is smaller than expected for its body mass. However, the data reported in the literature and formerly used to calculate the sperm whale EQ suffered from a potential bias due to the tendency to measure mostly larger males of this extreme sexually dimorphic species. Accordingly, we found that the brains of female sperm whales are close to the absolute weight range of the males, but, given the much lower body mass of females, their EQ results more than double of what reported before for the whole species, and is thus nearly into the primate range (female EQ = 1.28, male EQ = 0.56). This sexual dimorphism is unique among mammals. Female sperm whales live in large families in which social interactions and inter-individual communication are essential, while adult males live solitarily. Thus the particular sex-specific behavior of SWs may have led to a maternally-driven social evolution, and eventually contributed to achieve female EQ values (but not male EQs) among the highest ever calculated for mammals with respect to their large body mass.