The genesis of giants: behavioural ontogeny of male northern elephant seals

Acoustic behavior of humpback whale calves on the feeding ground: Comparisons across age and implications for vocal development

Studying sound production at different developmental stages can provide insight into the processes involved in vocal ontogeny. Humpback whales (Megaptera novaeangliae) are a known vocal learning species, but their vocal development is poorly understood. While studies of humpback whale calves in the early stages of their lives on the breeding grounds and migration routes exist, little is known about the behavior of these immature, dependent animals by the time they reach the feeding grounds. In this study, we used data from groups of North Atlantic humpback whales in the Gulf of Maine in which all members were simultaneously carrying acoustic recording tags attached with suction cups. This allowed for assignment of likely caller identity using the relative received levels of calls across tags. We analyzed data from 3 calves and 13 adults. There were high levels of call rate variation among these individuals and the results represent preliminary descriptions of calf behavior. Our analysis suggests that, in contrast to the breeding grounds or on migration, calves are no longer acoustically cryptic by the time they reach their feeding ground. Calves and adults both produce calls in bouts, but there may be some differences in bout parameters like inter-call intervals and bout durations. Calves were able to produce most of the adult vocal repertoire but used different call types in different proportions. Finally, we found evidence of immature call types in calves, akin to protosyllables used in babbling in other mammals, including humans. Overall, the sound production of humpback whale calves on the feeding grounds appears to be already similar to that of adults, but with differences in line with ontogenetic changes observed in other vocal learning species.

Social regulation of reproduction: control or signal?

Traditionally, dominant breeders have been considered to be able to control the reproduction of other individuals in multimember groups that have high variance in reproductive success/reproductive skew (e.g., forced sterility/coercion of conspecifics in eusocial animals; sex-change suppression in sequential hermaphrodites). These actions are typically presented as active impositions by reproductively dominant individuals. However, how can individuals regulate the reproductive physiology of others? Alternatively, all contestants make reproductive decisions, and less successful individuals self-downregulate reproduction in the presence of dominant breeders. Shifting perspective from a top-down manipulation to a broader view, which includes all contenders, and using a multitaxon approach, we propose a unifying framework for the resolution of reproductive skew conflicts based on signalling rather than control, along a continuum of levels of strategic regulation of reproduction.

Equivalence classification, learning by exclusion, and long-term memory in pinnipeds: cognitive mechanisms demonstrated through research with subjects under human care and in the field

Comparative cognition, as an interdisciplinary field, should utilize a holistic approach for studying cognitive mechanisms. We suggest that research with species of interest should employ both work with animals under human care and in the field. This complimentary approach allows for a better understanding of functional cognitive mechanisms themselves (i.e., comparative cognition regarding processes), and how these skill sets can relate to a particular species' ecological niche. We suggest that research evidence for equivalence classification, learning by exclusion, and long-term memory in pinnipeds can provide a foundation for discussion and implementation of a two-pronged methodological approach utilizing 'lab' and field' work. First, we describe evidence from research with pinnipeds under human care supporting each of these cognitive abilities, then follow this with evidence for implications of these mechanisms from complimentary field research. Lastly, we provide a brief discussion of implementation of a purposeful and two-pronged research approach as an understanding of pinnipeds' high levels of cognitive flexibility may underlie their success for navigating the ever-changing, and often human-altered, natural environment.

Phenotype constrains the vocal tract in the most dimorphic mammal, the southern elephant seal

The study of mammal acoustic communication was revolutionized by the application of the source-filter theory, originally developed for human speech. The theory states that the vocal tract is constrained by body anatomy and, therefore, creates a structural link between phenotype and acoustic formants, providing a basis for honest signalling. The phenotype-formants link was validated in many species, but the phenotype-vocal tract link was rarely assessed. We used 2D videogrammetry to estimate the vocal tract length of wild southern elephant seal males (Mirounga leonina Linnaeus, 1758) during their normal vocalization behaviour. We showed that: 1) the vocal tract can be measured non-invasively in a wild large mammal; 2) the vocal tract depends on the structural phenotype (age, body length, and skull size); 3) the nasal tract is more related to the structural phenotype than the buccal tract; 4) the dependence on size, and body length in particular, is stronger than the dependence on age. All together, the phenotypic constraint on vocal tract provides the anatomical basis for honest signalling in elephant seals.

Territorial behaviour and conflict management in a semi-social cichlid fish, Neolamprologus caudopunctatus

Conflict solving strategies can prevent fights from escalating and reduce the costs of aggressive encounters. Having the capacity to efficiently assess an opponent’s fighting abilities before fully committing to a fight is a useful social skill. Here, we conducted two experiments to investigate how a colony living, cichlid species, Neolamprologus caudopunctatus, changes its aggressive behaviour when faced with familiar vs unfamiliar opponents. First, we staged size matched, same-sex, dyadic resource contests and found that fights were always of low-intensity with neither familiarity nor sex influencing how quickly the conflict ended. Second, we explored the dual defence of mated territorial pairs together defending their territory boundaries against other pairs, either familiar or unfamiliar ones, and discovered that fights between two pairs were more vigorous, and that unfamiliar neighbouring pairs were attacked significantly more often than familiar pairs. We also observed that dark bars sometimes appeared on the sides of contestant’s bodies, and that these bars were far more common in winners than in losers, suggesting that these might be visual signals of dominance. However, conflicts where contestants displayed bars were of longer duration than those without. Taken together, our results further advance our understanding of territoriality and conflict resolution strategies and set the stage for future studies focusing on how animals manage to co-exist in closely aggregated breeding territories and to form colonies.

Comparative examination of pinniped craniofacial musculature and its role in aquatic feeding

Secondarily aquatic tetrapods have many unique morphologic adaptations for life underwater compared with their terrestrial counterparts. A key innovation during the land-to-water transition was feeding. Pinnipeds, a clade of air-breathing marine carnivorans that include seals, sea lions, and walruses, have evolved multiple strategies for aquatic feeding (e.g., biting, suction feeding). Numerous studies have examined the pinniped skull and dental specializations for underwater feeding. However, data on the pinniped craniofacial musculoskeletal system and its role in aquatic feeding are rare. Therefore, the objectives of this study were to conduct a comparative analysis of pinniped craniofacial musculature and examine the function of the craniofacial musculature in facilitating different aquatic feeding strategies. We performed anatomic dissections of 35 specimens across six pinniped species. We describe 32 pinniped craniofacial muscles-including facial expression, mastication, tongue, hyoid, and soft palate muscles. Pinnipeds broadly conform to mammalian patterns of craniofacial muscle morphology. Pinnipeds also exhibit unique musculoskeletal morphologies-in muscle position, attachments, and size-that likely represent adaptations for different aquatic feeding strategies. Suction feeding specialists (bearded and northern elephant seals) have a significantly larger masseter than biters. Further, northern elephant seals have large and unique tongue and hyoid muscle morphologies compared with other pinniped species. These morphologic changes likely help generate and withstand suction pressures necessary for drawing water and prey into the mouth. In contrast, biting taxa (California sea lions, harbor, ringed, and Weddell seals) do not exhibit consistent craniofacial musculoskeletal adaptations that differentiate them from suction feeders. Generally, we discover that all pinnipeds have well-developed and robust craniofacial musculature. Pinniped head musculature plays an important role in facilitating different aquatic feeding strategies. Together with behavioral and kinematic studies, our data suggest that pinnipeds' robust facial morphology allows animals to switch feeding strategies depending on the environmental context-a critical skill in a heterogeneous and rapidly changing underwater habitat.

Feel the beat: cape fur seal males encode their arousal state in their bark rate

The cape fur seal is one of the most colonial mammal species in the world. Breeding colonies are composed of harems held by mature males (older than 10 years) with up to 30 females and their pups, while roaming subadult males (younger and socially immature) are kept away from bulls' territories. As in other pinnipeds, cape fur seals are highly vocal and use acoustic signals in all their social interactions. Males produce barks-short vocalizations always produced in sequences-for territorial defense, mating behaviors, and agonistic interactions. These calls convey information about the sex, age class, and individual identity. This study investigated whether motivational cues such as the arousal state can be encoded in territorial males' barks and whether these cues are decoded by listening sub-adult males. The rate (number of calls per unit of time) and fundamental frequency of barks were found to significantly increase during high arousal state interactions (i.e., male-male confrontation) compared to spontaneous barks. Playback experiments revealed that subadult males responded with a higher level of vigilance when territorial males' barks had a faster bark rate. This mechanism of decoding the bulls' arousal state from barks will likely constitute an advantage for both bulls and the subadult males, by avoiding or reducing physical conflicts, and thereby reducing energy expenditure and the risk of injury. This study is the first experimental evidence of cape fur seals' using vocal rhythmic patterns to modulate their social interactions.

Maternal responses to pup calls in a high-cost lactation species

Bonding between mothers and their young is fundamental to mammalian reproductive behaviour and individual fitness. In social systems where the risk of confusing filial and non-filial offspring is high, mothers should demonstrate early, strong and consistent responses to their kin throughout the period of offspring dependence, irrespective of maternal traits. We tested this hypothesis through playback experiments in the northern elephant seal Mirounga angustirostris, a phocid species that breeds in high-density colonies. We found that mothers recognized their offspring throughout lactation and as early as 1-2 days after parturition. Measures of experience (age) and temperament (aggressivity) did not predict their response strength to filial playback treatments, nor did pup age or sex. Some mothers showed great consistency in behavioural responses throughout the lactation period, while others were less predictable. The strength of a female's response did not influence her pup's weaning weight; however, more consistent females weaned pups of higher mass. This is a rare demonstration of individual recognition among phocid mothers and their offspring, and suggests that consistency in maternal responsiveness may be an important social factor influencing the pup's growth and survival.