Otariid Pinnipeds Research Articles

Whiskers as a novel tissue for tracking reproductive and stress-related hormones in North Pacific otariid pinnipeds.

Keratinized tissues, including whiskers, are ideal for acquiring a record of physiological parameters. Most tissues provide a snapshot of physiological status; however, whiskers may support longitudinal sampling for reproductive and stress-related hormones, if hormones are incorporated as whiskers grow and concentrations change with physiological state. Whiskers from female Steller sea lions (Eumetopias jubatus) and northern fur seals (Callorhinus ursinus) were serially sectioned and pulverized and steroid hormones were extracted. Standard methods were used to validate enzyme immunoassay kits for cortisol, progesterone, 17β-estradiol and testosterone. All hormones were measurable in whisker segments from both species with progesterone concentrations showing cyclical patterns, which appear to signify previous pregnancies or luteal phases. Yearly progesterone concentrations were greater in years a pup was produced compared with years when no pup was observed. Free-ranging female Steller sea lions had reproductive rates between 0 and 1.0 (0.53 ± 0.33, n = 12) using a yearly progesterone concentration of 30pg/mg or greater to classify a reproductive year as producing a pup and below 30pg/mg as non-reproductive. Cortisol concentrations were greater near the root and rapidly declined, lacking any obvious patterns, throughout the rest of the whisker. Progesterone and testosterone concentrations were able to help determine sex of unknown individuals. Immunohistochemistry revealed that steroid hormones most likely do not leach out of whiskers based on the deposition patterns of progesterone and cortisol being present throughout the whisker length. Overall, measuring steroid hormones in whiskers can reveal individual reproductive histories over multiple years in sea lions and fur seals. Cyclical patterns of δ15N were useful for identifying periods of up to ~10years of growth within whiskers, and measuring both stable isotopes and hormones may be useful for differentiating periods of active gestation from diapause and potentially track multi-year reproductive histories of female otariids.

First record of tuberculosis lesions in zooarchaeological samples of otariid pinnipeds. New aspects of the pre-european origin of human tuberculosis in South America and dissemination mechanisms of Mycobacterium pinnipedii in the Southern Hemisphere

In 2003 Mycobacterium pinnipedii was described as responsible for producing tuberculosis (TB) in living otariid pinnipeds from Argentina and Australia. It is the only member of marine origin within the Mycobacterium tuberculosis Complex (MTBC), which also affects other domestic and wild mammals, and humans. Based on several pre-Columbian records of human tuberculosis in South America, in 2010-2011 a new hypothesis about the origin of this zoonosis through otariid pinnipeds arose. In 2014, this hypothesis was confirmed based on the study of ancient DNA from three mummies (700-1,000 years BP) of the Chiribaya culture (Peru). Since there were no records of TB bone lesions in zooarchaeological samples of otariid pinnipeds from South America and the rest of the world, our study aimed at examining zooarchaeological samples of pinnipeds from coastal sites of the Beagle Channel (Tierra del Fuego, Argentina), being the oldest Túnel I (6,400-4,300 years BP). A total of 4,138 vertebrae were analyzed, of which 0.46 % showed lesions compatible with TB. In addition, we propose a new hypothesis on possible mechanisms of Mycobacterium pinnipedii dissemination that would explain the transmission routes to the different otariid pinniped species of the Southern Hemisphere. Mycobacterium pinnipedii is one of the most aggressive mycobacteria of the MTBC and of high risk for humans.

Filarial infections in California sea lions vary spatially within the Gulf of California, Mexico.

At least two species of filarial worms, Dirofilaria immitis and Acanthocheilonema (Dipetalonema) odendhali, infect otariid pinnipeds, including the California sea lion (Zalophus californianus). To date, evidence of infection in sea lions has come from dead or captive animals, and little is known about filariasis in free-living populations. We sampled 45 California sea lion adults and 197 pups captured at 12 rookeries from different ecological regions within the Gulf of California and detected and quantified D. immitis and A. odendhali microfilariae in blood smears. We investigated differences in prevalence and parasite load (intensity of infection) among ecological regions. Microfilariae were detected in the blood of 35 of the 45 (77.78%) adult females and in 1 of the 197 (0.51%) pups examined. The average burden of A. odendhali per microlitre of blood was nearly twice that of D. immitis. Prevalence and intensity of infection differed significantly among regions, being highest for colonies within the northern and northcentral regions and lowest in the southern region. Dirofilaria immitis and A. odendhali infections displayed a similar spatial pattern of prevalence. Colony density inversely predicted the prevalence of microfilariae. Based on the clinical parameters typically associated with filarial infections in carnivores and physical examinations, none of the sea lions appeared to have evidence of disease. This is a first approximation to investigate the prevalence of microfilaria infections in free-ranging California sea lions and to explore their relevance to population health.

Effects of inhalational anesthesia on blood gases and pH in California sea lions

AbstractDespite the widespread use of inhalational anesthesia with spontaneous ventilation in many studies of otariid pinnipeds, the effects and risks of anesthetic‐induced respiratory depression on blood gas and pH regulation are unknown in these animals. During such anesthesia in California sea lions (Zalophus californianus), blood gas and pH analyses of opportunistic blood samples revealed routine hypercarbia (highest PCO2 = 128 mm Hg [17.1 kPa]), but adequate arterial oxygenation (PO2 > 100 mm Hg [13.3 kPa] on 100% inspiratory oxygen). Respiratory acidosis (lowest pH = 7.05) was limited by the increased buffering capacity of sea lion blood. A markedly widened alveolar‐to‐arterial PO2 difference was indicative of atelectasis and ventilation‐perfusion mismatch in the lung secondary to hypoventilation during anesthesia. Despite the generally safe track record of this anesthetic regimen in the past, these findings demonstrate the value of high inspiratory O2 concentrations and the necessity of constant vigilance and caution. In order to avoid hypoxemia, we emphasize the importance of late extubation or at least maintenance of mask ventilation on O2 until anesthetic‐induced respiratory depression is resolved. In this regard, whether for planned or emergency application, we also describe a simple, easily employed intubation technique with the Casper “zalophoscope” for sea lions.

Marine mammal hearing and the effects of noise

Marine mammals use sound for critical life functions through passive listening and, in some species, active sonar. Major recent advances have been made in understanding what sounds they make, how they hear, and how noise affects their hearing, behavior, and physiology. While data are lacking for whole groups, notably baleen whales, direct hearing measurements are available for most taxa. Some species, such as odontocete cetaceans, hear extremely broad frequency ranges, well into ultrasonic biosonar frequencies. Pinnipeds have more limited frequency ranges but are adapted for aerial and underwater hearing. Indirect data suggest whales use and are more sensitive to low frequencies. For most species, quite high received sound levels are required to temporarily or permanently affect hearing. Direct experimental measurements of behavioral responses have revealed strong species and context dependencies, with factors such as proximity, novelty, and spatial relationships influencing response probability as well as relative loudness. For active surveillance sonar systems operating in the 30-200 kHz range, whales and otariid pinnipeds are of much less concern in terms of audibility and potential behavioral disturbance than phocid pinniped and odontocete cetaceans, notably porpoises. Hearing loss or other injuries from such systems are quite unlikely in almost all practical scenarios.

Low Spatial Genetic Differentiation Associated with Rapid Recolonization in the New Zealand Fur Seal Arctocephalus forsteri.

Population declines resulting from anthropogenic activities are of major consequence for the long-term survival of species because the resulting loss of genetic diversity can lead to extinction via the effects of inbreeding depression, fixation of deleterious mutations, and loss of adaptive potential. Otariid pinnipeds have been exploited commercially to near extinction with some species showing higher demographic resilience and recolonization potential than others. The New Zealand fur seal (NZFS) was heavily impacted by commercial sealing between the late 18th and early 19th centuries, but has recolonized its former range in southern Australia. The species has also recolonized its former range in New Zealand, yet little is known about the pattern of recolonization. Here, we first used 11 microsatellite markers (n = 383) to investigate the contemporary population structure and dispersal patterns in the NZFS (Arctocephalus forsteri). Secondly, we model postsealing recolonization with 1 additional mtDNA cytochrome b (n = 261) marker. Our data identified 3 genetic clusters: an Australian, a subantarctic, and a New Zealand one, with a weak and probably transient subdivision within the latter cluster. Demographic history scenarios supported a recolonization of the New Zealand coastline from remote west coast colonies, which is consistent with contemporary gene flow and with the species' high resilience. The present data suggest the management of distinct genetic units in the North and South of New Zealand along a genetic gradient. Assignment of individuals to their colony of origin was limited (32%) with the present data indicating the current microsatellite markers are unlikely sufficient to assign fisheries bycatch of NZFSs to colonies.

Enamel ultrastructure of fossil and modern pinnipeds: evaluating hypotheses of feeding adaptations in the extinct walrus Pelagiarctos.

This study aimed to assess the enamel ultrastructure in modern otariid pinnipeds and in the extinct walrus Pelagiarctos. Teeth of the New Zealand fur seal (Arctocephalus forsteri), sea lion (Phocarctos hookeri), and fossil walrus Pelagiarctos thomasi were embedded, sectioned, etched, and analyzed via scanning electron microscopy. The enamel of NZ otariids and Pelagiarctos was prismatic and moderately thick, measuring 150-450μm on average. It consisted of transversely oriented Hunter-Schreger bands (HSBs) from the enamel-dentine junction (EDJ) to near the outer surface, where it faded into prismless enamel less than 10μm thick. The width of HSB was variable and averaged between 6 and 10 prisms, and they presented an undulating course both in longitudinal and cross sections. The overall organization of the enamel was similar in all teeth sampled; however, the enamel was thicker in canines and postcanines than in incisors. The crowns of all teeth sampled were uniformly covered by enamel; however, the grooved incisors lacked an enamel cover on the posterior side of the buccal face. Large tubules and tuft-like structures were seen at the EDJ. HSB enamel as well as tubules and tufts at the EDJ suggest increased occlusal loads during feeding, a biomechanical adaptation to avoid enamel cracking and failure. Despite overall simplification in tooth morphology and reduced mastication, the fossil and modern pinnipeds analyzed here retained the complex undulating HSB structure of other fossils and living Carnivora, while other marine mammals such as cetaceans developed simplified radial enamel.

Morphological variability and molecular identification of Uncinaria spp. (Nematoda: Ancylostomatidae) from grizzly and black bears: new species or phenotypic plasticity?

The hookworms Uncinaria rauschi Olsen, 1968 and Uncinaria yukonensis ( Wolfgang, 1956 ) were formally described from grizzly ( Ursus arctos horribilis) and black bears ( Ursus americanus ) of North America. We analyzed the intestinal tracts of 4 grizzly and 9 black bears from Alberta and British Columbia, Canada and isolated Uncinaria specimens with anatomical traits never previously documented. We applied morphological and molecular techniques to investigate the taxonomy and phylogeny of these Uncinaria parasites. The morphological analysis supported polymorphism at the vulvar region for females of both U. rauschi and U. yukonensis. The hypothesis of morphological plasticity for U. rauschi and U. yukonensis was confirmed by genetic analysis of the internal transcribed spacers (ITS-1 and ITS-2) of the nuclear ribosomal DNA. Two distinct genotypes were identified, differing at 5 fixed sites for ITS-1 (432 base pairs [bp]) and 7 for ITS-2 (274 bp). Morphometric data for U. rauschi revealed host-related size differences: adult U. rauschi were significantly larger in black bears than in grizzly bears. Interpretation of these results, considering the historical biogeography of North American bears, suggests a relatively recent host-switching event of U. rauschi from black bears to grizzly bears which likely occurred after the end of the Wisconsin glaciation. Phylogenetic maximum parsimony (MP) and maximum likelihood (ML) analyses of the concatenated ITS-1 and ITS-2 datasets strongly supported monophyly of U. rauschi and U. yukonensis and their close relationship with Uncinaria stenocephala (Railliet, 1884), the latter a parasite primarily of canids and felids. Relationships among species within this group, although resolved by ML, were unsupported by MP and bootstrap resampling. The clade of U. rauschi, U. yukonensis, and U. stenocephala was recovered as sister to the clade represented by Uncinaria spp. from otariid pinnipeds. These results support the absence of strict host-parasite co-phylogeny for Uncinaria spp. and their carnivore hosts. Phylogenetic relationships among Uncinaria spp. provided a framework to develop the hypothesis of similar transmission patterns for the closely related U. rauschi, U. yukonensis, and U. stenocephala.

Exploring the Consequences of Species Interactions Through the Assembly and Disassembly of Food Webs: A Pacific-Atlantic Comparison

Species interactions are best seen and understood through perturbations of the species themselves. These perturbations can be accomplished on small scales through controlled manipulations, but they sometimes happen on larger scales through more fortuitous contrasts over space or time. We used similarities and differences between the North Atlantic and Pacific oceans in an effort to improve understanding of the key dynamic processes that regulate the abundance and distribution of species. The North Atlantic biota is impoverished; many of its recent elements are derived from the North Pacific through transarctic biotic interchange. Sea otters, absent from the North Atlantic, drive a trophic cascade in the North Pacific by limiting herbivorous sea urchins, in turn allowing kelp and other macroalgae to flourish. Otariid pinnipeds are also absent from the North Atlantic. We suggest that the absence of sea otters as competitors and of otariid pinnipeds as predators favored the evolution of large body size in Atlantic cod. Cod therefore assumed the ecological role of sea otters in the North Atlantic, creating a functionally analogous ecosystem in which the collapse of cod has led to a kelp-to-urchin phase shift. The decline of cod and other finfish has also been instrumental in driving neritic/oceanic habitats in the North Atlantic from domination by fishes to domination by shrimps and other crustaceans. We hypothesize that the recent collapse of North Pacific pinnipeds helped drive that ecosystem in the opposite direction-from domination by shrimp and other crustaceans before the collapse to domination by fish today.

Monaural and binaural aerial hearing thresholds in a California sea lion (Zalophus californianus)

Though studies have focused on some aspects of binaural hearing in otariid pinnipeds, none have directly measured monaural hearing thresholds. Recent auditory evoked potential (AEP) testing with a young male California sea lion suggested that hearing threshold differences potentially existed between the subject's two ears. To further investigate these findings, aerial psychophysical audiograms were collected for this sea lion. Using headphones to deliver stimuli, monaural and binaural thresholds were measured at seven frequencies from 0.5 to 32 kHz in one-octave steps. The binaural and two monaural (left and right ears) psychophysical audiograms were all similar in that they had the typical mammalian U-shape, with peak sensitivity from 8 to 16 kHz. The right ear showed generally elevated psychophysical thresholds when compared to the left, although all differences were less than 7 dB. The psychophysical and AEP audiograms were similar in terms of shape; however, the directions of monaural threshold differences determined with AEP methods were not always consistent with those obtained using psychophysical methods. [Work supported by ONR.]

New approaches for studying the perception of vocal signals in otariid pinnipeds.

The pioneering work of Ronald Schusterman and David Kastak considered the acoustic signaling of pinnipeds from the perspectives of both sender and receiver. Their work often highlighted the otariid pinnipeds (sea lions and fur seals), which are known for their frequent, loud aerial vocalizations. Numerous aerial vocalizations have been described for otariids, and research has demonstrated that a number of call characteristics are sufficient for individual and species recognition. In contrast to the marked interspecific variability found in these vocalizations, laboratory studies with pure-tone stimuli suggest that the aerial hearing capabilities of otariids are nearly identical among species. This is a surprising result, as many components in the calls of otariids exist at frequencies well below the range of best hearing sensitivity. Future studies can potentially examine the relevance of frequency composition and other vocalization parameters using psychophysical methods that include complex acoustic stimuli and an analysis of subject response latencies. Such methods have proven useful for studying the manner in which birds perceptually categorize vocalizations [Dooling et al., J. Comp. Psychol. 101, 367–381 (1987)] and they may provide novel tools for investigating the link between vocalization structure and auditory perception in otariids. [Work supported by ONR.]

Psychophysical and electrophysiological aerial audiograms of a Steller sea lion (Eumetopias jubatus)

A within-subject comparison of auditory steady-state response (ASSR) and psychophysical measurements of aerial hearing sensitivity was conducted with an individual of the largest otariid species, the Steller sea lion. Psychophysical methods were used to obtain an unmasked aerial audiogram at 13 frequencies, spanning a range of 0.125-34 kHz. The subject had a hearing range (frequencies audible at 60 dB(rms) re 20 microPa) of about 0.250-30 kHz, and a region of best hearing sensitivity from 5-14.1 kHz. The psychophysical aerial audiogram of this Steller sea lion was remarkably similar to aerial audiograms previously obtained for California sea lions and northern fur seals, suggesting that the otariid pinnipeds form a functional hearing group. ASSR thresholds, measured at frequencies of 1, 2, 5, 10, 20, and 32 kHz, were elevated relative to corresponding psychophysical thresholds, ranging from +1 dB at 20 kHz, to +31 dB at 1 kHz. The ASSR audiogram accurately predicted the subject's high-frequency cutoff, and provided a reasonable estimate of hearing sensitivity at frequencies above 2 kHz. In testing situations where psychophysical methods are not possible, ASSR methods may provide an objective and efficient estimate of behavioral hearing sensitivity in otariid pinnipeds.

Aerial audiograms of Steller and California sea lions measured using auditory steady-state response methods.

Detection of aerial vocal signals by conspecifics is important in the reproductive behavior of the otariid pinnipeds. However, aerial hearing sensitivity measurements have only been obtained for a few otariid individuals that were trained to participate in behavioral experiments. In order to expand upon this small data set, auditory steady-state response (ASSR) methods were used to examine the aerial hearing sensitivity of Steller and California sea lions. Although ASSR thresholds were elevated relative to behavioral thresholds reported for otariids, the ASSR audiograms of the majority of individuals were similar to each other and to behavioral audiograms in terms of relative sensitivity. A marked reduction in sensitivity with increasing frequency regularly occurred between 16 and 32 kHz, indicating a consistent high-frequency cutoff. The reliability of the ASSR audiograms for both species suggests that behavioral aerial audiograms that exist for a few Steller and California sea lion individuals can be appropriately extrapolated to larger populations. The similarity of the ASSR audiograms among the Steller and California sea lions supports the notion that the otariid pinnipeds form a functional hearing group, with similar aerial hearing in terms of sensitivity and frequency range of hearing. [Work supported by ONR and NOAA Ocean Acoustics Program.]

Hookworm infection, anaemia and genetic variability of the New Zealand sea lion

Hookworms are intestinal blood-feeding nematodes that parasitize and cause high levels of mortality in a wide range of mammals, including otariid pinnipeds. Recently, an empirical study showed that inbreeding (assessed by individual measures of multi-locus heterozygosity) is associated with hookworm-related mortality of California sea lions. If inbreeding increases susceptibility to hookworms, effects would expectedly be stronger in small, fragmented populations. We tested this assumption in the New Zealand sea lion, a threatened otariid that has low levels of genetic variability and high hookworm infection rates. Using a panel of 22 microsatellites, we found that average allelic diversity (5.9) and mean heterozygosity (0.72) were higher than expected for a small population with restricted breeding, and we found no evidence of an association between genetic variability and hookworm resistance. However, similar to what was observed for the California sea lion, homozygosity at a single locus explained the occurrence of anaemia and thrombocytopenia in hookworm-infected pups (generalized linear model, F = 11.81, p < 0.001) and the effect was apparently driven by a particular allele (odds ratio = 34.95%; CI: 7.12–162.41; p < 0.00001). Our study offers further evidence that these haematophagus parasites exert selective pressure on otariid blood-clotting processes.

A comparison of behavioral and electrophysiological measures of aerial hearing sensitivity in a Steller sea lion (Eumetopias jubatus).

A number of studies with odontocete cetaceans have demonstrated that hearing sensitivity measurements using electrophysiological auditory steady‐state responses (ASSRs) can provide an efficient means of estimating a subject’s behavioral audiogram. Expansion of ASSR methods to another marine mammal group, the otariid pinnipeds (sea lions and fur seals), holds the potential to increase the number of otariid individuals and species for which hearing sensitivity data are available. A within‐subject comparison of ASSR and behavioral measures of aerial hearing sensitivity was conducted with an individual of the largest otariid species, the Steller sea lion. Psychophysical methods were used to obtain an unmasked aerial audiogram at 13 frequencies spanning a range of 0.125 to 34 kHz. Corresponding ASSR thresholds measured at frequencies of 1, 2, 5, 10, 20, and 32 kHz had differences (relative to behavioral thresholds) ranging from 1 dB at 20 kHz to 30 dB at 1 kHz. Overall, the ASSR audiogram was a fairly accurate predictor of the behavioral audiogram at frequencies of 2 kHz and above. Our results suggest that ASSR methods can be appropriately applied to otariid pinnipeds in estimating aerial sensitivity at frequencies of approximately 2 kHz and above.

Inertial and cochlear constraints for high‐frequency hearing in phocid and otariid pinnipeds

In air‐borne hearing, mammals rely on sound transmission through the tympanic membrane and middle ear ossicles between the surrounding air and the cochlea. The high‐frequency hearing limit (HFHL) is determined by the ossicular inertia, and also by the cochlear sensitivity. Due to coevolution, the sensitivity ranges of the middle and inner ear structures generally overlap, and the roles of inertial and cochlear constraints for the HFHL are difficult to discern. For studying this question we considered anatomical and experimental data for two phocid and two otariid pinnipeds. While any detailed mechanism for pinniped underwater hearing remains unclear, an underwater HFHL exceeding that in air is possible. Published in‐air and underwater audiograms provide an opportunity for comparing the roles of ossicular mass inertia and cochlear sensitivity in HFHL. Phocid ossicles are very heavy, and their inertia explains the lower HFHLs in air ‐ according to underwater audiograms the phocid cochlea is sensitive to higher frequencies. Otariids have normal‐sized mammalian ossicles, and their inertia should allow underwater hearing at higher frequencies than in air. However, the HFHL is approximately equal in air and water for otariids, hence their underwater HFHL is apparently set by the cochlea alone.

Evoked potential audiometry in sea lions

Auditory sensitivity in the otariid pinnipeds (sea lions and fur seals) has traditionally been studied using a relatively small number of trained subjects and psychophysical techniques. Recent refinement of auditory evoked potential techniques with odontocete cetaceans has elevated interest in adapting these methods for sea lion subjects, with the goal of increasing sample size and efficiency in audiometric studies. To date, several basic electrophysiological characteristics of the California sea lion (Zalophus californianus) auditory system have been described, and these findings have allowed for the development of more advanced techniques in investigations of sea lion hearing. Most notable is the recording of the envelope following response (EFR) evoked by narrow‐band, sinusoidally amplitude‐modulated tones. This method can provide significant advantages in the detection of low‐amplitude electrophysiological signals in noise using Fourier analysis and objective statistical detection of responses. Curren...

Acoustic determination of activity and flipper stroke rate in foraging northern fur seal females

Foraging effort for lactating female otariid pinnipeds is largely a function of the energy expended swimming to a site and diving in search of prey. This is especially true for northern fur seal Callorhinus ursinus females, which predictably punctuate their suckling with 7 to 12 d foraging trips at sea, with swimming distances often exceeding 400 km. In the present study we tested a unique approach (flow noise from onboard acoustic dataloggers) to empirically measure swim effort in free ranging female northern fur seals, the first such field measurements on an otariid pinniped. We first measured behavioural activity budgets of seals from a combination of satellite telemetry, pressure (depth), and onboard acoustic data. From these data we were able to quantify the time spent in each of 4 mutually exclusive forms of behaviour: locomoting, diving, resting, and surface activity. Second, flipper stroke rates and stroke rate patterns were measured from the acoustic data for each seal dur- ing 3 dive types (i.e. locomoting, shallow and mid/deep dives) and during 3 dive parts (descent, bot- tom time and ascent). Although stroke rates during each of the 3 dive types were similar (ca. 0.5 Hz), they were distinct during the different parts of a dive. In each case, variation among individuals was significant. Stroke rate patterns were distinct for the different dive types and dive parts. Overall, in addition to applying a unique technique to measure foraging effort in a declining population, the results emphasize the importance of accounting for individual variation to obtain accurate estimates of foraging cost.

High-frequency hearing in phocid and otariid pinnipeds: An interpretation based on inertial and cochlear constraints

Audiograms in air and underwater, determined by previous workers for four pinniped species, two eared seals (Otariidae) and two phocids (Phocidae), are supplemented here by measurements on their middle ear ossicular mass, enabling mechanistic interpretations of high-frequency hearing and audiogram differences. Otariid hearing is not largely affected by the medium (air/water). This indicates that cochlear constraints limit high-frequency hearing in otariids. Phocids, however, have massive middle ear ossicles, and underwater hearing has radically shifted towards higher frequencies. This suggests that the high-frequency hearing of phocids in air is constrained by ossicle inertia.

Pupillometry in seals and sea lions: ecological implications

Phocid and otariid pinnipeds forage almost exclusively under water, where observing them is difficult. As a result, little is known of how their senses are used while diving to locate and capture prey. In this study we used pupillometric methods to describe the relative visual abilities of three pinniped species: the northern elephant seal (Mirounga angustirostris), the harbor seal (Phoca vitulina), and the California sea lion (Zalophus californianus). The range of pupillary area and lower limit of pupillary adjustment were determined for each species. The northern elephant seal exhibited the largest range of pupillary area of the species examined. Furthermore, the elephant seal's pupil was found to reach maximum size only under extremely dim conditions. Both the harbor seal and California sea lion exhibited maximum pupillary dilation in conditions several log units brighter. These results indicate that the elephant seal's visual system is designed to function in dimmer conditions and to respond to greater changes in light levels than those of the harbor seal and sea lion. Such findings support the hypothesis that the visual systems of these pinnipeds are adapted for use in their respective foraging environments, and thus suggest that the visual sense is probably an important sensory modality used to locate and capture prey while diving.