Markus Horning

The cost of foraging by a marine predator, the Weddell seal Leptonychotes weddellii: pricing by the stroke.

SUMMARY Foraging by mammals is a complex suite of behaviors that can entail high energetic costs associated with supporting basal metabolism, locomotion and the digestion of prey. To determine the contribution of these various costs in a free-ranging marine mammal, we measured the post-dive oxygen consumption of adult Weddell seals (N=9) performing foraging and non-foraging dives from an isolated ice hole in McMurdo Sound, Antarctica. Dives were classified according to behavior as monitored by an attached video-data logging system (recording activity, time, depth, velocity and stroking). We found that recovery oxygen consumption showed a biphasic relationship with dive duration that corresponded to the onset of plasma lactate accumulation at approximately 23 min. Locomotor costs for diving Weddell seals increased linearly with the number of strokes taken according to the relationship: locomotor cost =– 3.78+0.04 × stroke number (r2=0.74, N=90 dives), where locomotor cost is in ml O2 kg–1. Foraging dives in which seals ingested Pleuragramma antarcticum resulted in a 44.7% increase in recovery oxygen consumption compared to non-foraging dives, which we attributed to the digestion and warming of prey. The results show that the energy expended in digestion for a free-ranging marine mammal are additive to locomotor and basal costs. By accounting for each of these costs and monitoring stroking mechanics, it is possible to estimate the aerobic cost of diving in free-ranging seals where cryptic behavior and remote locations prevent direct energetic measurements.

Key Questions in Marine Megafauna Movement Ecology

It is a golden age for animal movement studies and so an opportune time to assess priorities for future work. We assembled 40 experts to identify key questions in this field, focussing on marine megafauna, which include a broad range of birds, mammals, reptiles, and fish. Research on these taxa has both underpinned many of the recent technical developments and led to fundamental discoveries in the field. We show that the questions have broad applicability to other taxa, including terrestrial animals, flying insects, and swimming invertebrates, and, as such, this exercise provides a useful roadmap for targeted deployments and data syntheses that should advance the field of movement ecology.

Lunar cycles in diel prey migrations exert a stronger effect on the diving of juveniles than adult Galapagos fur seals

In our study of the development of diving in Gal pagos fur seals, we analysed changes in diving activity and body mass trends over the lunar cycle. Based on previously observed lunar cycles in colony attendance patterns, we hypothesized a greater impact of prey migrations of deep scattering layer organisms on younger fur seals. Using electronic dive recorders, we determined that seals dived less and deeper on moonlit nights than at new moon, and incurred body mass losses. These changes in foraging over the lunar cycle correlate with the suppression of the vertical migration of prey by lunar light. All effects were more pronounced in juveniles than adult females, with greater relative mass loss during full moon, which must (i) negatively affect long-term juvenile growth rates, (ii) lengthen periods of maternal dependence, and (iii) contribute to the lowest reproductive rate reported for seals. This underlines the importance of studying ontogeny in order to understand life histories, and for determining the susceptibility of animal populations to fluctuations in food availability.

Diving into old age: muscular senescence in a large-bodied, long-lived mammal, the Weddell seal (Leptonychotes weddellii).

SUMMARY Classic aging theory postulates the absence of pronounced organismal senescence in wild animals since mortality probably occurs first. Large-bodied, long-lived mammals are a recognized exception to this tenet, yet organismal senescence has not been investigated to date in such mammals in the wild. Furthermore, oxidative stress theory of aging supports the suggestion that exercise hypoxia, as regularly incurred during apneustic foraging in diving mammals might lead to cellular dysfunction and accelerated aging. To determine if an aspect of organismal senescence occurs in wild marine mammals, we examined the pattern of skeletal muscle aging (contractile and connective tissue components of longissimus dorsi and pectoralis muscles) in free-ranging adult Weddell seals (9–26 years). The average myocyte cross-sectional area was 22% greater with age in the longissiums dorsi, but no significant increase occurred in the pectoralis. Cross-sectional area was not related to body mass. Changes in myocyte number per area were consistent with the 35–40% age-increase in extracellular space in both muscle groups. Also consistent with extracellular space remodeling, total and relative collagen contents were significantly elevated in older seals (115% in longissimus dorsi; 65% in pectoralis). The ratio of muscle myocyte to collagen declined with age (50–63%) at both sites. Additionally, a shift towards a higher ratio of type I to type III collagen occurred with advancing age in both muscle groups (79% increase in pectoralis; 49% in longissimus dorsi). We reject the classic tenet and null-hypothesis that Weddell seals do not survive to an age where muscular senescence becomes detectable.

Temporary Captivity as a Research Tool: Comprehensive Study of Wild Pinnipeds Under Controlled Conditions

A new approach to the study of free-ranging, endangered western stock Steller sea lions (Eumetopias jubatus) was implemented at the Alaska SeaLife Center in Seward, Alaska. Groups of up to four juvenile (n = 16) Steller sea lions were held in temporary quarantine for research periods of up to three months. Hematological and blood chemistry parameters were collected at the beginning and end of captivity and compared to free-ranging juvenile controls to determine if animals in temporary captivity can provide accurate physiological data representative of their wild counterparts. Free-ranging pups and juveniles were compared for hematological differences related to developmental stage. Overall, temporarily captive animals did not differ from free-ranging juveniles. Seven of 17 blood parameters measured changed significantly during captivity, likely as a function of a regular schedule and low-impact nutritional studies (e.g., increased mass, cholesterol, total protein, and globulins). A decrease in white blood cells during the study period (10.4 ± 0.59 to 7.9 ± 0.33 m/mm 3 ) to levels lower than that of free-ranging animals (10.7 ± 0.40 m/mm 3 ) indicated a drop in overall stress during captivity despite research and handling procedures. Calcium increased with captivity duration, suggesting that physiological changes can begin in even limited time frames. Eight parameters related to immune status and diet differed significantly between juveniles and pups from the same geographical region. A strategy that combines the benefits of an extended research design with temporary holding of free-ranging animals is proposed as an alternative to traditional field methods for some types of focused physiological studies.

Designing an archival Satellite transmitter for life-long deployments on oceanic vertebrates: the life history transmitter

Despite the widespread use of sophisticated telemetry transmitters in behavioral, physiological and ecological studies, few studies on population dynamics of oceanic vertebrates use such technology, primarily due to the difficulty of obtaining multi-year records from individual animals. We present the first telemetry transmitter specifically designed for collecting vital data from marine vertebrates over extended periods, up to a decade. The implantable Life History Transmitter records data throughout the life of a host animal. After the host animal dies, the tag is extruded, and, while floating on the ocean or lying on a beach, transmits previously stored data to orbiting satellites. For tags relying solely on end-of-deployment transmission, reliability and proper recognition of tag state is crucial. The Life History Transmitter uses heuristic tag state determination, in combination with simple error detection and fault tolerance measures, to increase tag reliability and likelihood of data recovery. We used a computer simulation of tag deployments and various sensor failures on a PC platform, in combination with time-accelerated simulations running on the actual deployment platform, to test the functionality of fault tolerance and error detection protocols

ASSESSMENT OF ULTRASOUND IMAGING AS A NONINVASIVE MEASURE OF BLUBBER THICKNESS IN PINNIPEDS

Abstract Portable ultrasound imaging was examined as a noninvasive measure of skin including blubber thickness in captive subadult Steller sea lions (Eumetopias jubatus) and adult harbor seals (Phoca vitulina). This method was validated through comparison with blubber biopsy. Ultrasound images provided depth measurements that were accurate to 99.8% of the actual. This method allowed clear images of the epidermis, dermis, and blubber layer to be rapidly obtained, with minimal animal restraint, and allowed differentiation of phocid and otariid blubber structure.

SEASONAL AND SPATIAL BLUBBER DEPTH CHANGES IN CAPTIVE HARBOR SEALS (PHOCA VITULINA) AND STELLER'S SEA LIONS (EUMETOPIAS JUBATUS)

Abstract Pinniped blubber supports multiple functions including thermoregulation, reproduction, and buoyancy. Although blubber depth is frequently used as an indicator of health, the effect of sample site and seasonality are rarely taken into account. We monitored blubber depth via imaging ultrasound at 10 sites monthly for 1 year in 2 adult Stellers sea lions (Eumetopias jubatus) and 3 adult harbor seals (Phoca vitulina). Blubber of harbor seals was thicker and more variable than blubber of sea lions, and was thickest in winter. Changes in harbor seal blubber at all sites tracked variation due to season and mass. Sea lion blubber changed with mass only at specific sites, with no seasonal effect. The differing effects of season, mass, and location on the body must be carefully considered in both species before any interpretation of condition.

Predation on an upper trophic marine predator, the Steller sea lion: evaluating high juvenile mortality in a density dependent conceptual framework.

The endangered western stock of the Steller sea lion (Eumetopias jubatus) – the largest of the eared seals – has declined by 80% from population levels encountered four decades ago. Current overall trends from the Gulf of Alaska to the Aleutian Islands appear neutral with strong regional heterogeneities. A published inferential model has been used to hypothesize a continuous decline in natality and depressed juvenile survival during the height of the decline in the mid-late 1980s, followed by the recent recovery of juvenile survival to pre-decline rates. However, these hypotheses have not been tested by direct means, and causes underlying past and present population trajectories remain unresolved and controversial. We determined post-weaning juvenile survival and causes of mortality using data received post-mortem via satellite from telemetry transmitters implanted into 36 juvenile Steller sea lions from 2005 through 2011. Data show high post-weaning mortality by predation in the eastern Gulf of Alaska region. To evaluate the impact of such high levels of predation, we developed a conceptual framework to integrate density dependent with density independent effects on vital rates and population trajectories. Our data and model do not support the hypothesized recent recovery of juvenile survival rates and reduced natality. Instead, our data demonstrate continued low juvenile survival in the Prince William Sound and Kenai Fjords region of the Gulf of Alaska. Our results on contemporary predation rates combined with the density dependent conceptual framework suggest predation on juvenile sea lions as the largest impediment to recovery of the species in the eastern Gulf of Alaska region. The framework also highlights the necessity for demographic models based on age-structured census data to incorporate the differential impact of predation on multiple vital rates.

Monitoring glucocorticoid response to rehabilitation and research procedures in California and Steller sea lions

We used serum and fecal corticosteroid analysis to study the physiological response to a range of invasive and non-invasive procedures in sea lions. Four experimental groups of California sea lions (Zalophus californianus; Group A: restraint only [n=9], Group B: gas anesthesia without surgery [n=10], Group C: minimally invasive surgery [n=10], and Group D: invasive surgery [n=5]) were monitored for adrenal response. Feces were collected opportunistically from 72 hr before 72 hr post procedure for corticosterone analysis. All experimental groups showed substantial individual variation and no significant change in corticosterone levels after the procedures. Additional fecal and serum corticoid samples were collected from six free-ranging Steller sea lions (Eumetopias jubatus) in temporary captivity undergoing abdominal implantation of long-term telemetry devices. Only one sea lion exhibited a significant fecal corticosterone increase in response to the surgery. Capture and restraint appear to elicit a greater glucocorticoid response than invasive or non-invasive procedures.