Calibrating the time span of longitudinal biomarkers in vertebrate tissues when fine‐scale growth records are unavailable

Living in human-modified landscapes narrows the dietary niche of a specialised mammalian scavenger

Anthropogenic impacts on carnivores can be complex, posing numerous threats to many species, yet also benefits to those able to exploit certain resources. This balancing act is particularly precarious for those adapters that exploit dietary resources provided by humans, but still require other resources only available in native habitat. Here we measure the dietary niche of one such species, the Tasmanian devil (Sarcophilus harrisii), a specialised mammalian scavenger, across an anthropogenic habitat gradient stretching from cleared pasture to undisturbed rainforest. Populations inhabiting areas of greater disturbance showed restricted dietary niches, suggesting that all individuals fed on similar food items, even within regenerated native forest. Populations in undisturbed rainforest habitats had comparatively broad diets and showed evidence of niche partitioning by body size, which may reduce intraspecific competition. Despite the potential benefits of reliable access to high-quality food items in anthropogenically-modified habitats, the constrained niches we observed may be harmful, indicating altered behaviours and potentially increasing the rate of fights between individuals over food. This is of particular concern for a species at risk of extinction due to a deadly cancer primarily transmitted through aggressive interactions. The lack of diversity in devil diets within regenerated native forest compared to those in old-growth rainforest also indicates the conservation value of the latter for both the devil and the species which they consume.

Whiskers provide time-series of toxic and essential trace elements, Se:Hg molar ratios, and stable isotope values of an apex Antarctic predator, the leopard seal

In an era of rapid environmental change and increasing human presence, researchers need efficient tools for tracking contaminants to monitor the health of Antarctic flora and fauna. Here, we examined the utility of leopard seal whiskers as a biomonitoring tool that reconstructs time-series of significant ecological and physiological biomarkers. Leopard seals (Hydrurga leptonyx) are a sentinel species in the Western Antarctic Peninsula due to their apex predator status and top-down effects on several Antarctic species. However, there are few data on their contaminant loads. We analyzed leopard seal whiskers (n = 18 individuals, n = 981 segments) collected during 2018-2019 field seasons to acquire longitudinal profiles of non-essential (Hg, Pb, and Cd) and essential (Se, Cu, and Zn) trace elements, stable isotope (ẟ15N and ẟ13C) values and to assess Hg risk with Se:Hg molar ratios. Whiskers provided between 46 and 286 cumulative days of growth with a mean ~ 125 days per whisker (n = 18). Adult whiskers showed variability in non-essential trace elements over time that could partly be explained by changes in diet. Whisker Hg levels were insufficient (<20 ppm) to consider most seals being at "high" risk for Hg toxicity. Nevertheless, maximum Hg concentrations observed in this study were greater than that of leopard seal hair measured two decades ago. However, variation in the Se:Hg molar ratios over time suggest that Se may detoxify Hg burden in leopard seals. Overall, we provide evidence that the analysis of leopard seal whiskers allows for the reconstruction of time-series ecological and physiological data and can be valuable for opportunistically monitoring the health of the leopard seal population and their Antarctic ecosystem during climate change.

Effects of intraspecific competition and body mass on diet specialization in a mammalian scavenger

Animals that rely extensively on scavenging rather than hunting must exploit resources that are inherently patchy, dangerous, or subject to competition. Though it may be expected that scavengers should therefore form opportunistic feeding habits in order to survive, a broad species diet may mask specialization occurring at an individual level. To test this, we used stable isotope analysis to analyze the degree of specialization in the diet of the Tasmanian devil, one of few mammalian species to develop adaptations for scavenging. We found that the majority of individuals were dietary specialists, indicating that they fed within a narrow trophic niche despite their varied diet as a species. Even in competitive populations, only small individuals could be classified as true trophic generalists; larger animals in those populations were trophic specialists. In populations with reduced levels of competition, all individuals were capable of being trophic specialists. Heavier individuals showed a greater degree of trophic specialization, suggesting either that mass is an important driver of diet choice or that trophic specialization is an efficient foraging strategy allowing greater mass gain. Devils may be unique among scavenging mammals in the extent to which they can specialize their diets, having been released from the competitive pressure of larger carnivores.

Complementary use of stable isotopes and fatty acids for quantitative diet estimation of sympatric predators, the Antarctic pack-ice seals

The quantitative use of stable isotopes (SIs) for trophic studies has seen a rapid growth whereas fatty acid (FA) studies remain mostly qualitative. We apply the Bayesian tool MixSIAR to both SI and FA data to estimate the diet of three sympatric predators: the crabeater (Lobodon carcinophaga), Weddell (Leptonychotes weddellii) and leopard seal (Hydrurga leptonyx). We used SI data of their vibrissae and FA data of their outer blubber to produce comparable diet estimates for the same individuals. Both SI and FA models predicted the same main diet components, although the predicted proportions differed. For the crabeater seal, both methods identified krill, Euphausia superba, as the main, and almost exclusive, food item, although the FA model estimated a slightly lower proportion, potentially due to the low lipid content of krill compared to the fish species used in the model. For the Weddell seal the FA model identified the fish Pleuragramma antarcticum as the most important prey, whereas the SI model was not able to distinguish among prey species, identifying a ‘fish-squid’ group as the main diet component. For the leopard seal, both models identified krill as the main contributor; however, the predicted proportions for the secondary sources differed. Although vibrissae and outer blubber may not represent the same timeframe, the use of MixSIAR with FA data provides diet estimates comparable to those obtained with SI data, thus, both approaches were complimentary. The use of both biotracers offers a feasible option to study diets of wild animals in a quantitative manner.

Leopard seal diets in a rapidly warming polar region vary by year, season, sex, and body size

Background

Resolving the preferred prey items and dietary proportions of leopard seals is central to understanding food-web dynamics in the rapidly-warming Antarctic Peninsula region. Previous studies have identified a wide range of prey items; however, due to anecdotal or otherwise limited information, leopard seal diets remain unresolved by seal sex, individual, body size, region, and season. Over the 2013, 2014, and 2017 field seasons we collected scat, tissue samples (red blood cells and plasma; n = 23) for stable isotope analyses, and previously-reported animal-borne video from 19 adult leopard seals foraging near mesopredator breeding colonies at Cape Shirreff, Livingston Island. We summarized a priori diet information from scat and video analysis and applied a three-isotope (δ¹³C, δ¹⁵N, δ³⁴S), four-source (fish, fur seal, krill, penguin) Bayesian mixing model to examine temporal variability in both prey sources and leopard seal tissues.

Results

The austral spring diets of males and females focused on Antarctic krill (31.7–38.0%), notothen fish (31.6–36.5%), and penguin (24.4–26.9%) and were consistent across all 3 years. Several lines of evidence suggest the transition to summer foraging was distinct for males and females. Female diets transitioned rapidly to higher δ¹⁵N values (+2.1‰), indicating increased consumption of penguin (29.5–46.2%) and energy-dense Antarctic fur seal pup (21.3–37.6%).

Conclusions

The seasonal increase in leopard seal δ¹⁵N values, and thus fur seal in their diet, was predictably related to larger body size; it may also be forcing reductions to the largest Antarctic fur seal colony in the Antarctic Peninsula. Our ensemble sampling approach reduces historical biases in monitoring marine apex predator diets. Further, our results are necessary to best inform regional fisheries management planning.

Foraging behaviour of the South American sea lion (Otaria byronia) in two disparate ecosystems assessed through blubber fatty acid analysis

Fatty acids have been widely used as trophic biomarkers in marine mammals. However, for the South American sea lion, the most abundant otariid in the eastern South Pacific, there is no information about blubber fatty acids and their link to diet. Here, we compare fatty acid profiles of sea lions from two distinct oceanographic regions in northern and southern Chile. Their fatty acids vary greatly between regions, suggesting dietary differences at a spatial scale. The fatty acid C22:6ω3 was more abundant in sea lions from the northern region, likely associated with consumption of anchovy, cephalopods, and crustaceans, which are rich in that fatty acid, and have been reported as their main prey items. Sea lions from the southern region were richer in C22:1 and C20:1, characteristic of teleost fish, suggesting a piscivorous diet. Males displayed a more diverse fatty acid composition than females, suggesting a wider trophic niche. Few individual sea lions within the southern region had unusually high levels of C18:2ω6, commonly found in terrestrial environments. This suggests consumption of farmed salmon, whose diet is usually based on terrestrial sources. This demonstrates how human intervention is being reflected in the tissues of a top predator in a natural environment.

What's in a whisker? Disentangling ecological and physiological isotopic signals

RATIONALE

Stable isotope analysis of keratinized tissues is an informative tool for quantifying foraging ecology that can address questions related to niche specialization and temporal variation in behavior. Application of this approach relies on an understanding of tissue growth and how isotope ratios relate to physiological and ecological processes, data that are lacking for many species.

METHODS

We collected paired whisker length measurements from northern elephant seals to estimate growth and shedding patterns (n = 16). A subset of seals (n = 5) carried a satellite tag and time-depth recorder across the 7+ month foraging trip following the annual pelage molt. Stable isotopes of carbon and nitrogen were measured in whisker segments grown across the 6+ week fasting on land and the subsequent foraging trip; profiles were combined with growth parameters to timestamp each segment and investigate relationships with foraging behavior.

RESULTS

Whisker loss and initial regrowth primarily occurred during the annual pelage molt, but newly grown whiskers exhibited active, nonlinear growth across the foraging trip. The δ¹³ C and δ¹⁵ N values were higher in segments grown on land than at sea and exhibited a characteristic decline upon departure from the rookery. There was a relationship between latitude and longitude and δ¹⁵ N values, and individual whisker segments grown at sea could be classified to the correct ecoregion with 81% accuracy.

CONCLUSIONS

Fasting affected both δ¹³ C and δ¹⁵ N values and the ability to exclude these values from ecological investigations is crucial given the temporal overlap with tissue growth. The rapid decline in isotope ratios upon departure can be used to isolate portions of the whisker with a strong physiological signal, even for whiskers with unknown growth histories. The active growth across the foraging trip combined with the ability to identify differences in foraging behavior validates the utility of this approach for addressing ecological questions.

Vibrissa growth rate in California sea lions based on environmental and isotopic oscillations

Pinniped vibrissae provide information on changes in diet at seasonal and annual scales; however, species-specific growth patterns must first be determined in order to interpret these data. In this study, a simple linear model was used to estimate the growth rate of vibrissae from adult female California sea lions (Zalophus californianus) from San Esteban Island in the Gulf of California, Mexico. The δ¹⁵N and δ¹³C values do not display a marked oscillatory pattern that would permit direct determination of the time period contained in each vibrissa; thus, time (age) was calculated in two ways: 1) based on the correlation between the observed number of peaks (Fourier series) in the δ¹⁵N profile and the length of each vibrissa, and 2) through direct comparison with the observed number of peaks in the δ¹⁵N profile. Cross-correlation confirmed that the two peaks in the δ¹⁵N profile reflected the two peaks in the chlorophyll-a concentration recorded annually around the island. The mean growth rate obtained from the correlation was 0.08 ± 0.01 mm d^-1, while that calculated based on the observed number of peaks was 0.10 ± 0.05 mm d^-1. Both are consistent with the rates reported for adult females of other otariid species (0.07 to 0.11 mm d^-1). Vibrissa growth rates vary by individual, age, sex, and species; moreover, small differences in the growth rate can result in significant differences over the time periods represented by the isotopic signal. Thus, it is important to assess this parameter on a species-by-species basis.