Adaptability of Narwhal Foraging Patterns: A Comparative Analysis of Three Populations

Introduction

• Rapid climate change can cause adaptation, range shift, and evolution in mammals.

• Phenotypic plasticity, the ability of a genotype to produce different forms in response to variable conditions, increases a species' survival chance.

• Foraging behavior plasticity can result from spatial segregation.

• Evolutionary change in foraging without physiological or morphological changes can account for differences in preferred prey.

• Behavioral modifications can precede genetic differentiation.

• Narwhals (Monodon monoceros L.) live exclusively in Arctic waters.

• The species experienced range contraction during the Little Ice Age.

• Narwhals have extremely low genetic diversity, which may reduce their ability to evolve.

• Narwhals are considered sensitive to environmental changes due to their limited distribution, selective diet, and reduced behavioral modification capability.

• The sensitivity of narwhals is based on information from only one of the three populations.

• Different populations within a species can demonstrate different adaptations, especially if spatially segregated.

Research Focus

• Investigated foraging behavior plasticity among narwhal populations.

• Goal: to understand if narwhals can modify their foraging behavior in a changing Arctic environment.

Narwhal Populations

• Three spatially and genetically segregated narwhal populations:

  • East Greenland (EG): inhabits eastern shores of Greenland and the Greenland Sea, estimated at 6,000 individuals.

  • Baffin Bay (BB): overwinters in the Davis Strait, summers in northern Canada and western Greenland, estimated at 60,000 individuals.

  • Northern Hudson Bay (NHB): winters in the Hudson Strait, summers in northern Hudson Bay, estimated at 12,500 individuals.

• Narwhals are culturally and economically important to the Inuit.

• Annual subsistent harvest:

  • EG: approximately 110

  • BB: approximately 800

  • NHB: approximately 100

• Narwhals are considered one of the most sensitive Arctic marine mammals to climate change because of:

  • Limited distribution (Atlantic region of the Arctic)

  • Small population size (ca. 100,000)

  • Dietary specialization

• Current knowledge about narwhal diet comes primarily from stomach content analysis on hunted narwhals from the BB population.

• No studies have been conducted on the NHB or EG narwhals.

• Stomach contents from BB narwhals identified primary prey:

  • Greenland halibut (Reinhardtius hippoglossoides)

  • Arctic cod (Boreogadus saida)

  • Polar cod (Arctogadus glacialis)

  • Squid (Gonatus fabricii)

Other Prey

• Other species identified in smaller amounts:

  • Snailfishes (Liparis spp.)

  • Redfish (Sebastes marinus)

  • Sculpins (Cottidae)

  • Eelpout (Lycodes spp.)

  • Skate egg sacs (Raja spp.)

  • Shrimps (Pasiphaea tarda and Hymenodora glacialis)

  • Shrimp Pandalus borealis (abundant)

  • Capelin (Mallotus villosus)

  • Wolffish (Anahichas lupus and A. minor) (minor prey items)

Sexual Size Dimorphism

• Sexual size dimorphism exists: males and females differ in size.

• Males have an erupted left tooth that results in a spiraled tusk.

• Few studies have compared diet between male and female narwhals because of low sample size of females.

• One study found no evidence of dietary segregation between sexes based on stomach contents (small female sample size, n=29).

• Hunter-collected samples are typically biased towards males, limiting the ability to detect differences in diet between males and females.

• Diving capacity is greater for larger animals.

• Male narwhals are likely capable of making deeper dives and potentially foraging more in the benthos in deep waters.

• Males, because of their larger size, may be able to manipulate larger prey than females.

Limitations of Stomach Content Analysis

• Stomach content analysis can result in biased estimates because of:

  • Varying digestion rates

  • Provides information only on what was consumed, not what was assimilated into tissues

  • Provides information on the organism’s last meal

• This is particularly a problem in Arctic marine mammals because:

  • Seasonal changes in food availability and diet

  • High-energy requirements

• Stomach contents may not be representative of typical feeding events.

Stable Isotope Analysis

• More recent studies use stable isotope analysis to determine differences in:

  • Trophic level

  • Pelagic versus benthic feeding

  • Proportions of prey consumed

• Stable isotope analysis is a powerful tool because predator tissues are directly related to the ratios found in their prey with a progressive enrichment factor.

• The stable nitrogen isotope ratio (\frac{15N}{14N}, expressed as d15N) provides information on an organism’s trophic level.

• The stable carbon ratio (\frac{13C}{12C}, expressed as d13C) reflects its spatial foraging distribution (pelagic versus benthic, or offshore versus coastal).

• Previous studies found differences in d15N and d13C between narwhals from two different locations in West Greenland.

• No study has conducted a large-scale comparison of narwhal stable isotope values across the three populations.

• Stable isotope ratios vary with geography because of variable d15N and d13C at the base of the food web.

• There is known to be a large gradient in d13C across the narwhal’s geographic span.

• An organism’s isotopic niche can provide insight into an animal’s dietary niche.

• Animals with very small isotopic niches are often dietary specialists, whereas dietary generalists have a much larger isotopic niche.

• This depends on the isotopic variability of the environment and the prey sources they consume.

• Stable isotope niche width is not directly comparable to an animal’s trophic niche.

• Niche analysis can provide information on dietary variation at the population level if isotopic variation in prey is taken into account.

Hypotheses

1. Narwhals display phenotypic plasticity, resulting in populations having different d13C and d15N values.

   • Expected d13C and d15N would differ due to geographic expanse; however, differences should be greater if populations have distinct foraging behavior.

2. Dietary niche sizes for the three populations would differ.

   • Predicted the isotopic niche of the BB population would be larger than the NHB and EG populations because of increased competition for resources.

   • Alternatively, narwhals in East Greenland may have a larger isotopic niche due to a larger range and different baseline d13C and d15N values.

3. Males, having a larger body size, would be able to exploit a greater range of resources than females, resulting in them having different d13C and d15N, as well as a larger isotopic niche.

   • Expected they might be able to dive deeper and spend greater time at depth, resulting in their skin tissue expressing a higher d13C value.

   • Could forage on larger prey, which may increase the d15N value of their tissues.

Additional Goals

• Estimate the importance of primary prey components for each of the three populations.

• Discuss the potential for narwhal to adjust foraging behavior in the face of ecosystem shifts occurring with climate warming.

Methods

• Inuit hunters and researchers collected narwhal skin samples from:

  • Pond Inlet, Nunavut, Canada (Baffin Bay population)

  • Repulse Bay, Nunavut, Canada (Northern Hudson Bay population)

  • Ittoqqortormiut, Greenland (East Greenland population)

• All samples are collected opportunistically, resulting in a varying number of male and female samples across various temporal scales.

• Analyses included only samples collected in the summer months (June through September).

• Samples from other seasons were rare across populations.

• Narwhal skin tissue was sub-sectioned and a 0.5 g piece of skin was diced, freeze-dried for 48 hours, homogenized, and lipid extracted using a 2:1 chloroform:methanol solution.

Isotope Ratio Mass Spectrometer (IRMS)

• A continuous flow isotope ratio mass spectrometer (IRMS, Finnigan MAT Deltaplus, Thermo Finnigan, San Jose, CA, USA) was used to determine d13C, d15N and % C and N of 400–600 lg of tissue.

• The standard reference material was Vienna Pee Dee Belemnite carbonate for CO2 and atmospheric nitrogen N2.

• Every 12th sample was run as a triplicate to assess precision; the mean standard deviation of these samples was 0.1 \%% for d13C and 0.4 \%% for d15N.

• Internal lab and National Institute of Standards and Technology (NIST) standards were analyzed after every 12 samples for quantification of samples and to assess analytical precision.

• Analytical precision (standard deviation) for NIST standard 8414 (bovine muscle, n = 152) and an internal lab standard (tilapia muscle, n = 152) for d13C was 0.07 \%% and 0.09\%%, and for d15N was 0.15\%% and 0.19 \%%.

• Certified NIST standards were analyzed during sample analysis to assess accuracy.

NIST Standards Results

• For d13C:

  • The mean value for NIST 8542 ($-10.48 \pm 0.03 \%%$; n = 10) was within 0.01 of the certified value of $-10.47$.

  • For NIST 8573 ($-26.26 \pm 0.04 \%%$; n = 10) was within 0.13 of the certified value of $-26.39 \%%$.

• For d15N:

  • The mean value for NIST 8542 ($4.58 \pm 0.11 \%%$; n = 10) was within 0.12 of the certified value of 4.70\%%.

  • For NIST 8548 ($20.11 \pm 0.38 \%%$; n = 9) was within 0.30 of the certified value of $20.41 \%%$.

Additional Method Details

• All stable isotope analyses were conducted at the University of Windsor, Great Lakes Institute for Environmental Research.

• d13C values were corrected for the ‘‘Suess effect’’ by applying a correction of 0.02 \%%, per year beyond 1982.

• Normal quantile plots for d13C and d15N were normally distributed across populations and between sexes.

• Variances were also homogenous for the three populations across both sexes for d13C (Levene’s test: F5, 211 ¼ 0.38, P ¼ 0.86) and d15N (Levene’s test: F5, 211 ¼ 1.31, P ¼ 0.26), thus no data transformations were required.

• A generalized linear model, which included population, sex and the interaction between the two factors, was used to assess if d13C and d15N values differed among populations and between sexes.

• Tukey’s HSD tests were used to determine which populations differed when significant differences were detected in the full model.

Niche Width and Statistical Comparison

• Niche widths were calculated and statistically compared using a Bayesian framework.

• A multivariate ellipse-based metric was used to compare populations of different sizes.

• Ellipse standard areas were calculated using the SIBER package within SIAR in R (Parnell et al. 2010).

Predator and Prey Stable Isotope Values

• Predator and prey stable isotope values to understand what prey may be contributing to the differences in stable isotope ratios among populations.

• Potential prey was identified based on stomach content analyses from the BB population:

  • Arctic cod (B. saida)

  • Polar cod (A. glacialis)

  • Greenland halibut (R. hippoglossoides)

  • Shrimp (P. borealis)

  • Squid (Gonatus spp.)

  • Capelin (M. villosus)

• Discrimination factors of 2.6 \%% for d15N and 1.9 \%% for d13C were added to the potential prey.

• These values were calculated by taking an average of fractionation factors reported by studies of killer whales (Orcinus orca) and bottlenose dolphins (Tursiops truncatus).

• SIAR was used to assess the contribution of each prey to the diet of narwhals for the three populations (Parnell et al. 2010).

Results

• The three populations had significantly different d13C (F2, 211 ¼ 319.88, P , 0.0001) and d15N (F2, 211¼201.10, P , 0.0001).

  • NHB had the highest mean d13C ($-17.0 \%%$)

  • BB had the highest mean d15N ($16.6 \%%$)

  • EG had the lowest mean d13C ($-19.1\%%$) and d15N (14.6%).

Sex Differences

• Males and females differed in d13C (F1, 211 ¼ 9.72, P , 0.01) with males having significantly higher d13C ($-17.9 \%%$) than females ($-18.1\%%$).

• Sexes did not differ significantly in their d15N values (F1, 211 ¼ 1.28, P ¼ 0.26).

• There was no significant interaction between sex and population for d13C (F2, 211 ¼ 0.69, P ¼ 0.50) or d15N (F2, 211 ¼ 0.37, P ¼ 0.69).

Standard Ellipse Area

• Standard ellipse area of the EG population (n ¼ 25) was larger than the ellipse for the BB population (n ¼ 127), although this result was not quite significant (P ¼ 0.06), and significantly larger than the NHB ellipse (n ¼ 65; P ¼ 0.04).

• There was no significant difference between ellipse area for narwhals from NHB and BB (P ¼ 0.66).

• Male and female narwhals did not differ in their isotopic niche size in the BB (n ¼ 69 and 58, respectively; P ¼ 0.15), EG (n ¼ 17 and 8, respectively; P ¼ 0.78) or NHB (n¼41 and 24, respectively; P¼0.90) populations.

Primary Prey

• Cod (A. glacialis and/or B. saida), Greenland halibut (R. hippoglossoides), shrimp (P. borealis), squid (Gonatus spp.), and capelin (M. villosus) were all considered potential prey for narwhals in each population.

• Results from the stable isotope mixing models revealed that narwhals from EG consume significantly more capelin than other populations, and less shrimp.

• Narwhals from BB consumed slightly more Arctic and polar cod than NHB narwhals, and NHB narwhals consumed more Greenland halibut.

• Male and female narwhals typically had similar diets within a population.

• In BB, males appeared to consume more shrimp than females, while females ate more cod.

• In NHB, males ate more halibut and less capelin and squid compared to females.

• Males and females in EG were difficult to distinguish based on their prey proportions.

• Male and female narwhals from EG feed in the pelagic zone to a greater extent, while narwhals in NHB forage more in the benthos.

• Males in BB spend more time foraging in a benthic food web, while females in this population forage similarly in a benthic and pelagic food web.

Discussion

• The world’s three narwhal populations have very different d13C and d15N values, suggesting they have different preferred prey and, thus, may be more flexible in their foraging behavior than previously thought.

• Narwhal flexibility in preferred prey may help them face changing food web structure and prey distribution that are accompanying climate change.

d13C Value

• d13C values change as you move from benthic or inshore food webs to pelagic food webs, with organisms in the benthic environment displaying a higher d13C value.

• Mean d13C was lowest in the EG population, suggesting narwhals in this region feed within a more pelagic food web compared to the NHB and BB populations.

• Stable isotope mixing model results confirmed that EG narwhals preferentially feed on capelin.

• The NHB population had the highest d13C values, suggesting they feed in a more benthic food web, which is consistent with them inhabiting a much shallower ecosystem compared to BB and EG.

• NHB narwhals consumed a large proportion of shrimp, while pelagic prey such as capelin and squid played a minor role in their diet.

• If necessary, narwhals may be able to switch primary prey and monopolize on the increase in capelin abundance in NHB, which may mitigate the negative impacts of reduced cod and benthic species.

• BB narwhals had intermediate d13C values and the highest d15N values, indicating they feed at a higher trophic level than the other two populations.

• These values are consistent with narwhals in this population consuming high proportions of Greenland halibut; however, the mixing models did not show that Greenland halibut is the primary prey.

• This may be indicative of the timing of sample collection (restricted to summer months) and the time frame that this tissue represents in the diet.

Stable Isotope Turnover

• Stable isotope turnover rates are unknown in this species.

• Complete tissue turnover in the skin of beluga whales (Delphinapterus leucas) was approximately 2–3 months.

• The stable isotope values in narwhal skin in BB would not necessarily reflect a large proportion of Greenland halibut (primarily consumed in the winter), because it would only reveal foraging in the late spring and summer.

• Alternatively, much longer turn over rates (>1 year) have been determined in large mammals, which may indicate that narwhals in BB have a more variable diet than stomach content studies have revealed and Greenland halibut is not the major prey item.

• Stable isotope mixing models provide best estimates of diet for the three narwhal populations; however, there is uncertainty surrounding the discrimination factor used, and these models are known to be sensitive to the specified discrimination factor.

• It is possible that some potential prey were excluded from the models, especially for the EG and NHB populations.

• Many other prey stable isotope values were investigated for the two populations; however, none of these prey had stable isotope values within the range seen for narwhals in the two populations and, thus, they were excluded from the stable isotope mixing models.

• Overall, the model results are insightful but should be interpreted with caution.

Isotopic Niche Size

• The size of a species' isotopic niche can provide insight into the extent of their dietary diversity.

• The isotopic niche, although related to the ecological niche, is not directly comparable.

• To interpret the variance seen within narwhal populations, we have to consider the variability of the stable isotope values within the prey.

• The EG narwhal population had the largest niche width, which is consistent with these whales having the greatest geographical range.

• Although it is generally assumed that a larger isotopic niche can be interpreted as a larger trophic niche where organisms are typically generalists that feed on an array of prey, it has been shown that populations confined to one site may display greater isotopic variances within their population due to individual specialization.

• The BB population had a relatively small isotopic niche despite its population being vastly larger than the other two, which suggests that there is little intraspecific competition to result in individual specialists; however, there may be strong interspecific competition resulting in specialization at the population level.

• The supply of Greenland halibut in the winter must be substantial enough to sustain the current population in this region; however, an expanding halibut fishery in the Arctic has the potential to compete with narwhal feeding.

Sexual Segregation in Diet

• There is sexual segregation in diet for all populations, which may be related to the diving ability of males and females.

• Studies investigating dive behavior in narwhal sexes have been limited by sample size and, perhaps, as a result, have documented conflicting results.

• It has been shown that female narwhals have significantly lower dive rates than males and they typically make dives ,400 m, but another study reported no difference in diving performance between female narwhals and their male counterparts.

• Given the sexual size dimorphism, males are likely capable of making deeper dives and, therefore, capable of foraging in the benthos even in deep waters.

• Increased benthic foraging for males would explain their increased d13C value.

Beluga Whales

• Beluga whales are considered to eat a much more diverse range of prey than narwhals.

• Overall, belugas consume a greater range of prey than narwhals, but based on our results, narwhal may also be flexible in their preferred prey.

• Although there is some range overlap between narwhals and belugas, typically the species have different preferred habitats, which has reduced competition for food.

• Our study and a study conducted by Thiemann et al. (2008) suggest there is substantial overlap in preferred prey, and competition may become more intense when both prey and the predators shift their distribution with changing climate.

Isotope Ratios Fluctuation

• Stable isotope ratios fluctuate with geography as a result of variable d15N and d13C at the base of the food web, and, therefore, we have to consider that differences among populations may be a result of the isotopic differences across the large geographical span.

• Comparisons were restricted to narwhal samples collected in summer months, which eliminated any confounding that may be caused by seasonal changes in diet, but annual changes in diet could also have impacted differences among the populations.

Possible outcomes for a species experiencing climate change (Fuller et al., 2010):

1. The species can shift distribution.

2. Genetically adapt.

3. Go extinct.

4. Employ phenotypic plasticity.

• We showed that stable isotope values differ among the populations, and this difference is related to differences in preferred prey and foraging behavior among the populations.

• Future studies should monitor changes in preferred prey of narwhals in NHB, the most southerly narwhal population, that is currently experiencing documented food web modifications as a result of changes in climate.

• Additionally, investigations of stomach contents and fatty acids from narwhals in the EG and NHB populations would provide a clearer picture of how flexible narwhal are in their foraging behavior.