Discussion

Using the earplug from a single blue whale, we have been able to demonstrate that: (i) lipophilic compounds accumulate in cerumen; (ii) contaminants and hormones that accumulate in the cerumen lamina are chronologically archived; (iii) trace analysis techniques can be used to measure contaminants and hormones in individual lamina; and (iv) by combining chemical concentrations from individual lamina, we can reconstruct lifetime profiles for an individual whale (i.e., birth to death).

This study reports previously uncharacterized lifetime cortisol profiles in a baleen whale (Fig. 2A). Cortisol is a biomarker of stress in mammals with concentrations directly associated with responses to a combination of environmental, physical, chemical, and social factors (21⇓⇓–24). During this response, glucocorticoid steroid hormones are released into the blood stream with the amount reflecting the severity of the stressor (25). This cortisol profile highlights several prominent peaks as well as episodic variability over the animal’s lifetime. Baseline cortisol concentrations were 45 pg⋅g⁻¹ (6–12 mo), whereas cortisol peaked at 420 pg⋅g⁻¹ (126–132 mo). This change in cortisol corresponds to an ∼800% increase over the initial baseline. This peak in cortisol concentration immediately follows the largest peak in testosterone (114–126 mo) (Fig. 2A). This suggests that the cortisol maximum was due to breeding competition or social bonds formed during sexual maturity (26). Interestingly, the mean cortisol concentrations doubled over the life of this blue whale (Fig. 2A). The general increase in cortisol over the animal’s lifetime could be associated with a multitude of factors including weaning, development, sexual maturity, migration, food availability, environmental conditions, changes in social status, accumulated contaminant exposure, and/or environmental noise.

The increase of androgens during postnatal development is a key factor defining sexual maturity in male mammals (27). The significant peak (400-fold over baseline) in testosterone observed during this study at ∼114–126 mo provides a strong indication of sexual maturity (Fig. 2A). This unique approach provides chemical verification of sexual maturity (via lifetime testosterone profiles) and offers improved resolution over historical methods such as age–length estimates (20, 28), ear plug lamina counts (14), and ovarian corpora counts–length data (29), which cumulatively have previously estimated sexual maturity of male blue whales to be between 60 and 180 mo (30). Testosterone concentrations sequestered in the cerumen from this male blue whale ranged from ∼230 to 93,000 pg⋅g⁻¹. The lifetime testosterone profile reported here from a single animal was in agreement with a study by Kjeld et al. (3) who measured serum testosterone concentration in 278 male fin whales at varying ages, where testosterone concentration also ranged over two orders of magnitude (35–14,000 pg⋅g⁻¹). Kjeld et al. (3) used earplugs for aging and histological and anatomical indices to determine the medium age of sexual maturity in male fin whales to be 120 mo (10 y).

The reconstructed POP profiles of this male blue whale demonstrate that a substantial maternal transfer occurred during gestation and/or lactation. A review by Wagemann and Muir (31) highlighted similar maternal transfer of contaminants in a large number of marine mammals throughout the north hemisphere. The maternal transfer of POPs for this individual blue whale was equal to ∼20% of its total lifetime burden. This substantiates previous hypotheses regarding the ability of organic contaminants to undergo maternal transfer during gestation and lactation in large marine mammals (32, 33). Further, our lifetime POP burden measured in this blue whale is in agreement with the lifetime physiologically based pharmacokinetic fugacity model developed by Hickie et al. (1999) (Fig. 2E) (32). In our study, the lifetime accumulative lipophilic contaminants burden describes the uptake, metabolism, and excretion of POPs by the organism over its entire lifetime. The lifetime accumulative POP burden was recorded in the lamina of the earplug and totaled 5,200 ng⋅g⁻¹. Blubber tissue collected from the same male blue whale provided a total POP burden of 4,700 ng⋅g⁻¹ or 90% of the total accumulative burden recorded in the earplug (Fig. 2E, red vertical line). This 10% decrease may be due to differences in uptake, metabolism, and excretion process specific to blubber compared with cerumen. Regardless, under the assumption that both the whale blubber and earplug can provide estimates of accumulative contaminant burden, we see striking similarities between these two matrices. Specifically, o,p′-DDE, p,p′-DDE, p,p′-DDT, cis- and transnonachlor, PCB 105, 118, 138, 153, 156, and 187, as well as PBDE 47, 99, and 100 were identified in both blue whale earplug cerumen and blubber (Fig. 3). The major difference between the two matrices is that s.c. blubber cannot estimate when the exposure transpired, whereas earplugs provide lifetime accumulative contaminant burden with, in this case, a 6-mo resolution.

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Fig. 3.

Male blue whale blubber (left axis) and earplug (right axis) contaminant profiles. Blubber was sampled near muscle blubber interface and compared with the lamina corresponding to 72–78 mo in the earplug, which represents a midpoint in the organism lifespan.

The POP burden measured in each lamina suggests that contaminants are recirculating throughout the body during both periods of feeding and fasting. The impact from the chronic and acute POP exposure on baleen whales is largely unknown, but may potentially be positively correlated with cortisol. This relationship has been difficult to identify using inconsistent sampling strategies (34⇓–36). Contaminant and hormone profiles reconstructed from this earplug suggest that there may be a weak positive correlation between contaminant burden and cortisol concentrations as a function of time; however, with a sample size of n = 1, this should be considered a tentative assessment.

Anthropogenic mercury is ubiquitous in the environment and has received much attention among ecologists, environmental chemists, and toxicologists because of its ability to bioaccumulate and impair neurological development. Well-documented research involving humans reveals maternal transfer of mercury in utero and then to the neonate during lactation (37). Mercury profiles in this blue whale do not mirror maternal transfer to the same degree as the POPs (Fig. 2F). The mercury profile also highlights two pulse events ranging from 60 to 72 mo and from 120 to 126 mo. Because this blue whale appeared to routinely traverse the coast of California (ship strike near Santa Barbara, CA), we speculate that these pulse events may be associated with regional environmental and/or anthropogenic increases of mercury (38).

This article highlights significant advantages and research opportunities in the fields of biology and chemistry, specifically the reconstruction of lifetime chemical profiles (i.e., birth to death) in baleen whales. Lifetime profiles offer significant improvements over costly ship time and conventional intermittent sampling techniques that use blood (39), feces (40), blubber (41⇓–43), morphometric measurements, and/or exhalations (4) as well as conservation advantages in the reduction in the samples (blood, blubber, etc.) required to address a specific research question (3, 41). Using earplugs to age and reconstruct lifetime chemical profiles allows for a more comprehensive examination of stress, development, and contaminants. In addition, earplugs allow for the simultaneous assessment of multiple research questions (e.g., concerning contaminants and hormones) thereby expanding opportunities to address more complex and integrated questions, such as the impact of POP burden on the lifetime stress of an animal. Finally, earplugs allow for examination of both persistent compounds (i.e., POPs) as well as compounds that are metabolized in the body (i.e., hormones). Earplugs may provide a unique opportunity to reconstruct exposure profiles for compounds such as polycyclic aromatic hydrocarbons, which typically undergo rapid biodegradation in tissues such as liver and blubber (44).

One of the most profound advantages offered by earplugs is the ability to retrospectively examine critical issues through the analysis of archived museum samples, some of which were harvested in the 1950s. A comprehensive database could be derived by combining the analysis of multiple earplugs harvest over multiple generations. Further, this innovative tool increases the feasibility of accurately assessing anthropogenic impact on everything from an individual organism to marine ecosystems. Without such data, there is no context with which to interpret the biological significance or anthropogenic impact of individuals or populations.