Zinc concentrations in teeth of female walruses reflect the onset of reproductive maturity

Patterns of trace element deposition in beluga whale teeth reflect early life history

Determination of trace element concentrations in continuously growing biological structures such as otoliths, whiskers, and teeth can provide important insight into physiological and ontogenetic processes. We examined concentrations of 11 trace elements (Li, Mg, Mn, Cu, Zn, Se, Rb, Sr, Cs, Ba, Pb) in the annual dentine growth layer groups (GLGs) of teeth of 66 Eastern Canadian Arctic belugas (Delphinapterus leucas). Several of these trace elements displayed clear and consistent patterns in early life, though few longer term trends or signals were present in trace element data for either females or males. Large changes in Sr and Ba concentrations in fetal dentine reflected in utero shifts in element deposition in the teeth of developing belugas. Marked changes in these elements during the first years after birth were likely associated with the onset of nursing and subsequent weaning. Mg, Mn, and Zn also displayed clear and consistent patterns in early life that correlated strongly with dentine stable nitrogen isotope (δ15N) data, suggesting these elements merit further study as potential tools for studying nursing and weaning. Depositional patterns of Zn and Pb, which have been linked to sexual maturation in female Pacific walruses (Odobenus rosmarus divergens), were inconsistent in beluga teeth. Some individuals (including males) displayed patterns strongly resembling those observed in female walruses, whereas many animals did not, perhaps because they had not yet reached sexual maturity. The lack of clear patterns in trace element deposition after the first few years of life may have resulted from pooling samples from multiple populations/regions collected across more than two decades, but may also indicate that elemental concentrations are primarily driven by other, extrinsic processes later in life, and might be useful as biomonitors of environmental element concentrations or tools for delineating population structure.

Reading the biomineralized book of life: expanding otolith biogeochemical research and applications for fisheries and ecosystem-based management

Chemical analysis of calcified structures continues to flourish, as analytical and technological advances enable researchers to tap into trace elements and isotopes taken up in otoliths and other archival tissues at ever greater resolution. Increasingly, these tracers are applied to refine age estimation and interpretation, and to chronicle responses to environmental stressors, linking these to ecological, physiological, and life-history processes. Here, we review emerging approaches and innovative research directions in otolith chemistry, as well as in the chemistry of other archival tissues, outlining their value for fisheries and ecosystem-based management, turning the spotlight on areas where such biomarkers can support decision making. We summarise recent milestones and the challenges that lie ahead to using otoliths and archival tissues as biomarkers, grouped into seven, rapidly expanding and application-oriented research areas that apply chemical analysis in a variety of contexts, namely: (1) supporting fish age estimation; (2) evaluating environmental stress, ecophysiology and individual performance; (3) confirming seafood provenance; (4) resolving connectivity and movement pathways; (5) characterising food webs and trophic interactions; (6) reconstructing reproductive life histories; and (7) tracing stock enhancement efforts. Emerging research directions that apply hard part chemistry to combat seafood fraud, quantify past food webs, as well as to reconcile growth, movement, thermal, metabolic, stress and reproductive life-histories provide opportunities to examine how harvesting and global change impact fish health and fisheries productivity. Ultimately, improved appreciation of the many practical benefits of archival tissue chemistry to fisheries and ecosystem-based management will support their increased implementation into routine monitoring.

Graphical abstract

Life history in primate teeth is revealed by changes in major and minor element concentrations measured via field-emission SEM-EDS analysis

Overcoming the non-specificity of histological accentuated growth lines in hard tissues is an ongoing challenge. Identifying season at death and reproductive events has profound implications for evolutionary, ecological and conservation studies. Dental cementum is a mineralized tissue with yearly periodicity that continues deposition from tooth formation until death, maintaining a record spanning almost the entire life of an individual. Recent work has successfully employed elemental analysis of calcified incremental tissues to detect changes in extrinsic conditions such as diet and climate and to identify two important life-history milestones: weaning and sexual maturity. Here, we employ field-emission scanning electron microscopy and energy-dispersive X-ray analysis to measure the relative concentrations of calcium, phosphorous, oxygen, magnesium and sodium in the cementum of 34 teeth from seven male and female rhesus macaques with known medical and life-history information. We find that changes in relative magnesium concentrations correspond with reproductive events in females and breastfeeding in infants. Additionally, we observe seasonal calcium patterns in 77.3% of the samples.

Growth, maturity, reproduction, and life expectancy in ex-situ pacific walruses (Odobenus rosmarus divergens)

Background

Pacific walruses are found in Arctic regions of the Chukchi and Bering Sea where rapid changes in environmental conditions resulting in loss of sea ice are occurring. Therefore, accurate life history data are crucial for species management plans and longitudinal data collected over the lives of individual walruses housed in zoos and aquaria provide otherwise difficult to obtain biological information.

Results

While similar at birth, Gompertz regression curves indicated that males grew faster than females (p < 0.0001) in weight (99 kg vs 57.6 kg/y) and length (26.9 cm vs 26.3 cm/y) with physical differences being detected by age 3 for weight and age 7 for length. Males reached adult weight at 13.5 ± 3.3 y and females by age 12.3 ± 2.3 y. The mean age at first ovulation and at first conception occurred at 8.8 y and 9.6 y. Greater than 75% of all conceptions and calving occurred between February and March and from May to June, respectively. Mean gestation lasted 423 d and false pregnancies lasted at least 169 d with a decrease (p < 0.05) in serum progesterone concentration between false pregnancy and pregnancy occurring within 6 months after ovulation. Based on these results, we estimated embryonic diapause to last from 120 to 139 days, and fetal growth last ~ 284 days. All males older than 8 y had an increase in serum testosterone and body weight that was highest in February and lowest in July. Overall, no differences were observed between male and female survival, with a mean (± SEM) life expectancy of 19.5 ± 1.5 y, respectively. Currently, the oldest male and female captive walruses are 40 and 43 y, respectively.

Conclusions

Data provided herein include details of life history characteristics of zoo and aquaria housed walruses that are useful for wild population recovery models. In particular, results on survivorship and the identification of the most vulnerable period for calf survival can help with model development and suggests that for recovery to occur birthing locations for this species must be protected.

Elemental composition of primary lamellar bone differs between parous and nulliparous rhesus macaque females

Extracting life history information from mineralized hard tissues of extant and extinct species is an ongoing challenge in evolutionary and conservation studies. Primary lamellar bone is a mineralized tissue with multidien periodicity that begins deposition prenatally and continues until adulthood albeit with concurrent resorption, thus maintaining a record spanning several years of an individual's life. Here, we use field-emission scanning electron microscopy and energy-dispersive X-ray analysis to measure the relative concentrations of calcium, phosphorous, oxygen, magnesium and sodium in the femora of seven rhesus macaque with known medical and life-history information. We find that the concentration of these elements distinguishes parous from nulliparous females; that in females calcium and phosphorus are lower in bone formed during reproductive events; and that significant differences in relative magnesium concentration correlate with breastfeeding in infants.

Otolith geochemistry reflects life histories of Pacific bluefin tuna

Understanding biological and environmental factors that influence movement behaviors and population connectivity of highly migratory fishes is essential for cooperative international management and conservation of exploited populations, like bluefin tuna. Pacific bluefin tuna Thunnus orientalis (PBT) spawn in the western Pacific Ocean and then juveniles disperse to foraging grounds across the North Pacific. Several techniques have been used to characterize the distribution and movement of PBT, but few methods can provide complete records across ontogeny from larvae to adult in individual fish. Here, otolith biominerals of large PBT collected from the western, eastern, and south Pacific Ocean, were analyzed for a suite of trace elements across calcified/proteinaceous growth zones to investigate patterns across ontogeny. Three element:Ca ratios, Li:Ca, Mg:Ca, and Mn:Ca displayed enrichment in the otolith core, then decreased to low stable levels after age 1-2 years. Thermal and metabolic physiologies, common diets, or ambient water chemistry likely influenced otolith crystallization, protein content, and elemental incorporation in early life. Although similar patterns were also exhibited for otolith Sr:Ca, Ba:Ca and Zn:Ca in the first year, variability in these elements differed significantly after age-2 and in the otolith edges by capture region, suggesting ocean-specific environmental factors or growth-related physiologies affected otolith mineralization across ontogeny.

Exploring the Use of SEM-EDS Analysis to Measure the Distribution of Major, Minor, and Trace Elements in Bottlenose Dolphin (Tursiops truncatus) Teeth

Dolphin teeth contain enamel, dentin, and cementum. In dentin, growth layer groups (GLGs), deposited at incremental rates (e.g., annually), are used for aging. Major, minor, and trace elements are incorporated within teeth; their distribution within teeth varies, reflecting tooth function and temporal changes in an individual's exposure. This study used a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) to determine the distribution of major (e.g., Ca, P), minor (e.g., Cl, Mg, Na), and trace elements (e.g., Cd, Hg, Pb, Zn) in teeth from 12 bottlenose dolphins (Tursiops truncatus). The objective was to compare elemental distributions between enamel and dentin and across GLGs. Across all dolphins and point analyses, the following elements were detected in descending weight percentage (wt %; mean ± SE): O (40.8 ± 0.236), Ca (24.3 ± 0.182), C (14.3 ± 0.409), P (14.0 ± 0.095), Al (4.28 ± 0.295), Mg (1.89 ± 0.047), Na (0.666 ± 0.008), Cl (0.083 ± 0.003). Chlorine and Mg differed between enamel and dentin; Mg increased from the enamel towards the dentin while Cl decreased. The wt % of elements did not vary significantly across the approximate location of the GLGs. Except for Al, which may be due to backscatter from the SEM stub, we did not detect trace elements. Other trace elements, if present, are below the detection limit. Technologies with lower detection limits (e.g., laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)) would be required to confirm the presence and distribution of trace elements in bottlenose dolphin teeth.

Walrus teeth as biomonitors of trace elements in Arctic marine ecosystems

Effective biomonitoring requires an understanding of the factors driving concentrations of the substances or compounds of interest in the tissues of studied organisms. Biomonitoring of trace elements, and heavy metals in particular, has been the focus of much research; however, the complex roles many trace elements play in animal and plant tissues can make it difficult to disentangle environmental signals from physiology. This study examined the concentrations of 15 trace elements in the teeth of 122 Pacific walruses (Odobenus rosmarus divergens) to investigate the potential for walrus teeth as biomonitors of trace elements in Arctic ecosystems. Elemental concentrations were measured across cementum growth layer groups (GLGs), thereby reconstructing a lifetime history of element concentrations for each walrus. The locations of GLGs were used to divide trace element time series into individual years, allowing each GLG to be associated with an animal age and a calendar year. The elements studied exhibited a great deal of complexity, reflecting the numerous factors responsible for generating tooth trace element concentrations. Generalized linear mixed models were used to investigate the importance of age and sex in explaining observed variation in trace element concentrations. Some elements exhibited clear physiological signals (particularly zinc, strontium, barium, and lead), and all elements except arsenic varied by age and/or sex. Pearson's correlations revealed that elements were more strongly correlated among calendar years than among individual walruses, and correlations of trace elements within individual walruses were generally inconsistent or weak. Plots of average elemental concentrations through time from 1945 to 2014 further supported the correlation analyses, with many elements exhibiting similar patterns across the ~70-year period. Together, these results indicate the importance of physiology in modulating tooth trace element concentrations in walrus tooth cementum, but suggest that many trace elements reflect a record of environmental exposure and dietary intake/uptake.

Weaning, parturitions and illnesses are recorded in rhesus macaque (Macaca mulatta) dental cementum microstructure

Many open questions in evolutionary studies relate to species' physiological adaptations, including the evolution of their life history and reproductive strategies. There are few empirical methods capable of detecting and timing physiologically impactful events such as weaning, parturition and illnesses from hard tissue remains of either extant or extinct species. Cementum is an incremental tissue with post eruption annual periodicity, which covers the tooth root and functions as a recording structure of an animal's physiology. Here we test the hypothesis that it is possible to detect and time physiologically impactful events through the analysis of dental cementum microstructure. Our sample comprises 41 permanent and deciduous teeth from male and female rhesus macaques (Macaca mulatta) with known medical, lifestyle and life history information. We develop a semi-automated method of cementum histological analysis for the purpose of event detection and timing, aimed at significantly reducing the amount of intra- and interobserver errors typically associated with histological analyses. The results of our work show that we were able to detect known events including weaning, parturition, illness and physical trauma with high accuracy (false negative rate = 3.2%; n = 1), and to time them within an average absolute difference of 0.43 years (R²  = .98; p < .05). Nonetheless, we could not distinguish between the several types of stressful events underlying the changes in cementum microstructure. While this study is the first to identify a variety of life history events in macaque dental cementum, laying foundations for future work in conservation and evolutionary studies of both primates and toothed mammals at large, there are some limitations. Other types of analyses (possibly chemical ones) are necessary to tease apart the causes of the stressors.

Steroid hormones in Pacific walrus bones collected over three millennia indicate physiological responses to changes in estimated population size and the environment

The Pacific walrus (Odobenus rosmarus divergens) is an iconic Arctic marine mammal and an important resource to many Alaska Natives. A decrease in sea ice habitat and unknown population numbers has led to concern of the long-term future health of the walrus population. There is currently no clear understanding of how walrus physiology might be affected by a changing Arctic ecosystem. In this study, steroid hormone concentrations (progesterone, testosterone, cortisol and estradiol) were analysed in walrus bones collected during archaeological [3585-200 calendar years before present (BP)], historical [1880-2006 common era (CE)] and modern (2014-2016 CE) time periods, representing ~ 3651 years, to track changes in reproductive activity and cortisol concentrations (biomarker of stress) over time. Our results show that modern walrus samples have similar cortisol concentrations (median = 43.97 ± standard deviation 904.38 ng/g lipid) to archaeological walruses (38.94 ± 296.17 ng/g lipid, P = 0.75). Cortisol concentrations were weakly correlated with a 15-year average September Chukchi Sea ice cover (P = 0.002, 0.02, r 2 = 0.09, 0.04, for females and males, respectively), indicating a possible physiological resiliency to sea ice recession in the Arctic. All steroid hormones had significant negative correlations with mean walrus population estimates from 1960 to 2016 (P < 0.001). Progesterone in females and testosterone in males exhibited significant correlations with average September Chukchi Sea ice cover for years 1880-2016 (P < 0.001 for both, r2  = 0.34, 0.22, respectively). Modern walruses had significantly lower (P = < 0.001) reproductive hormone concentrations compared with historic walruses during times of rapid population increase, indicative of a population possibly at carrying capacity. This is the first study to apply bone as a tool to monitor long-term changes in hormones that may be associated with changes in walrus population size and sea ice cover.

Detection of steroid and thyroid hormones in mammalian teeth

Endocrine tools can provide an avenue to better understand mammalian life histories and predict how individuals and populations may respond to environmental stressors; however, few options exist for studying long-term endocrine patterns in individual marine mammals. Here, we (i) determined whether hormones could be measured in teeth from four marine mammal species: narwhal (Monodon monoceros), beluga (Delphinapterus leucas), killer whale (Orcinus orca) and Atlantic walrus (Odobenus rosmarus rosmarus); (ii) validated commercially available enzyme immunoassay kits for use with tooth extracts; and (iii) conducted biological validations for each species to determine whether reproductive hormone concentrations in teeth correlated with age of sexual maturity. Tooth extracts from all species had measurable concentrations of progesterone, testosterone, 17β-estradiol, corticosterone, aldosterone and triiodothyronine (T3); however, cortisol was undetectable. Parallelism between the binding curves of assay kit standards and serially diluted pools of tooth extract for each species was observed for all measurable hormones. Slopes of accuracy tests ranged from 0.750 to 1.116, with r² values ranging from 0.977 to 1.000, indicating acceptable accuracy. Biological validations were inconsistent with predictions for each species, with the exception of female killer whales (n = 2), which assumed higher progesterone and testosterone concentrations in mature individuals than immature individuals. Instead, we observed a decline in progesterone and testosterone concentrations from infancy through adulthood in narwhal (n = 1) and walruses (n = 2) and higher reproductive hormone concentrations in immature individuals than mature individuals in belugas (n = 8 and 10, respectively) and male killer whales (n = 1 and 2, respectively). While unexpected, this pattern has been observed in other taxa; however, further analytical and biological validations are necessary before this technique can be used to assess individual mammalian endocrine patterns.

Evaluating tooth strontium and barium as indicators of weaning age in Pacific walruses

Lactation length and weaning age provide important information about maternal investment, which can reflect the health and nutritional status of the mother, as well as broader reproductive strategies in mammals. Calcium-normalized strontium (Sr) and barium (Ba) concentrations in the growth layers of mammalian teeth differ for nursing animals and those consuming non-milk foods, thus can be used to estimate age-at-weaning. To date, this approach has been used only for terrestrial animals, and almost exclusively for primates.The goal of this study was to determine whether Sr and Ba concentrations in the cementum of Pacific walrus Odobenus rosmarus divergens teeth can be used to estimate weaning age. Teeth from 107 walruses were analysed using laser ablation inductively coupled plasma mass spectrometry, and calcium-normalized 88Sr and 137Ba concentrations were quantified.For most walruses, both Sr and Ba concentrations exhibited rapid changes in early life. Ba concentrations matched closely with expected patterns in the published literature, rapidly declining from high to low concentrations (typically from ~10 ppm to ~5 ppm). In contrast, Sr exhibited a pattern opposite to that presented in studies of terrestrial mammals, appearing nearly identical to Ba (typically declining from ~400 ppm to ~200 ppm). To explain these findings, we present conceptual models of the factors generating weaning signals in Sr and Ba for terrestrial mammals, as well as a new, hypothetical model for walruses. Both a visual and mathematical approach to weaning age estimation indicated a median weaning age of walruses at the end of the second year of life (in the second dark layer of the tooth cementum), with many walruses estimated to have weaned in their third year of life, and a smaller group weaning in their fourth or fifth year. This is later than expected, given a published estimate of walrus weaning at 18-24 months.These results do not conclusively support the use of tooth Sr and Ba for estimating weaning age in walruses, and further research is warranted to better understand the drivers of the observed patterns of Ba and Sr accumulation in walrus teeth.