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1. Reproductive Steroid Hormone Patterns in Baleen of Two Pregnant Humpback Whales ( Megaptera novaeangliae )

   https://doi.org/10.1093/icb/icac070  

   Lowe, Carley L; Hunt, Kathleen E; Neilson, Janet L; Gabriele, Christine M; Teerlink, Suzie S; Buck, C Loren (June 2022, Integrative and Comparative Biology)

   Abstract Understanding reproductive physiology in mysticetes has been slowed by the lack of repeated samples from individuals. Analysis of humpback whale baleen enables retrospective hormone analysis within individuals dating back 3–5 years before death. Using this method, we investigated differences in four steroid hormones involved in reproduction and mating during confirmed pregnant and non-pregnant periods in two female humpback whales (Megaptera novaeangliae) with known reproductive histories based on sightings and necropsy data. Cortisol, corticosterone, testosterone, and estradiol concentrations were determined via enzyme immunoassay using subsamples of each baleen plate at 2 cm intervals. There were no significant differences in cortisol or corticosterone during pregnancy when compared to non-pregnancy (inter-calving interval), but there were significant differences between the two whales in average glucocorticoid concentrations, with the younger whale showing higher values overall. For testosterone, levels for the younger female peaked at parturition in one pregnancy, but also had spikes during non-pregnancy. The older female had three large spikes in testosterone, one of which was associated with parturition. Estradiol had large fluctuations in both whales but had generally lower concentrations during non-pregnancy than during pregnancy. There were peaks in estradiol before each pregnancy, possibly coinciding with ovulation, and peaks coinciding with the month of parturition. Both estradiol and testosterone could be useful for determining ovulation or impending birth. Using baleen to investigate retrospective steroid hormone profiles can be used for elucidating long-term patterns of physiological change during gestation. Lay summary Case studies of two pregnant humpback whales whose hormones were analyzed in baleen may illuminate when humpback whales ovulate, gestate, and give birth. These physiological metrics could assist in accurate population growth assessments and conservation of the species. This study shows that baleen hormone analysis can be a useful tool for understanding whale reproductive physiology. 

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2. Multi-year progesterone profiles during pregnancy in baleen of humpback whales ( Megaptera novaeangliae )

   https://doi.org/10.1093/conphys/coab059  

   Lowe, Carley L; Hunt, Kathleen E; Rogers, Matthew C; Neilson, Janet L; Robbins, Jooke; Gabriele, Christine M; Teerlink, Suzie S; Seton, Rosemary; Buck, C Loren (January 2021, Conservation Physiology)

   Narayan, Edward (Ed.)

   Abstract Understanding calving rates of wild whale populations is critically important for management and conservation. Reproduction of humpback whales (Megaptera novaeangliae) is difficult to monitor and, even with long-term sighting studies, basic physiological information such as pregnancy rates and calving intervals remain poorly understood in many populations. We hypothesized that pregnant whales have sustained elevations in baleen progesterone that temporally correlate with gestation. To test this hypothesis, baleen progesterone profiles from two adult female North Pacific humpbacks, both with extensive sighting records and documented pregnancies, were compared to those of a nulliparous female (adult female never seen with a calf) and a juvenile male. Baleen specimens recovered during necropsy were subsampled every 2 cm from the base to the tip of the plate, with each interval representing 30–45 days of growth. Homogenized baleen powder was assayed for progesterone using enzyme immunoassays. The date of growth of each sampling location on the baleen plate was estimated based on stable isotope analysis of annual δ15N cycles. Progesterone profiles from both pregnant whales showed sustained high progesterone content (>350 ng/g) in areas corresponding to known pregnancies, inferred from calf sightings and post-mortem data. The younger female, estimated to be 13 years old, had higher progesterone during pregnancy than the 44.5 year old, but levels during non-pregnancy were similar. The nulliparous female and the male had low progesterone throughout their baleen plates. Baleen hormone analysis can determine how progesterone concentrations change throughout gestation and has potential for estimating age at first reproduction, pregnancy intervals, failed pregnancies and early calf mortality. Understanding rates of calving and current and historic reproductive patterns in humpbacks is vital to continuing conservation measures in this species. 

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3. Prolonged baleen hormone cycles suggest atypical reproductive endocrinology of female bowhead whales

   https://doi.org/10.1098/rsos.230365  

   Lysiak, NSJ; Ferguson, SH; Hornby, CA; Heide_Jørgensen, MP; Matthews, CJD (July 2023, Royal Society Open Science)

   Serial measurements of hormone concentrations along baleen plates allow for reconstructions of mysticete whale reproductive histories. We assessed gestation and calving interval in bowhead whales (Balaena mysticetus) by measuring progesterone, oestradiol, corticosterone and nitrogen stable isotope ratios (δ¹⁵N) along baleen of 10 females from the eastern Canada-west Greenland population. Three immature females (body size < 14.32 m) had uniformly low progesterone concentrations across their baleen, while seven mature females (body size ≥ 14.35 m) had repeated, sustained elevations of progesterone indicative of pregnancies. The mean duration of progesterone elevations (23.6 ± 1.50 months) was considerably longer than the approximately 14 month gestation previously estimated for this species. We consider several possible explanations for this observation, including delayed implantation or sequential ovulations prior to gestation, strategies that would allow females to maximize their fitness in variable Arctic conditions, as well as suggest modified criteria defining gestation as a shorter component of the entire progesterone peak. Calving intervals varied within and among individuals (mean = 3.7 years; range = range 2.8–5.7 years), providing population-specific reproductive estimates for growth models used in bowhead whale management and conservation. 

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4. Male Bowhead Whale Reproductive Histories Inferred from Baleen Testosterone and Stable Isotopes

   https://doi.org/10.1093/iob/obac014  

   Hunt, Kathleen E; Buck, C Loren; Ferguson, Steven H; Fernández Ajo, Alejandro; Heide-Jørgensen, Mads Peter; Matthews, Cory J (January 2022, Integrative Organismal Biology)

   Synopsis Male mammals of seasonally reproducing species typically have annual testosterone (T) cycles, with T usually peaking during the breeding season, but occurrence of such cycles in male mysticete whales has been difficult to confirm. Baleen, a keratinized filter-feeding apparatus of mysticetes, incorporates hormones as it grows, such that a single baleen plate can record years of endocrine history with sufficient temporal resolution to discern seasonal patterns. We analyzed patterns of T every 2 cm across the full length of baleen plates from nine male bowhead whales (Balaena mysticetus) to investigate occurrence and regularity of T cycles and potential inferences about timing of breeding season, sexual maturation, and reproductive senescence. Baleen specimens ranged from 181–330 cm in length, representing an estimated 11 years (smallest whale) to 22 years (largest whale) of continuous baleen growth, as indicated by annual cycles in stable isotopes. All baleen specimens contained regularly spaced areas of high T content (T peaks) confirmed by time series analysis to be cyclic, with periods matching annual stable isotope cycles of the same individuals. In 8 of the 9 whales, T peaks preceded putative summer isotope peaks by a mean of 2.8 months, suggesting a mating season in late winter / early spring. The only exception to this pattern was the smallest and youngest male, which had T peaks synchronous with isotope peaks. This smallest, youngest whale also did not have T peaks in the first half of the plate, suggesting initiation of T cycling during the period of baleen growth. Linear mixed effect models suggest that whale age influences T concentrations, with the two largest and oldest males exhibiting a dramatic decline in T peak concentration across the period of baleen growth. Overall, these patterns are consistent with onset of sexual maturity in younger males and possible reproductive senescence in older males. We conclude that adult male bowheads undergo annual T cycles, and that analyses of T in baleen may enable investigation of reproductive seasonality, timing of the breeding season, and life history of male whales. 

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5. Ecology and life history of baleen whales inform climate change vulnerabilities and priorities for future monitoring

   https://doi.org/10.1093/icesjms/fsag042  

   Thorne, Lesley H; Meyer-Gutbrod, Erin (March 2026, ICES Journal of Marine Science)

   Baleen whales are key consumers in marine habitats that serve as vectors of nutrients, but their populations have been slow to recover from past commercial whaling due to their low reproductive rates and ongoing anthropogenic threats. Climate impacts have become central to the demography and habitat use of baleen whales, and conservation efforts must account for these impacts to be effective. However, knowledge of baleen whale climate responses is lacking, and current survey effort is insufficient to capture changes in migration and habitat use for many species. Due to their unique combination of ecological and life history characteristics, baleen whales are particularly vulnerable to climate change and their climate responses should be expected to differ fundamentally from those of other marine consumers. These characteristics include the need for both large quantities and high densities of prey, the need to accumulate large energy reserves seasonally, as well as highly migratory movements and the reliance on high-latitude foraging areas with narrow seasonal windows in resource availability. Climate responses in baleen whales may involve abrupt changes in foraging habitat in contrast to more gradual poleward shifts observed in other species, changes in the timing, extent or tendency for migration, and changes in energy accumulation or fitness. Recognizing that climate impacts may differ from other species is critical to measuring and anticipating changes to baleen whale populations in the face of ongoing climate change, and to effectively managing their populations in the future. North Atlantic right whales (Eubalena glacialis), arguably the best studied baleen whale species at a population scale, exemplify expectations for baleen whale climate responses and the implications for future management and conservation. Right whales have shown abrupt changes to traditional patterns of habitat use and migratory behavior, with major implications for the effectiveness of existing protections. Understanding and detecting baleen whale climate responses and effectively guiding management in the face of ongoing change will require increased survey effort and novel methods to survey remote habitats; improved knowledge of mechanisms of prey aggregation and climate impacts on these processes; and continued development and refinement of mechanistic models and dynamic management strategies. 

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