Our understanding of vibrissal function in pinnipeds is poor due to the lack of comparative morphological, neurobiological, and psychophysical performance data. In contrast, the function of terrestrial mammalian vibrissae is better studied. Pinnipeds have the largest vibrissae of all mammals, and phocids may have the most modified vibrissae. The tactile performance for pinniped vibrissae is well known for harbor seals (Phoca vitulina). Harbor seals display at least two types of tactile behavior involving their mystacial vibrissae: a fine discriminatory capability using active touch and hydrodynamic trail following (the ability to detect and follow turbulent trails). This study investigated innervation patterns of harbor seal follicle‐sinus complexes (F‐SCs) to test the hypothesis that the whiskers used in hydrodynamic trail following possess increased innervation investment compared to other phocids. Therefore, the most lateral vibrissae from five harbor seals were histologically processed so that morphometric measurements and axon counts could be collected. Vibrissae from one harbor seal were immunolabeled with anti‐protein gene product (PGP 9.5) to document the pattern of deep vibrissal nerve innervation of the F‐SCs. Overall, harbor seals showed an innervation pattern (axons/F‐SC and axons/muzzle) similar to other phocids. The ventrolateral vibrissae, involved in hydrodynamic trail following, have greater axon density in harbor seals than harp seals, suggesting harbor seal F‐SC innervation patterns could explain their performance at trail following. The combination of microstructural, innervation investment, and behavioral data provides a foundation for functional inference regarding this tactile behavior in harbor seals and also facilitates future comparative work for other pinniped species. Anat Rec, 302:1837–1845, 2019. © 2019 American Association for Anatomy
Several populations of North Pacific pinnipeds are currently listed as depleted under the Marine Mammal Protection Act, endangered under the Endangered Species Act, of the United States or with unknown status, highlighting the need for new methods to assess the reproductive rates of these populations. Most phocids are annual breeders with estrus and parturition occurring on terrestrial or ice platforms. In phocids, serum progesterone concentrations remain elevated during late gestation, supporting identification of pregnancy after implantation (Gardiner, Boyd, Follett,
Keratinized tissues, including whiskers, are ideal for acquiring a record of physiological parameters. Most tissues provide a snapshot of physiological status; however, whiskers may support longitudinal sampling for reproductive and stress-related hormones, if hormones are incorporated as whiskers grow and concentrations change with physiological state. Whiskers from female Steller sea lions (Eumetopias jubatus) and northern fur seals (Callorhinus ursinus) were serially sectioned and pulverized and steroid hormones were extracted. Standard methods were used to validate enzyme immunoassay kits for cortisol, progesterone, 17β-estradiol and testosterone. All hormones were measurable in whisker segments from both species with progesterone concentrations showing cyclical patterns, which appear to signify previous pregnancies or luteal phases. Yearly progesterone concentrations were greater in years a pup was produced compared with years when no pup was observed. Free-ranging female Steller sea lions had reproductive rates between 0 and 1.0 (0.53 ± 0.33, n = 12) using a yearly progesterone concentration of 30 pg/mg or greater to classify a reproductive year as producing a pup and below 30 pg/mg as non-reproductive. Cortisol concentrations were greater near the root and rapidly declined, lacking any obvious patterns, throughout the rest of the whisker. Progesterone and testosterone concentrations were able to help determine sex of unknown individuals. Immunohistochemistry revealed that steroid hormones most likely do not leach out of whiskers based on the deposition patterns of progesterone and cortisol being present throughout the whisker length. Overall, measuring steroid hormones in whiskers can reveal individual reproductive histories over multiple years in sea lions and fur seals. Cyclical patterns of δ15N were useful for identifying periods of up to ~10 years of growth within whiskers, and measuring both stable isotopes and hormones may be useful for differentiating periods of active gestation from diapause and potentially track multi-year reproductive histories of female otariids.
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