Abstract
Hairs are fundamental structures for mammals, serving crucial functions such as thermal insulation and hydrophobicity. In domestic animals, hair is also a valuable source of high-performance fibers for the textile industry, which has led to intensive study. However, there is limited comparative knowledge about the physical properties of hair across different wild mammalian species. In our lab, we are investigating the physical properties of hairs from a diverse range of wild mammalian species, laying the groundwork for an in-depth comparative study. These physical properties can be linked to the internal structures of the hairs. Using polarized light microscopy, we can visualize the internal structure of hairs, which are composed of a hollow channel (medulla) surrounded by a cortex and a keratin cuticle(1). By examining the brown hairs of three distinct mammals—the Patagonian mara, the brown bear, and the Amur tiger—we observe striking differences in their internal structures. We speculate that these structural differences correspond to varying physical properties, which we are currently investigating.
METHODS
The hairs were collected after being clipped at the Kolmården zoo (Norrkoping, Sweden) during routine veterinary procedures and stored in dry conditions at ambient temperature (15% < RH < 70%, 20–22 °C). The hairs were collected from the animal back. Since the results of polarized light microscopy are affected by the hair color, all hairs presented in this study were light brown. The hairs were mounted on paper frame with a 1 cm squared window and placed under the microscope in the middle part of the shaft. The brown bear is a female born in 2019, the amur tiger is a female born in 2016, and the Patagonian mara is a female born in 2023. The hairs were visualized with white polarized light using a Microscope Axioscope 5/7 KMAT, Zeiss. This microscope is equipped with a rotary stage, 360° with 45° click-stops.
The hair sample in the paper frame was placed on this rotary stage and rotated 45° from extinction (normal) position to reach the diagonal position where the brightness is at its maximum, thus enhancing the birefringence effect (Fig. 1). The advantage of using polarized light microscopy to study the internal structure of hairs is that it is a noninvasive technique that allows the measurement of quantitative information, such as the birefringence index. This quantitative information can then be correlated with other physical properties, such as the mechanical properties.1
FIG. 1.
The hairs of a Patagonian mara (Dolichotis patagonum, 69 ± 15 μm in diameter), a brown bear (Ursus arctos, 77 ± 7 μm in diameter), and an Amur tiger (Panthera tigris altaica, 74 ± 14 μm in diameter) visualized under polarized light microscopy in the middle part of the shaft. The Amur tiger and brown bear hairs display a clear medulla cavity, while the hair of the Patagonian mara lacks a visible medulla. The varying birefringence effects in the keratin cortex suggest differences in structure.
ACKNOWLEDGMENTS
We would like to extend our heartfelt gratitude to the Kolmården staff for their invaluable assistance with hair collection. Special thanks go to Matilda, Ida, and Nora for their help with the Patagonian mara. We are also deeply grateful to Josefin Larsson for arranging the sampling and to Sunna Edberg for providing brown bear and tiger hair as well as for her inspiring enthusiasm that significantly contributed to this work. We would like to dedicate this work to the bear Freja, may she rest in peace.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Gabriele Greco: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Anna Rising: Project administration (supporting); Supervision (supporting); Writing – review & editing (supporting).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
- 1. Hausman L. A., “ Structural characteristics of the hair of mammals,” Am. Nat. 54, 496–523 (1920). 10.1086/279782 [DOI] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.