Structure and properties of baleen in the Southern right (Eubalaena australis) and Pygmy right whales (Caperea marginata)

Abstract

Baleen is a resilient and keratinised filter-feeding structure attached to the maxilla of mysticete whales. It is strong and tough, yet a pliant and resilient material, that withstands extreme pressures in the oral cavity during feeding. We investigated the structure, water content, wettability and mechanical properties of baleen of the Southern right (SRW) and Pygmy right whales (PRW), to understand the effects of hydration on the physical and mechanical properties of baleen. Sixty 25 × 15mm baleen subsamples were prepared from one individual of SRW and PRW. Half were hydrated in circulated natural seawater for 21 days and half were dry. Water content analysis showed that SRW baleen was 21.2% water weight and PRW was 26.1%. Wettability testing indicated that surfaces of both hydrated and dried SRW and PRW baleen were hydrophilic, with hydrated samples of both species having lower contact angle values. For the SRW, the average contact angle of hydrated baleen was 40° ± 13.2 and 73° ± 6 for dried samples. Hydrated PRW baleen had an average contact angle of 44° ± 15.3, which was lower than in dried samples (74° ± 2.9). Three-point bending mechanical tests showed that the average maximum flexural stress of dried SRW (134.1 ± 34.3 MPa) and PRW samples (117.8 ± 22.3 MPa) were significantly higher than those of hydrated SRW (25.7 ± 6.3 MPa) and PRW (19.7 ± 4.8 MPa) baleen. Scanning electron microscope images showed the stratification of the outer cortical layer, with cross-linked keratin fibres observed within and between baleen keratin sheets. Hydrated baleen, as in its natural and functional behaviour, has greater flexibility and strength, attributes necessary for the complex filter feeding mechanism characteristic of whales. Hydration must be considered when addressing the physical and mechanical properties of baleen, especially when using dried museum specimens.