Hydration sensitivity of ostrich claw keratin

Abstract

In a recent paper [1] I reported the first measurements of the elastic properties of the keratinous covering of the ostrich claw. Of course, as well as having to transmit and withstand the substantial [2] forces during locomotion the claw material too must resist abrasive wear from contact with substrates. Ostriches (Struthio camelus L.) naturally inhabit arid desert or savannah regions of the African continent, however since antiquity [3] they have become extensively domesticated and now occur in temperate environments where the climate is cooler and more damp. Indeed, it has been shown that ostriches will alter their behavior when faced with a cold, damp northern European climate [4]. Under such conditions it is likely that the claw material will absorb water from the environment and its water content will increase. We know that increasing the water content of the α-keratin, from which the claws and hairs of mammals are composed, leads to reduced stiffness, strength and hardness [5–8] but toughness is highest at intermediate hydration levels [9]. We do not have a comparable body of literature relating to the β-keratin occurring in birds; only one study has identified that increased water content leads to decreases in stiffness and strength of feather keratin [10]. The effect of two levels of hydration on microhardness and Young’s modulus was determined on samples of claw keratin from six ostriches. For microhardness testing small cubes (∼9 mm3) were tested after polishing with 600 and 1200 grit “wet and dry” paper and a final napping with 0.5 micron diamond cutting paste. Vickers microhardness tests were carried out using a Zwick 3212 test machine with a test load of 20 g. Indenter descent time and dwell time of 15 s were allowed and a 45 s period allowed prior to measuring the indentation. Specimens were first tested “dry” (conditioned at 50% RH) and then “wet” after 72 h immersion in distilled water. Six specimens were tested, their hardness being the mean of ten indentations at random points over their surfaces. The tensile Young’s modulus was determined for specimens having a cross-sectional area of approximately 1 mm2 and a mean length of 9.77 (SEM = 0.37) mm. Specimens for testing “wet” and “dry” were cut from adjacent areas of claw to minimise the effect of any regional variation in properties. Tests were conducted by using a Davenport-Nene test machine and were loaded using a low test speed (1 mm min−1). The test machine and grips had been calibrated for compliance and a correction factor applied to measured