Improving the quality of Australian crocodile skins

S.C. Manolis; G. Webb; K.C. Richardson

The number of crocodile farms around the world has increased greatly over the last decade. As a result of increasing numbers of farmed crocodile skins on the international market, grading standards have become very strict. Saltwater Crocodiles (Crocodylus porosus) have held their reputation as having the best skin of all of the crocodilian species, but this counts for little if skin quality is compromised. The presence and/or extent of physical damage (eg scratches, bites, processing cuts) contributes significantly to the downgrading of skins, and is usually related to husbandry regimes. In some cases it may be related to disease (eg brown spot, pox virus), the effects of which sometimes may not be apparent until after processing (tanning). Farmed skins invariably are wider per unit length than wild skins, the result of fast growth rates and types of food in a captive situation. The relatively high fat contents of captive diets, coupled with the highly efficient crocodilian system for storing fat, causes farm animals to be more obese and ‘wider’ than their wild counterparts. The relatively high value of C. porosus skins exists because of the characteristics of the species, namely: there is no bone in the belly scales; the ventral scales are of similar size; and, there is a high number of scale rows on the belly surface (i.e. more scales). We found considerable variation in the number of scale rows in C. porosus, ranging from 27-37 rows (mean = 31.2 rows). From the product manufacturers point of view, the more scales on the belly, the better the skin for many high quality fashion products. However, a very low proportion (<2%) of animals examined possess the highest numbers of rows (35-37), and so a skin with this characteristic does not have a premium price. Some manufacturers indicated that a regular supply of skins with higher numbers of rows would be very well received. Manipulation of the incubation environment was considered a way to increase the number of scale rows on an animal (the number of rows is set by the time of hatching). There was a significant effect of incubation temperature on the mean number of scale rows, with higher temperatures producing a higher mean number of rows. Scale formation is visibly evident by 30-35 days (at 30C), about one- third the way through incubation, and so pulse and switch experiments were undertaken to determine the sensitive period at which scale formation could be affected. These experiments revealed that the sensitive period was between 26 and 30 days for high temperature (33C) incubation, and 10-30 days for cool temperature (30C) incubation. A genetic component was also identified as affecting the number of scale rows. Examination of the progeny of known nesting pairs of adult C. porosus indicated that the female was not significant, but the male was. That is, there was a significant (although highly variable) relationship between the number of scale rows on the male and that on his progeny. With farms having an increasing reliance on captive-bred hatchlings (there is no other option in Queensland), this result indicates that selection of male breeding stock is more important than is currently thought. The embryonic development of the skin of C. porosus occurs in four stages. The first stage (14-21 days) is characterised by a single layered epidermis, below which lies a poorly defined dermis. Stage 2 (22-34 days) is characterised by a thickened multilayered epidermis, and the dermis consists of loose fine fibers. Both epidermis and dermis thicken during this period, to 11 μ m and 50 μ m respectively. Small elevations in the epidermis correspond to integumentary sense organs of older embryos. By Stage 3 (35-66 days) the dermis appears as two discrete types - loose dermis underlying the epidermis, and dense dermis. The dense dermis thickens rapidly at a steady rate, and continues to do so until hatching. The loose dermis decreases in thickness, from about 38 to 18 μ m. The epidermis does not change thickness between 26 and 40 days, remaining at about 11 μ m. This was different to the situation observed in Australian Freshwater Crocodiles (C. johnstoni), where the epidermis grows steadily throughout the incubation period. The significance of this difference is unknown, but the period of stable epidermal thickness corresponds to the sensitive period indicated by incubation experiments. Stage 4 (67 days to hatching) is characterised by a thick dermis, the presence of birefringent fibers/granules in the loose dermis and the formation of a definite keratin layer. The periderm becomes progressively thinner after about 60 days, and is totally lost before hatching. At hatching the epidermis is about 28 μ m thick, and consists of four strata (s. germanitivum, s. spinosum, s. granulosum, s. corneum). The dense dermis from one scale traverses below the interscalar cleft to embed into the dense dermis of the adjacent scale. The development of all the major skin components was quantified. The growth of the skin as a whole (to about 250 μ m at hatching) proceeds in a linear fashion. The skin continues to grow in a linear fashion after hatching, with general thickening of the epidermis and dermis. As a high proportion of the skin thickness is attributable to dermis (generally greater the 80%), the development of the dermis reflects that of the total skin. At hatching skin thickness from different sites on the body was very similar, but differences become apparent by one year of age. The skin on the ventral surfaces (e.g. belly, tail) was thicker than sites elsewhere (e.g. neck, wrist), a difference more apparent after one year of age. In addition, keratin made up a higher proportion of the epidermis in the ventral surfaces. For a given sized crocodile, from the wild or farmed, skin thickness was highly variable. The skin from the chin of wild crocodiles was thicker (33%) than that of farmed crocodiles, confirming the information from skin buyer s and tanners that wild skins tend to be thicker than farmed skins. Interestingly, the increase in overall thickness was the result of thicker dermis and thinner epidermis. In the wrist the wild skin was s lightly thinner than farmed ones, but the epidermis was still relatively thinner (by about 50%). Within different types of skin, four patterns of dermal architecture were identified. These varied on the basis of the arrangement of the collagen bundles making up the dermis. These longtitudinal bundles tended to be narrower and more compact than in farmed animals, and it is felt that changes in these orthogonal layers may reflect differences between wild and farmed animals. Growth rates on crocodile farms far exceed those of wild crocodiles, and together with a fattier diet, may alter the dermal structure of the skin. Certainly, skin buyers and tanners were largely ignorant of the morphological characteristics of crocodile skins. Saltwater Crocodiles are widely distributed and extensively farmed and/or harvested (e.g. Malaysia, Thailand, Papua New Guinea, Indonesia, Palau, Solomon Islands, Singapore, India) throughout their range. There appears to be a high degree of variation between C. porosus from different countries and farming operations, and there are probably interaction of factors influencing the quality of skins from them.

Improving the quality of Australian crocodile skins

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