1. The normal sweat glands of twenty-eight calves, nineteen live adult cattle and eighteen slaughtered cattle from temperate breeds were examined. In every animal the lumen of the sweat glands contained a fluid-like material.

2. It appeared on examination and rough measurement that the size of the sweat glands taken from animals after slaughter was smaller than that of the glands taken from live animals even when the same sampling technique was used for both.

3. The sweat glands of sixteen calves were examined before and at intervals after the animals had been subjected to a hot environment. The glands were always seen full of a fluid-like material.

4. It is concluded that if the calf sweat gland is stimulated by heat stress it must function either (a) by simple diffusion through the sweat gland wall and hence to the skin surface, or (b) by a secretory process not involving degeneration of the glandular epithelium. It is probable that at least part of the contents of the lumen of the sweat glands is derived from the epithelium by a process of active secretion.

Sebum was collected from pre-cleaned areas on the skin of Ayrshire calves at a fixed time using a cup and solvent technique, extracted and weighed. The rate of sebum output decreased with time and after 18 h there was very little increase in the weight of sebum on the skin surface. Sebum output over a period of 3 h was not statistically different between animals of different ages but tended to be higher in castrated males than in females and was lower in winter than throughout the remainder of the year.

A noticeable feature of mammalian skin surface lipids is the considerable variation in lipid and fatty acid compositions that exists between species (Nicolaides, Fu & Rice, 1968). The sebaceous lipids of oxen are characterized by the presence of relatively high levels of triglycerides, which, in contrast to those of other bovine tissues, contain a significant proportion (> 20%) of linoleic acid (Noble, Crouchman & Moore, 1974; Smith, Noble & Jenkinson, 1975; O'Kelly, Reich & Mills, 1980). The uniqueness of these highly unsaturated triglycerides has indicated possible roles in the chemical and biological defence systems of the skin surface for linoleic acid released by hydrolysis (Jenkinson, 1980). The skin surface triglycerides of calves do not attain the high levels of linoleic acid displayed by adults until 4–5 weeks after birth (Noble et al. 1975). It is, however, not known if a new-born calf compensates for this difference by an increase in sebum triglyceride concentration and output or, in view of the decreased availability of linoleic acid at this Stage, produces a sebum of widely different composition from the adult.

Comparative evidence on the topography, climate and chemical nature of the mammalian skin surface ecosystem is reviewed. Potential and established surface defence systems have been identified and their significance and interactions discussed in an attempt to provide a basis for further multidisciplinary study of the protective role of the skin surface, a function which is more extensive than hitherto recognised.

The collected evidence of the Symposium illustrates that sufficient information is now available to enable initial broad definition of the skin surface ecosystems for the different animal classes. As may be expected more is known about the ecosystems of classes (mammal, fish) and genera (e.g. Bos, Salmo) of commercial interest. A notable exception is the skin surface of the bird, about which, despite the development of intensive poultry husbandry, surprisingly little is known. The work to date on the morphology, chemical nature and inhabitants of the different ecosystems, however, provides only limited data on the dynamic processes, actions and co-actions occurring within them and the interchange of inhabitants between them. In particular, the role of the surface micro-climates in the development, survival and transfer of micro-organisms has still to be determined. Our present knowledge is thus almost entirely limited to recognition of the basic elements of the skin surface ecosystems and there is still a paucity of data on the variation between species, individuals and body regions, some of which appear to have specialised functions or like the joints of arthropods may be more susceptible to colonisation and invasion. Nonetheless, although the definition of the various surface ecosystems has by no means been completed, a stage has been reached where identification, classification and comparison of the defence mechanisms operating at the skin surface of different classes of animals can be attempted.

The effects of freeze-branding young beef animals using two coolants were investigated and an attempt was made to avoid hide damage.

1. Branding with irons cooled in liquid nitrogen gave good markings but resulted in severe hide damage.

2. CO2-branding caused much less hide damage but gave fewer legible brands.

3. Legible brands with both coolants always resulted in some marking of the hide.

4. Branding on the jaw provided a satisfactory means of identification without loss of hide value.

The possibility of using a skin measurement as an index of potential milkproducing capacity in cattle was re-examined. Skin specimens were obtained by biopsy from 7 dairy herds, and each of 5 anatomical measurements was correlated with (1) milk yield and (2) butterfat percentage.

Only one of the pooled within-herd relationships was statistically significant. This relationship indicated that within the breeds studied, cows with the least hair follicle depth would be expected to give the highest milk yield. It is suggested that a more extensive investigation of this relationship might yield results of practical value in assessing milk-producing capacity and in progeny testing.