It has been known for some time that there are a number of ductless glands in the body and that the secretions of these glands (and from others having ducts) play a very important role in the growth and development of the body. The action exerted by the ductless glands and from certain glands having ducts (the endocrine glands) is equally important in the functioning of the fowl's body as with the human species.
At the outset if must be admitted that our knowledge of the working of the endocrine glands is fragmentary – and moreover, that what knowledge we possess is not all, as yet, of practical importance to the poultry keeper. Nevertheless, the application of a small part of that knowledge has already indicated that substantial changes in certain commercial methods of management may take place. A brief sketch of other possible lines work on which these possibilities may be based will, no doubt, be of interest to the poultry keeper whose horizon stretches beyond the immediate problems of the day.
A survey of the whole field – limited as our knowledge is – would be too extensive for the scope of a single article, and the following account is therefore concerned with one aspect of endocrinology – the effect of the secretions of the sex glands, since it is in their field that practical application of our knowledge appears to have made the greatest advance.
The secretions of the sex glands appear to control the sexual characteristics of the bird. Thus, those secretions from the male organs result in the copulative habits of the male bird, the development of male plumage, the comb, wattles and the male voice. In the case of the female, the feminine habits and sexual characteristics derive from the secretions of the female sex organs.
A simple demonstration is the caponizing of the male bird leading to certain feminine habits and the shrinkage or depression of the male attributes, such as the large size of the male comb and wattles.
The secretions, or hormones as they are collectively called, are known as androgens in the case of the male, while those of the female are known as estrogens.
These terms cover several substances, but of major importance is the fact that a large number of substances having a similar chemical composition – and more important – having the same biological properties, can by synthetically prepared. Now the knowledge of the role played by these sex hormones suggested that some advantage might result from the treatment of fowl by androgens or estrogens.
It seemed possible that the injection of female sex hormones into hens or pullets might stimulate the female habit of egg production. A similar treatment directed towards the male might lead to feminine bodily characteristics of value in the table poultry industry, while treatment of the incubating egg with estrogenic substances (female sex hormone) might lead to the production of female chicks only.
It is probably known that the gonads or sex glands are very similar in shape until the sixth day of embryonic development. With the male the two gonads develop into two equally active testes or male gonads, but in the case of the female it is only the left gonad that becomes active on maturity as the ovary – the right remaining rudimentary. What the scientist endeavors t do is to change a genetic male, i.e., an embryo that would normally hatch as the production of inter-sexes – chicks showing the attributes of both mal and female. The several investigations seem to indicate, however, that some of the male hormone materials lead to ambi-sexual activities.
Treatment of genetic males with female hormone materials, i.e., estrogens, seems more successful. The genetic males produced from incubated eggs so treated appear to have developed the sex organs of a female to a lesser or greater degree. In the case of genetic females examples have been produced showing two incompletely developed oviducts, and in a case cited by Greenwood, the bird laid shell-less eggs.
It seems not at all impossible that in the not too distant future treatment of the embryo with female sex hormones (estrogens) may lead to the production of female chicks only – an obvious advantage from the viewpoint of the table egg producer.
No great attempt appears to have been made to ascertain whether a male bird can be turned into a female. The variation between the gonads in the case of the female appears to lead to some complication in trying to carry out this work. Injections with certain male hormone substances into the incubating egg have certainly led to stimulation of egg production and can be achieved by the treatment of the hen with female hormone material. One of the reasons may be the extremely complicated nature of the problem. A stimulation of one activity alone may not result in the desired end if other activities do not receive an equally strong stimulus, and it will be borne in mind that egg production is an extremely complicated process.
Nevertheless there is some evidence that the injection of one estrogenic substance leads to increased secretion of albumen, while another hormone – prolactin – the secretion of the pituitary gland that induces broodiness.
Cow manure apparently contains an androgenic (male hormone) substance, and the inclusion of dried cow manure in a normal mash has been shown to lead to depression of egg production. Dried cow manure with the androgenic substances destroyed included in a mash containing no animal protein did, however, materially improves hatchability.
One important point that is apparent throughout the work of most investigators of the problems is the fact that in most instances the changes induced are temporary. This is not surprising since in the normal bird the various hormones are being continually secreted. Until this difficulty can be overcome, it may from the financial viewpoint prove a limiting factor over the practical application of the work.
This short term effect, no doubt, led to the belief that short term feminisation of males would be of practical value for the table poultry industry, and might do away with the need for caponisation. The theory held was obviously that the stimulation of feminine behavior would lead to results identical with those reached by caponisation. Several treatments with estrogens – notably diethylstilbestrol – have been attempted. The treatment has been carried into effect with both male and female birds.
The work is still in the experimental stage and it is not surprising to find some differences of view held by the many workers concerned. With old hens opinion seems generally in accord with the view that no change in the carcass quality takes place.
With male birds varying opinions are held, some workers maintaining that no change in weight takes place but appearance and texture of skin is improved, others take the view that deposition of the fat – but not necessarily amount of fat – is affected, while still other investigators state that an increase in carcass weight is achieved.
Quite possibly these varying views arise from the different estrogenic materials used, different methods of implantation, and the varying ages of the birds. The length of treatment normally extends over about four weeks.
Obviously much more investigation is necessary and it must not be forgotten that complete absorption of the estrogen must be assured for the substances may be effective with the human consumer of the bird!
To sum up: In spite of our limited knowledge of the subject it is clear that the use of sex hormones may be of great advantage to poultry keeping in the future. Three attractive fields of research have been indicated and no doubt they will be of interest to the workers at the research centres recently set up in this country. The practical application of fresh knowledge in this field may be a possibility far sooner than many imagine.