LIGHT AND SEASONAL REPRODUCTION IN ANIMALS

Summary Some fifty species of animals have provided the material for the investigations reviewed. The majority have been birds, but mammals, reptiles, amphibia, fish and invertebrates have also been used. A list is provided. Most of the animals investigated have been species with short breeding seasons restricted to the spring in the northern hemisphere. In their main aspects the results have shown considerable uniformity, exceptions coming primarily from species that normally have no breeding periodicity, such as domestic rabbits, or animals of tropical origin. Species hibernating during the winter, or aestivating in summer, and in their natural state removed from exposure to daylight for several months of the year, have also provided exceptions. The underlying principle is to be found in the fact that when such periodically breeding species are subjected to artificially lengthened days in the winter or late autumn they can be brought from the sexually quiescent condition characteristic of winter into the breeding condition typical of spring. The basic factor appears to be the length of day to which they are exposed. The analogous phenomenon in plants has been termed photoperiodism by botanists. A brief review of this phase is given. Until 1924 the reproductive rhythm in birds was attributed to the rising temperatures of spring, yet in the practice of yogai in Japan, many centuries old, and the induction of the muit in Holland, also going back a long way in history, light had been applied practically to induce winter singing. The underlying principle was unrecognized. Increased egg production in hens through the use of artificial light, first practised a century ago in Spain, is shown to be essentially different from the principle here dealt with. It may be stated in general that animals (primarily birds) of the northern hemisphere, with short reproductive periods, show analogous responses in the following respects: Days increased to spring duration by means of artificial illumination in mid‐winter induce development of the sex organs. Spring days artificially curtailed to winter length induce regression. Various birds may be brought into breeding condition two or even three times in a year, but a specific period of rest is required by the organs between one developmental peak and another. It has been shown by numerous investigators that the reproductive rhythm depends on pituitary activity and that upon removal of this organ atrophy of the sex organs follows. The effect of extended lengths of day thus appears to be secondary as far as the gonads are concerned, ceasing after hypophysectomy. Methods of administering artificial light seem to be important. Small graduated increases result in highly developed organs completely normal in size and histology. Longer increases induce irregularities and premature maturation of the spermatic elements in the testes (of birds) and have, in an extreme case, resulted in a significant rate of mortality in the animals employed (starlings). Wave‐length, beyond certain limits, is of doubtful significance, the visible rays generally being effective provided they are of sufficient intensity. Intensity, once a certain quite low value has been reached, appears to be of no further significance, and increased intensity, if duration remains the same, has no augmenting effect. Ultra‐violet light, administered in the form of “sun‐lamp”, induces development in birds at the same rate as an ordinary electric light bulb (devoid of ultraviolet) of similar intensity, if given in comparable doses. The case is somewhat doubtfully analogous in mammals. Temperatures, with birds, appear to be immaterial, but low temperatures in fish may exert a retarding effect. The most highly favoured theory put forward to account for the observed facts is that light falling on the eye stimulates the pituitary (presumably through nervous channels) which in turn induces gonadal development. Numerous authors believe light, qua light, to be an integral factor in the results obtained. Experiments to determine the issue have taken the form of blinding the experimental animals either by the use of opaque hoods over the head, or by severance of the optic nerve, or by total removal of the eye, or by lesion of certain nerve tracts of the brain. The results have been largely contradictory and the question remains open. An alternative viewpoint is that light is concerned only in so far as it provides a means of keeping the animals awake and physiologically active, but is in itself of no further significance, and that increasing diurnal activity, induced by increasing increments of illumination, is the stimulating factor that activates the pituitary. This hypothesis is supported by experiments in which birds (juncos) have been brought to full breeding condition by mechanical disturbance in feeble light far below the intensities required to bring about development when light alone is employed as the agent to induce wakefulness. Also by the fact that when (in the experiments alluded to in the previous paragraph) blinded birds have been kept together with unblinded individuals, the gonads of all have developed at the same rate, yet if the blinded birds are kept by themselves, and then given additional light, development of the organs does not take place. It is suggested that the disturbance forced on the blinded individuals by the unblinded kept with them is, in effect, similar to the mechanical disturbance imposed on juncos. When the source of disturbance (unblinded birds) is removed, there is no gonadal development. It may reasonably be assumed that with the eyes removed light can no longer be perceived and that only disturbance remains as the means of inducing wakefulness over the allotted period of time. London starlings, roosting in the city nightly in light derived from street lamps below the minimum required to induce sexual development, may attain breeding condition in early February. The birds roost over main traffic thoroughfares and are kept in a state of wakefulness until the theatre crowds have subsided some time after midnight. Traffic disturbance is an undoubted factor in forcing these birds to a daily period of wakefulness beyond normal winter duration. How far an inherent reproductive rhythm has influenced the numerous experiments involved in this review is open to question. It has been shown with more than one species that if the animal be kept on a short length of day into the spring, a small increase of the gonads occurs nevertheless and evidently reflects an incipient rhythm independent of the immediate effects of length of day. Little is known of this factor but its presence has been demonstrated on various occasions and may add greater significance to the time of year at which experiments are undertaken. Adaptation to light conditions, of such profound importance in the matter of successful reproduction, appears to be the keynote to reproductive cycles in the northern hemisphere. Wholly unrelated animals have inevitably developed similar reactions. The method by which such adaptations have become established in various groups or species is, however, not necessarily the same, and generalizations, in our present state of knowledge, may be entirely fallacious.