The Reproductive Cycle of the Male Weasel (Mustela nivalis)
Authors: HILL MARGARET 
Published: August 2010

SummaryThree hundred and twenty‐seven male weasels were collected in England and Wales during 1931 to 1935. They have been used to study the limits of the breeding season, the seasonal changes undergone by the reproductive organs, and the development of the reproductive organs in the young animal.The structure of the male reproductive organs is simple, being similar to those of the ferret and the stoat. There are no seminal vesicles and no obvious prostate gland. The testes descend at an early age and remain in the scrotum throughout life.In the development of the skull, jaw, and teeth, and of the os penis, the young male weasel reaches in about six to seven months a condition indistinguishable from that of the adult animal. One hundred and thirty‐four of the weasels have been classified as young animals and one hundred and sixty‐six as adults seven months of age and older.The young are born over an extensive period, from April to possibly as late as September. Males born early in the breeding season, in April and May, reach adult body weight very quickly, and become sexually mature when about four months old. Sperms are present in the testes of the most advanced young towards the end of July and during August. Such testes weigh from 0–107‐0‐222 gm. Young males born later in the season do not attain sexual maturity until the year following that in which they were born. Their rate of growth generally is not so rapid, and by September their testes weigh less than 0–09 gm. Arrest of development and regression begin towards the end of September and the beginning of‐October. By December the testes of all the young animals have reached ancestrous quiescence. At the beginning of the breeding season there is no difference in the time taken to arrive at spermatogenesis between the young and previous adult group of animals, both being active by March.The following points in the development of the reproductive organs of the young male in its first six months of life have been noted:–1. The testis weight is more closely correlated with tubule diameter than it is in the adult.2. After the maximum diameter of the seminiferous tubule has been reached there is no continued lengthening of the tubules and no increase in testis weight.3. Both the absolute growth of the epididymis and its growth relative to its testis weight is less in the young male than in the adult.4. The interstitial cells are smaller than in the normal breeding season adult.5. During the winter months two types of testis can be separated histologically–those which resemble the adult group and belong to early born young and those of the late born young.The testes of adults (animals over seven months) are fully active from March to about the end of August, when signs of regression become apparent, though fertile males have been found as early as February 17th and as late as the middle of October, showing that they may occur over a period of about eight months.Retrogression is rapid, and comparative quiescence is reached at the end of October and lasts until the beginning of the next year. From January to February the testes are preparing actively for the breeding season, the onset of spermatogenesis being perhaps even more rapid than that of ancestrous regression.At no time does the production of spermatogonia and spermatocytes cease entirely. Ancestrous quiescence is therefore slightly less marked than in the ferret and the stoat. It thus appears that in the weasel the anterior pituitary gland is partially active throughout the winter, though not so active as that of the hedgehog, in the testis of which at least two to three rows of spermatocytes are generally found throughout the winter.Seasonal variation in the size of the testis is not quite so great as in the ferret, and there is correspondingly less change in the diameter of the seminiferous tubule.There is a definite cycle in the size of the interstitial cells. They are large during the breeding season, the average cell area being from 180 to 250 sq. p, whilst they are small and crowded together during the non‐breeding season, when the average cell area is 80–90 sq. μ.The cycle in the epididymis is closely related to that of the testis.The vas deferens shows seasonal changes correlated with the cyclic activity of the testis.This work was begun during my tenure of the Keddey Fletcher‐Warr Studentship of the University of London.I wish to express my grateful thanks to Sir Henry Dale, F.R.S., for providing me with facilities at the National Institute for Medical Research, and to Dr. A. S. Parkes, F.R.S., for the use of the material, for reading the paper, and much helpful criticism.I am also indebted to Professor F. W. Rogers Brambell for his most generous assistance in collecting material, to Mr. J. Thornton Carter for his kind help in working out the dentitional characters of the skulls, and to Dr. Matthew Young for his advice on the statistical treatment of certain of the data. I have also to thank Mr. H. B. de Montmorency for pointing out to me the significance of the distortion of the frontal and ethmoid bones present in so many of the weasel skulls.The photo‐micrographs are the work of Mr. F. J. Pittock, to whom my cordial thanks are due.