inhibition of spermatogenesis and sperm sus- 
tentation in birds. Experimentally spermato- 
genesis has been shown to be inhibited by re- 
duction of light, deficiency of vitamin E, 
altered hormonal balance, hybridization, short- 
wave irradiation and by the action of drugs. 
The reduction of light obviously is imprac- 
tical for operational use in the natural habitat. 
Mammals are more sensitive to vitamin E 
deficiency than are birds, Carroll and Noble 
(1957) reported that male rats raised with a 
supplement of 10 percent or more by weight of 
erucic acid suffered a progressive reduction 
in spermatogenesis and became completely 
sterile after about 5 months despite normal 
growth and goodhealth. A vitamin E antagonism 
is suspected. However, the use of most nutri- 
tive deficiencies in inhibiting spermatogenesis 
is impractical, as the diets in wild populations 
cannot be restricted where natural foods are 
available ad libitum. 
The genetic isolation conferred upon the first 
generation of hybrid crosses is well known. 
Mules, be they mammalian or avian, are 
sterile, but who can enforce hybridization in 
nature, and (in contrast to work with insects) 
it is impractical to release large numbers of 
vertebrates propogated from the laboratory. 
One of the management problems infisheries 
is that fish tend to overpopulate or exceed the 
carrying capacity of the brooks and ponds. 
Where there are too many fish for the amount 
of food available, the fish grow in numbers but 
not in size. Anglers would rather have larger 
even though fewer quarry. In a closed system, 
such as in a fishpond, it is possible, although 
expensive, to kill by rotenone all the fish and 
restock with individuals of one sex only. This 
type of facultative sterility has actually been 
practiced effectively in the reclamation pro- 
gram of the Massachusetts Division of Fish- 
eries and Game. 
The hormonal balance, which regulated the 
formation of spermatozoa in the testis, has 
been a natural site of attack for reproduction 
prohibitionists. The administration of prolactin 
is antagonistic to gametogenesis and also in- 
hibits gonadotropic hormone secretion (Bates 
et al., 1937). However, proteinaceous hormones 
cannot be administered orally and it is im- 
practical to capture and inject enough animals 
from any appreciable fraction of a wild popula- 
tion. Androgens and estrogens and progestins, 
106 
natural or synthetic, have metabolic potency 
when ingested by depressing the testes by way 
of the pituitary. Diethylstilbestrol (D.E.S)- 
induced capons that were once available onthe 
poultry market bear testimony to the effective- 
ness of the hormonal approach to sex reversal, 
but those capons were produced by implantation 
of longlasting hormonal pellets under the skin. 
The ingestion of hormones (the ethinylated 
steroids are more effective orally than the non- 
ethinylated) is ineffective unless continued over 
an extended period, and most of these hormones 
are prohibitively expensive. In rats and mice 
an acute androgen-sensitive vulnerability to 
induction of permanent sterility exists within 
a period between birth and the 10th day of age. 
The vulnerability (not the sterility) disappears 
after the 10th day of age. Barraclough (1961) 
thought the physiological mode of action was 
through the crippling of pituitary growth. 
Viviparous fishes (of no importance in fish- 
eries management) can be sex directed by 
hormones, aS can many species of frogs 
(Witschi and Crown, 1937). 
Paralleling the familiar medical immunity 
response that develops from vaccination in 
the practice of disease prevention, vertebrates 
can be made immune to fertilization by inocula- 
tion with a vaccine containing gonadal or 
germinal tissues or fluids. Immunologic pro- 
cedures include the formation of antigonado- 
trophic substances, the formation of antibodies 
in the agglutination process, the formation of 
antibodies against spermatogenesis, and even 
the formation of antihormone antibodies. Nelson 
(1961) has summarized some of this work in 
its application to humans. In birds, Wentworth 
and Mellen (1964) at the University of Massa- 
chusetts have recently induced active immunity 
to fertilization and suppression of spermato- 
genesis by means of testicular antigen. 
Freund et al, (1953) showed aspermatogenesis 
in a rodent. Until an orally effective immunity 
can be induced, this line of approach will have 
little but theoretical value to control of large 
wildlife population. Inoculation of elk by means 
of propelled syringes, however, may be apos- 
sible contemporary use of the injection tech- 
nique. 
The effects of ionizing radiation usually come 
to mind when one begins to search for a 
weapon of sterilization. The radiosensitivity of 
birds has recently been reviewed by Wetherbee 

