J. theor. BioL (1992) 155, 271-272
LEYrEg TO TrIE EDITOR
Evolutionary Aspects of Mammalian Secondary Sexual Characteristics The recent publication by Scott Freeman (1990) of the University of Washington raises some interesting considerations with regard to the scrotal position of the testicles in male mammals. Clearly it is of considerable evolutionary disadvantage to allow the reproductive organs to be placed in such a vulnerable position, and must be counterbalanced by some advantage. Female mammals of course have always protected their gonads well with the bodily cavity, so their fecundity and fertility may be considered to be of more importance by nature, and this gives clues regarding the observed anomaly in males. The current temperature-dependant concept does indeed fail critically as the author espouses. Not least as there are approximately 1500 ascrotal mammalian species that do produce sperm at body temperature (Carrick & Setchell, 1977). Furthermore external testes appear to be almost unique to mammals an observation which requires explanation. Scott Freeman's explanation is partly flawed in a number of aspects, firstly it does not offer an explanation for this mammalian uniqueness. Secondly, the author's suggestion that external testes result in a poor blood supply to the testes may in a contradictory fashion allow the sperm top become "fitter" by developing more mitochondria, is questionable. It is quite reasonable to suppose that mammals could have evolved with internal testes with a reduced blood supply if the latter, could offer some evolutionary advantage. In addition if temperature dependance was important, a channel to the external environment would have offered a safer means of cooling the testes. This is not in agreement with current observations. External testes may have evolved as a result of more direct reproductive considerations. Throughout species some secondary sexual characteristics appear to have been adopted despite immediate disadvantages conferred on the individual. The peacock, for example, has a considerably curtailed ability to fly as a result of his exotic plumage, however, he is more capable of attracting a female mate. Ritual mating habits of marked complexity have been preserved in evolution. Unique to most mammals is the means by which sexual intercourse itself occurs; this is an important clue. In terms of evolution there should be a strong drive to mate, motivated by the pleasure stimulus from mating. It is likely that the pelvic muscular contractions associated with orgasm in the female assist the passage of spermatozoa to the site of fertilization. Furthermore, in some mammalian species such as the rabbit, cat, ferret and camel, ovulation in the female is dependant on the stimulus derived from coitus (Karsch, 1984). The persistance of the external female clitoris itself in mammals attests to the importance that the stimulation of this organ plays in reproduction. The only way in which stimulation of this organ, in most mammals, may occur during intercourse is by the presence of scrotai testes. In simple terms external testes may increase clitoral stimulation as a result of the method of intercourse employed 271
0022-5193/92/060271 + 02 $03.00/0
© 1992 Academic Press Limited
272
A. P. W O J C E I C H O W S K I
by most mammals. Not unexpectedly, therefore the presence of a clitoris is closely associated with that of external testes. Humans are not restricted, as mammals are, in the mode of intercourse and this mechanism may not be of such importance in Homo sapiens. This simple explanation may therefore have eluded us. The strength of a theory depends on its ability to explain the observations, and on inspection this idea appears to satisfy the criteria. In the first instance most mammals which have evolved in such a way as to make this mammalian mechanism of intercourse impossible have also lost their scrotal testicles. An obvious example would be the whale. An additional observation which supports the idea, relates the relative size of the scrotal sack which appears to be in many instances of the required length to provide the stimulation. It is not unreasonable to suppose that the evolution of secondary sexual characteristics may in some way be involved in the mode of coitus, and that sexual drives may be increased by increased sexual stimulation. The solution adopted by any one species is likely to be a compromise influenced by its particular mating system, the animals evolutionary ancestry, its ecology and the relative importance of male reproductive vulnerability over female fecundity. The theory proposed, requires further investigation and should not be dismissed on "prudish" grounds; nature has no such preformed prejudices. Department of Medicine, Cobbold Laboratories, The Middlesex Hospital, Mortflner Street, London W I N 8 A A , U.K.
A . P . WOJCIECHOWSK1
(Received on 19 September 1990, Accepted in revised form on l August 1991) REFERENCES
CARRICK,F. N. & SETCItELL,B. P. (1977). The evolution of the scrotum. In: Reproduction and Eoolution (Calaby, J. H. & Tyndale-Biscoe, T. H., eds) pp. 165-170. Canberra: Australian Academy of Science. FREEMAN, S. (1990). The evolution of the scrotum: a new hypothesis. J. theor. Biol. 145, 429-445. KARSCH, F. J. (1984). The hypothalamus and anterior pituitary. In : Hormonal Cono'ol of Reproduction, Book 3, Reproduction in Mammals 2nd ed. (Austin, C. R. & Short R. V., eds) pp. 14. Cambridge: Cambridge University Press.
LEYrEg TO TrIE EDITOR
Evolutionary Aspects of Mammalian Secondary Sexual Characteristics The recent publication by Scott Freeman (1990) of the University of Washington raises some interesting considerations with regard to the scrotal position of the testicles in male mammals. Clearly it is of considerable evolutionary disadvantage to allow the reproductive organs to be placed in such a vulnerable position, and must be counterbalanced by some advantage. Female mammals of course have always protected their gonads well with the bodily cavity, so their fecundity and fertility may be considered to be of more importance by nature, and this gives clues regarding the observed anomaly in males. The current temperature-dependant concept does indeed fail critically as the author espouses. Not least as there are approximately 1500 ascrotal mammalian species that do produce sperm at body temperature (Carrick & Setchell, 1977). Furthermore external testes appear to be almost unique to mammals an observation which requires explanation. Scott Freeman's explanation is partly flawed in a number of aspects, firstly it does not offer an explanation for this mammalian uniqueness. Secondly, the author's suggestion that external testes result in a poor blood supply to the testes may in a contradictory fashion allow the sperm top become "fitter" by developing more mitochondria, is questionable. It is quite reasonable to suppose that mammals could have evolved with internal testes with a reduced blood supply if the latter, could offer some evolutionary advantage. In addition if temperature dependance was important, a channel to the external environment would have offered a safer means of cooling the testes. This is not in agreement with current observations. External testes may have evolved as a result of more direct reproductive considerations. Throughout species some secondary sexual characteristics appear to have been adopted despite immediate disadvantages conferred on the individual. The peacock, for example, has a considerably curtailed ability to fly as a result of his exotic plumage, however, he is more capable of attracting a female mate. Ritual mating habits of marked complexity have been preserved in evolution. Unique to most mammals is the means by which sexual intercourse itself occurs; this is an important clue. In terms of evolution there should be a strong drive to mate, motivated by the pleasure stimulus from mating. It is likely that the pelvic muscular contractions associated with orgasm in the female assist the passage of spermatozoa to the site of fertilization. Furthermore, in some mammalian species such as the rabbit, cat, ferret and camel, ovulation in the female is dependant on the stimulus derived from coitus (Karsch, 1984). The persistance of the external female clitoris itself in mammals attests to the importance that the stimulation of this organ plays in reproduction. The only way in which stimulation of this organ, in most mammals, may occur during intercourse is by the presence of scrotai testes. In simple terms external testes may increase clitoral stimulation as a result of the method of intercourse employed 271
0022-5193/92/060271 + 02 $03.00/0
© 1992 Academic Press Limited
272
A. P. W O J C E I C H O W S K I
by most mammals. Not unexpectedly, therefore the presence of a clitoris is closely associated with that of external testes. Humans are not restricted, as mammals are, in the mode of intercourse and this mechanism may not be of such importance in Homo sapiens. This simple explanation may therefore have eluded us. The strength of a theory depends on its ability to explain the observations, and on inspection this idea appears to satisfy the criteria. In the first instance most mammals which have evolved in such a way as to make this mammalian mechanism of intercourse impossible have also lost their scrotal testicles. An obvious example would be the whale. An additional observation which supports the idea, relates the relative size of the scrotal sack which appears to be in many instances of the required length to provide the stimulation. It is not unreasonable to suppose that the evolution of secondary sexual characteristics may in some way be involved in the mode of coitus, and that sexual drives may be increased by increased sexual stimulation. The solution adopted by any one species is likely to be a compromise influenced by its particular mating system, the animals evolutionary ancestry, its ecology and the relative importance of male reproductive vulnerability over female fecundity. The theory proposed, requires further investigation and should not be dismissed on "prudish" grounds; nature has no such preformed prejudices. Department of Medicine, Cobbold Laboratories, The Middlesex Hospital, Mortflner Street, London W I N 8 A A , U.K.
A . P . WOJCIECHOWSK1
(Received on 19 September 1990, Accepted in revised form on l August 1991) REFERENCES
CARRICK,F. N. & SETCItELL,B. P. (1977). The evolution of the scrotum. In: Reproduction and Eoolution (Calaby, J. H. & Tyndale-Biscoe, T. H., eds) pp. 165-170. Canberra: Australian Academy of Science. FREEMAN, S. (1990). The evolution of the scrotum: a new hypothesis. J. theor. Biol. 145, 429-445. KARSCH, F. J. (1984). The hypothalamus and anterior pituitary. In : Hormonal Cono'ol of Reproduction, Book 3, Reproduction in Mammals 2nd ed. (Austin, C. R. & Short R. V., eds) pp. 14. Cambridge: Cambridge University Press.
