December 29, 192 1] 



NATURE 



5«>5 



Letters to the Editor. 



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The Accessory Nature of many Structures and Habits 

 Associated with Courtship. 



A STUDY of mating-habits in birds and other animals 

 has led me to conclusions which perhaps have a 

 general interest as bearing upon the whole theory of 

 sexual selection. 



As is well known, the difficulty of understanding 

 how sexual selection could operate in monogamous 

 animals has, together with other objections, led many 

 naturalists to reject it wholly or almost wholly as a 

 factor in evolution. However, whether we reject it 

 or not, there remains a large body of observed facts 

 which is in need of some evolutionary explanation. 

 The facts are (i) that elaborate displays and other 

 ceremonies occur in many animals in connection with 

 mating, and (2) that brilliant colours and special struc- 

 tures are generally developed in these animals, and 

 are employed chiefly or solely in these displays and 

 ceremonies. 



Our first and immediate conclusion is that, in view 

 of their elaborate nature and widespread distribution, 

 it is impossible to maintain that all these ceremonies 

 and associated structures are biologically meaning- 

 less. Further, in many species the employment of 

 such special structures is invariably associated with 

 sexual ceremonies ; for example, the ruff and ear- 

 tufts in both sexes of Podiceps cristatus, the crested 

 grebe, the elongated scapulars of egrets, or the '"tail " 

 of the male peacock. In these cases of invariable 

 association, then, we can be sure, so far as observa- 

 tion alone permits certainty, that both structures and 

 ceremonies have a biological significance in connection 

 with mating. We can, with an almost equal approach 

 to certainty, assign biological significance to such 

 bright colours as are placed so as to be especially 

 conspicuous during sexual ceremonies, e.g. the wing 

 colours and markings of the blackcock and crested 

 grebe, the brightlv coloured markings on male spiders, 

 the brilliance of the heads and necks of male birds 

 of paradise, etc. The problem is then to discover 

 iL'hat the biological significance behind these colours, 

 structures, and ceremonies may be. 



That in a large number of instances they cannot 

 have arisen through any process of sexual selection 

 is shown by the fact that in many monogamous birds 

 there exist mating ceremonies, often elaborate, which 

 occur only after pairing-up has taken place for the 

 season. Examples of these are to be seen in the 

 Sylviidae (see E. Howard, "The British Warblers," 

 1907-14), the crested grebe (Huxley, Proc. Zool. Soc., 

 1914), apparently the Fringillidae (E. Howard, '" Terri- 

 tory in Bird Life," 1920), and in divers and egrets 

 (unpublished observations of my own). It is obvious 

 that sexual selection, i.e. selection bv a female as 

 between several f)otential mates, ca^nnot operate here, 

 since there is only one male and one female involved. 



Further light is thrown on the question bv the 

 observation quoted in Pycraft's "Courtship of 

 Animals " (Hutchinson, London, 1913) that male newts 

 deposit their spermatophore before beginning their 

 "courtship" antics. As a result of the performance, 

 apparently, the female picks up the spermatophore 

 with the lips of her cloaca. It is obvious here that 

 the special ceremony and coloration of the male can 



NO. 2722, VOL. 108] 



be thought of only as an excitant to induce the female 

 to perform her part in effecting the union of the 

 gametes ; if several males were to dejx>sit their 

 spermatophores before a single female, and then all 

 perform their mating antics, there could be no possible 

 agency through which a male with a particularly 

 effective display could succeed in making the female 

 pick up his particular sjjermatophore rather than 

 another. 



This conclusion is fully borne out by the experi- 

 mental results of Sturtevant upon the dipteran Droso- 

 phila (quoted by Morgan, Carnegie Institution Pub- 

 lication No. 285, 1919, p. 61). The male of Droso- 

 phila has a mating ceremony in which he vibrates his 

 wings in front of the female. Sturtevant showed that 

 single males whose wings were cut off would succeed 

 in mating when isolated with single females, but only 

 after an average time considerably longer than that 

 required by normal males. But when a normal male 

 and one deprived of wings were imprisoned together 

 in company with a single fem.ale, the wingless male 

 succeeded in mating almost as often as his normal 

 competitor. The so-called "courtship," therefore, is 

 again a stimulant ; once the fema'e has been stimu- 

 lated by the display she is equally ready to mate with 

 any male, whether normal or so mutilated as to be 

 unable to perform the "courtship" action. As 

 Morgan says (loc. cit., p. 61), "This critical test puts 

 the problem in a different relation from that which 

 Darwin's theory of female choice was meant to throw 

 light on." 



It appears, therefore, that in these examples — 

 newts, Drosophila, and the p>ost-mating ceremonies of 

 monogamous birds — sexual selection cannot be opera- 

 tive. The effect of the ceremonies in these and many 

 other species is not the selection of one rather than 

 another out of several possible mates, but simply a 

 facilitation of the union of the gametes. 



Now copulatory organs have exactly this same 

 function. When internal fertilisation is employed, 

 copulatory organs are in the majority of cases de- 

 veloped. They are an adaptation to a particular 

 method of fertilisation. In precisely similar fashion 

 I would say that when organisms possess a certain 

 grade of mental (nervous) development one of two 

 further adaptations is required to facilitate the union 

 of the gametes. Either, as in mammals, a hormone 

 is periodically produced which causes the female at 

 certain times to be ready to take any male, while at 

 others she refuses all males ; in this case displays and 

 ceremonies will be useless, and fighting between males 

 will be the rule. Or else the stimulus to make the 

 female ready to take the male must come from with- 

 out ; it must, therefore, affect the nervous system 

 through the sense-organs, and will generally take the 

 form of a display by the male. 



If the female were at all times ready to take the 

 male, excessive coition would occur, which would be 

 deleterious ; thus it is desirable that some stimulus, 

 whether endocrine or sensory, internal or external, 

 should be necessary in order that coition may take 

 place. 



I have so far purposely simplified the problem by 

 dealing only with those species in which it is the 

 female who needs stimulation, the male who is alwavs 

 or usually readv for coition and performs the special 

 ceremony. The reasoning, however, will equallv 

 applv to cases of "reversed sexual selection." such 

 as the Phalarope, where the female is more brightly 

 coloured and the male requires stimulation, or to the 

 numerous cases of "mutual courtship," such as is 

 found in grebes, divers, egrets, herons, the kagu, the 

 fulmar, etc., in which both sexes are brightly coloured. 



