Do ring doves copulate more than necessary for fertilization?

Physiology & Behavior, Vol. 27, pp. 659-662. Pergamon Press and Brain Research Publ., 1981. Printed in the U.S.A.

Do Ring Doves Copulate More Than Necessary for Fertilization? 1 MEI-FANG

CHENG,

MARY PORTER AND GREGORY

BALL

Institute o f A n i m a l Behavior Rutgers-The State University, N e w a r k , N J 07102 R e c e i v e d 7 M a y 1981 CHENG, M.-F., M. PORTER AND G. BALL. Do ring doves copulate more than necessaryfor fertilization? PHYSIOL. BEHAV. 27(4) 659-662, 1981.--It has been suggested that in monogamous species a female will protect herself from mate desertion by withholding copulation until she is assured of the male's parental care. The ring dove (Streptopelia risoria) is a monogamous species whose females copulate with males frequently during prelaying phase. In this study we demonstrate that copulations did not result in fertilization unless they took place contiguous to the pre-ovulatory hormonal state of the female which was brought about only if the male courted the female for a sufficient period of time. Thus, in this species, ovulation and therefore fertilization was dependent on the male's presence regardless of how often copulation took place, insuring that the male must make a time commitment to guarantee paternity. Ring dove

Reproductive strategy

Copulation

D A R W I N ' S theory of sexual selection [10] and more recently Triver's concept of parental investment [20] and Dawkin's concept of the selfish gene [ l l ] all focus on how each sex develops characteristics and strategies to maximize chances of spreading its own genes successfully. Thus, males' tendency to be promiscuous, may be counteracted by the female reproductive synchrony [12,14] or by female's withholding commitment (copulation) until she is assured of her mate's commitment to stay with her. In monogamous avian species, in particular, where both parents participate in incubation and rearing the young, natural selection would favor females that develop characteristics which enable them to retain their mates after copulation. One might also expect that the underlying mechanisms can vary from species to species, In this paper, we will report how female ring doves, a species in which both sexes share parental care, effectively achieve this end through hormonal mechanisms. Ring doves when paired in a laboratory setting will copulate after a few hours of pairing and continue to copulate periodically during the entire preovulatory courtship period of 7-10 days [4,17]. (Typically a pair will copulate more than a dozen times before oviposition; we will designate them as accumulated multiple copulations.) This is in marked contrast to the majority of female behavioral responses to male courtship, such as approaching or avoiding the male, or nest-cooling with the male which occurs in an orderly sequence and in a specified time course in relation to the female's ovarian stage and subsequent egg-laying [4]. This striking exception to the usual sequence of courtship behavior could be the key to the role which copulations play in the female strategy o f keeping the male with her. Since multiple copulations are also observed in the field at least in the Afri-

Hormonal mechanisms

Fertilization

Breeding success

can Black Duck [18], it would be both interesting and theoretically important to understand their functions. In rats, multiple copulations (with intromissions) result in a greater amount of sperm storage [16] and a higher level of progesterone secretion [l] which respectively increase chances of successful fertilization and successful pregnancy. Are accumulated multiple copulations in ring doves also instrumental to successful fertilization? It seems unlikely for the reason that copulations in ring doves are well distributed over several days of different ovarian stages, whereas, in rats, all copulations occur in a short period of time (minutes) at the height of the female's estrus. However, the possibility remains that the copulations accumulated over days might be critical in some manner to successful fertilization in ring doves. To test this possibility, in the first experiment, we limited the time the mates were in contact with one another during the pre-laying period so that we could observe all copulations and then relate the number and timing of copulations to egg fertilizations. If the quantity of copulation is critical for bringing about successful fertilizations, one would expect more fertile eggs in the pairs with higher accumulated multiple copulations than those with low level of copulations. Conversely, one or two copulations (regardless of the ovarian stage in which the copulations take place) should not result in fertile eggs. The second experiment carried out after the completion of the first experiment was designed to separate the copulation frequency effect from the effect of the ovarian stage in which these copulations take place; both variables were present in the first experiment. This was done by keeping constant the number of copulations that occur. In theory, one should also include a group where copulations are tested

1Contribution No. 249 from the Institute of Animal Behavior, Rutgers University.

C o p y r i g h t © 1981 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/81/100659-04502.00/0

660

CHENG, PORTER AND BALL

during the same ovarian stage, allowing different numbers of copulations to occur. In practice, this is not feasible because ring doves rarely copulate more than twice during the 2 to 4 hrs of an observation period.

LATENCY OF LAYING

from lost co~lon

from ~lt~l poE'In

I

12

EGGS F- Fie'tile I - Infertile

METHOD

Ring doves raised in the breeding colony of the Institute of Animal Behavior were given one successful breeding experience prior to use in this study. In the first experiment, 64 individuals comprising 32 pairs of ring doves were permitted access to prospective mates for 2 hour periods per day for 1, 2, 3, 5, or 7 consecutive days. The number of copulations during each observation period was noted. In the second experiment, male and female ring doves were placed in breeding cages with glass partitions separating them. Under this condition, the male and female engage only in visual and auditory courtship. At the end of 7 days, the partition was removed to allow at least one but not more than two copulations to occur; glass partitions were then replaced to separate the pairs. For pairs that failed to copulate on that day, the test procedure was repeated daily until copulation(s) was observed. In another group, pairs of doves were allowed to copulate on the first and second day during 2-hour sessions, at the end of which the glass partitions were replaced so that no further copulations were possible. A third intermediate group was allowed to copulate once or twice after 3 to 4 days of exposure with glass partitions in place. Pairs which did not copulate during mating session were replaced by other pairs. In both experiments, latency of egg-laying of each pair was recorded and the fertility of the eggs was determined by candling the eggs 3-4 and 7-8 days after laying. Copulation was recorded when a male made a coaca contact with a female lasting 3-5 sec followed by "laugh" ("kah" vocalization). Unsuccessful copulation was also noted when a male dismounted before making the contact and did not "'laugh." In this study, unsuccessful copulations occurred only twice, both by the same male in Experiment 1 (pair no. 31, Fig. 1).

I

I

~¾//,,Y%J

J I

z~ is UA~/A [

I

z2 ///////#///~//Y~/~U/~/~/~ ~/~ I

I

V/Z//E/~Z/Zf ,/CE//')U~ "//~/,",, Y?////C///ClII~//I{///¢/II~, 5 ~#SI,~ I

2

3

4

5

6

12 8 8 8 8 8 8 8 9 IC 14 14 7 6 7 7 8 8 7 7 8 8

7

DAY OF COURTSHIP CYCLE

FIG. 1. Number of copulation and fertility of eggs. Shaded blocks indicate the days of courtship cycle when male-female pairs were allowed to interact and copulate during 2-hr observation, at the end of which the pairs were separated by a glass partition. The number inside the shaded blocks indicates the frequency of copulation which occurred during that observation period. For example, pair no. 10 was allowed to interact for 7 days and they copulated once on day 4 and once on day 7. Eighty percent of the pairs laid a clutch of 2 eggs and they were concordant for fertility, with the exception of pair no. 8 (indicated by arrow).

RESULTS AND DISCUSSION

The results of Experiment 1 are presented in Fig. 1. While multiple copulations increase the probability of eggs being fertile (approximately 86% of the multiple-copulation pairs in contrast to 30% of the one- or two-copulation pairs laid fertile eggs; multiple vs single copulation: X2=3.92, p<0.05), chances of successful fertilization depend very much on the day of the courtship cycle (first 3 vs last 4 days of courtship cycle: X2=I0.1 I, p<0.01) when the copulations take place. For example, when copulations took place at the later phase of courtship, as few as 2 copulations resulted in fertilization (pairs no. 10 and no. 15). On the other hand, during the early phase of courtship, three or more copulations did not result in fertilization (pairs nos. 13, 18, 19). These results suggest therefore that the ovarian stage in which copulation takes place could be the key to the fertility of eggs. This is clearly substantiated by the results in the second experiment, where number of copulations was limited to at least one but not more than two. It should be pointed out that the number of days of courtship in the first experiment cannot be compared directly with those in the second experiment, where interactions were limited to visual and auditory cues and hence required extra

days of courtship before females oviposit. However, the results of Experiment 1 and 2 can be compared in terms of fertility of eggs in relation to the stage of ovarian development when copulations take place. Copulation by the pairs that had been exposed to each other for some time (Table ], Group A) resulted in a higher percentage of successful fertilizations. Based on what we know about the time course of the secretion of ovarian hormones in this species [6], we are reasonably sure that the females of Group A that were pre-exposed to males (with glass partition) and some got to spend more time together (without glass separation) were in an optimal condition for ovulation. The projection is in good agreement with our actual observations that when partitions were removed for a mating session, these pre-exposed females showed the "'proceptive sexual c r o u c h " - - t h e female's active display of sexual crouch which has been shown to depend on synergistic action of gonadotropin releasing hormone and estrogen [5], and which is characteristically shown by females nearing ovulation (two or three days before ovulation or from four to

REPRODUCTIVE STRATEGY IN RING DOVE

661

TABLE 1 LAYING LATENCYAND PERCENTAGEOF EGG FERTILITYRESULTINGFROMTHE PAIRS ALLOWEDTO COPULATEAT DIFFERENTSTAGESOF THE COURTSHIPPHASE

Group

Conditions

N

% Laying

Laying Latency (Days) From Copulation Median Range

% Eggs Fertilized

A

Male-famale w/glass partition for 7-25 days then copulated

10

60*

5t

3-5

100

B

Male-female w/glass partition for 3-4 days then copulated

10

40*

14t

14--20

0

C

Male-female allowed to copulate on 1st or 2nd day 2 hr session

10

30*

15t

15-25

0

*Group A differs from Group C but not from Group B at the level of a=0.05 by Tocher's modification of the Fisher test. The median and range of the number of days required before copulation takes place in Group A are 8 and 7-25 respectively. tGroup A differs from Group B and Group C respectively at the level of a=0.05 by X2 test for independent samples.

five days before oviposition [4]). On the other hand, most copulations that occurred during the first four days under the condition we described (Group B and C, Table 1) were characterized by a pre-ovulatory, ritualized sequence of displays. It consists of the male's and female's self-preening in synchrony with each other while the male circles around the female. At some point the female approaches the male and they engage in mutual billings which lead to the male's feeding the female and the female's sexual crouch [4]. Ovariectomy abolishes all components of the female sexual behavior except the sequential displays of the preovulatory behavior [4], indicating therefore that the latter behavioral pattern is relatively free of hormonal influence. It is not unusual to observe ovariectomized doves engaged in copulations following this ritualized sequence of preovulatory displays, but never in the "proceptive sexual crouch." Female ring doves can, therefore, exhibit the copulatory act in the absence of an apparent influence of ovarian hormones. But, only the copulations mediated by hormones are likely to result in fertilization. What these findings indicate is that the female will copulate whether or not she has ova in store or is about ready to release her eggs. The male, therefore, cannot use the female's willingness to copulate as a sure sign that she is in a period when insemination can be successful. The finding that the copulation which took place more than 6 days before egg laying rarely resulted in fertile eggs (Fig. 1) is in agreement with the report that ring dove sperm are viable for 6--8 days [21]. It is conceivable the male relies on this characteristic and desert the female after early copulations. Our study shows, however, that the male cannot depend solely on the sperm's life span for successful fertilization since his success also depends very much on the

female's ovarian stage. Unless a male continuously courts the female, the ovarian development will not reach an ovulatory stage, and hence, there will be no egg released for the viable sperms to inseminate. In addition, a deserted male faces the disadvantage of being superseded by a second male in sperm competition. This has been clearly documented in a study where dwarf hens were inseminated with semen of one phenotype and then reinseminated 4 hr later with semen of the other phenotype; high numbers of progeny were from the second insemination [9]. Males are, therefore, forced to stay with females after copulations at least until oviposition in order to insure fertilization of their sperm. Another possible male strategy to maximize his reproductive success is to shorten the time it takes a female to ovulate by ardently courting her for a shorter period [3] and then to desert her to court a new female. However, male courtship does not have complete control over the rate of follicular development. Recent studies in our laboratories have shown that the female controls the rate of her follicular development by her own vocalizations (nest-coos) and her ovulatory process by her nest-building activity. Females made experimentally incapable of displaying nest-coos (an ovarian hormone dependent behavior) either by electrolytically lesioning the specific region in the midbrain (ICo: [8]) or bilaterally sectioning the motor nerves innervating the syrynx (the hypoglossal nerves: [7]) failed to show a follicular growth even in the presence of vigorous courtship by their males. Breeding females normally nest-coo continuously until about the time of oviposition, and there is a significant positive correlation between the female's ability to nest-coo and the level of follicular growth. In another study, we showed that everything being equal, the female engaged in high level of nestbuilding ovulated sooner by stimulating the onset of pre-

662

CHENG, PORTER AND BALL

ovulatory depression of follicular-stimulating-hormone secretion (Cheng and Balthazart, in preparation). Both female nest-coos and nest-building are b e h a v i o r induced by male courtship. H o w e v e r , the female controls the intensity of her response. There is a limit therefore to the m a l e ' s control o v e r the rate of follicular d e v e l o p m e n t and his ability to shorten his stay. After oviposition, male and female ring d o v e s take turns incubating the eggs until they hatch and then they share in rearing the young. The present study does not explain why the males stay once eggs are laid. We know, h o w e v e r , that the release of h o r m o n e s which support incubation and rearing behavior in males are induced by their m a t e ' s behavior [13], and furthermore, this parental b e h a v i o r is not compatible with the initiation of courtship b e h a v i o r with another female [6]. The males' remaining continuously with his mate after egg laying appears to be, ironically a c o n s e q u e n c e of his own endocrine response to his m a t e ' s laying. That is, his hormonal stage obliges him to stay with his mate. It seems that the d e v e l o p m e n t o f this characteristic (broodiness in response to the m a t e ' s laying) is also closely related to the strength of mutual pair-bonding. Males are strongly bonded to their females after egg laying. The breed-

ing cycle would be seriously disrupted if mates are switched during incubation, e v e n though the switched mates were physiologically synchronized [2,19]; a similar mate-switch before laying, on the other hand, has absolutely no effect on the pairs' on-going cycle (Cheng, unpublished observation). In other words, the male and female are fully " c o m m i t t e d " to each other around the time of egg laying. The selective pressure for developing these characteristics in birds are discussed by Lack [15] and clearly these conditions insure continuous stay o f males. In summary, the fact that the ring d o v e pair seems to require pair bonding as well as physiological synchrony to successfully c o m p l e t e hatching and squab rearing, and the present finding that copulation must be a c c o m p a n i e d by a significant time investiment on the part of the male for it to lead to successful fertilization probably constitutes the female ring d o v e ' s protection against male desertion. ACKNOWLEDGEMENTS We would like to thank R. Dawkins for the suggestion of the title and J. S. Rosenblatt for suggestions in the preparation of this manuscript. This work was supported by NIMH Grant No. MH-02271 and Career Development Award No. K02 MH-70897 to M.-F. Cheng.

REFERENCES

I. Adler, N. T. Effects of the males' copulatory behavior on successful pregnancy of the female rat. J. comp. physiol. Psy~hol. 69: 613-622, 1969. 2. Ball, G. and R. Silver. Sex roles in incubation behavior in the ring dove (Streptopelia risoria). Paper presented at the Northeast Regional Meeting of the Animal Behavior Society, Monmouth, NJ, 1978. 3. Barfield, R, Gonadotropic hormone secretion in the female ring dove in response to visual and auditory stimulation by the male. J. Endocr. 49: 205-210, 1971. 4. Cheng, M.-F. Effect of ovariectomy on the reproductive behavior of female ring doves (Streptopelia risoria). J. c'omp, physiol. Psyc'hol. 82: 221-233, 1973. 5. Cheng, M.-F. Role of gonadotrophin releasing hormone in the reproductive behaviour of female ring doves. J. Endocr. 74: 37-45, 1977. 6. Cheng, M.-F. Progress and prospects in ring dove research: A personal view. Adv. Study Behav. 9: 97-129, 1979. 7. Cohen, J. and M.-F. Cheng. Role of vocalizations in the reproductive cycle of ring doves (Streptopelia risoria): Effects of hypoglossal nerve section on the reproductive behavior and physiology of the female. Hormones Behav. 13: 113-127, 1979. 8. Cohen, J. and M.-F. Cheng. The role of the midbrain in courtship behavior of the female ring dove (Streptopelia risoria): Evidence from radiofrequency lesion and hormone implant studies. Bruin Res. 207: 27%301, 1981. 9. Compton, M. M., H. P. Van Krey and P. B. Siegel. The filling and emptying of the uterovaginal sperm-host glands in the domestic hen. Poultry Sci. 57: 1696-1700, 1978. 10. Darwin, C. The Descent o f Man and Selection in Relation to Sex. London: John Murray, 1871. II. Dawkins, R. The Selfish Gene. Oxford: Oxford University Press, 1976.

12. Emlen, S. T. and L. W. Oring. Ecology, sexual selection, and the evolution of mating systems. Science 197: 215-223, 1977. 13. Friedman, M. C. Physiological conditions for the stimulation of prolactin secretion by external stimuli in the ring dove. Ph.D. dissertation. Rutgers---The State University, New Brunswick, NJ, 1966. 14. Knowlton, N. Reproductive synchrony, parental investment and the evolutionary dynamics of sexual selection. Anim. Behay. 27: 1022-1033, 1979. 15. Lack, D. Ecological Adaptations for Breeding in Birds. London: Methuen & Co., Ltd.. 1968. 16. Lanier, D. L., D. G. Estep and D. A. Dewsbury. Role of prolonged copulatory behavior in facilitating reproductive success in a competitive mating situation in laboratory rats. J. comp. physiol. Psychol. 93: 781-792, 1979. 17. Martinez-Vargas, C. and C. J. Erickson. Social and hormonal determinants of nest-building in the ring dove. Behaviour 45: 12-37, 1973. 18. McKinney, F., W. R. Spiegfried, J. Ball and P. G. H. Frost. Behavioral specializations for river life in the African Black duck (Anas sparsa Eyton). Z. Tierpsychol. 48: 34%400, 1978. 19. Sims, E. and M.-F. Cheng. Pair-bonds, reproductive synchrony, and breeding success. Presented at the 12th Eastern Conference on Reproductive Behavior, Rockefeller University. New York, NY, 1980. 20. Trivers, R. Parental investment and sexual selection. In: Sexual Selection and the Descent o f Man, edited by B. Campbell. Chicago: Aldine Publishing Company, 1972. 21. Zenone, P. G., M. E. Sims and C. J. Erickson. Male ring dove behavior and the defense of genetic paternity. Am. Nat. 114: 615-626, 1980.