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Male age: effect on mating behaviour and success in the cockroach Diploptera punctata
Anita. Behav., 1986, 34, 1874-1879
Male age: effect on mating hehaviour and success in the cockroach Diploptera punctata A N D R E A P. W O O D H E A D
Program in Evolutionary Ecology and Behavior, Department of Biology, University of Iowa, Iowa City, IA 52242, U.S.A.
Abstract. Older cockroach, Diploptera punctata, males (28 days old) transferred more sperm and larger spermatophores to females than 8- or 14-day-old males. In one male:one female behavioural tests, no significant correlations were found between either how rapidly males initiated courtship with females (response time), or intensity or duration of courtship, and the number of sperm or size of spermatophore subsequently transferred. Eight-day males were slower to initiate courtship than 28-day males, There was no significant difference in the duration of courtship for 8-, 14- and 28-day males, even though the mean duration of courtship was more than twice as long for 8-day males compared with 14- and 28-day males. In competitive mating tests with one female and three males, one of each age, 28-day males were significantly more successful than 8- and 14-day males. It is concluded that the greater success of 28-day males is due to differences in male behaviour rather than to female discrimination.
Much work in sexual selection has focused on the identification of factors that contribute to male mating success. For animals in which females require some courtship from males before they will mate with them, it is likely that males that initiate courtship quickly and court actively will be successful in obtaining mates. Further, it has been proposed that females may choose mates based on the quality of courtship performed by males (Trivers 1972). An important question is, then, what factors affect the courtship performance of males? A number of factors that can potentially affect male courtship have been noted by Thornhill & Alcock (1983). These include the size and strength of males, their health and energy reserves and also their age and mating history. There is some evidence that courtship behaviour may be correlated with the ability of a male to transfer sperm or spermatophore materials. Selection would favour females that respond to actively courting males, since discrimination by females would ensure that they receive gametes from a courting male. Moreover, in those species in which males transfer sperm in a nutritious spermatophore (e.g. Boggs & Gilbert 1979), discriminating females would thus receive energy for themselves or their offspring. In newts, the rate at which males perform three courtship displays is positively correlated with the
number of spermatophores they subsequently transfer (Halliday 1976). Similarly, Rutowski (1979) found that male courtship persistence dropped after an initial mating by males and also that the second spermatophore transferred was smaller. Male age may be another factor that influences courtship behaviour. Long et al. (1980) found that older Drosophila melanogaster males were more successful in mate tests than younger males. Males of the cockroach Diploptera punctata that do not have the opportunity to mate accumulate sperm and accessory gland secretions; therefore older males transfer more sperm and larger spermatophores than younger males (Stay & Roth 1958; Tobe et al. 1979; Woodhead 1984). The goal of the present study was to determine whether differences could be detected in the courtship behaviour, female response to courtship behaviour and mating success of Diploptera males 8, 14 and 28 days old (adult age). However, it is not sufficient to document variation in male behaviour and female response, and from this to conclude that differential mating success occurs via female choice. Female discrimination may or may not play a role in competitive mate tests. Therefore, in this study, the results from one male:one female behavioural tests in which male behaviour and female response
1874
Woodhead: Male age and mating success
were assessed were compared with the outcome of competitive mating tests in which three males, one of each age, competed for one female.
METHODS
1875
determined by mating them with females and then counting the number of developing embryos in the brood sacs of these females 28 days after mating. At this stage, embryos can be readily distinguished from eggs that have not developed (Stay & Coop 1973).
Study Animal
Behavioural Tests
Stock colonies of Diploptera were reared as described previously (Woodhead 1985). Virgin females were obtained by isolating last instar female larvae. Small groups of newly eclosed adult males were removed from stock cultures daily and reared together. The age of males is given in days after adult eclosion. All matings in this study were between virgin females and males of known age. During copulation, a male transfers sperm to a female in a container, the spermatophore. Females mate once, soon after adult eclosion, while still teneral. About 7 days after mating, females oviposit 10 14 eggs that subsequently develop in the brood sac with nutrient secretions provided by the mother. Gestation (the period of embyronic development) is about 60 days. A female is not receptive to mating throughout pregnancy. Females will mate again, once only, immediately after giving birth.
All females used in behavioural tests were day 0 virgins 2-4 h old (estimated by cuticular coloration). Thus males were provided with a uniform stimulus. Behavioural tests were conducted in a walk-in chamber that was maintained at 25~ A plastic box 25 • 32 • 8.5 cm deep with an entrance tube near the bottom and with a glass cover was used for tests. On a test day, 8-, 14- and 28-day males were placed in separate glass vials. To eliminate observer bias the age of the male used in each test was concealed until tests were completed. For each test a female was introduced into the chamber through the inlet tube. Ten minutes later a male was introduced into the chamber. Typically, after male-female contact, the male began to rapidly move his antennae over the female. Then the male oriented alongside the female, but in the opposite direction, and began courting. Courtship behaviour consists of a series of bouts of wing flutters by the male followed by a raised wing position. A female responds to a courting male by climbing up on the male's back when the male is in the wing-up position. The male then rapidly engages the female's geIfitalia. If the female does not respond, the male continues to court. For each test the duration of time from malefemale contact to the initiation of courtship by the male (response time) was measured. A maximum value of 120 s was assigned for response time. The number of bouts of wing flutters performed by each male was counted and the duration of courtship was timed. The duration of courtship is a measure of how quickly a female responds to a male's courtship. A maximum value of 300 s was assigned for courtship duration. The rate of courtship display is the number of bouts of wing flutters/ duration of courtship. The rate of courtship display was calculated only for those males with courtship duration less than 60 s long because it was anticipated that the rate of display would slow as a result of fatigue.
Measurement Techniques The wet weight, head capsule width, length of spermatophore and number of sperm transferred, and the duration of copulation were measured in a subset of males used in the behavioural tests (see below). Head capsule width and spermatophore length were measured using an ocular micrometer. For sperm counting, a spermatophore was crushed in a known amount of saline. The number of sperm in an aliquot of this solution was counted using a haemocytometer. Another set of males, 8, 14 and 28 days old, was mated to females. Spermatophores were removed from the females and their length was measured. The spermatophores were dried for 48 h at 70~ and weighed with an electrobalance.
Male Fertility The fertility of males 8, 14 and 28 days old was
1876
Animal Behaviour, 34, 6
Competitive Mating Tests In addition to the one male:one female behavioural tests described above, competitive mating tests were carried out in which one female was introduced into the chamber with three males, one of each age. Before the test, the males were individually marked with a small dot of paint on one wing. Ten minutes after the males were placed in the chamber, a virgin female, 2 4 h old, was introduced. Observations of behaviour, including whether the female contacted all three males before mating, were recorded.
RESULTS Table I shows that 28-day males transferred significantly more sperm and larger spennatophores than 8- and 14-day males. Length is a representative measure of size for spermatophores. The correlation between spermatophore length and dry weight was r=0-944 (P<0"001). The spermatophores transferred by a group of 8-, 14- and 28-day males ranged from 1.24 to 1.80 mm in length and from 0"138 to 0-426 mg in dry weight (N=30). Table I also shows that older males transferred reproductive materials more quickly than younger males; the duration of copulation was significantlyshorter for 28-day males. There was no significant difference in the size of males in the different age groups.
Male Fertility Male age did not affect the number of eggs oviposited by females; however, none of the eggs in a batch developed in four of 33 (12%) females mated to 8-day males, two of 58 (3%) females
mated to 14-day males and zero of 38 (0%) females mated to 28-day males.
Behavioural Tests The data were first analysed without regard to male age. No significant correlations were found between the time to initiation of courtship (response time), or the intensity of courtship, and the number of sperm or size of spermatophore transferred by males. There were also no significant correlations between the duration of courtship, a measure of the female's response to a courting male, and the number of sperm or size of spermatophore transferred (Spearman rank correlations, N = 34-45 for each correlation calculated). There was a small positive correlation, r~=0.194 (P=0-05, N=106) between response time and duration of courtship. Males that initiated courtship quickly tended to be accepted more quickly by females. The response time, courtship intensity and duration of courtship for 8-, 14- and 28-day males is shown in Table II. Fourteen- and 28-day males began courting sooner and females responded more quickly to their courtship. The response time of 8-day males was significantlylonger than that of 28-day males. Courtship intensity was similar for males of each age. There was no significant difference in the duration of courtship for males in the three age groups (Kruskal-Wallis test), even though the mean duration of courtship was more than twice as long for 8-day males compared with 14- and 28-day males. Response time and duration of courtship were divided into three classes: 1-10 s, 11-59 s and 60 s and greater (Fig. 1). Males responded quickly to females; more than half the males in each age group
Table 1. Male size, spermatophore length and number of sperm transferred, and duration of copulation for 8-, 14and 28-day males Male age (days) 8 14 28
Wet weight Head capsule width Spermatophorelength (rag) (mm) (mm) 152_+16(24) 153_+11(25) 147_+16 (24)
3.24_+0.10 (23) 3.26_+0.10 (23) 3.20_+0.10 (22)
1.32__+0.08"**(17) 1.49_+0.08"**(17) 1.62_+0.07 (16)
Number of sperm Copulation duration ( x 103) (min) 13.3_+10.2"** (15) 19.2_+9.7 * (14) 29.3___10.4 (14)
65_+15"** (21) 54-+11"* (17) 42_+9 (22)
Values given are the mean _+SD for the sample size indicated in parentheses. Individual Mann-Whitney tests compared values for 8- and 14-day males with those of 28-day males: *P < 0.05; **P< 0.01; ***P< 0.001.
Woodhead." Male age and mating success
1877
Table lI. Mating behaviour of 8-, 14- and 28-day males Male age Response time Courtship intensity Duration ofcourtship (days) (s) (No. wing flutter bouts/s) (s) 8 14 28
24_+31" (44) t7_+26 (45) 16+26 (40)
0.287_+0.116 (32) 0-288_+0-113(36) 0.307_+0.174 (35)
58_+103 (37) 28-t-68 (38) 22-1-57 (35)
See Table I for explanation of values and statistical tests.
R e s p o n s e time
80
60
40
20" m E
0
g
i1
8-day mates []
1 4 - d a y males
[]
2 8 - d a y mates
o0
o ID 13_
Duration of courtship
80
for 8-day males is not explained by a larger proportion of males in this age group being assigned the maximum value of 120 s. Six of 129 males were assigned the maximum value; two from each age group. F o r duration of courtship, the difference in mean values can in part be explained by a larger proportion of 8-day males being assigned the maximum value of 300 s. Out of 110 behavioural tests in which the duration of courtship was measured, five 8-day males, two 14-day males and one 28-day male were assigned the maximum value. O f these males, three 8-day males and one 14-day male did not mate.
60
Competitive Mating Tests 40
20
1-10
s
11-59
s
60§ s
Figure 1. Response time and duration of courtship for 8-, 14- and 28-day malts. Bars represent the mean percentage of males of a given age that are assigned to one of three time classes for each behaviour. See Table I1 for sample sizes.
In competitive mating tests 28-day males were more successful than 8- and 14-day males (Table III). When all mating tests are considered, 28-day males mated with the female in 18 o f 28 tests (P<0"01, g2-test). In those tests in which all three males contacted the female before mating occurred, the 28-day male mated with the female in 15 of 22 tests (P<0-01, x2-test). Eight- and 14-day
Table IIL Mating success of 8-, 14- and 28-day males initiated courtship within 10 s of contacting a female. Most females accepted males after a short period of courtship. The duration of courtship was 10 s or less in the majority of behavioural tests. Although there was no significant difference in the duration of these behaviours with male age (Zztest), a smaller proportion of 8-day males initiated courtship within 10 s and were accepted by females within 10 s compared with 14- and 28-day males. Also, for a larger proportion of 8-day males, response time and duration of courtship were 60 s or longer. The longer mean response time (Table II)
Male age (days)
3-M testst (N=22) All tests (N= 28)
8
14
28
P
4 (18.2)
3 (13.6)
15 (68,2)
*
6 (21-4)
4 (14-3)
18 (64.3)
*
The number of tests (percentage of tests) in which males of each age mated with the female. t Tests in which all three males contacted the female. * P < 0.01; Z2-test.
Animal Behaviour, 34, 6
1878
males achieved approximately an equal number of the remaining matings. In a typical mate test, the female ran off after contacting one or more males. Twenty-eight-day males were more successful at following females, and they were more persistent in their courtship. In some tests male-male interference was observed.
DISCUSSION There has been remarkably little work done with insects on the relationship between the amount of reproductive material males have to transfer and their courtship behaviour, and on the effect of male age on mating behaviour and success. In Diploptera, there was no correlation between response time or courtship intensity of males and the amount of sperm or spermatophore material transferred subsequently. A striking finding of this study is that older Diploptera males were more successful than younger males in competitive mating tests (Table III). Greater mating success of older males has also been reported for Drosophila (Long et al. 1980), and it was suggested that subtle differences in the courtship of older males contributed to their success or that auditory or olfactory cues might be involved. A pheromone cue has been postulated to be the basis for female discrimination between virgin males and males that have recently mated in Drosophila (Markow et al. 1978). It is possible that Diploptera males release a sex pheromone during courtship that attracts females. Secretions, which may include pheromones, are secreted by tergal glands in male cockroaches (Roth 1969). If some 8day males have not reached complete sexual maturity, and it appears that this is the case since a substantial portion (12%) of matings with 8-day males yielded no offspring, and if these males produce only small amounts of a pheromone, then this could explain the slower response by females to the courtship of 8-day males in the behavioural tests (Table II, Fig. 1). An advantage to females that respond quickly to mature courting males is that they are likely to receive gametes from these males. Mating with an infertile male has serious consequences for the fitness of Diploptera females since they will not mate again until after aborting the batch of unfertilized eggs some 3~4 weeks later (Woodhead 1985). The prediction from the results of the beha-
vioural tests is that 8-day males should be less successful than 14- and 28-day males in competitive mating tests if female discrimination based on courtship behaviour is an important factor in determining the outcome of these tests. Eight-day males were slower to initiate courtship and females were less attracted to their courtship. However, it was found that 28-day males were significantly more successful than both 8- and 14-day males. It is this lack of agreement with prediction that leads to the conclusion that behaviours of 28-day males, other than the courtship behaviours assessed, are responsible for their greater mating success. Twenty-eight-day males mated with the female in approximately two-thirds of the mating tests whether or not the female contacted all three males before mating (Table III). This suggests that the success of 28-day males was not due to their ability to find the female first. If this were so, then 28-day males would have achieved a greater proportion of the matings in all tests compared with tests in which all three males contacted the female. In this study it is not possible to determine whether male age alone or an interaction between male age and the amount of reproductive material that males have to transfer leads to mating success. To distinguish between these alternatives, it would be necessary to test males of the same age but with different amounts of reproductive material to transfer. This study demonstrates that the results of mating tests need to be interpreted carefully. Females may be capable of responding differentially to male courtship, yet female discrimination may not play a role in the outcome of competitive mating tests.
ACKNOWLEDGMENTS I thank Ola Fincke and Barbara Stay for their helpful comments on the manuscript.
REFERENCES Boggs, C. L. & Gilbert, L. E. 1979. Male contribution to egg production in butterflies: evidence for transfer of nutrients at mating. Science, N.Y., 206, 83-84. Halliday, T. R. 1976. The libidinous newt, an analysis of variations in the sexual behaviour of the male smooth newt, Triturus vulgaris. Anim. Behav., 24, 398414. Long, C. E., Markow, T. A. & Yaeger, P. 1980. Relative male age, fertility, and competitive mating success in Drosophila melanogaster. Behav. Genet., 10, 163 170.
Woodhead: Male age and mating successMarkow, T. A., Quaid, M. & Kerr, S. 1978. Male mating experience and competitive courtship success in Drosophila melanogaster. Nature, Lond., 276, 821-822. Roth, L. M. 1969. The evolution of male tergal glands in the Blattaria. Ann. Entomol. Soc. Am., 62, 176-208. Rutowski, R. L. 1979. The butterfly as an honest salesman. Anim. Behav., 27, 1269-1270. Stay, B. & Coop, A. 1973. Developmental stages and chemical composition in embryos of the cockroach, Diploptera punctata, with observations on the effect of diet. J. Insect Physiol., 19, 147 171. Stay, B. & Roth, L. M. 1958. The reproductive behavior of Diploptera punctata (Blattaria: Diplopteridae). Proc. Tenth Int. Congr. Entomol., 2, 547--552. Thornhill, R. & Alcock, J. 1983. The Evolution of Insect Mating Systems. Cambridge, Massachusetts: Harvard University Press.
1879
Tobe, S. S., Musters, A. & Stay, B. 1979. Corpus allatum function during sexual maturation of male Diploptera punctata. Physiol. Entomol., 4, 79-86. Trivets, R. L. 1972. Parental investment and sexual selection. In: Sexual Selection and the Descent of Man 1871-1971 (Ed. by B. Campbell), pp. 136-179. Chicago: Aldine. Woodbead, A. P. 1984. Effect of duration of larval development on sexual competence in young adult male Diploptera punctata. Physiol. Entomol., 9, 473 477. Woodhead, A. P, 1985. Sperm mixing in the cockroach Diploptera punetata. Evolution, 39, 159-164.
(Received 22 August 1985; revised 24 Novembe? 1985; MS. number." A4599)
Male age: effect on mating hehaviour and success in the cockroach Diploptera punctata A N D R E A P. W O O D H E A D
Program in Evolutionary Ecology and Behavior, Department of Biology, University of Iowa, Iowa City, IA 52242, U.S.A.
Abstract. Older cockroach, Diploptera punctata, males (28 days old) transferred more sperm and larger spermatophores to females than 8- or 14-day-old males. In one male:one female behavioural tests, no significant correlations were found between either how rapidly males initiated courtship with females (response time), or intensity or duration of courtship, and the number of sperm or size of spermatophore subsequently transferred. Eight-day males were slower to initiate courtship than 28-day males, There was no significant difference in the duration of courtship for 8-, 14- and 28-day males, even though the mean duration of courtship was more than twice as long for 8-day males compared with 14- and 28-day males. In competitive mating tests with one female and three males, one of each age, 28-day males were significantly more successful than 8- and 14-day males. It is concluded that the greater success of 28-day males is due to differences in male behaviour rather than to female discrimination.
Much work in sexual selection has focused on the identification of factors that contribute to male mating success. For animals in which females require some courtship from males before they will mate with them, it is likely that males that initiate courtship quickly and court actively will be successful in obtaining mates. Further, it has been proposed that females may choose mates based on the quality of courtship performed by males (Trivers 1972). An important question is, then, what factors affect the courtship performance of males? A number of factors that can potentially affect male courtship have been noted by Thornhill & Alcock (1983). These include the size and strength of males, their health and energy reserves and also their age and mating history. There is some evidence that courtship behaviour may be correlated with the ability of a male to transfer sperm or spermatophore materials. Selection would favour females that respond to actively courting males, since discrimination by females would ensure that they receive gametes from a courting male. Moreover, in those species in which males transfer sperm in a nutritious spermatophore (e.g. Boggs & Gilbert 1979), discriminating females would thus receive energy for themselves or their offspring. In newts, the rate at which males perform three courtship displays is positively correlated with the
number of spermatophores they subsequently transfer (Halliday 1976). Similarly, Rutowski (1979) found that male courtship persistence dropped after an initial mating by males and also that the second spermatophore transferred was smaller. Male age may be another factor that influences courtship behaviour. Long et al. (1980) found that older Drosophila melanogaster males were more successful in mate tests than younger males. Males of the cockroach Diploptera punctata that do not have the opportunity to mate accumulate sperm and accessory gland secretions; therefore older males transfer more sperm and larger spermatophores than younger males (Stay & Roth 1958; Tobe et al. 1979; Woodhead 1984). The goal of the present study was to determine whether differences could be detected in the courtship behaviour, female response to courtship behaviour and mating success of Diploptera males 8, 14 and 28 days old (adult age). However, it is not sufficient to document variation in male behaviour and female response, and from this to conclude that differential mating success occurs via female choice. Female discrimination may or may not play a role in competitive mate tests. Therefore, in this study, the results from one male:one female behavioural tests in which male behaviour and female response
1874
Woodhead: Male age and mating success
were assessed were compared with the outcome of competitive mating tests in which three males, one of each age, competed for one female.
METHODS
1875
determined by mating them with females and then counting the number of developing embryos in the brood sacs of these females 28 days after mating. At this stage, embryos can be readily distinguished from eggs that have not developed (Stay & Coop 1973).
Study Animal
Behavioural Tests
Stock colonies of Diploptera were reared as described previously (Woodhead 1985). Virgin females were obtained by isolating last instar female larvae. Small groups of newly eclosed adult males were removed from stock cultures daily and reared together. The age of males is given in days after adult eclosion. All matings in this study were between virgin females and males of known age. During copulation, a male transfers sperm to a female in a container, the spermatophore. Females mate once, soon after adult eclosion, while still teneral. About 7 days after mating, females oviposit 10 14 eggs that subsequently develop in the brood sac with nutrient secretions provided by the mother. Gestation (the period of embyronic development) is about 60 days. A female is not receptive to mating throughout pregnancy. Females will mate again, once only, immediately after giving birth.
All females used in behavioural tests were day 0 virgins 2-4 h old (estimated by cuticular coloration). Thus males were provided with a uniform stimulus. Behavioural tests were conducted in a walk-in chamber that was maintained at 25~ A plastic box 25 • 32 • 8.5 cm deep with an entrance tube near the bottom and with a glass cover was used for tests. On a test day, 8-, 14- and 28-day males were placed in separate glass vials. To eliminate observer bias the age of the male used in each test was concealed until tests were completed. For each test a female was introduced into the chamber through the inlet tube. Ten minutes later a male was introduced into the chamber. Typically, after male-female contact, the male began to rapidly move his antennae over the female. Then the male oriented alongside the female, but in the opposite direction, and began courting. Courtship behaviour consists of a series of bouts of wing flutters by the male followed by a raised wing position. A female responds to a courting male by climbing up on the male's back when the male is in the wing-up position. The male then rapidly engages the female's geIfitalia. If the female does not respond, the male continues to court. For each test the duration of time from malefemale contact to the initiation of courtship by the male (response time) was measured. A maximum value of 120 s was assigned for response time. The number of bouts of wing flutters performed by each male was counted and the duration of courtship was timed. The duration of courtship is a measure of how quickly a female responds to a male's courtship. A maximum value of 300 s was assigned for courtship duration. The rate of courtship display is the number of bouts of wing flutters/ duration of courtship. The rate of courtship display was calculated only for those males with courtship duration less than 60 s long because it was anticipated that the rate of display would slow as a result of fatigue.
Measurement Techniques The wet weight, head capsule width, length of spermatophore and number of sperm transferred, and the duration of copulation were measured in a subset of males used in the behavioural tests (see below). Head capsule width and spermatophore length were measured using an ocular micrometer. For sperm counting, a spermatophore was crushed in a known amount of saline. The number of sperm in an aliquot of this solution was counted using a haemocytometer. Another set of males, 8, 14 and 28 days old, was mated to females. Spermatophores were removed from the females and their length was measured. The spermatophores were dried for 48 h at 70~ and weighed with an electrobalance.
Male Fertility The fertility of males 8, 14 and 28 days old was
1876
Animal Behaviour, 34, 6
Competitive Mating Tests In addition to the one male:one female behavioural tests described above, competitive mating tests were carried out in which one female was introduced into the chamber with three males, one of each age. Before the test, the males were individually marked with a small dot of paint on one wing. Ten minutes after the males were placed in the chamber, a virgin female, 2 4 h old, was introduced. Observations of behaviour, including whether the female contacted all three males before mating, were recorded.
RESULTS Table I shows that 28-day males transferred significantly more sperm and larger spennatophores than 8- and 14-day males. Length is a representative measure of size for spermatophores. The correlation between spermatophore length and dry weight was r=0-944 (P<0"001). The spermatophores transferred by a group of 8-, 14- and 28-day males ranged from 1.24 to 1.80 mm in length and from 0"138 to 0-426 mg in dry weight (N=30). Table I also shows that older males transferred reproductive materials more quickly than younger males; the duration of copulation was significantlyshorter for 28-day males. There was no significant difference in the size of males in the different age groups.
Male Fertility Male age did not affect the number of eggs oviposited by females; however, none of the eggs in a batch developed in four of 33 (12%) females mated to 8-day males, two of 58 (3%) females
mated to 14-day males and zero of 38 (0%) females mated to 28-day males.
Behavioural Tests The data were first analysed without regard to male age. No significant correlations were found between the time to initiation of courtship (response time), or the intensity of courtship, and the number of sperm or size of spermatophore transferred by males. There were also no significant correlations between the duration of courtship, a measure of the female's response to a courting male, and the number of sperm or size of spermatophore transferred (Spearman rank correlations, N = 34-45 for each correlation calculated). There was a small positive correlation, r~=0.194 (P=0-05, N=106) between response time and duration of courtship. Males that initiated courtship quickly tended to be accepted more quickly by females. The response time, courtship intensity and duration of courtship for 8-, 14- and 28-day males is shown in Table II. Fourteen- and 28-day males began courting sooner and females responded more quickly to their courtship. The response time of 8-day males was significantlylonger than that of 28-day males. Courtship intensity was similar for males of each age. There was no significant difference in the duration of courtship for males in the three age groups (Kruskal-Wallis test), even though the mean duration of courtship was more than twice as long for 8-day males compared with 14- and 28-day males. Response time and duration of courtship were divided into three classes: 1-10 s, 11-59 s and 60 s and greater (Fig. 1). Males responded quickly to females; more than half the males in each age group
Table 1. Male size, spermatophore length and number of sperm transferred, and duration of copulation for 8-, 14and 28-day males Male age (days) 8 14 28
Wet weight Head capsule width Spermatophorelength (rag) (mm) (mm) 152_+16(24) 153_+11(25) 147_+16 (24)
3.24_+0.10 (23) 3.26_+0.10 (23) 3.20_+0.10 (22)
1.32__+0.08"**(17) 1.49_+0.08"**(17) 1.62_+0.07 (16)
Number of sperm Copulation duration ( x 103) (min) 13.3_+10.2"** (15) 19.2_+9.7 * (14) 29.3___10.4 (14)
65_+15"** (21) 54-+11"* (17) 42_+9 (22)
Values given are the mean _+SD for the sample size indicated in parentheses. Individual Mann-Whitney tests compared values for 8- and 14-day males with those of 28-day males: *P < 0.05; **P< 0.01; ***P< 0.001.
Woodhead." Male age and mating success
1877
Table lI. Mating behaviour of 8-, 14- and 28-day males Male age Response time Courtship intensity Duration ofcourtship (days) (s) (No. wing flutter bouts/s) (s) 8 14 28
24_+31" (44) t7_+26 (45) 16+26 (40)
0.287_+0.116 (32) 0-288_+0-113(36) 0.307_+0.174 (35)
58_+103 (37) 28-t-68 (38) 22-1-57 (35)
See Table I for explanation of values and statistical tests.
R e s p o n s e time
80
60
40
20" m E
0
g
i1
8-day mates []
1 4 - d a y males
[]
2 8 - d a y mates
o0
o ID 13_
Duration of courtship
80
for 8-day males is not explained by a larger proportion of males in this age group being assigned the maximum value of 120 s. Six of 129 males were assigned the maximum value; two from each age group. F o r duration of courtship, the difference in mean values can in part be explained by a larger proportion of 8-day males being assigned the maximum value of 300 s. Out of 110 behavioural tests in which the duration of courtship was measured, five 8-day males, two 14-day males and one 28-day male were assigned the maximum value. O f these males, three 8-day males and one 14-day male did not mate.
60
Competitive Mating Tests 40
20
1-10
s
11-59
s
60§ s
Figure 1. Response time and duration of courtship for 8-, 14- and 28-day malts. Bars represent the mean percentage of males of a given age that are assigned to one of three time classes for each behaviour. See Table I1 for sample sizes.
In competitive mating tests 28-day males were more successful than 8- and 14-day males (Table III). When all mating tests are considered, 28-day males mated with the female in 18 o f 28 tests (P<0"01, g2-test). In those tests in which all three males contacted the female before mating occurred, the 28-day male mated with the female in 15 of 22 tests (P<0-01, x2-test). Eight- and 14-day
Table IIL Mating success of 8-, 14- and 28-day males initiated courtship within 10 s of contacting a female. Most females accepted males after a short period of courtship. The duration of courtship was 10 s or less in the majority of behavioural tests. Although there was no significant difference in the duration of these behaviours with male age (Zztest), a smaller proportion of 8-day males initiated courtship within 10 s and were accepted by females within 10 s compared with 14- and 28-day males. Also, for a larger proportion of 8-day males, response time and duration of courtship were 60 s or longer. The longer mean response time (Table II)
Male age (days)
3-M testst (N=22) All tests (N= 28)
8
14
28
P
4 (18.2)
3 (13.6)
15 (68,2)
*
6 (21-4)
4 (14-3)
18 (64.3)
*
The number of tests (percentage of tests) in which males of each age mated with the female. t Tests in which all three males contacted the female. * P < 0.01; Z2-test.
Animal Behaviour, 34, 6
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males achieved approximately an equal number of the remaining matings. In a typical mate test, the female ran off after contacting one or more males. Twenty-eight-day males were more successful at following females, and they were more persistent in their courtship. In some tests male-male interference was observed.
DISCUSSION There has been remarkably little work done with insects on the relationship between the amount of reproductive material males have to transfer and their courtship behaviour, and on the effect of male age on mating behaviour and success. In Diploptera, there was no correlation between response time or courtship intensity of males and the amount of sperm or spermatophore material transferred subsequently. A striking finding of this study is that older Diploptera males were more successful than younger males in competitive mating tests (Table III). Greater mating success of older males has also been reported for Drosophila (Long et al. 1980), and it was suggested that subtle differences in the courtship of older males contributed to their success or that auditory or olfactory cues might be involved. A pheromone cue has been postulated to be the basis for female discrimination between virgin males and males that have recently mated in Drosophila (Markow et al. 1978). It is possible that Diploptera males release a sex pheromone during courtship that attracts females. Secretions, which may include pheromones, are secreted by tergal glands in male cockroaches (Roth 1969). If some 8day males have not reached complete sexual maturity, and it appears that this is the case since a substantial portion (12%) of matings with 8-day males yielded no offspring, and if these males produce only small amounts of a pheromone, then this could explain the slower response by females to the courtship of 8-day males in the behavioural tests (Table II, Fig. 1). An advantage to females that respond quickly to mature courting males is that they are likely to receive gametes from these males. Mating with an infertile male has serious consequences for the fitness of Diploptera females since they will not mate again until after aborting the batch of unfertilized eggs some 3~4 weeks later (Woodhead 1985). The prediction from the results of the beha-
vioural tests is that 8-day males should be less successful than 14- and 28-day males in competitive mating tests if female discrimination based on courtship behaviour is an important factor in determining the outcome of these tests. Eight-day males were slower to initiate courtship and females were less attracted to their courtship. However, it was found that 28-day males were significantly more successful than both 8- and 14-day males. It is this lack of agreement with prediction that leads to the conclusion that behaviours of 28-day males, other than the courtship behaviours assessed, are responsible for their greater mating success. Twenty-eight-day males mated with the female in approximately two-thirds of the mating tests whether or not the female contacted all three males before mating (Table III). This suggests that the success of 28-day males was not due to their ability to find the female first. If this were so, then 28-day males would have achieved a greater proportion of the matings in all tests compared with tests in which all three males contacted the female. In this study it is not possible to determine whether male age alone or an interaction between male age and the amount of reproductive material that males have to transfer leads to mating success. To distinguish between these alternatives, it would be necessary to test males of the same age but with different amounts of reproductive material to transfer. This study demonstrates that the results of mating tests need to be interpreted carefully. Females may be capable of responding differentially to male courtship, yet female discrimination may not play a role in the outcome of competitive mating tests.
ACKNOWLEDGMENTS I thank Ola Fincke and Barbara Stay for their helpful comments on the manuscript.
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Woodhead: Male age and mating successMarkow, T. A., Quaid, M. & Kerr, S. 1978. Male mating experience and competitive courtship success in Drosophila melanogaster. Nature, Lond., 276, 821-822. Roth, L. M. 1969. The evolution of male tergal glands in the Blattaria. Ann. Entomol. Soc. Am., 62, 176-208. Rutowski, R. L. 1979. The butterfly as an honest salesman. Anim. Behav., 27, 1269-1270. Stay, B. & Coop, A. 1973. Developmental stages and chemical composition in embryos of the cockroach, Diploptera punctata, with observations on the effect of diet. J. Insect Physiol., 19, 147 171. Stay, B. & Roth, L. M. 1958. The reproductive behavior of Diploptera punctata (Blattaria: Diplopteridae). Proc. Tenth Int. Congr. Entomol., 2, 547--552. Thornhill, R. & Alcock, J. 1983. The Evolution of Insect Mating Systems. Cambridge, Massachusetts: Harvard University Press.
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Tobe, S. S., Musters, A. & Stay, B. 1979. Corpus allatum function during sexual maturation of male Diploptera punctata. Physiol. Entomol., 4, 79-86. Trivets, R. L. 1972. Parental investment and sexual selection. In: Sexual Selection and the Descent of Man 1871-1971 (Ed. by B. Campbell), pp. 136-179. Chicago: Aldine. Woodbead, A. P. 1984. Effect of duration of larval development on sexual competence in young adult male Diploptera punctata. Physiol. Entomol., 9, 473 477. Woodhead, A. P, 1985. Sperm mixing in the cockroach Diploptera punetata. Evolution, 39, 159-164.
(Received 22 August 1985; revised 24 Novembe? 1985; MS. number." A4599)