Sexual behaviour of male rats rehabilitated after early protein-calorie malnutrition.

The effect of early protein-calorie malnutrition (from birth to the age of 55 days) followed by nutritional rehabilitation (from the 58th day) on sexual behaviour was studied in male rats aged about 125 days. The sexual stimulation conditions on the part of the oestrus female were made as optimal as possible and were fully controlled. About half the malnourished males displayed precopulatory and copulatory behaviour, while in the control group these values were almost 100%. When malnourished males copulated, their copulatory performance did not differ very greatly from that of the well-nourished controls. The other experimental males displayed no signs of sexual behaviour during the testing period. Exposure of the males to the scent of a female in the latter's absence greatly stimulated their interest in the odour (sniffing the floor of the experimental box) in both the control and the malnourished animals which afterwards copulated. The results indicate that the sexual behaviour of males subjected to protein-calorie deficiency in early ontogenesis is at the very least delayed, if not completely suppressed.     

Size-dependent male mating behaviour and sexual harassment in a population of Atlantic mollies (Poecilia mexicana) from a sulphur creek

The Atlantic molly Poecilia mexicana inhabits a variety of habitats. In previous studies, small males from clear-water populations showed considerably more sexual behaviours than large males. Males from a sulphur creek, the El Azufre, or from a sulphur cave (cave molly) showed comparatively low sexual activity, and typical size-dependent mating behaviour was either absent or large males showed even more sexual behaviour than smaller ones. Sexually deprived cave molly males increased their sexual activity, but small males still did not show more sexual behaviour than large ones, suggesting that the genetic basis for typical size-dependent mating behaviour has been lost. Likely, the extreme abiotic conditions (hypoxia and toxic hydrogen sulphide) have selected against energetically costly behaviours. In this paper, we examined how El Azufre males would respond to sexual deprivation. We found these males to also increase their sexual activity after sexual deprivation but, unlike cave mollies, small males now showed more sexual behaviour than large ones, suggesting that the genetic basis for this behaviour is not lost in this population. Differences between populations may be due to the less harsh characteristics of El Azufre (e.g. less H₂S), resulting in weaker selection against energy-consuming behaviours. Furthermore, we tested for potential costs of male sexual harassment for females in terms of reduced feeding efficiency in the presence of a male, as was shown for P. mexicana from a clear-water population, but was not found in cave mollies or the El Azufre fish. Test females were either starved for 1 week to increase hunger levels or male sexual deprivation and female starvation were combined; however, we found no evidence for male sexual harassment even after these treatments. Our results parallel previous findings in the cave molly and suggest that El Azufre males have lost specific behavioural traits that cause sexual harassment.     

Seminal ribonuclease synthesis in bull reproductive organs during ontogenesis

Seminal ribonuclease (AS RNase) is synthesized in the epididymis, ampullary glands and seminal vesicles of sexually mature bulls. During sexual maturation of Czech red-spotted bulls it first begins to be synthesized in the seminal vesicles and ampullary glands, after the age of 20 weeks. At this age practically none is synthesized in the epididymis. As soon as the epithelial cells of the seminal vesicles start to synthesize the enzyme and secretion of the fluids of this organ begins, synthesis per ml fluid is almost the same as in sexually mature bulls. AS RNase synthesis in the cauda epididymidis begins after the age of 27 weeks and is individually variable. AS RNase synthesis in the reproductive organs depends on the testosterone concentration in the blood plasma of the bull.     

Seminal ribonuclease of bulls - biosynthesis in normal and orchitic bulls and correlation with blood plasma hormone levels

In sexually mature and healthy bulls, seminal ribonuclease (AS RNase) is synthesized in the distal part of the corpus epididymidis, the cauda epididymidis, the ampullary glands and the seminal vesicles. Indirect immunofluorescence demonstrated AS RNase binding to the cytoplasmic droplets of bull spermatozoa.In bulls with orchitis, AS RNase synthesis decreases in accordance with the degree of damage to the Leydig cells and the drop in the blood plasma testosterone level. The organ most sensitive to decreased testosterone levels, from the aspect of AS RNase synthesis, is the epididymis and the least sensitive are the seminal vesicles. Hypertrophy of the adrenal cortex (in particular of the zona fasciculata and the zona reticularis) and elevated adrenocortical secretion - demonstrated by a raised cortisol concentration in the blood plasma of severely orchitic bulls - failed to inhibit AS RNase synthesis. Injections of Gn RH-LH and HCG raised the blood plasma cortisol, but not testosterone, concentrations in bulls with very severe orchitis. This also indicates serious damage to the Leydig cells in these bulls.     

Cytotoxic effect of structurally modified bull seminal ribonuclease (AS RNase)

Chemical modifications of bull seminal ribonuclease (AS RNase) cause considerable changes in its cytotoxic activity. However, binding of AS RNase on cells has been changed little or not at all. Every modification (oxidation, reduction, carbomethylation, succinylation and maleylation) inhibited ribonuclease and aspermatogenic and embryotoxic activity of AS RNase. The common cytotoxic effect was also changed to some degree. Inhibition of incorporation of ³H-thymidine to BP-8 tumor cells in culture was observed after application of native, oxidized, reduced, and carboxymethylated AS RNase.     

Sexual behaviour of female rats subjected to early protein energy malnutrition

Young rats (left with the mother until weaned) were exposed from birth to the age of 49 days to a low protein (malnutritional) diet. At 80 days the young females were ovariectomized and were subjected to standard long-term treatment with oestradiol and progesterone. The females' sexual behaviour was studied in interaction tests with an intact male. It was evaluated according to a scale of sexual responsiveness comprising copulatory (a lordosis posture) and precopulatory (a presenting posture, hopping, ritualized darting) patterns. All the experimental females displayed a high copulatory readiness and precopulatory behaviour; a tendency to lower sexual responsiveness, interpreted as diminished sensitivity to the given doses of the hormones, was manifested only after their repeated administration. Compared with the control females, the number of experimental animals in which the given doses of the hormones induced complete precopulatory behaviour--ritualized darting--was smaller. The results contrast with findings on the sexual behaviour of males subjected to malnutrition from an early age.     

Inhibition of sexual behaviour among subordinate stumptail macaques,Macaca arctoides

A series of four experiments was conducted to examine rank-related differences in copulatory behaviour among male stumptail macaques, and to investigate the proximal mechanisms responsible for the reduced sexual activity among subordinate males. In the first experiment observations were made for 34 months on a group of 97 individuals. Lower-ranking males usually copulated surreptitiously and copulations attempted in view of higher-ranking males were more likely to be interrupted. Lower-ranking males also copulated more frequently when higher-ranking males were removed from their view; however, even the presence of a restrained higher-ranking male was sufficient to inhibit their copulatory behaviour. Three experiments were performed on a second social group of 30 stumptails to replicate and extend the findings of experiment 1. Increases in sexual activity in the absence of higher-ranking males were specific to sexual behaviour, and were not caused by general excitement due to removal or handling of animals. Female proceptive behaviour also increased in the absence of high-ranking males suggesting that the suppression of copulatory activity may not be solely due to behavioural changes in the males. These findings provide the first experimental support for the hypothesis that the copulatory behaviour of subordinate male stumptail macaques is suppressed by the presence of dominant males and that the absence of these males removes the suppression.     

Testosterone, cuckoldry risk and extra-pair opportunities in the Seychelles warbler

In male birds, testosterone (T) plays an important role in aggressive and mate-attraction behaviour. In the cooperatively breeding Seychelles warbler, Acrocephalus sechellensis, extra-group copulations (EGCs) occur frequently, but are not accompanied by sexual courtship displays as in within-pair copulations. Paternity is nearly always gained by primary males. We investigated whether T levels and sperm storage capability (cloacal protuberance (CP)) in adult primary and subordinate males were related to timing of egg laying, levels of cuckoldry and extra-group paternity (EGP) opportunities. During the sexually active period before egg laying, T levels and CP were only elevated or enlarged (respectively) in primary males, and some suggestion was found that subordinate males do not invest in elevated T levels. The peak in T occurred during the fertile period of the female partner and corresponded to the peak period of male sexual displays and mate guarding, but was independent of cuckoldry risk (density of neighbouring primary males). CP was also enhanced during this period; however, CP but not T remained elevated after egg laying by their mates, and CP but not T was positively related to EGP opportunities (density of neighbouring fertile females). We conclude that T is involved in sexual courtship displays and mate guarding, but not in gaining EGCs. These findings contrast with those in other species where EGP involves elaborate sexual displays.     

Precopulatory behaviour of male rats: developmental aspects and dependence on female's solicitation

Sexual communication between sex partners can involve just one sensory modality or a combination, depending upon such factors as species, habitat, and context. Ethological aspects of sexual communication has been widely documented in rodents. In rats, sexual communication between male and female varies according to the production of signals by a female that signal receptivity, proceptivity, and attractivity. However, in the laboratory experiment, such approach is often neglected. In the present study, two types of stimulus female--Lordotic and Darting--were used with the aim to examine developmental changes in precopulatory behaviour of males. Besides the dependence of the male's precopulatory repertoire on the strength of proceptive stimuli emitted by the female was studied. Male rats ranging from 30 to 175 days of age were observed under the dyadic interaction. It was found: (a) Precopulatory behaviour of the 30-day-old males was not clear-cut, the males devoted more time to social investigation of adult (Darting) female. (b) All the 40-day-old males exhibited precopulatory behaviour in the range of the repertoire displayed by adult animals. (c) Precopulatory activity of juvenile (45-day-old) as well as of adult (90-day-old) males exposed to Lordotic female was significantly lower as compared with that exhibited by males toward Darting female. (d) Copulatory readiness of males increased with the age, in fact, all animals aged from 75-135 day were able to pass from the precopulatory into the copulatory phase of sexual interaction. (e) Although the 175-day-old males exhibited pronounced precopulatory activity, they did not initiate copulations. The implications of each of these findings are discussed from the point of view of both the developmental aspects and the stimulus-response relationships. To sum up, the development of normal flow of sexual behaviour of male rats proceeds simultaneously with the development of physiological and morphological parameters. The appearance of precopulatory behaviour is less dependent on the internal (hormonal) readiness than copulatory behaviour. The connection of both phases, i.e. precopulatory and copulatory, is terminated at about Day 75, and is the key moment of sexual interaction from the point of view of reproductive success. In fact, a successful course of sexual interaction is codetermined by the intensity of behavioural stimuli and/or by the completeness of proceptive patterns provided by the female partner. It is clear that the reproductive process cannot be completed without some degree of communication. The deterioration of sexual interaction found in males aged 175 day seems to be the consequence of their absolute heterosexual abstinence.(ABSTRACT TRUNCATED AT 400 WORDS)     

Role of the medial preoptic area in sexual behaviour of the male rat: a study using repeated cycloheximide infusions

The effect of cycloheximide (CHX), an inhibitor of proteosynthesis, on sexual behaviour was studied in adult male rats in which it was infused into the medial preoptic area of the hypothalamus (MPOA). Sexual interaction took place under control and modification of the precopulatory behaviour of the female. Among the various bilaterally infused amounts of CHX--20 micrograms in 1 microliter, 40 micrograms in 1 microliter, 80 micrograms in 2 microliters--the effect of the largest dose corresponded to a hypothetical state of copulatory readiness of the males; 2 h after administration the males were not capable of initiating copulatory behaviour with a passively receptive female, but did so successfully with a highly soliciting female. The dependence of copulatory readiness of the males on the precopulatory behaviour of the females was confirmed repeatedly after this dose of CHX. At the same time, the precopulatory activity of the males towards a passively receptive female was unimpaired. The effect of CHX was reversible; 48 h after infusion the males displayed high copulatory readiness. Only half the males (n = 7) given bilateral infusions of 80 micrograms CHX fulfilled criterion of copulatory performance. Histological control demonstrated that the MPOA was affected bilaterally by CHX infusion in 10 males out of 14. The results are discussed from the aspect of participation of the MPOA in the regulation of male sexual behaviour. The method allows changes in sexual behaviour to be studied in the same individual in a chronic experiment.     

Social behaviour and heart rate in tammar wallabies (Macropodidae: Macropus eugenii)

In an enclosed group of tammar wallabies the behaviour of all individuals and the heart rate (HR) of three males were recorded simultaneously. The social structure was characterized by the dominance relationships between the males and the sexual preference of all males for the oldest female. During non-oestrus, dominance was obvious when a male intervened terminating the sexual behaviour of a subdominant one. The relatively low amount of agonistic behaviour between the males is assumed to be due to the clear dominance relationships. Locomotion as well as sexual and agonistic behaviour were accompanied by an acceleration of HR. The HR pattern depended on the intensity of the behaviour, its duration and the animal's identity. Commonly performed sexual behaviour and approach/retreat encounters gave rise to HR values not different from HR values during locomotor activity without interactive components. Social events that occurred infrequently—like fights, and the mating of an oestrous female—were accompanied by strong accelerations of HR indicating high cardiac effort. The mild cardiac activity during common social encounters is consistent with a strategy of maximizing energy conservation. HR patterns during specific behaviour could not entirely be accounted for by the energetic costs of activity. Spreading the forelimbs in response to specific stimuli—like the fly-over of raptors—was indicative of a strong HR response even if the animal was motionless. During specific social encounters like the sexual behaviour of the alpha-male following an intervention—HR responses revealed that arousal might exceed motor activity in affecting HR. Individual differences of these HR responses are attributed to the age, experience and social status of a male.     

Incubation Behaviour in Northern Lapwings: Nocturnal Nest Attentiveness and Possible Importance of Individual Breeding Quality

Previous studies of sex roles in the polygynous Northern Lapwing Vanellus vanellus have shown that males incubate less than females, perhaps suggesting that sexual selection is important in shaping the parental behaviour of this species. The purpose of this study was to (1) examine for the first time the possibility that males compensate for low diurnal nest attentiveness by increasing their nocturnal assistance and (2) evaluate the hypotheses that sexual selection and individual breeding quality determines incubation behaviour in lapwings. Males were never found incubating at night in 19 nests, although median diurnal male attentiveness was 15.3% in 16 of the same pairs. Nor were there any differences among monogamous and polygynous males in the time spent in four categories of behaviour (incubation, guarding, maintenance and mating activities). The time males spent in mating and incubation behaviours was weakly negatively correlated, and the time spent incubating varied considerably among males (0–74%). Further, female body condition was positively related with male nest attentiveness and there was a negative relationship between nest attentiveness and date of arrival to the study area in monogamous, but not in polygynous, males. We argue that sexual selection could not alone explain all sides of Northern Lapwing incubation, and suggest that individual differences in breeding quality may also be important.     

Body size dimorphism and sexual segregation in polygynous ungulates: an experimental test with Soay sheep

Sexual segregation in Soay sheep (Ovis aries) was investigated using an experimental approach in order to test the sexual dimorphism-body size hypothesis. Two corollaries of the sexual dimorphism-body size hypothesis were tested: (1) in dimorphic species males, the larger sex, have relatively smaller bite sizes on short swards because of the scaling of incisor arcade with body weight, and (2) they move off earlier to feed on taller but poorer-quality swards when such swards are patchily distributed on a scale which enables the spatial segregation of individuals. Patch choice between sexes was estimated using a matrix of grass patches which differed in both quality and biomass of grass on offer (HQ: high-quality-low-biomass; LQ: low-quality-high-biomass). Sex differences in patch choice and grazing behaviour were tested in short-term preference trials. Incisor breadth showed no significant difference between sexes. On the other hand, muzzle width was dimorphic, with females having a narrower muzzle than males. Bite size was significantly different between the sexes, being smaller in females than in males, although it was not significantly different between sward types. Females had a higher bite rate than males and the bite rate was higher in the HQ sward type than the LQ sward type. When the effect of body mass was removed, no sex differences in muzzle size, bite size or bite rate were found. The intake rate did not differ between the sexes or between sward types. Whilst both sexes preferred the HQ sward type, females spent a significantly longer time feeding on the LQ sward type than did males. The difference detected between the sexes in patch choice was not consistent directly with the sexual dimorphism-body size hypothesis. Alternative explanations based on sex differences in foraging behaviour in relation to body mass sexual dimorphism are discussed to explain the result. Received: 1 February 1999 / Accepted: 12 May 1999     

Sex-Specific Aggression and Antipredator Behaviour in Young Brown Trout

Sex differences in adult behaviour are often interpreted as consequences of sexual selection and/or different reproductive roles in males and females. Sex-specific juvenile behaviour, however, has received less attention. Adult brown trout males are more aggressive than females during spawning and juvenile aggression may be genetically correlated with adult aggression in fish. We therefore tested the prediction that immature brown trout males are more aggressive and bolder than immature females. Because previous work has suggested that precocious maturation increases dominance in salmonids, we included precocious males in the study to test the prediction that early sexual maturation increase male aggression and boldness. Aggression and dominance relations were estimated in dyadic contests, whereas boldness was measured as a response to simulated predation risk using a model heron. Independent of maturity state, males initiated more than twice as many agonistic interactions as females in intersexual contests. However, males were not significantly more likely to win these contests than females. The response to a first predator attack did not differ between sex categories, but males reacted less to a second predator attack than females. Sexual maturity did not affect the antipredator response in males. Since there is no evidence from field studies that stream-living immature male and female salmonids differ in growth rate, it appears unlikely that the sex differences demonstrated are behavioural consequences of sex-specific investment in growth. It seems more likely that sex-specific behaviour arises as a correlated response to sexually selected gene actions promoting differential behaviour in adult males and females during reproduction. Alternatively, sex differences may develop gradually during juvenile life, because a gradual developmental program should be less costly than a sudden behavioural change at the onset of sexual maturity.     

Experimental manipulation of male behaviour during copulation in Stenomacra marginella (Heteroptera: Largidae): effect on copula duration, female remating and oviposition

Apparently stimulatory male copulatory behaviour (MCB) is widespread among arthropods and it could help males to increase their fitness by inducing favourable behavioural and physiological changes in females. The empirical study of female responses to MCB is hindered because its experimental manipulation is difficult. We have developed a technique for reducing, with minimal disturbance, the frequency of MCB in the true bug Stenomacra marginella. Here, we test the idea that, in a polygamous species like S. marginella, sexual selection favours males whose MCB induces females to increase copula duration (thereby increasing the amount of sperm and accessory substances transferred), reduce their sexual receptivity to additional males and increase their rate of oviposition. Males prevented from performing MCB increased their rate of attempts to perform MCB. Copulations with previously mated females were of longer duration than those with virgin females, probably as a male adaptation for sperm competition, and MCB could have played a role in inducing this effect. Partial or total experimental reduction of MCB frequency had no effect on remating rates, because most females accepted remating at the first opportunity (1 day after their first copula). The probability of egg laying was reduced in females whose first mate was partially prevented from performing copulatory courtship, but not in females whose first mate was completely prevented from performing copulatory courtship. This is an intriguing result and further experiments are needed to understand its causes. We hypothesize that MCB evolved as a result of sexual selection.     

Enhancing mating performance after juvenile hormone treatment in Anastrepha fraterculus: a differential response in males and females acts as a physiological sexing system

Methoprene (a mimic of juvenile hormone) treatment can reduce the time required for sexual maturation in Anastrepha fraterculus (Wiedemann) (Diptera: Tephritidae) males under laboratory conditions, supporting its use as a treatment for sterile males within the context of the sterile insect technique (SIT). We evaluated sexual behaviour, mating competitiveness of methoprene-treated males, and female readiness to mate after methoprene-treatment in field cages. The study involved two strains of A. fraterculus from Argentina and Peru, which show several polymorphisms in relation to their sexual behaviour. We also analyzed whether methoprene treatment affected male and/or female behaviour in the same way in these two strains. Methoprene-treated males were equally competitive with untreated mature males, and became sexually competitive 6 days after emergence (3–4 days earlier than untreated males). In contrast, methoprene did not induce sexual maturation in females or, at least, it did not induce a higher rate of mating in 7-day-old females. These results were observed both for the Argentina and the Peru strains. Altogether, our results indicate that methoprene treatment produces sexually competitive males in field cages. In the absence of a genetic sexing system, and when sterile males and females of A. fraterculus are released simultaneously, the fact that females do not respond as do males to the methoprene treatment acts as a physiological sexing effect. Therefore, in the presence of mainly sexually immature sterile females, released sexually mature sterile males would have to disperse in search of wild fertile females, thereby greatly reducing matings among the released sterile insects and thus enhancing sterile insect technique efficiency.     

Social dominance and female reproductive behaviour in rhesus monkeys (Macaca mulatta)

The relationship between dominance rank and female sexual behaviour was examined in rheusus monkeys (Macaca mulatta) living in a social group. High-ranking females engaged in copulatory series as frequently as lower-ranking females. Furthermore, lower-ranking females copulated with as many available males as did high-ranking females. Social rank did appear to influence the pattern of sexual activity exhibited, in that copulatory series were more often initiated by the higher-ranking animal of the mating pair. Copulatory series involving high-ranking females were characterized by more mounts by males and were longer in duration. Higher-ranking animals interfered more often with copulatory series involving other animals, but such interference was not effective in preventing completion of the series. These data indicate that any reproductive advantage conferred to high-ranking females is not the result of sexual competition in social-living rhesus monkeys.     

Effect of ribonucleases on cell-mediated lympholysis reaction and on GM-CFC colonies in bone marrow culture

Natural dimer of bovine seminal ribonuclease (AS RNase) suppressed markedly DNA synthesis in allogeneic mixed lymphocyte culture (MLC) of normal human lymphocytes and simultaneously inhibited induction of cytotoxic effector cells within the sensitization phase of indirect cell-mediated lympholysis (CML) reaction. The last purification step of the AS RNase isolation procedure did not increase the suppressive activity of AS RNase compared to a less purified preparation (ZS RNase), thus, the later preparation was mostly used. ZS RNase (10 micrograms/ml) caused 50% inhibition of MLC reaction whereas pancreatic ribonuclease (A RNase) was 10 times less effective. The suppressive effect of RNases added in the beginning of the sensitization phase of the CML reaction correlated with that observed in the MLC reaction. The concentrations of ZS RNase (10 micrograms/ml), A RNase (100 micrograms/ml), and additionally tested cyclosporin A (0.5 microgram/ml) resulted in nearly total abrogation of cytolysis in CML. ZS RNase added after the sensitization of effector cells did not influence their cytolytic action on target cells within the destruction phase of CML. Natural killer and killer cell activities in normal peripheral lymphocytes were not inhibited by ZS RNase at the concentration of 330 micrograms/ml. ZS RNase (20 micrograms/ml), cocultivated 1 h with normal human bone marrow cells and then washed off, enhanced formation of GM-CFC colonies in semisolid agar culture up to 200%. Simultaneously tested antilymphocyte globulin increased the number of GM-CFC colonies at the average of 128%. This stimulating effect on colony formation appeared also in bone marrow culture of patients suffering with various hematological disorders. The possibility of utilizing the preparations gained from seminal plasma in clinical bone marrow transplantation is discussed.     

RNase L and double-stranded RNA-dependent protein kinase exert complementary roles in islet cell defense during coxsackievirus infection

Coxsackievirus (CV) is an important human pathogen that has been linked to the development of autoimmunity. An intact pancreatic beta cell IFN response is critical for islet cell survival and protection from type 1 diabetes following CV infection. In this study, we show that IFNs trigger an antiviral state in beta cells by inducing the expression of proteins involved in intracellular antiviral defense. Specifically, we demonstrate that 2',5'-oligoadenylate synthetases (2-5AS), RNase L, and dsRNA-dependent protein kinase (PKR) are expressed by pancreatic islet cells and that IFNs (IFN-alpha and IFN-gamma) increase the expression of 2-5AS and PKR, but not RNase L. Moreover, our in vitro studies uncovered that these pathways play important roles in providing unique and complementary antiviral activities that critically regulate the outcome of CV infection. The 2-5AS/RNase L pathway was critical for IFN-alpha-mediated islet cell resistance from CV serotype B4 (CVB4) infection and replication, whereas an intact PKR pathway was required for efficient IFN-gamma-mediated repression of CVB4 infection and replication. Finally, we show that the 2-5AS/RNase L and the PKR pathways play important roles for host survival during a challenge with CVB4. In conclusion, this study has dissected the pathways used by distinct antiviral signals and linked their expression to defense against CVB4.     

Sexual behaviour of immature male eastern mosquitofish: a way to measure intensity of intra-sexual selection?

Immature males of eastern mosquitofish Gambusia holbrooki start to be sexually active well before their copulatory organ (gonopodium) has completely developed and before they become able to transfer sperm. Sexual activity of males, consisting of copulatory attempts tending to bypass female acceptance, is intense (one attempt per minute) and is likely to be energetically very costly. The sexual behaviour of immature males relative to their maturation stage is described and tested against two possible adaptive explanations. Sexual activity was present in males from the beginning of the development of their gonopodium and increased during the following stages of maturation. Two to three weeks before gonopodium development was completed, sexual activity of immatures was as high as that of adults. Adult males showed aggressive behaviour against a male attempting a copulation, irrespective of the maturity of the latter. Since previous studies have shown that the reproductive success in this species is negatively correlated with male size when male–male competition is low (i.e. when the sex ratio is female biased), but decreases with male size when competition is high, the hypothesis was tested that sexual activity of immature males functions as a way to predict their future reproductive success if they mature at a given size. A second hypothesis tested was that precocious sexual experience improves the efficiency of copulatory attempts. Results were more in agreement with the first hypothesis, since size at maturity of males was influenced by the sex ratio experienced during maturation and precocious experience gave very little advantage.