Female territoriality and the function of scent-marking in a monogamous antelope (Oreotragus oreotragus)

In monogamous species, females often choose between males according to the quality of the territories they defend, but the extent to which females themselves contribute to territory defence is frequently underestimated. Here we test for differences in male and female roles during paired scent-marking bouts, a key component of territorial defence, in a monogamous antelope. In two populations (Kenya, Zimbabwe) of klipspringer, Oreotragus oreotragus, both males and females usually scent-marked at the same site, but there were significant differences between sexes in terms of investment within bouts. Females initiated most bouts, thus dictating the marking strategy of the pair. Males initiated relatively few bouts, but deposited more scent marks per bout than females and were usually the last to scent-mark before leaving the site; they marked on the same branches as the female and thus overmarked her scent. Both sexes deposited more marks during paired than solo visits. Immediately preceding and following scent-marking bouts, males approached females and females left males more often than expected. Female scent-marking rates were higher when they were receptive than at other times, and this increase was matched by elevated marking rates of males. Females may increase marking rates when they are receptive in order to test the quality of their mate or to incite male competition. However, these ideas are unlikely to explain female scent-marking behaviour outside the mating season, which appears to be related primarily to territorial defence. We suggest that these differences in investment in scent-marking bouts are consistent with predictions that females may be autonomously territorial and that overmarking of female scent by males is a form of mate-guarding. Received: 17 November 1999 / Received in revised form: 24 February 2000 / Accepted: 13 March 2000     

To whom it may concern: the transmission and function of chemical signals in Lemur catta

The goals of this study were to investigate the transmission and possible functions of chemical signals in intragroup communication among ring-tailed lemurs, Lemur catta. In particular, I examined the effects of sex on these processes because sexual selection theory predicts specific functions for chemical signals. I recorded all interactions with 214 scent marks of 11 male and 9 female ring-tailed lemurs during the first 10 min following their deposition. I found that 62% of these scent marks were investigated with a median latency of 30 s and that 89% of investigated scents were also counter-marked by the receiver. The type of focal scent mark (male or female anogenital and male antebrachial mark) had a significant effect on both the timing and type of response. Males investigated and counter-marked female scents more often than vice versa, but significant second-order transitions suggested that the behavior of an animal was not only influenced by the immediately preceding scent mark and that a scent is not completely masked by a counter-mark. There was no evidence for an audience effect, and only social rank of female senders had an effect on receivers. Variation in the response of receivers across reproductive seasons as a function of senders' sex indicated that female scents may function in mate attraction and competition among females, whereas male scents may be primarily used in intrasexual competition. Three main conclusions emerged. First, the exchange of olfactory signals within groups was highly structured and surprisingly efficient. Second, olfactory signals may constitute general mesasages whose transfer is partly controlled by the receiver. Finally, sexual selection theory provides a useful theoretical framework for functional examinations of mammalian olfactory communication. Received: 13 June 1997 / Accepted after revision: 20 March 1998     

Effect of pair-bonding with adults on the ontogenetic manifestation of aggressive behavior in a primate,Saguinus fuscicollis

Summary Injurious aggression, scent marking and some visual displays shown in situations analogous to territorial defense were studied in young saddleback tamarins who had lived with nonrelated adult and juvenile social partners from 6 months of age under the experimental conditions outlined below. Twenty male and 20 female tamarins were removed from their natal families when 6 months old, and were used to form pairs and trios of the following composition: (1) one juvenile female and one juvenile male (10 pairs); (2) one juvenile female and one adult male (4 pairs); (3) one juvenile female, one juvenile male, and one adult male (6 trios); (4) one adult female and one juvenile male (4 pairs). These groups cohabited permanently and when the subjects were between 10 and 12 months old, their social interactions with adult conspecific strangers were studied, using series of 10-min social encounter tests. During these tests injurious aggression (attacks, fights, chases), a number of visual hostile displays (threat face, ruffle, arch) and scent marking were recorded.Young males and females living with adults of the opposite sex showed more injurious aggression against strangers and scent marked more frequently than young subjects living with partners of their own age. Males living with adult females also showed more facial threat than males living with young females (Figs. 1 and 2). Young males living in trios with adult males and young females scent marked less frequently than males living only with a young female. Aggression, scent marking and facial threat shown by subjects living with adult sex partners were at the level of socially and sexually experienced, pair-bonded adults (Fig. 3). No differences in ruffling and arching scores were found. The injurious aggression scores of the subjects were correlated with those of their mates. These results show that tamarins can fulfill the behavioral roles of pair bonded adults and also breed (Epple and Katz 1980) at an unexpectedly early age, when they live under the influence of an adult sex partner. Young animals cohabiting with an adult of the same sex, on the other hand, show a trend toward inhibition of adult-like behaviors.     

Manipulation of Olfactory Signaling and Mate Choice for Conservation Breeding: a Case Study of Harvest Mice

Abstract:  Mate choice by females can introduce difficulties to captive breeding programs because there may be a conflict between the conservation manager's choice of mate (based on random allocation or maximizing heterogeneity) and the females' own preferences, often resulting in incompatibility and aggression. Similar effects are caused by inappropriate social contexts at the time of pairing. We manipulated the social experience of male and female harvest mice ( Micromys minutus ) to investigate whether we could enhance compatibility between randomly allocated mates by altering female preferences. In one experiment, we used a choice test to identify female preferences between two males and then varied the competitive context of unpreferred males by transferring competitor's scent marks into their cages. The manipulation caused them to increase their investment in a form of olfactory signaling (scent marking), which female rodents use as an indicator of male quality when choosing mates. The manipulation increased their attractiveness relative to the initially preferred male when the choice test was repeated. In a second experiment, we tested the effect of females' familiarity with the odor of males by transfer of male scent marks to female cages. Females preferred familiar males in choice tests and were less aggressive toward them when pairs were introduced than females paired with unfamiliar males. This kind of approach can influence mate choice, and transferring scent marks between cages or collections is an effective and practical behavioral means of improving success in conservation breeding programs.     

The scent of sun worship: basking experience alters scent mark composition in male lizards

Signals used in female choice should honestly advertise the benefits that males can provide, with direct benefits often argued as being more important than indirect benefits. However, the nature of direct benefits in species without paternal care or nuptial gifts is poorly understood. Previous studies on lizards suggest that females decide where to settle and assumedly who to mate with based on information contained in scent marks from territorial males. Access to high-quality thermal resources is crucial for female reproductive success. Females may therefore be able to detect and exploit thermal-induced variation in the chemical composition of male scent marks when assessing the quality of his territory. We show that the amount of time male wall lizards (Podarcis muralis) are allowed to bask significantly alters the chemical composition of their femoral secretions used in scent marking. The direction of the change is consistent with adaptive plasticity to maintain signalling efficacy under warm conditions that increase evaporation of femoral secretions. The compounds affected by basking experience included those previously associated with male quality or shown to mediate male–male competition in lizards. However, whilst female lizards could discriminate between scent marks of males that had experienced different basking conditions, they did not preferentially associate with the scent from males from high-quality thermal conditions. These results highlight the potential importance of a previously neglected environmental effect on chemical signalling. We suggest thermal effects may have significant consequences for scent-mark composition in variable environments, with potential repercussions on olfactory communication in lizards.     

Scent-marking behavior in wild groups of common marmosets (Callithrix jacchus)

Scent-marking in mammals has been frequently related to within-group social and reproductive dominance and to defense of territory and resources. We studied the scent-marking behavior of five wild groups of common marmosets, Callithrix jacchus, during 5 months of the fruiting season in northeastern Brazil. Circumgenital marking was the most common type of marking. Marks were distributed throughout the home range and were deposited mainly during travel and intergroup encounters. Although marks were commonly deposited at gum trees, there was no evidence that the animals used scent marks to label fruiting trees or sleeping sites. Contrary to expectations, reproductively dominant females did not mark more than reproductively subordinate females. Moreover, during intergroup encounters, reproductively subordinate females displayed higher frequencies of scent-marking than the reproductively dominant females of their group. Our results suggest that scent-marking is not strictly tied to reproductive dominance or territorial (or resource) defense in common marmosets. Because marks provide information about individual identity and reproductive condition, scent marks could serve different functions when used by different individuals. The high frequency of marking by reproductively subordinate females during intergroup encounters suggests that scent-marking might be used to signal to individuals of neighboring groups. Our data highlight the importance of social and ecological variables in scent-marking behavior. Received: 26 October 1998 / Received in revised form: 20 May 1999 / Accepted: 30 May 1999     

Responses to conspecific scent marks and the ontogeny of territorial marking in immature terrestrial salamanders

We investigated the ontogeny of responses to scent marks in immature terrestrial salamanders (Salamandra lanzai) that inhabit the southwest Alps. In this species, sexual maturity is usually reached at about 8 years, and adults exhibit territoriality. One should expect territoriality to take place largely before the acquisition of sexual maturity if sexual competition is not the main force driving territoriality. However, both the difficulties for inexperienced animals to find a suitable territory and size-related competition may delay the acquisition of territoriality in juveniles. We performed choice tests with juveniles belonging to two age groups (1±1 years old versus 4±1 years old). Each focal animal was offered, in random order, the choice between two shelters, one without scent and the other containing one of the following scents: own, same-aged animal, a juvenile belonging to the other age group, or adult female. We also performed choice tests with adult females for which the scents of two juveniles belonging to a different age group were successively offered in a random order. Older juveniles were strongly attracted toward their own shelter and mostly avoided the shelters that contained the scent of juveniles of about the same age and of adult females. Adult females avoided the scents of older juveniles but not younger juveniles. These results therefore suggest that older juveniles use territorial marking. Conversely, young juveniles behaved randomly with respect to their own scents and to those of a same-aged juvenile, and they were significantly attracted towards the odor of an older animal, especially adult females. Both older juveniles and adult females displayed a random behavior toward the scents of young juveniles. Our results suggest that young juveniles do not defend territories but use spaces occupied by older individuals.Communicated by A. Mathis     

Female sensory bias may allow honest chemical signaling by male Iberian rock lizards

Some mate choice theories propose that only male signals that are honest and condition-dependent can be stable, while another hypothesis states that males evolve signals that exploit the sensory system of females. However, sensory traps might evolve into honest signals if they are differentially costly for males. We tested whether a pre-existing sensory bias for food chemicals explained chemosensory preferences of female Iberian rock lizards for male scents. We manipulated hunger levels of females and found that food-deprived females had increased chemosensory responses to chemical stimuli from both invertebrate prey and femoral secretions of males, but not to control water. Further tests suggested that cholesta-5,7-dien-3-ol (provitamin D₃), a lipid found in both prey and males’ scent, may be one of the chemicals eliciting these responses. Moreover, hungry females spent more time on scent marks of males that had experimentally increased cholesta-5,7-dien-3-ol than on scent marks of males alone, whereas for control females this effect was not significant. We suggest that preexisting sensory bias for essential nutrients (i.e., provitamin D) may be the origin of similar female responses to male chemicals. However, previous studies have suggested that the allocation of these chemicals to ornaments is costly and only high quality males can afford it. Therefore, preexisting sensory bias for essential nutrients may further allow the evolution and maintenance of honest sexual displays.     

Female tactics to reduce sexual harassment in the Sumatran orangutan (Pongo pygmaeus abelii)

Orangutan males demonstrate intrasexual dimorphism with corresponding alternative mating strategies. Sexual harassment is the predominant feature of the mating strategy that subadult males pursue. This study investigated the countertactics that females employ to reduce sexual harassment by subadult males. I observed 207 copulations during more than 9,000 h of focal follows of wild Sumatran orangutans at the Suaq Balimbing Research Station over a 23-month period. Rates of copulations initiated by subadult males increased during months of high fruit abundance, and most mating attempts were directed toward females with weaned infants. Simultaneous harassment by multiple subadult males increased significantly during months of high fruit abundance, and nearly all adult female-adult male consortships occurred during periods of high fruit abundance. Females who maintained spatial association with adult males, either via consortship or by nonmating temporary parties, received lower rates of harassment, as measured by the success rate of subadult male mating attempts. Adult female-adult male parties did not always result in mating between the associating dyad. Female initiation of protective services by adult males is one social tactic that female orangutans employ to reduce sexual harassment. Females therefore receive direct services from adult males, which may be one factor that influences female mate choice in Sumatran orangutans.     

Olfactory cues,genetic relatedness and female mate choice in the agile antechinus (<Emphasis Type="Italic">Antechinus agilis</Emphasis>)

Females show mate preferences for males that are genetically dissimilar to themselves in a variety of taxa, but how females choose these males is not clearly understood. In this study, we examined the effects of olfactory stimuli and genetic relatedness on female mate choice in a small carnivorous marsupial, the agile antechinus (Antechinus agilis), during two breeding seasons. Captive female antechinus in oestrus were provided with a combination of male urine and body scent from two novel males, one more genetically similar and one more dissimilar to the females, in a Y-maze olfactometer. Genetic relatedness between females and pairs of males was determined using highly polymorphic, species-specific, microsatellite markers. Females consistently chose to visit the scents of males that were genetically dissimilar to themselves first, spent significantly more time near the source of those scents and showed more sexual and non-exploratory behaviours near those scents. These data demonstrate that chemosensory cues are important in mate choice in the agile antechinus and that females prefer males that are genetically dissimilar to themselves.     

Male Iberian rock lizards may reduce the costs of fighting by scent matching of the resource holders

Animals should adopt strategies to minimize the costs of intraspecific aggressive interactions. For example, individuals should be able to identify resource holders in advance and avoid fighting with them because residents are generally more likely than intruders escalate aggression. It has been suggested that scent marks function mainly to allow competitor assessment by conveying the costs of entering a scent-marked area. Individuals may identify territory owners by comparing the scent of substrate marks with the scent of any conspecific they encounter nearby, assessing whether these two scents match or not, a mechanism known as scent matching. Here, we examined the response of male Iberolacerta cyreni lizards to areas scent-marked by other males and the potential role of scent matching in agonistic interactions. We designed a laboratory experiment where we allowed a male to explore the scent-marked substrate of another male, and then we immediately staged agonistic encounters in a nearby clean neutral area with either the male that had produced the scent marks (matching treatment) or with a different non-matching individual male. The higher chemosensory exploratory rates of substrate scent marks in comparison to clean substrates suggested that males detected and spent more time exploring scent marks to obtain information on the donor male. Moreover, this information was later used to decide the fighting strategy. Intruding males delayed time until the first agonistic interaction, reduced the intensity of fights and the number of aggressive interactions, and won less interactions with males which scent matched that of scent marks (because they would be considered as the territory owners) than with other non-matching individuals. Our results show that male I. cyreni lizards use scent matching as a mechanism to assess the ownership status of other males, which could contribute to modulate intrasexual aggression, reducing costs of agonistic interactions.     

Female association preferences based on olfactory cues in the guppy,<Emphasis Type="Italic"> Poecilia reticulata</Emphasis>

Guppies are highly sexually dimorphic. Females have been shown to mate preferentially with males with various visible traits and previous studies investigating mate choice have focussed on these visual cues. However, guppies have been shown to respond behaviourally to odour cues from conspecifics as well as from predators. Therefore, sexual selection in guppies may act on multiple sensory modalities, but so far only visual cues have been investigated. Here, we used a specially designed olfactory choice tank to determine whether females could detect other guppies on the basis of their olfactory cues only, and to determine whether females preferred to associate with males or females and whether they could distinguish between different males. We show that female guppies associated preferentially with other females when given a choice between water containing the odour cues from another fish and water containing no odour cues. When females were presented with olfactory cues from a male and a female, they investigated the female initially, but then most reversed their decision and swam to the male. Furthermore, we demonstrate that females associated preferentially with certain males based upon olfactory cues alone, but that this choice was in direct opposition to that made when the females received only visual cues from the same males.Communicated by: M. Abrahams     

Scent communication by female Columbian ground squirrels, Urocitellus columbianus

Scent-marking is a frequent behaviour of highly social ground squirrels and might play an important role in their social dynamics. Female Columbian ground squirrels exhibit considerable scent-marking during the reproductive period. We examined how gestating and lactating females responded to jugal gland scent-marks of same-sexed and opposite-sexed conspecifics with attention to genetic relatedness and the geographical location of the territory of individuals. We tested the dear-enemy, threat-level and kin-discrimination hypotheses to explain patterns of scent-marking. Females sniffed the scent of non-neighbouring males significantly longer than other types of scent categories and tended to over mark the scent of females more than the scent of males. Furthermore, females sniffed significantly longer at scents during gestation than during lactation. We concluded that scent-marking mainly functioned in the defence of female territories and for protection of pups against infanticidal females (threat-level hypothesis). Our results were also in accordance with the kin-discrimination hypothesis, because greater attention was paid to the marks of non-kin females. Kin females might not pose an infanticidal threat, perhaps explaining greater tolerance among related reproductive females. We concluded that scent-marking may be a relatively low-cost means of territorial defence, as well as a means of communication of aspects of individual identity.     

Countermarking by male pygmy lorises (<Emphasis Type="Italic">Nycticebus pygmaeus</Emphasis>): do females use odor cues to select mates with high competitive ability?

Competitive countermarking occurs when animals compete to ensure that their scent marks are in the top-most position and more recently deposited than those of rivals. Because it takes a great deal of time and energy, and perhaps dominant or territorial status, to patrol the area and rapidly countermark rivals' marks, assessors may use the presence of countermarks as a reliable cue of the signaler's competitive ability. Selection on assessors, therefore, should favor the evolution of mechanisms to determine which scent is the countermark. Sexual selection theory predicts that females select high-quality males with whom to mate; therefore, females should mate preferentially with countermarking males. We examined the role of countermarking in intra-male competition and female mate choice in the pygmy loris. In experiment 1, we found that males deposit significantly more urine when countermarking other males' urine, but countermark female urine no more than control stimuli. In experiment 2, we exposed females to the urine of two males for several weeks. One male always deposited his urine first, whereas the second male always deposited his urine later and on top of the first male's urine. When reaching peak estrus, females were given a simultaneous choice test between the two males, and showed a significant preference for the countermarking male with regard to: (1) her location and orientation to the male, (2) her chemosensory interest in the male and his cage, and (3) her affiliative socio-sexual behaviors. These results are consistent with predictions from the competitive countermarking hypothesis, and demonstrate that chemosignals play a profound role in governing female reproductive behavior in the species.     

Offspring number in pygmy marmosets, Cebuella pygmaea *, in relation to group size and the number of adult males

In many species of birds and mammals with a co-operative breeding and rearing system, offspring survival is positively related to the number of helpers. In the New World callitrichine primates (marmosets and tamarins), adult males are considered as particularly valuable helpers, and female reproductive success may depend strongly on the males' contribution to infant care. We analysed the number of offspring (infants, juveniles) in groups of wild pygmy marmosets, Cebuella pygmaea (Callitrichinae, Cebidae, Primates), in relation to the number of adult males and to the number of adult and subadult group members. In contrast to other callitrichines with a co-operative system of infant care, no relationship was found between the number of adult males and the number of infants and offspring. However, there was a significant positive relationship between the number of juveniles and the number of adult and subadult group members. The lack of a relationship between infant and adult-male number is interpreted as resulting from the reduced importance of adult males as helpers in pygmy marmosets in comparison to other callitrichines, probably due to the reduced costs of infant care. The relationship between the number of juveniles and the number of adult and subadult group members is in accordance with increased offspring survival in larger groups, as observed in other primates. Received: 1 February 1999 / Received in revised form: 5 June 1999 / Accepted: 29 June 1999     

Sexual selection and species recognition in the pygmy swordtail, Xiphophorus pygmaeus: conflicting preferences

Sexual selection and species recognition play important roles in mate choice; however, sexual selection preferences may overlap with traits found in heterospecifics, producing a conflict between sexual selection and species recognition. We examined female preferences in Xiphophorus pygmaeus for male traits that could provide both types of information to determine how females use multiple cues when preferences for these cues would conflict. We also examined X. pygmaeus behavior in the field to determine if females have the opportunity to choose mates. As no male-male competition was observed in the field, and females occasionally chased males from feeding areas, females apparently have the opportunity to exercise mate choice in their natural habitat. In the laboratory, female X. pygmaeus used body size as a sexual selection cue, preferring large heterospecifics (X. cortezi) to small conspecifics. Females also preferred barless X. cortezi over barred X. cortezi when males were size matched. Because X. pygmaeus males do not have bars, this preference suggests that X. pygmaeus females use vertical bars in species recognition, and that large body size and vertical bars are conflicting cues. However, X. pygmaeus females did not have a preference for males of either species when sexual selection and species recognition cues were presented concurrently. This result was surprising, because preferences for species recognition cues are often assumed to be stronger than sexual selection cues. We suggest that females may be using additional species-specific cues in mate choice to prevent hybridization.     

The special role of male Cebus monkeys in predation avoidance and its effect on group composition

Summary Vigilance behavior, predator detection abilities, and responses to real and model predators were studied in two species of capuchin monkeys (Cebus albifrons and C. apella) in a Peruvian lowland rain forest. Adult males were more vigilant than adult females in both species, mainly because the males spent less time feeding and foraging and partly because they were at the periphery more often than the females. The increased vigilance of adult males is reflected in their superior performance in the detection of (model) predators. Adult and subadult males were also far more likely to approach and mob real and model predators. Adults that were outside the center of the group increased foraging activities but cut back an feeding, much of which was done in exposed tree crowns. Current theory suggests that primate groups are multi-male when a single male is unable to defend sexual access to the group of females. In these small capuchin groups, which are multimale, the enhanced safety of females and young provided by extra adult males furnishes a more plausible explanation. A comparison of the two capuchins with the ecologically similar Southeast Asian Macaca fascicularis suggests that the high predation risk outside the group may also have caused the unusual male career profile in capuchins, which have a long tenure of dominants and a very long potential lifespan. Further predictions of this hypothesis are developed.     

Selection for male choice based primarily on mate compatibility in the oldfield mouse, Peromyscus polionotus rhoadsi

Despite the consensus that mate choice acts as a mechanism for selection of secondary sexual traits, the evolutionary forces affecting mate preferences themselves remain controversial. In this study, we first demonstrated selection acting directly on the mate preferences of monogamous male oldfield mice, Peromyscus polionotus rhoadsi. One group of male oldfield mice were allowed to express a social preference between two potential mates, and were subsequently paired with either their preferred or rejected female. Among these pairs, those containing preferred females produced more offspring than did those containing rejected females. We next demonstrated that this fitness advantage depended primarily on compatibility between the members of a mated pair. A second group of male oldfield mice were not allowed the opportunity to express a social preference between potential mates. Rather, these males were paired with females that had been either preferred or rejected by males in the first group. Among these pairs, those containing preferred females did not produce more offspring than those containing rejected females. In other words, individual mate preferences had fitness consequences only for those males that expressed them, demonstrating that these preferences were based primarily on compatibility between mates.     

Polyandry and paternity in a wild population of the swordtail Xiphophorus nigrensis

Genetic analyses of parentage provide crucial information about the prevalence of polyandry and the potential for sexual selection to operate in wild populations. In the swordtail Xiphophorus nigrensis, large males are thought to have a competitive advantage due to their superiority in male–male contests and attractiveness to females, who are presumed to mate multiply. I examined the distribution of paternity within broods, the relationship between male body size and paternity and the effect of sire number on fecundity from females collected in the field. Sixty-one percent of females produced offspring from two to four males, with 70% of the offspring typically sired by one of the males represented in the brood. Male body size did not affect paternity share or whether females were multiply mated, as predicted if precopulatory sexual selection has a strong effect on the outcome of postcopulatory sexual selection. Female fecundity increased with the number of sires; however, this relationship was not observed when the smallest broods, where multiple mating is more difficult to detect, were excluded from the analysis. The high levels of multiple paternity and reproductive skew suggest that postcopulatory sexual selection has important evolutionary consequences in X. nigrensis. Traits important in precopulatory sexual selection, such as male body size, however, are more likely to affect sexual selection by increasing the number of mates obtained rather than paternity share within broods.     

Sex ratios,mating behavior and sexual size dimorphism of the northern water snake,Nerodia sipedon

Competition among males to mate is generally associated with male-biased size dimorphism. In this study we examine mating behavior in the northern water snake (Nerodia sipedon), a species in which males are much smaller than females despite substantial competition among males to mate. Competition among males was a consequence of a male-biased operational sex ratio due to slightly higher female mortality from a birth sex ratio of 1 : 1, and, in 1 year, more synchronous and longer mating activity by males. Approximately one-third of both males and females appeared not to mate in a given year. Larger males were generally more likely to attempt mating, but size did not explain the variance in the number of aggregations in which individual males participated. Within aggregations, males that were successful at achieving intromission were larger than unsuccessful males in 1 of 2 years. Variation in condition (mass relative to length) and relative tail length were not generally useful predictors of either mating effort or success in males. Because large size was often advantageous to males, sexual size dimorphism appeared not to be a consequence of sexual selection favoring smaller males. Because sexual dimorphism was evident at birth, and both males and females matured sexually at about 4 years, sexual dimorphism was not simply a consequence of one sex growing at the maximum rate for longer. Female fecundity increased with size, and sex differences in size-fecundity relations may underly the pattern of sexual size dimorphism. However, because multiple mating by females is common, sperm competition is likely to be important in determining male reproductive success. Therefore, allocation of energy to sperm rather than growth may also prove to be an important influence on male growth rates and sexual size dimorphism.