Ecological constraints on group size: an analysis of spider monkey and chimpanzee subgroups

The social organization of spider monkeys (Ateles geoffroyi) and chimpanzees (Pan troglodytes) appear remarkably similar. In this paper, field studies of these two species were used to (1) test a model of ecological constraints on animal group size which suggests that group size is a function of travel costs and (2) assess ecological and social factors underlying the social organization of these two species. Spider monkeys were studied over a 6-year period in Santa Rosa National Park, Costa Rica, and chimpanzees were studied for 6 years in Kibale National Park, Uganda. Adults of both species spent their time in small subgroups that frequently changed size and composition. Thus, unlike most primate species, spider monkeys and chimpanzees were not always in a spatially cohesive social group; each individual had the option of associating in subgroups of a different size or composition. Both species relied on ripe fruit from trees that could be depleted through their feeding activity. However, spider monkey food resources tended to occur at higher densities, were more common, less temporally variable, and did not reach the low levels experienced by chimpanzees. Analyses of the relationship between subgroup size and the density and distribution of their food resources suggested that travel costs limit subgroup size. However, these ecological factors did not influence all age/sex classes equally. For example, the number of adult males in a subgroup was a function of food density and travel costs. However, this was not the case for female chimpanzees, suggesting that the benefits of being in a subgroup for females did not exceed the costs, even when ecological conditions appeared to minimize subgroup foraging costs. Therefore, it seems likely that social strategies influenced the relationship between food resource variables and subgroup size.     

Mating system and mating success of the desert spider Agelenopsis aperta

Field studies of the desert spider Agelenopsis aperta revealed a primarily monogamous mating system. However polygyny, polyandry and polygynandry were superimposed upon the primary system, with 9% of the marked males and 11% of the marked females in a field population mating more than once. In the laboratory males commonly mated multiply with fertile offspring resulting, while females were less likely than males to mate multiply. Monogamy under field conditions was enforced by two factors: (1) high travel costs to males, and (2) a significant decline in female receptivity after the first mating. Heavy males were more likely to be accepted by females both in the field, and in female choice experiments conducted in the laboratory. Finally, male weight determined the outcome of male-male agonistic interactions over females. One possible explanation for female choice in this system which lacks male parental investment is that females may be using male size as an indicator of future success of their offspring.     

Vigilance in wild Thomas's langurs (Presbytis thomasi ): the importance of infanticide risk

This study examines vigilance as a behavioural indicator of the importance of infanticide risk by comparing the infanticide avoidance hypothesis with the predation avoidance and mate defence hypotheses for wild Thomas's langurs (Presbytis thomasi) in Sumatra. We found that all individuals were more vigilant in situations of high predation risk, i.e. lower in the trees and in the absence of neighbours. Females were also more vigilant on the periphery of the group. However, there were variations in vigilance levels that could not be accounted for by the predation avoidance hypothesis. Males without infants showed higher levels of vigilance in areas of home range overlap than in non-overlap areas during the early phase of their tenure, strongly suggesting mate defence. In these areas of home range overlap where Thomas's langur groups can interact, males may attack females and infants, and so the infanticide risk for males and females with infants is likely to be high in these areas. Only females with infants, but not males with infants or females without infants, showed higher vigilance levels in overlap areas than in non-overlap areas; in addition, in overlap areas, females with an infant were more vigilant than females without an infant, while this was not the case in non-overlap areas. Both females and males with infants were more vigilant high in the trees than at medium heights in overlap areas but not elsewhere. These findings can only be explained by the infanticide avoidance hypothesis. In contrast to predator attacks, infanticidal male attacks come from high in the canopy, and only occur in overlap areas. There was a significant sex difference in vigilance, but males were only more vigilant than females without an infant, and not more vigilant than females with an infant. We conclude that vigilance varied mainly in relation to the risk of predation and infanticide. Mate competition only played a role for males during the early phase of their tenure. Predation risk seems to offer the best explanation for vigilance for all individuals in the absence of infants. Both predation risk and infanticide risk played a role for females and males with infants. Received: 4 April 1998 / Accepted after revision: 6 September 1998     

Daring females,devoted males,and reversed sexual size dimorphism in the sand-dwelling spider <Emphasis Type="Italic">Allocosa brasiliensis</Emphasis> (Araneae,Lycosidae)

Sexual selection theory predicts that a higher investment in offspring will turn females into the selective sex, while males will compete for accessing and courting them. However, there are exceptions to the rule. When males present a high reproductive investment, sex roles can reverse from typical patterns, turning males into the choosy sex, while females locate males and initiate courtship. In many spiders, males are smaller than females, wandering in search of sedentary females and maximizing the number of copulations. In the present study, we present findings on the sand-dwelling wolf spider, Allocosa brasiliensis, evidencing a reversal in typical courtship roles reported for the first time in spiders. Males were bigger than females. Females located males and initiated courtship. Copulation always occurred in male burrows and took place mainly in long burrows. Males donated their burrows to the females after copulation, closing the entrance before leaving with female cooperation from inside. Males would provide females with a secure place for ovipositing, being exposed to predation and diminishing their future mating possibilities until constructing a new burrow. The cost of vacating the burrow and losing the refuge in an unpredictable habitat, such as sand dunes, would explain the courtship roles reversal in this spider species. Results turn A. brasiliensis as a promising model for discussing the determinants of sex roles and the pressures that drive their evolution and maintenance. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Lévy walk patterns in the foraging movements of spider monkeys (<Emphasis Type="Italic">Ateles geoffroyi</Emphasis>)

Scale invariant patterns have been found in different biological systems, in many cases resembling what physicists have found in other, nonbiological systems. Here we describe the foraging patterns of free-ranging spider monkeys (Ateles geoffroyi) in the forest of the Yucatan Peninsula, Mexico and find that these patterns closely resemble what physicists know as Lévy walks. First, the length of a trajectorys constituent steps, or continuous moves in the same direction, is best described by a power-law distribution in which the frequency of ever larger steps decreases as a negative power function of their length. The rate of this decrease is very close to that predicted by a previous analytical Lévy walk model to be an optimal strategy to search for scarce resources distributed at random. Second, the frequency distribution of the duration of stops or waiting times also approximates to a power-law function. Finally, the mean square displacement during the monkeys first foraging trip increases more rapidly than would be expected from a random walk with constant step length, but within the range predicted for Lévy walks. In view of these results, we analyze the different exponents characterizing the trajectories described by females and males, and by monkeys on their own and when part of a subgroup. We discuss the origin of these patterns and their implications for the foraging ecology of spider monkeys.Communicated by D. Watts     

Sex ratio and maternal rank in wild spider monkeys: when daughters disperse

Summary Data from a long-term field study of the spider monkey, Ateles paniscus, in Peru indicate that a strongly female-biased sex ratio exists from birth in this population. Of 46 infants born between July 1981 and June 1986, 12 were male, 32 were female and 2 were of undetermined sex. This effect is consistent between years as well, with more females than males born in each year of the study (Table 1). This bias is driven by the fact that low-ranking females produce daughters almost exclusively, while high-ranking females bias their investment somewhat less strongly towards sons (Table 2). The unusual pattern of female-biased maternal investment observed in this population of Ateles probably occurs for a combination of the following reasons: (1) maternal investment in individual male offspring is somewhat greater than in individual female offspring; (2) males remain with their natal groups, and the sons of high-ranking females are likely to be competitively superior to the sons of low-ranking females; (3) males compete for mates, and only the one or two most dominant males within a community are likely to achieve significant reproductive success. Two possible mechanisms of sex-ratio adjustment and the evidence for each are discussed.     

Fissioning minimizes ranging costs in spider monkeys: a multiple-level approach

The adjustment to deal with intragroup food competition is probably the most plausible explanation of high levels of fission–fusion dynamics. However, studies did not always support expected relations between food availability, ranging costs, and subgroup size. We used several levels of analysis differing in the time and spatial scale in order to investigate this explanation in spider monkeys. In our study, subgroups were larger when food availability was higher across most levels of analyses used. We also found a fine-scale adjustment: compared to the food patch previously visited, spider monkeys traveled to larger patches just after fusions. This was not without an immediate travel cost: the interpatch distance and travel time after a fusion were longer than that before the fusion. This rapid adjustment shows the flexibility that fission–fusion dynamics can offer. Spider monkeys are in large subgroups only when food conditions are favorable, as evidenced by the fact that at all the other time-scale levels larger subgroups did not experience greater ranging costs than smaller subgroups. Our results indicate that on the whole spider monkeys successfully minimize ranging costs by fission and fusion of subgroups.     

Experimental evidence for female-driven monandry in the wolf spider, <Emphasis Type="Italic">Pardosa astrigera</Emphasis>

Females of many species mate multiply, yet some taxon females mate with only one male, also known as monandry. Although the underlying mechanism behind female monandry is poorly understood relative to female polyandry, there are two contrasting hypotheses, male control and female control, for the maintenance of monandry. Since females generally benefit from multiple mating for material and/or genetic benefits, cases of monandry may reflect male manipulation on female remating at the expense of female fitness (male control). Alternatively, monandry may be favored by females, if females maximize their fitness by mating once (female control). Here, we tested two hypotheses by manipulating the number of mating (repeated mating and polyandry) on female fitness in a largely monandrous wolf spider, Pardosa astrigera. We allowed females to be inseminated once, twice with the same males (repeated mating) or with two males (polyandry) and determined female fitness consequences. The number of female mating, regardless of a single mating, repeated mating, or polyandry, had no significant effects on female fecundity, fertility, and survival and size of their spiderlings. However, the fitness cost of female multiple mating may to some extent be underestimated under laboratory conditions. In addition, female survival was adversely affected by induced multiple mating. Therefore, our results suggest that monandry of the wolf spider (P. astrigera) may be under the control of females, rather than under the control of males.     

Mating patterns of woolly spider monkeys,Brachyteles arachnoides: implications for female choice

Summary Woolly spider monkeys show a promiscuous, polygynous mating system in which a receptive female mates with several males, often in rapid succession. Copulation is prolonged with an average duration of 4.1±1.5 (SD) min. Coitus consists of a stationary phase with the male in intromission, followed by stercotypic behaviours of the female which appear to cue and/or accompany male climax and ejaculation. Subadult and adult males show different association patterns with individual females. Subadult males form long-term consortships with particular females while adult males appear strongly attracted to a particular female only when the is receptive. These different behavior patterns of males are viewed as age-specific mating tactics. Males in a mating aggregation show little intermale aggression for sexual access to a receptive female. Large testis size in this species suggests that much intermale competition for reproductive success may be carried out at a postcopulatory level, perhaps by sperm competition. The copulatory pattern of woolly spider monkeys may function primarily as a mechanism of female choice, aiding a female in assessing the quality of males in hei mating aggregation while helping to ensure that maximal high quality spermatozoa will be available at the proper time for fertilization.     

Male takeover,infanticide, and female countertactics in white-headed leaf monkeys (<Emphasis Type="Italic">Trachypithecus leucocephalus</Emphasis>)

Infanticide by males is common in mammalian species such as primates in which lactation lasts much longer than gestation. It frequently occurs in one-male groups following male takeovers and is likely a male reproductive strategy. Reported female countertactics include abrupt weaning of infants, dispersal, or paternity confusion. Here, we estimated costs of female countertactics in terms of weaning ages and interbirth intervals. We observed a population of white-headed leaf monkeys (Trachypithecus leucocephalus) in Nongguan Nature Reserve, China (1995–2006) mainly composed of one-male groups. Takeovers (N = 11) coincided with the peak conception period. Detailed data are presented for five takeovers (34 females, 29 infants, and 47 group-years) leading to six infant disappearances (42.9% of infant mortality). All presumed infanticides were in accordance with the sexual selection hypothesis. Following a takeover, females without infants or with old infants stayed with the new males, incurring no or low costs (via abrupt weaning). Females with young infants dispersing with the old males also experienced low costs. High costs (due to infant loss) were incurred by pregnant females and those with young infants who stayed with the new males indicating that paternity was not confused. Costs in terms of long interbirth intervals were also high for females leaving with the old males to later join the new males, despite infant survival. Female countertactics reflected female philopatry mediated by infant age. Presumably due to the seasonal timing of takeovers, most countertactics seemed successful given that 32.3% of females apparently incurred no costs and 41.2% incurred only low costs.     

Alternative reproductive tactics in the spider Meta segmentata

Summary In the spider Meta segmentata web site quality and competitive ability determine female distributions, and ultimately male mating strategy. Large, fecund females position their webs in aggregations in high quality habitats, whereas small females live alone in lower quality sites. Competition for web sites is intense. Large females have a fighting advantage, but web ownership can offset size disadvantages of about 10%. Males compete for access to female webs. The smallest males are excluded from aggregations of females but have a very high probability of mating monogamously with females in poor habitats. The largest males move about within aggregations and by mating with many females achieve the highest reproductive gains. Moderately sized males only have a low probability of mating in an aggregation, but for those that do succeed the reproductive gains are higher than for those that mate monogamously in low quality areas. On average, moderately sized males obtain reproductive gains from the high risk, high gain strategy of defending webs in aggregations that are equal to those from definding solitary females in poor areas. That 18 of 21 moderately sized males defend females in aggregations suggests that M. segmentata behaves in a riskprone fashion.     

Male mating preference is associated with risk of pre-copulatory cannibalism in a socially polymorphic spider

We performed male attraction experiments and staged courtship sequences to test for non-random mating with respect to social behavioral phenotype in the comb-footed spider, Anelosimus studiosus. While asocial behavior is the dominant phenotype in all A. studiosus populations examined to date, a social phenotype approaches a frequency of 15% in colder environments. We collected test subjects from higher latitude polymorphic populations and scored all individuals as to their behavioral phenotype prior to their use in these trials. Males of both phenotypes differentially approached and courted social females over asocial females and no-spider controls. By offering males different numbers of females of one type vs. the other in subsequent trials, we determined that the difference in attractiveness between the two phenotypes social/asocial is 1.5/1. Both the web produced by a female and a female that has been removed from its web attract males. We suggest that the male attracting pheromone is present on females and is also attached to silk threads. Staged encounters completed between males and females of the respective phenotypes demonstrated that courting males suffer significantly less pre-copulatory sexual cannibalism with social females than with asocial ones, and thus, female social tendency is phenotypically linked to sexual aggression. We propose that the male preference for social females is adaptive because of the observed asymmetry in courtship success.     

Delayed maturation in plumage colour: Evidence for the female-mimicry hypothesis in the kestrel

Summary In many sexually dichromatic species, young males have female-like plumage during their first potential breeding year. The female-mimicry hypothesis (FMH) supposes that by possessing female-like plumage young males deceive older conspicuous males into believing that they are females, thus reducing competition from adult males. The status-signalling hypothesis (SSH) supposes that adult males can distinguish sex, but postulates that young males reduce competition from adult males by reliably signaling low status with their dull plumage. We tested these hypotheses in the European kestrel (Falco tinnunculus). Female-like young males settled to breed closer to adult males than did other adult males (Figs. 1a, b). By settling near adult males, young males seemed to increase their chance of mating with adult females. Adult female-young male pairs had better reproductive success than yearling-yearling pairs. These results suggest that there is an adaptive value in possessing a female-like plumage colour in the breeding season. To test the FMH, we measured sexual preference of adult males when adult females and young males were simultaneously shown in an aviary. Adult males were unable to recognize sex, because in half the cases they preferred young males (Fig. 3). However, when adult males and females were shown simultaneously, males preferred females (Fig. 2). Our results support the FMH rather than the SSH, because young males successfully deceived older males by their plumage.     

Male care of infants in a siamang (Symphalangus syndactylus) population including socially monogamous and polyandrous groups

While male parental care is uncommon in mammals, siamang (Symphalangus syndactylus) males provide care for infants in the form of infant carrying. I collected behavioral data from a cohort of five wild siamang infants from early infancy until age 15–24 months to identify factors affecting male care and to assess the consequences of male care for males, females, and infants in a population including socially monogamous groups and polyandrous groups. There was substantial variation in male caring behavior. All males in polyandrous groups provided care for infants, but males in socially monogamous groups provided substantially more care than males in polyandrous groups, even when the combined effort of all males in a group was considered. These results suggest that polyandry in siamangs is unlikely to be promoted by the need for “helpers.” Infants receiving more care from males did not receive more care overall because females compensated for increases in male care by reducing their own caring effort. There was no significant relationship between indicators of male–female social bond strength and male time spent carrying infants, and the onset of male care was not associated with a change in copulation rates. Females providing more care for infants had significantly longer interbirth intervals. Male care may reduce the energetic costs of reproduction for females, permitting higher female reproductive rates.     

Non-independent mate choice in a fiddler crab: a case of stimulus enhancement

Non-independent mate selection occurs when the choice behavior of a female is altered by the interactions between other females and males. In the fiddler crab Uca mjoebergi, males court mate-searching females by waving their one greatly enlarged claw. When a female approaches a male, he initiates high-intensity waving. We conducted one natural mate choice experiment and two mate choice experiments using custom-built robotic crabs. We show that the decision of one female to approach a group of males increases the probability that another female will approach and visit a male from the same group. We suggest that this behavior is best explained by the ‘stimulus enhancement’ hypothesis, where the presence of a female near a group of males makes them more likely to be detected by other females due to an increase in male display rate.     

Reproductive consequences of male body mass and aggressiveness depend on females’ behavioral types

Relatively few investigations explicitly test for concordant versus conflicting selection pressures from intrasexual versus intersexual selection. Here, we examine the effects of male body mass and behavioral type (BT) on reproductive success in the spider Anelosimus studiosus, with emphasis placed on the potential interaction between intrasexual and intersexual selection influences. Female A. studiosus exhibit either an aggressive-active or docile-passive BT, both of which co-occur in multifemale colonies. Males, in contrast, exhibit a more continuous distribution of behavioral tendencies. We investigated the male traits favored by females in five trial types: one docile female, one aggressive female, four docile females, four aggressive females, and two docile and two aggressive females. Male reproductive success was estimated by the number eggs produced by females following staged mating trials. In previous work, it was established that large aggressive males are favored in male–male contests, an intrasexual effect. However, large aggressive males were not universally favored here. We failed to detect an effect of male body mass or aggressiveness on reproductive success in trials with all docile females; however, in situations involving aggressive females, large aggressive males experienced diminished reproductive success relative to small docile males. Large, aggressive males were also more likely to be attacked and killed by aggressive females in the first 20 min of staged encounters and were more likely to be found dead after 72 h of unobserved interactions. Taken together, our data suggest that the reproductive consequences of male traits differ based on (1) the aspect of sexual selection being considered (intrasexual versus intersexual) and (2) the BT of their prospective mates: large aggressive males enjoy advantages in intrasexual selection and when courting docile females and small docile males experience reduced risk of cannibalism and increased reproductive success with aggressive females.     

Temporal constraints and female preference for burrow width in the fiddler crab, <Emphasis Type="Italic">Uca mjoebergi</Emphasis>

We studied sampling behaviour and mate choice in the fiddler crab Uca mjoebergi. Once a female selects a mate, she copulates in his burrow and remains there until releasing her aquatic larvae. U. mjoebergi occurs in habitats that are inundated only by the highest amplitude spring tides. Females can only release their larvae during these tides, and release before or after will result in complete failure of reproductive effort. Matings occur over a 5-day period near the end of neap tides. Our results suggest that within the mating period, females adjust their larval developmental rates by selecting specific burrows in which to incubate their clutches. We found that at the start of the mating period, females chose larger males as mates. Since male size was positively correlated to burrow width, females were selecting wide burrows and effectively incubating at lower temperatures. This would slow down the developmental rates of larvae. In contrast, females that mated late in the mating period selectively chose small males. By incubating in narrower, warmer burrows, these females may increase the developmental rates of larvae. We propose that females are selecting burrows to influence incubation rate and ensure timely release of their larvae. Female U. mjoebergi appear to adjust their preference for the direct benefits of mate choice to increase their reproductive success.     

“Friendships” between new mothers and adult males: adaptive benefits and determinants in wild baboons (<Emphasis Type="Italic">Papio cynocephalus</Emphasis>)

Close associations between adult males and lactating females and their dependent infants are not commonly described in non-monogamous mammals. However, such associations [sometimes called “friendships” (Smuts 1985)] are regularly observed in several primate species in which females mate with multiple males during the fertile period. The absence of mating exclusivity among “friends” suggests that males should invest little in infant care, raising questions about the adaptive significance of friendship bonds. Using data from genetic paternity analyses, patterns of behavior, and long-term demographic and reproductive records, we evaluated the extent to which friendships in four multi-male, multi-female yellow baboon (Papio cynocephalus) groups in Amboseli, Kenya represent joint parental care of offspring or male mating effort. We found evidence that mothers and infants benefited directly from friendships; friendships provided mother–infant dyads protection from harassment from other adult and immature females. In addition, nearly half of all male friends were the genetic fathers of offspring and had been observed mating with mothers during the days of most likely conception for those offspring. In contrast, nearly all friends who were not fathers were also not observed to consort with the mother during the days of most likely conception, suggesting that friendships between mothers and non-fathers did not result from paternity confusion. Finally, we found no evidence that prior friendship increased a male’s chances of mating with a female in future reproductive cycles. Our results suggest that, for many male–female pairs at Amboseli, friendships represented a form of biparental care of offspring. Males in the remaining friendship dyads may be trading protection of infants in exchange for some resources or services not yet identified. Our study is the first to find evidence that female primates gain social benefits from their early associations with adult males. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sexual cohabitation as mate-guarding in the leaf-curling spider Phonognatha graeffei Keyserling (Araneoidea, Araneae)

The leaf-curling spider Phonognatha graeffei incorporates a twisted leaf into the central hub of its orb-web that is used as a retreat. This species is unusual among orb-weaving spiders because males cohabit in the leaf retreat with both immature and mature females, mating with the former shortly after the female molts. Cohabitation appears to be a form of mate-guarding because cohabiting males respond agonistically to rival males that venture onto the web, and their behaviour depends upon the reproductive status of the female; males defending immature females are more aggressive than those defending virgin, adult females. Males copulate with previously mated females for significantly longer than with virgin females. Females may cannibalise cohabiting males, which occurs independently of whether the female has been deprived of food. Females that cannibalise a single male do not have a higher fecundity than non-cannibalistic females. Received: 2 February 1996 / Accepted after revision: 27 October 1996     

Age-related female mating decisions are condition dependent in wolf spiders

Female mating behaviors are known to be sensitive to a variety of individual factors both external and internal to a female; however, mating decisions are likely due to a suite of interacting factors. By independently manipulating female and male diet in the wolf spider Rabidosa rabida and testing females across age groups, we demonstrate that, in addition to its independent effect, female nutritional condition interacts with female age to influence female mating behavior. Overall, high-quantity diet (HD) females were more likely to mate than low-quantity diet (LD) females. Within the LD females, older individuals were more likely to mate than younger individuals, while within HD females, mating probabilities were equal across females of different age classes. With respect to mate choice, only female age influenced the likelihood of mating based on male diet. Young females were choosier as they were more likely to mate with HD males than LD males; in contrast, older females were equally likely to copulate with males of each diet treatment. In addition, the likelihood of pre-sexual cannibalism was dependent on both female and male diet. High-quantity diet females were more likely to cannibalize than LD females, and attacks were directed towards LD males most often. We discuss our results in terms of costs versus benefits of female mate choice.