A mechanism for female choice of large males in the treefrog Hyla chrysoscelis

Summary In this study, we combine both field and laboratory experiments to address the effects of female preference for certain call characteristics on a large-male mating advantage in the treefrog H. chrysoscelis. In laboratory-choice experiments, females always chose the call with the lower fundamental frequency when call rate and call intensity were held constant and the difference in frequency between the two calls was 15%. The lower frequency call was preferred by 8 out of 12 females when the difference in fundamental frequency was 7.4%. These results are consistent with field comparisons of the size of unmated males calling within 2 m of a mated male: Male body size was negatively correlated with fundamental frequency and the greater the size difference, the more likely that the larger male mated. In field choice experiments, females preferred males with higher call rates. Since size differences between males used in this experiment averaged only 2.3 mm, we would not expect the fundamental frequency of a male's call to be the best predictor of mating success. Laboratory results demonstrated that call rate could override female preference for the low frequency call over the high frequency call, while intensity could at least dilute this preference. However, individual males in the field varied both call rate and the call intensity as perceived by the female. We suggest that the interaction between call rate, male size and mating success should be studied further through the use of field-choice experiments.     

Information transfer and gain in flocks: the effects of quality and quantity of social information at different neighbour distances

We assessed experimentally how the quality and quantity of social information affected foraging decisions of starlings (Sturnus vulgaris) at different neighbour distances, and how individuals gained social information as a function of head position. Our experimental set up comprised three bottomless enclosures, each housing one individual placed on a line at different distances. The birds in the extreme enclosures were labelled senders and the one in the centre receiver. We manipulated the foraging opportunities of senders (enhanced, natural, no-foraging), and recorded the behaviour of the receiver. In the first experiment, receivers responded to the condition of senders. Their searching rate and food intake increased when senders foraged in enhanced conditions, and decreased in no-foraging conditions, in relation to natural conditions. Scanning was oriented more in the direction of conspecifics when senders behaviour departed from normal. In the second experiment, responses were dose dependent: receivers increased their searching rate and orientated their gaze more towards conspecifics with the number of senders foraging in enhanced food conditions. In no-foraging conditions, receivers decreased their searching and intake rates with the number of senders, but no variation was found in scanning towards conspecifics. Differences in foraging and scanning behaviour between enhanced and no-foraging conditions were much lower when neighbours were separated farther. Overall, information transfer within starling flocks affects individual foraging and scanning behaviour, with receivers monitoring and copying senders behaviour mainly when neighbours are close. Information transfer may be related to predation information (responding to the vigilance of conspecifics) and foraging information (responding to the feeding success of conspecifics). Both sources of information, balanced by neighbour distance, may simultaneously affect the behaviour of individuals in natural conditions.Communicated by H. Kokko     

Testing the ‘agonistic buffering’ hypothesis

Summary Data are presented from a 15-month study on triadic male-infant interactions (agonistic buffering) among wild Barbary macaques, and the agonistic buffering hypothesis reevaluated. The sociometrics of triadic interactions derived from the distribution of 535 interactions among individually known adult and subadult males showed that there were significant individual male differences in the frequency of initating and/or receiving triadic interactions, but there were no such differences between the high and low ranking male subgroups (Tables 2–4).Males did not choose each other equally for triadic encounters. Each male had a different set of three other males (out of 11 possible) that he preferred to approach for a triadic encounter; and each male received triadic approaches essentially from only three males (Tables 4 and 5). Each male showed a specific preference for which infant to select for participation in the triad, and in fact, males who preferred one another for triads also preferred to use the same infant in these encounters. These males that preferred each other and the same infant both had a special care-taking relationship with that preferred infant (Tables 6–7).Possessing an infant per se seemed to be irrelevant in whether a male would approach or be approached for a triad. In 78% of all triads, males separated immediately after termination of the triad. When the agonistic buffering hypothesis is reexamined against the data in this study, it appears that it cannot adequately accommodate the thesis that it serves to regulate dominant/subordinate relations among males. Rather males choose to participate with each other in agonistic buffering because, and by means of, a shared, common, and special care-taking relationship with the same infant. The indiscriminate use of the terminology agonistic buffering to describe multiple-male/infant interactions in this species should be dropped.     

Song as an attractant for male and female European starlings,and the influence of song complexity on their response

Summary We tested the hypothesis that song attracts females and repels males in the European starling. We broadcast recorded song from speakers attached to nestboxes, while paired boxes with silent speakers served as controls. As predicted, females were attracted to the song boxes. Contrary to prediction, males were also attracted to the `song' boxes. Singing by male starlings may be costly because it attracts competitors for limited nesting sites, but the cost cannot be avoided due to the need to attract a mate. In a second experiment simple song (composed of 20 different phrase types) and complex song (40 phrase types) were played simultaneously. More males were captured at boxes where simple song was played. Song complexity may function as an indicator of male quality and be used by male starlings to assess potential competitors.     

Optimal foraging and territorial defence in the Great Tit (Parus major)

Summary We describe an experiment designed to investigate the trade-off between foraging and territorial vigilance in the Great Tit. Captive territorial male Great Tits were observed while foraging in a large indoor aviary. They obtained food from two operant patches in which the supply of food was gradually depleted during a visit. We predicted that during control sessions the birds would switch between patches in such a way as to maximise their overall feeding rate. In experimental sessions, we introduced briefly a rival male as an intruder at the start of the test. The foraging male could see the rival only when travelling between patches and not while feeding within patches. We predicted that during experimental sessions birds would switch between patches more often than in control tests, sacrificing food intake for territorial vigilance. Three of the four males tested behaved in approximately the predicted manner. We discuss the use of an inverse optimality argument to provide a calibration of feeding against the benefit resulting from territorial vigilance.     

Optimal foraging: Some simple stochastic models

Summary Some simple stochastic models of optimal foraging are considered. Firstly, mathematical renewal theory is used to make a general model of the combined processes of search, encounter, capture and handling. In the case where patches or prey items are encountered according to a Poisson process the limiting probability distribution of energy gain is found. This distribution is found to be normal and its mean and variance are specified. This result supports the use of Holling's disc equation to specify the rate of energy intake in foraging models. Secondly, a model based on minimization of the probability of death due to an energetic shortfall is presented. The model gives a graphical solution to the problem of optimal choices when mean and variance are related. Thirdly, a worked example using these results is presented. This example suggests that there may be natural relationships between mean and variance which make solutions to the problems of energy maximization and minimization of the probability of starvation similar. Finally, current trends in stochastic modeling of foraging behavior are critically discussed.     

Baboon loud calls advertise male quality: acoustic features and their relation to rank,age, and exhaustion

Free-ranging adult male baboons give loud two-syllable wahoo calls during dawn choruses, interactions between groups, when chasing females, and in aggressive interactions with other males. Previous research has shown that the rate and duration of these contest wahoos are correlated with a males competitive ability: high-ranking males call more often, call at faster rates, and call for longer bouts than do low-ranking males. Here we report that acoustic features of wahoos also reveal information about male competitive ability. High-ranking males give wahoos with higher fundamental frequencies (F0) and longer hoo syllables. Within-subject analyses revealed that, as males fall in rank, the hoo syllables tend to shorten within a period of months. As males age and continue to fall in rank, F0 declines, hoo syllables shorten, and formant dispersion decreases. Independent of age and rank, within bouts of calling F0 declines and hoo syllables become shorter. Because wahoos are often given while males are running or leaping through trees, variation in these acoustic features may function as an indicator of a males stamina. The acoustic features of contest wahoos thus potentially allow listeners to assess a males competitive ability.Communicated by C. Nunn     

The establishment of foraging flocks in house sparrows: risk of predation and daily temperature

Summary The foraging decisions of animals often reflect a trade-off between the risk of predation and efficient foraging. One way an animal may reduce the risk of predation, and hence exploit a resource patch in relative safety, is by foraging in a group. Solitary pioneer sparrows often recruit others to a food source by making chirrup calls in order to establish foraging flocks. This study describes the decisions of house sparrows that arrive at food resources of different risks of predation. Four feeding sites at different distances from a perching site and from an observer were presented to sparrows. When the feeder was adjacent to the perching site and far from the observer, the pioneers chirruped less frequently and were more likely to forage alone than when the feeder was in the other three positions. There were differences in the scanning behaviour of sparrows at these sites, suggesting that they were responding to different risks of predation. Furthermore, the chirrup rates of pioneer sparrows in this study and a previous study were found to be negatively correlated with maximum daily temperature. This is consistent with the hypothesis that energy requirements may affect the flock establishment decisions of sparrows, and that the benefits of foraging in flocks may be greater at lower temperatures.     

Aggregation and foraging behavior of whirligig beetles (Gyrinidae)

Summary Whirligig beetles aggregate in the daytime into dense single-and multispecies groups (rafts) of hundreds or thousands of individuals. On the 22km shoreline of Lake Itasca in northern Minnesota, these aggregations were on the average 0.8 km apart, and they were usually found day after day in the same ocations.Most beetles apparently do not home to the aggregation of their origin after dispersing at night because (a) the species composition of some aggregations changed greatly, and (b) paint-marked beetles (Dineutus horni) moved overnight from one aggregation as far as 4km, joining 11 of the 14 large (>300 beetles) D. horni groups on the lake.Throughout the night, the largest concentrations of beetles remained within 100m of the diurnal aggregation sites. Beetles reconvened into the compact rafts before daybreak, in part by following each other in sometimes long single files or trains. Their forward motion stopped after they joined large number of other beetles. We infer that following behavior enables those individuals that have dispersed from their original aggregations (during their nocturnal foraging) to find and join other aggregations before daylight.Naive fish ate the beetles despite their noxious secretions. However, fish living near rafting sites and feeding on insects on the water surface in daylight should soon learn to avoid the beetles. The rafting sites would then become safe places. We observed fish attacking only those beetles that had been either dispersed from their rafts or released into open water away from raft sites in the daytime. We speculate that the evolutionary significance of the aggregation behavior is related to predator (fish) avoidance.     

Graded recruitment in a ponerine ant

Summary (1) The giant tropical ant, Paraponera clavata, exhibits graded recruitment responses, depending on the type, quantity, and quality of a food source. More ants are initially recruited to a large prey or scavenge item than to a large quantity of sugar water. (2) Individual ants encountering prey items gauge the size and/or unwieldiness of the item, regardless of the weight, when determining whether to recruit. (3) The trail pheromone of this species is often used as an orientation device by individual ants, independent of recruitment of nestmates. (4) It is proposed that the foraging behavior of P. clavata represents one of the evolutionary transitions from the independent foraging activities of the primitive ants to the highly coordinated cooperative foraging activities of many higher ants.     

Trunk trails and the searching strategy of a leaf-cutter ant,Atta colombica

Summary Atta colombica uses chemical mass recruitment that allows the rapid exploitation of resources. Most foragers thus search only within patches. Accumulation of extra foragers at patches results in sampling of alternate food items and area-restricted search as patch resources are depleted.Individual workers have a higher probability of removing a leaf fragment the earlier they arrive at a bait. Workers that arrive when much of the resource is gone travel further on the bait (within the patch) but do not spend significantly more time at the patch. They give up after 50–80s.Foraging effort is centered on the extensive trail system, not on the nest a predicted by time and energy foraging models. Search effort is also trail centered. The probability that an item will be discovered decreases with distance from the trail and increasing litter depth. Trail traffic and trail quality together mave no significant effect although this may be because they act antagonistically.Economic considerations predict that trials should be built to high quality and very productive sites. If trails are built as a result of recruitment and recruitment reflects patch quality and productivity, characteristics of forage sites are physically embodied in the trail system.Leaf cutter foraging is better understood as a long term optimization that effectively exploits resources over the lifetime of the colony than as prudent predation that husbands resources.     

Reproductive effort in relation to maternal social rank in reindeer (<Emphasis Type="Italic">Rangifer tarandus</Emphasis>)

In polygynous mammals, high-quality females may increase their fitness by providing superior care to their offspring. Based on the agonistic interactions of female reindeer in an experimental herd during two consecutive years (1997 and 1998), we tested whether maternal social rank influenced: (1) winter body-mass change of females, (2) preparturition reproductive effort (measured as fecundity, the birth mass and the birth date of their calves), (3) preweaning maternal effort (measured as calves preweaning mortality, early preweaning and late preweaning growth rate and September body mass of calves), and (4) postweaning maternal effort (measured as calves body-mass change during their first winter). In the models, we included September females body mass as a covariate to separate the effects of maternal rank and body mass. We also tested whether the effect of social rank on maternal efforts was dependent on offspring sex. High-ranked females gained body mass whereas low-ranked females lost weight during the winter. Fecundity was higher and date of birth was earlier in high-ranked females than in subordinates, whereas no effect of females rank on birth mass of calves was found. Early preweaning growth rate and September body mass of calves increased with increasing females social rank, whereas late preweaning daily growth rate of calves was not influenced by females rank. Calves preweaning mortality was only influenced by year, which also explained most of the variance in the winter body-mass change of calves. The effects of females rank on the reproductive-efforts parameters studied were not specific to offspring sex. These findings suggest that females rank influences reproductive effort during the preparturition, as well as the preweaning, period, the effect being sex independent.Communicated by R. Gibson     

The erect ‘penis’ is a flag of submission in a female-dominated society: greetings in Serengeti spotted hyenas

In East Africa, spotted hyenas live in large clans in a highly structured society dominated by females. A clan is a fission-fusion society where members are often solitary or in small groups. Spotted hyenas have a ritualized greeting during which two individuals stand parallel and face in opposite directions. Both individuals usually lift their hind leg and sniff or lick the anogenital region of the other. The unique aspect of greetings between individuals is the prominent role of the erect penis in animals of both sex. Female spotted hyenas have fused outer labiae and a pseudo-penis formed by the clitoris which closely resembles the male penis and can be erected. During greetings subordinates signalled submission with gestures which were not necessarily reciprocated by the dominant participant. Asymmetries were most pronounced in greetings between adult females where the probability of asymmetries increased with the divergence in rank between partners. Greetings between adult females and males were uncommon and restricted to males above median rank, principally the alpha male. Models of primate affiliative behavior assume that benefits derived from social relationships with different individuals are not equal and that individuals are selected to maximize the benefits they receive from social relationships with others. The observed distribution of greetings between partners of different rank matched the predictions of these models. An examination of non-adaptive hypotheses on the evolution of the pseudo-penis demonstrated that the conventional scenario linking (initial) virilization of female genitalia with selection for female dominance does not explain either the initial virilization, nor the evolution of the pseudo-penis to its current form and use. We sketch a new scenario that links (1) initial virilization to the occurrence of neonatal siblicide amongst members of a twin litter, and (2) costs of maintenance, pseudo-penile control over copulation and male submission. Our analysis confirms previous adaptive hypotheses on the function of the pseudo-penis in greetings and suggests new hypotheses to account for hitherto unexplained features. Correspondence to: M.L. East     

The choice of two prey types that minimises the probability of starvation

Summary In this paper we investigate the optimal diet of a forager faced with two prey types. Classical optimal foraging theory, based on the maximization of the mean net rate of energetic gain , predicts that the optimal policy is either to take only the more profitable prey type or to take both prey types. The decision between these policies does not depend on the forager's energy reserves or the time available for foraging. We develop two alternative models, based on the minimization of the probability of starvation. In the first model, foraging occurs continuously, and it is optimal to take a prey type if and only if it increases the forager's energy reserves. In the second model foraging stops at dusk, and the forager dies during the night if its reserves at dusk are too low. The optimal policy, which has to be found numerically by dynamic programming, depends on the forager's reserves and the time left till dusk. In general the optimal policy is either to take both types or to take only the more profitable type. Taking both types is optimal when reserves are low, and there is some evidence that this occurs. The models show that factors that have been ignored in classical models may be of importance.     

Dwarf mongoose and hornbill mutualism in the Taru desert,Kenya

Summary Dwarf mongooses in the Taru desert region of Kenya form foraging communities with a variety of endemic bird species, especially hornbills. The prey spectra of the mongooses and hornbills overlap almost completely. For the other bird species forming the foraging community only partial overlap exists. The association between the birds and mongooses is actively sought by both parties. The birds wait in tress around the termite mound where the monogooses are sleeping for them to emerge and the mongooses delay their foraging departure if no birds are present. There is a positive relationship between the number of mongooses in the group and the number of birds accompanying them. A true mutualism only exists between the mongooses and the two hornbill species Tockus deckeni and T. flavirostris since their presence or arrival affects the subsequent start of foraging. These two hornbill species have also been observed to influence the start of foraging actively by means of two behaviour patterns termed chivvying and waking. Both the mongooses and birds are exposed to a high predator pressure from raptors with an overlap in the birds of prey predating the various species. This predator pressure is counteracted behaviourally by the mongooses by means of an altruistic behaviour pattern, guarding. Both mongooses and birds warn vocally and flee when a raptor is sighted. The mongooses modify their guarding behaviour to compensate for the warning behaviour of the birds in two ways: (a) fewer mongooses guard when large numbers of birds are present and vice versa, (b) the frequency of the mongooses' intraspecific warning calls is significantly reduced in cases where birds are present in comparison with those where they are absent. The birds also sight and respond to the raptor first on significantly more occasions than the mongooses. In addition, the birds also warn for raptor species which do not predate them but which are mongoose predators, not, however, for raptors which are not mongoose predators. This mutualistic association with its high degree of compensatory behaviour by both parties appears to be unique for free-living vertebrates and has its closest parallel in the trophobiosis described for ants and aphids.     

Correlates of boldness in three-spined sticklebacks (<Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis>)

Behavioural variation is known to occur between individuals of the same population competing for resources. Individuals also vary with respect to their boldness or shyness. An individuals position along the shy-bold axis may be defined as the extent to which it is willing to trade off potentially increased predation risks for possible gains in resources. Similarly, group living may be interpreted as a trade-off between anti-predatory tactics and foraging efficiency. The responses of three-spined sticklebacks (Gasterosteus aculeatus) were tested across four social contexts to assess relative boldness or shyness and to further examine whether their behaviour would be consistent within and between contexts. Individuals displayed consistent responses within and between the first two contexts: those individuals which resumed foraging rapidly after a simulated aerial predator attack also displayed low shoaling tendencies. Such fish were deemed to be bold, whilst those which displayed the converse behaviour, slow resumption of foraging and a high shoaling tendency, were deemed to be shy. In a third context, bold individuals out-competed shy conspecifics for food. Boldness was also positively correlated with growth over a 6-week period. The position adopted by an individual within a group is usually interpreted as a trade-off between predation risk and foraging efficiency—both are greater at the front of a mobile group. Bold individuals showed significantly stronger tendencies towards front positions than shy conspecifics. The results suggest that, contrary to some previous studies on other animals, bold or shy behaviour in sticklebacks is consistent between contexts.Communicated by T. Czeschlik     

Food threshold territoriality in a water strider (Gerris remigis)

Summary Summer generation 3rd, 4th and 5th instar nymphs plus adults of Gerris remigis were satiated for 2 days in a laboratory tray then deprived of food. Within 1/2–2 days, 19 of 27 nymphs and 10 of 30 adults began to exhibit territoriality, continued being so for 3–9 days, then ceased shortly before becoming quiescent. In the field, muscid flies fed to different territorial striders at a rate of 0, 1, 2, or 3 flies/day, resulted in 8 of 10 striders (at 2 flies/day) and 10 of 10 striders (at 3 flies/day) ceasing territoriality, whereas 5 of 8 controls (at 0 flies/day) remained territorial. Thus, lower and upper food thresholds were demonstrated, the upper threshold approached both from above (laboratory study) and below (field study).     

The evolution of cheating and selfish behavior

Summary We examine the necessary conditions for the spread of genes that determine selfish and cheating behaviors and the rate of spread of these genes through structured populations, in order to address the question of the invadability of altruistic systems by anti-social mutations. We find that, although cheaters always have a higher relative fitness than altruists within groups, population structures which permit the evolution of altruism also preclude invasion by anti-social mutations. These results are related to a discussion by Hamilton (1971) concerning the limits to the evolution of altruistic and selfish behaviors.     

Lead-based paint in dwellings: The potential for contamination of the home environment during renovation

The quantity and particle size characteristics of lead in dust released during three different paint removal techniques was determined under controlled conditions and in situ in a dwelling. Air-lead and deposited dust-lead levels were highest after sanding but burning-off and hot-air removal methods also produced significant contamination. The importance of dust particle-size and lead is discussed in relation to the potential hazard to home renovators via inhalation and to children via the hand-to-mouth route.This paper is also being published in Proceedings of the Conference on Lead in the Home Environment and is one of several selected from SEGH sponsored conferences for simultaneous publication in this journal.     

Habitat and age of the giant squid (<Emphasis Type="Italic">Architeuthis sanctipauli</Emphasis>) inferred from isotopic analyses

The age and habitat of the giant squid, Architeuthis sanctipauli Velain, 1877, were determined based on isotopic analyses of the statoliths of three female specimens captured off Tasmania, Australia, between January and March 1996. Assuming that the aragonite of the statoliths formed in equilibrium with seawater, ¹⁸O analyses indicated that the squid lived at temperatures of 10.5–12.9°C, corresponding to average depths of 125–250 m and maximum depths of 500 m. The capture records indicated that these squid may have occasionally ranged still deeper, to as much as 1000 m. All the statoliths were labeled with bomb ¹⁴C (¹⁴C=+22.9 to +44.6), consistent with the depths inferred from ¹⁸O. A thin section through one of the statoliths revealed 351 growth increments grouped into check-ring structures every 10–16 increments. A model for statolith growth and the pattern of temporal change in ¹⁴C in the water column was used to estimate the ages of the three specimens. These estimates were very sensitive to the choice of depth range over which ¹⁴C values were integrated. Assuming that the capture depths represented the maximum habitat depths of these individuals, the calculations suggested an age of 14 years or less. More refined age estimates require a better understanding of the variation of ¹⁴C and temperature with depth in the areas in which the squids live.Communicated by J.P. Grassle, New Brunswick