Acoustic orientation in the palmate newt, Lissotriton helveticus

Experiments reported here were carried out to investigate the use of acoustic cues by palmate newts (Lissotriton helveticus) for orientation and to study whether this behavior is learned, or whether two populations of palmate newts that cohabit with different frog species (Iberian green frog, Rana perezi, and European common brown frog, Rana temporaria) show different phonotactic preferences. The orientation tests consisted of presenting a control stimulus (white noise), a sympatric acoustic stimulus (calls of R. perezi or R. temporaria, depending on the origin of newts), or an allopatric stimulus (calls of natterjack toads, Bufo calamita, or R. perezi). Newts were released in a circular arena, while the acoustic stimuli were presented outside of the circular arena in four different compass orientation directions (0, 90, 180 and 270°). In this study, we show that L. helveticus performed positive phonotaxis toward the calls of R. perezi only when both species shared habitat, orienting randomly when R. perezi was absent from the newt’s natal population. Newts from both populations oriented randomly when exposed to the allopatric and control acoustic stimuli. These results suggest, for the first time, that recognition of the sympatric heterospecific calls could be learned. However, newts sharing the breeding pond with a population of R. temporaria oriented randomly when exposed to the calls of this species. The fact that the breeding seasons of R. temporaria and L. helveticus do not overlap in time does not allow the use of R. temporaria calls as a guidance mechanism for migrating individuals of L. helveticus.     

Selective phonotaxis by male wood frogs (<Emphasis Type="Italic">Rana sylvatica</Emphasis>) to the sound of a chorus

Frogs and toads commonly form large choruses around suitable breeding habitat during the mating season. Although often regarded as a constraint on the acoustic behavior of signalers and receivers, the sounds of a chorus could also serve as an acoustic beacon that allows some frogs to locate the breeding aggregation. Attraction to chorus sounds might be particularly important for explosively breeding frogs. In these species, which often mate just one or a few days during the year, the timing and location of breeding aggregations can be unpredictable because their formation often depends on local climatic factors, such as rainfall or a change in temperature. I used laboratory playback experiments to test the hypothesis that male wood frogs (Rana sylvatica), an explosively breeding frog, exhibit positive phonotaxis toward the sound of a conspecific chorus. Males were released at the center of a rectangular arena with a speaker positioned in each corner facing toward the release point. In a single-stimulus experiment, more males approached a speaker broadcasting a conspecific chorus than the three silent speakers in the arena. In a two-stimulus experiment, more males approached a speaker broadcasting a conspecific chorus compared to the two silent speakers or a fourth speaker simultaneously broadcasting the spectrally overlapping sound of a heterospecific (R. septentrionalis) chorus. These results are consistent with the hypothesis that male wood frogs could use the sound of a chorus as a beacon to locate a short-lived breeding aggregation.     

Larval behavior of predacious sister-species: orientation,molting site,and survival in Chrysopa

Field and laboratory studies compared two features of larval behavior in a pair of predacious sisterspecies of green lacewings: one (Chrysopa slossonae) a specialist on a single species of colonial aphids (the woolly alder aphid) that occur on branches and trunks of alder trees, the other (C. quadripunctata) a general aphid feeder whose primary prey is dispersed on foliage of diverse types of trees. First, a few hours after hatching, larvae of the two species develop significantly different phototactic responses; the differences correspond well with the spatial distributions of their prey. Most C. slossonae exhibited negative phototaxis, a response that helps move hatchlings inward on alder trees toward the woolly alder aphid colonies, whereas most C. quadripunctata hatchlings showed positive orientation to light, a response that tends to keep them in tree canopies with their prey. Second, in greenhouse experiments, a significantly greater proportion of C. slossonae larvae (second instars) molted within woolly alder aphid colonies and remained with the aphids than did C. quadripunctata larvae. These differences indicate that the specialist larvae have evolved a high degree of behavioral fidelity to their prey. However, larvae (second instars) of the two species that were released near ant-tended woolly alder aphid colonies in the field had similar recovery (= survival) rates. Consequently, natural selection may not act on behavioral traits that influence larval fidelity to prey during the late second and early third instars.     

The function of call alternation in the African reed frog (Hyperolius marmoratus): precise call timing prevents auditory masking

The function of call alternation in the African painted reed frog, Hyperolius marmoratus was studied. Males actively avoided call overlap with neighboring males in their natural habitat. Advertisement calls produced by groups of two or three males showed less overlap than expected if they called at random. In addition, isolated males significantly reduced their calling rate to match the periodicity of a playback of periodic tone pulses, with vocalizations given during the silent intervals between tones. Gap-detection experiments showed that males suppressed vocalizations during playbacks of constant-frequency tone bursts and gave more calls during silent periods than expected by chance. Females discriminated against conspecific advertisement calls with many pulses in favor of calls with few or no pulses. This suggests that there would be little selection pressure on males to alternate calls so that pulses are not obscured. To test if call alternation functions to make calls more conspicuous, females were presented with identical conspecific advertisement calls from two speakers in four different temporal patterns: simultaneous, overlapping, abutting, and alternating. Females did not discriminate when calls were presented in a simultaneous, abutting, or alternating time pattern. When calls overlapped with the trailing call delayed from 0.5 to 70 ms females discriminated in favor of the leading call. When the intensity of the leading call was reduced by 6 dB the preference for the leading call was maintained when calls overlapped by 2 and 40 ms but was abolished or reversed at 0.5 and 70 ms, respectively. These results support the notion that the second of two partially overlapping calls was acoustically masked, rendering female painted reed frogs unwilling to approach or unable to locate such calls. It is suggested that either simultaneous masking or the precedence effect are responsible for the observed behavior. Acoustic masking of the second call by the first when calls overlapped was maintained even when the frequency of the first call was altered by 150 Hz above and below the more preferred frequency of the second call. Received: 13 April 1995/Accepted after revision: 5 November 1995     

Sex, intimidation and severed limbs: the effect of simulated predator attack and limb autotomy on calling and emergence behaviour in the field cricket Gryllus bimaculatus

Crickets can autotomize a limb in order to evade predation; however, this autotomy compromises their escape speed and, therefore, their ability to avoid subsequent predation events. We recorded two measures of predator-avoidance behaviour in field crickets (Gryllus bimaculatus) exposed to varying levels of predation threat, with the most extreme level leading to autotomy of a limb. Our first measure of caution was time to emergence from cover, which was affected by perceived predation treatment, with both autotomized males and females being significantly more cautious than intact individuals. For males (but not females), the presence of a calling conspecific encouraged earlier emergence. Our second measure of caution was alteration of male calling behaviour. Autotomized males remained silent for significantly longer time after disturbance than intact males, but there was no difference in call rate once they had resumed calling, suggesting that behavioural changes were not merely a result of injury response but a controlled modification of behaviour as a result of autotomy. These data suggest that autotomy in field crickets results in altered calling behaviour, which was not significantly altered with different acoustic environments.     

Chick recognition and acceptance: a weakness in magpies exploited by the parasitic great spotted cuckoo

Hosts of brood parasites have evolved the ability to discriminate non-mimetic and even mimetic eggs, but not non-mimetic chicks. Here we demonstrate that the great spotted cuckoo Clamator glandarius does not provide its magpie Pica pica host with a super-normal stimulus that helps to avoid recognition, because single cuckoo chicks introduced into otherwise unparasitized magpie nests are not fed at a higher frequency than single magpie chicks introduced to parasitized magpie nests. Another series of experiments demonstrated that magpies have the ability to discriminate cuckoo chicks, mainly when these are introduced at the end of the nestling period, and especially when the cuckoo chick together with a magpie chick is presented to adult magpies outside the nest. This supports the idea that cuckoos exploit the obligatory reaction of magpies to feed all young that have been hatched in their nests and whose signatures they have learnt. Furthermore, the experimental cuckoo chicks in parasitized magpie nests were more likely to be accepted than they were in non-parasitized nests. This supports the hypothesis that magpies may learn to recognise their own nestlings as those present in the nest and may indicate that a comparison between cuckoo and magpie nestlings is the basis of discrimination.     

Repeated training does not improve the path integrator in desert ants

Desert ants, Cataglyphis fortis, return to their nest by means of path integration vectors. By using the reversal of these vectors, they approach previously visited feeding sites again. They adjust these vectors whenever outbound and inbound vector are set into conflict or when they make use of external cues. Here, we examine the influence of repeated training on the accuracy, precision, and straightness of outbound and inbound vectors. We trained desert ants to forage to and fro between their nest and a feeder and made sure that they relied exclusively on their path integrator. Neither the ants’ outbound nor their inbound runs, which, in general, are straighter than the outbound runs, become more accurate, precise, or straighter during repeated training. Hence, repeated training does not improve the path integrator in desert ants.     

Tardy females,impatient males: protandry and divergent selection on arrival date in the two sexes of the barn swallow

Protandry reflects the earlier arrival of males than females to the site of reproduction. Such protandry is hypothesised to arise from sex differences in costs and benefits of early arrival. I investigated temporal patterns of arrival date of male and female barn swallows Hirundo rustica and temporal patterns of selection to test the hypothesis that sex differences in selection account for sex differences in arrival date. Mean arrival date of male barn swallows but not of females advanced during the last 33 years, giving rise to an increasing sex difference in arrival date. Early arrival was favoured by increasingly better survival in males, while females showed an opposite pattern that did not reach significance, although the effect differed between sexes. Early arrival increased fecundity in both sexes equally.The sex difference in viability selection in relation to arrival date changed from positive to negative as the degree of protandry increased in recent years, although there was no similar significant relationship for fecundity selection. Furthermore, sex differences in viability selection in a given year affected the degree of protandry in the following year through differential survival of certain phenotypes over others. Finally, temporal changes in sex difference in viability selection and protandry were related to an increase in the interval between first and second clutches, as the duration of the breeding season increased because of climatic amelioration. These findings suggest that arrival date is under divergent selection in the two sexes, providing a mechanism for the evolution of protandry.     

Benefits of trenching behavior in the context of an inducible defense

Summary. Many species of insects sabotage the pressurized defense vessels of their host plants prior to feeding. This behavior, however, does not render leaves indefinitely suitable, as some species employing this behavior eventually abandon uneaten portions of sabotaged leaves. In this study, we examined whether and to what degree wild parsnip, Pastinaca sativa, is capable of restoring its pressurized defenses and whether cabbage loopers, Trichoplusa ni, which normally trench parsnip leaflets, benefit from their trenching behavior. The pressurized oil tubes of parsnip leaves are rich in toxic terpenoids and furanocoumarins. A disruption of the integrity of the tubes (via razor blade nicks) in leaflets revealed that that some of their contents were expelled at the break and that some movement of oil from outside the leaflet (i.e., the midvein) occurred, bolstering furanocoumarin levels in the leaflet within minutes. Pressure and chemical content in a leaflet’s oil tubes were also shown to be restored within 24 hours of depressurization. This recovery ability allowed parsnip leaflets to respond to daily depressurizations by mechanical damage for up to at least 5 assaults, cumulatively causing an approximate ten-fold increase in furanocoumarins. Cabbage loopers fed parsnip leaflets that were artificially trenched accumulated twice as much body mass as larvae fed leaflets augmented with furanocoumarins equivalent to the quantity that would be avoided through trenching, indicating that trenching does benefit the herbivore. Although parsnip recovers from trenching rapidly, it does not do so within the time that cabbage loopers consume trenched leaflets     

Individual recognition of neighbors by song in a suboscine bird, the alder flycatcher Empidonax alnorum

The ability of territorial males to discriminate between songs of their neighbors and songs of strangers has been demonstrated in 27 species of songbirds. Such experiments test only the ability of a subject to discriminate between two classes of stimuli, familiar (neighbors) and unfamiliar (strangers) songs. Individual recognition of neighbors is a finer, more complex type of discrimination. The ability of territorial males to recognize individual neighbors by song has been documented in 12 species of oscine passerines (Passeriformes, Passeri), but has never been demonstrated in suboscine passerines (Tyranni). We investigated recognition of songs of individual neighbors in a suboscine, the alder flycatcher (Empidonax alnorum). We performed a series of song playback experiments and recorded responses of territorial males to songs of neighbors and songs of strangers broadcast from two locations, the neighbor boundary and an opposite boundary. Subjects responded more aggressively to songs of a neighbor when played from the opposite boundary than when played from the neighbor boundary. They responded with equal aggression to songs of strangers regardless of location of playback. The difference in response to neighbor songs between speaker locations and the lack of a difference in response to stranger songs indicate that territorial males associate a particular song with a particular location (territory), and thus recognize individual neighbors.Communicated by I. Hartley     

Copulation behavior and paternity in the chaffinch

Summary We investigated two aspects of sperm competition in a wild population of chaffinches Fringilla coelebs: copulation behavior and genetic parentage determined by DNA fingerprinting. Pairs copulated at a high rate before laying, peaking at 4.4 copulation attempts per hour on day –3 relative to laying, with an average of 207 copulation attempts (83 behaviorally successful) per clutch. Extra-pair copulation attempts (n = 20) made up 7.8% of all copulation attempts that females were involved in. The rate at which pairs copulated was lower during egg-laying, but there was no evidence that this was due to the male of the pair timing copulations to coincide with an insemination window; it was more likely due to the fact that females solicited copulations at a lower rate. Both sexes solicited copulations but males solicited mainly extra-pair copulations. DNA fingerprinting showed that 17.0% of chicks (n = 47) in 23% of broods (n = 13) were fathered by a male other than the one paired to their mother. There was no evidence of intraspecific brood parasitism. In three cases where we were able to identify the father of extra-pair offspring it proved to be a neighboring male.Correspondence to: B.C. Sheldon     

Disease prevention and resistance in social insects: modeling the survival consequences of immunity,hygienic behavior,and colony organization

Understanding the origin of disease resistance in social insects is difficult due to the lack of well-established phylogenies of presocial and eusocial species and the absence of extant basal and intermediate forms. Moreover, comprehensive accounts of infection-control traits in social insect lineages are not available. Therefore, to explore the evolution of pathogen control in social insects we used cellular automata models to analyze the efficacy of immunity and nest hygiene, which we assumed were basal traits, and allogrooming, which likely followed the transition to eusociality, and their interactions with colony demography and patterns of worker spatial distribution. Models showed that nest hygiene provided an immediate survival benefit and that immunity lowered overall disease susceptibility under both constant and periodic exposure scenarios. Allogrooming increased survivorship in chronically challenged colonies but also increased pathogen transmission rates under conditions of periodic exposure. Colonies having demographies biased towards young or old individuals had slightly higher mortality than those with heterogeneous demographies. The distribution of older individuals relative to the nest center had no significant effect on susceptibility and provided only a minor survival advantage. Models indicated that nest hygiene and immunity function on different temporal scales and can interact with demography to lower disease risks. Our results suggest how infection control systems in social insects could have been built upon the inducible immune defenses and nest hygienic behaviors of solitary and presocial ancestors and served as important preadaptations to manage disease exposure and transmission in colonies of eusocial species.     

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.     

Asexuals looking for sex: conflict between species and mate-quality recognition in sailfin mollies (Poecilia latipinna)

When two closely related species are sympatric the process of species recognition (identifying conspecifics) and mate-quality recognition (increased fitness benefits) can yield a conflict when heterospecifics resemble high-quality conspecifics. Conflict in species versus mate-quality recognition may serve as a possible mechanism for the persistence of unisexual, gynogenetic Amazon mollies (Poecilia formosa). Amazon mollies require sperm from closely related species (e.g., sailfin mollies, P. latipinna) to start embryogenesis but inheritance is strictly maternal. When choosing mates, male sailfin mollies from populations sympatric with Amazon mollies may rely on traits indicating species identity rather than those indicating mate quality. Conversely, males from allopatric populations may rely more on traits indicating mate quality. Previous work has found that male sailfin mollies in sympatry exhibit a significantly greater mating preference for female sailfin mollies over Amazon mollies compared to males in allopatry. In addition, male sailfin mollies prefer to associate with and produce more sperm in the presence of larger conspecific females, which are more fecund. We hypothesized that male sailfin mollies experience a conflict in species recognition and mate-quality recognition in the presence of Amazon mollies that are relatively larger than female sailfin mollies. To test this hypothesis, we paired males from sympatric and allopatric populations with a larger Amazon molly and a smaller female sailfin molly. We scored the number of mating attempts that males directed to conspecific and heterospecific females. Males in most sympatric and allopatric populations demonstrate no clear preference for conspecifics. In addition, we found some evidence for a difference in mating preference between allopatric and sympatric populations with males from allopatry showing a greater heterospecific mate preference. These results indicate a conflict between species and mate-quality recognition. In sympatry this conflict may contribute to the persistence of gynogenetic Amazon mollies.     

Inter-specific variation in anti-predator behavior in sympatric species of kangaroo rat

Kangaroo rats, Dipodomys, occupy desert habitats with little cover and thus are under high predation risk from diverse predators. The behavior used to assess predation risk or to escape capture is unknown. We therefore compared anti-predator behavior of two sympatric species of kangaroo rat of different sizes, D. merriami and D. spectabilis. We first examined whether kangaroo rats use olfaction as a first line of defense against snake predation and tested the rats for their responses to scent extracted from two species of snake that live sympatrically with the kangaroo rats, the Mojave rattlesnake (Crotalus scutulatus) and the gopher snake (Pitophis melanoleucus). We also tested for species differences in anti-predator behavior through 15-min interactions between the kangaroo rats and free-moving gopher snakes. We found that D. spectabilis actively approached the scent of both rattlesnakes and gopher snakes more than controls of vegetable oil and evaporated solvent (Fig. 1). In contrast, D. merriami did not differentiate snake odors from controls in the experimental arena, but they sniffed the sand where a free-moving snake had passed more than D. spectabilis. Both species successfully avoided predation in encounters with live snakes. Although total numbers of approaches and withdrawals were similar (Fig. 2), D. spectabilis spent significantly more time within striking distance of the snake than D. merriami. D. spectabilis approached the head of the snake in 93% of its approaches and often engaged in nose to snout contact with the snake. If the snake struck, D. spectabilis jumped directly backward to avoid a strike and footdrummed at a safe distance. In contrast, D. merriami oriented to the snake more than D. spectabilis, but approached the head in only 41% of the approaches and rarely engaged in nose-to-snout contact. The snakes struck, hissed and decreased predatory approaches with D. spectabilis but not with D. merriami (Fig. 3). These results show that kangaroo rats can behaviorally influence the risk of being preyed on by snakes. The two species differ, however, in how they react to snakes. The larger D. spectabilis confronts snakes while the smaller D. merriami monitors snakes from a safe distance and avoids them.     

Probabilistic movement model with emigration simulates movements of deer in Nebraska, 1990–2006

Movements of deer can affect population dynamics, spatial redistribution, and transmission and spread of diseases. Our goal was to model the movement of deer in Nebraska in an attempt to predict the potential for spread of chronic wasting disease (CWD) into eastern Nebraska. We collared and radio-tracked >600 white-tailed deer (Odocoileus virginianus) and mule deer (Odocoileus hemionus) in Nebraska during 1990–2006. We observed large displacements (>10 km) for both species and sexes of deer, including migrations up to 100 km and dispersals up to 50 km. Average distance traveled between successive daily locations was 166 m for male and 173 for female deer in eastern Nebraska, and 427 m for male and 459 for female deer in western Nebraska. Average daily displacement from initial capture point was 10 m for male and 14 m for female deer in eastern Nebraska, and 27 m for male and 28 m for female deer in western Nebraska. We used these data on naturally occurring movements to create and test 6 individual-based models of movement for white-tailed deer and mule deer in Nebraska, including models that incorporated sampling from empirical distributions of movement lengths and turn angles (DIST), correlated random walks (CRW), home point fidelity (FOCUS), shifting home point (SHIFT), probabilistic movement acceptance (MOVE), and probabilistic movement with emigration (MOVEwEMI). We created models in sequence in an attempt to account for the shortcomings of the previous model(s). We used the Kolmogrov–Smirnov goodness-of-fit test to verify improvement of simulated annual displacement distributions to empirical displacement distributions. The best-fit model (D = 0.07 and 0.08 for eastern and western Nebraska, respectively) included a probabilistic movement chance with emigration (MOVEwEMI) and resulted in an optimal daily movement length of 350 m (maximum daily movement length of 2800 m for emigrators) for eastern Nebraska and 370 m (maximum of 2960 m) for western Nebraska. The proportion of deer that moved as emigrators was 0.10 and 0.13 for eastern and western Nebraska, respectively. We propose that the observed spread of CWD may be driven by large movements of a small proportion of deer that help to establish a low prevalence of the disease in areas east of the current endemic area. Our movement models will be used in a larger individual-based simulation of movement, survival, and transmission of CWD to help determine future surveillance and management actions.     

Relatedness,recognition errors,and colony fusion in the termite <Emphasis Type="Italic">Nasutitermes corniger</Emphasis>

Loss of aggression between social groups can have far-reaching effects on the structure of societies and populations. We tested whether variation in the genetic structure of colonies of the termite Nasutitermes corniger affects the probability of aggression toward non-nestmates and the ability of unrelated colonies to fuse. We determined the genotypes of workers and soldiers from 120 colonies at seven polymorphic microsatellite loci. Twenty-seven colonies contained offspring of multiple founding queens or kings, yielding an average within-colony relatedness of 0.33. Genotypes in the remaining 93 colonies were consistent with reproduction by a single queen and king or their progeny, with an average within-colony relatedness of 0.51. In standardized assays, the probability of aggression between workers and soldiers from different colonies was an increasing function of within-colony relatedness. The probability of aggression was not affected significantly by the degree of relatedness between colonies, which was near zero in all cases, or by whether the colonies were neighbors. To test whether these assays of aggression predict the potential for colony fusion in the field, we transplanted selected nests to new locations. Workers and soldiers from colonies that were mutually tolerant in laboratory assays joined their nests without fighting, but workers and soldiers that were mutually aggressive in the assays initiated massive battles. These results suggest that the presence of multiple unrelated queens or kings promotes recognition errors, which can lead to the formation of more complex colony structures.     

Prior encounters with the opposite sex affect male and female mating behavior in a wolf spider (Araneae,Lycosidae)

Mate availability can vary widely in nature depending upon population density and sex ratio and can affect the ability of individuals to be selective in mate choice. We tested the effects of prior encounters with the opposite sex (i.e., exposure to the opposite sex either with or without mating) on subsequent mating behavior in two experiments that manipulated mate availability for both males and females in the wolf spider, Hogna helluo. The probability of mating in the experimental trial depended upon whether the prior encounter involved mating or not, and males and females responded in opposite directions. Exposure without mating resulted in a higher subsequent frequency of mating for females and a lower subsequent frequency of mating for males, while prior mating experience resulted in a lower frequency of female remating and a higher frequency of male remating. Prior exposure without mating did not affect female aggression. However, mated females engaged in precopulatory cannibalism more frequently than virgins. Mated males escaped postcopulatory cannibalism more frequently than virgins. Our results show that males respond to exposure without mating in the expected manner. However, prior mating (1 week earlier) had unexpected effects on males, which may be due to mated males being of higher quality. There were little or no effects of the size of the prior exposure individual or mate on subsequent mating behaviors. Further research is needed to determine why different species use different degrees of prior information in mate choice.     

An experimental examination of female preference patterns for components of the male advertisement call in the quacking frog,<Emphasis Type="Italic"> Crinia georgiana</Emphasis>

Even though females prefer particular components of a male display, their preferences may not be expressed during mate choice. Here, we quantified female preference patterns in the frog Crinia georgiana for call rate, pulse number and dominant frequency using two-speaker trials. Females typically chose signals emitted at a higher rate, with more pulses (when variation was extreme) and with an average dominant frequency. The results for pulse number and call rate agree with a previous field study which found that these two call components explained significant variation in male mating success. In contrast, the strong preferences for average dominant frequencies detected here disagree with the previous field study which failed to find any relationship between dominant frequency and male mating success. To explain the discrepancy we investigated whether the patterns of female preference for dominant frequency changed when another property, call rate, was varied simultaneously. Most females chose the call with an average dominant frequency when offered a choice between a call with an average dominant frequency and call rate or a non-average dominant frequency (±2 SD) and high call rate. When the differences in dominant frequency were reduced (±1 SD), females showed no clear preference for either signal. Accordingly, female preference patterns for one call component can vary with the expression of another. These results do not explain why dominant frequency did not account for any variation in male mating success under field conditions.Communicated by T. Beschlitz     

Chick-a-dee call syntax,social context,and season affect vocal responses of Carolina chickadees (<Emphasis Type="Italic">Poecile carolinensis</Emphasis>)

Chick-a-dee calls in many chickadee (Poecile) species are common vocal signals used in a diversity of social contacts. The calls consist of four notes, A, B, C, and D, which follow simple rules of syntax (note ordering and composition) to generate many unique call types. We used field playbacks with Carolina chickadees, P. carolinensis, to ask whether violations of a syntactical rule affected their vocal responses. We show that chickadee responses to typical calls (e.g. AAAACCCC and CCCCDDDD) differ from responses to atypical calls (e.g. CACACACA and DCDCDCDC) depending on playback note composition, season, and social context (presence of heterospecifics). In the fall/winter, playbacks of typical calls with A and C notes elicited the greatest number of A and B notes in chick-a-dee call responses and typical calls with D notes elicited the greatest number of C notes, when in the presence of heterospecifics. In contrast, the corresponding atypical calls did not elicit similar responses. This suggests communicative significance is lost in calls that violate a rule of syntax in the fall/winter. In the spring, neither chickadee feebeefeebay song rate nor chick-a-dee calls responses differed by playback type. We suggest that call syntax is less salient for mated pairs in the spring than it is for fall/winter flocks that rely more on conspecific communication for foraging success and flock cohesion. This study represents the first experimental evidence that chickadees attend to both note composition and ordering in chick-a-dee calls.Communicated by W.A. Searcy