Melissa Hughes

The function of concurrent signals : visual and chemical communication in snapping shrimp

Animal signals are often used in more than one context or by more than one class of signaller. In the big-clawed snapping shrimp,Alpheus heterochaelis, the open chela display is a visual signal produced by both males and females. Thus, to respond appropriately to the open chela display in interactions, shrimp need information about the sex of the other individual. These experiments were performed to determine the effect of sex information from chemical signals on the response of shrimp to the visual signal of the open chela. First, chemical signals from males and females were presented alone. Shrimp did not respond differentially to chemical signals from males and females. Next, chemical signals were presented concurrently with the open chela display. Male shrimp showed a greater response to an open chela with male chemical signals than to an open chela with female chemical signals. Male response to an open chela with female chemical signals was also a function of the chelas relative size, and this size-relative response was less than the response to the open chela alone. Male response to an open chela with male chemical signals was independent of relative size. In contrast, females responded aggressively to open and closed chelae regardless of relative size, and regardless of concurrent chemical signals. The relative costs and benefits associated with aggressive and pair-forming interactions differ for males and females, and these differences are likely to contribute to the differences in assessment observed here.

Geographic Song Discrimination in Relation to Dispersal Distances in Song Sparrows

Whether geographic variation in signals actually affects communication between individuals depends on whether discriminable differences in signals occur over distances that individuals move in their lifetimes. We measure the ability of song sparrows (Melospiza melodia) to discriminate foreign from local songs using foreign songs recorded at a series of increasing distances and compare the results with previous measurements of dispersal distances. We test discrimination in males using playback of songs on territories and measuring approach and in females using playback to estradiol‐treated captives and measuring courtship display. Females fail to discriminate against foreign songs recorded at 18 km but do discriminate against foreign songs recorded at 34, 68, 135, and 540 km. Males fail to discriminate against foreign songs recorded at 18, 34, 68, 135, and 270 km but do discriminate against foreign songs from 540 km. Females are more discriminating, but even they do not discriminate at a distance three times the root‐mean‐square dispersal distance, as estimated from mtDNA variation. We suggest that female preference for local songs benefits females not because it allows them to reject foreign males but because accurate production of local song serves as a test of song‐learning ability.

The response of male and female song sparrows to geographic variation in song

We tested female and male Song Sparrows (Melospiza melodia) from a Pennsylvania site for discrimination between local songs and foreign songs recorded in New York. In Experiments 1 and 2 we measured the copulatory response of female Song Sparrows to playback of local and foreign songs. In Experiment 3 we measured the aggressive response of territorial males to playback. We used mean responses per subject as sample points in the statistical analysis in Experiment 1, but to avoid pseudoreplication we designed Experiments 2 and 3 with sufficient numbers of exemplars of local and foreign songs to use mean responses per exemplar as sample points. Responses in all three experiments were significantly stronger for local than for foreign songs. Song Sparrow songs show a great deal of variation within locales, and a pattern of gradual and subtle geographic change, so it is not obvious how or why our subjects performed the discrimination.

Individual variation in the strength of territory defense in male song sparrows: Correlates of age, territory tenure, and neighbor aggressiveness

In many species, the ability to defend a territory is essential for a male to obtain any reproductive success at all, and even among territorial individuals, variation in the strength of territory defense could have a significant impact on how much reproductive success is obtained. Previous studies have documented consistent individual differences in the vigor with which male song sparrows (Melospiza melodia) defend their territories, as measured by the strength of their reactions to territorial intrusions simulated through song playback. Variation in the strength of defense could reflect intrinsic differences among individuals in their resource holding potential (RHP), or variation in extrinsic factors. In this study, we examined whether variation in the strength of territory defense corresponds to differences in intrinsic factors such as the age or experience of the territory owner, the extrinsic factor of the level of aggression shown by neighbours, or both. Results indicate that males that previously held territories on the study site, regardless of whether they were holding the same territory as the previous year, show higher levels of territory defense than males that are new to the study site, and, assuming that returning males are older males, suggest that age is more important than experience on a specific territory in determining strength of territory defense. In addition, we found evidence that males with high levels of territorial aggression tend to be spatially clustered. The pattern observed suggests that a combination of intrinsic and extrinsic factors contribute to the expression of individual differences in territorial aggression.

Song-type sharing in song sparrows: implications for repertoire function and song learning

Abstract One hypothesis for the function of song repertoires is that males learn multiple song types so that they may share songs with neighbors, allowing them to match during territorial interactions. In at least one song sparrow population, in Washington, territorial males share a high proportion of song types with their neighbors and use these shared songs in matching. We recorded song sparrows in Pennsylvania and quantified sharing of whole songs and song segments. We found that song sharing is an order of magnitude less common in the Pennsylvania population. We found sharing of song segments to be significantly more common than the sharing of whole songs in three of the five fields we examined, while we found no significant differences between whole and partial song sharing in the remaining two fields. Finally, we found no evidence that sharing is greater between birds in the same field compared to birds in different fields. Taken with the data from Washington song sparrows, these results provide evidence for intraspecific geographic variation in the organization of song repertoires, and suggest that song sharing has not been a strong selective force in the evolution of song repertoires in song sparrows as a species. Furthermore, Washington and Pennsylvania song sparrows differ in how they learn song, in that Washington birds copy whole songs, while Pennsylvania birds appear to copy and recombine song segments, as has been found in laboratory studies of song learning. Thus both song learning and the function of song repertoires differ between populations of song sparrows. Such intraspecific geographic variation offers a unique opportunity to explore the ecological and historical factors which have influenced the evolution of song.

Acoustic sequences in non-human animals: a tutorial review and prospectus

Animal acoustic communication often takes the form of complex sequences, made up of multiple distinct acoustic units. Apart from the well‐known example of birdsong, other animals such as insects, amphibians, and mammals (including bats, rodents, primates, and cetaceans) also generate complex acoustic sequences. Occasionally, such as with birdsong, the adaptive role of these sequences seems clear (e.g. mate attraction and territorial defence). More often however, researchers have only begun to characterise – let alone understand – the significance and meaning of acoustic sequences. Hypotheses abound, but there is little agreement as to how sequences should be defined and analysed. Our review aims to outline suitable methods for testing these hypotheses, and to describe the major limitations to our current and near‐future knowledge on questions of acoustic sequences. This review and prospectus is the result of a collaborative effort between 43 scientists from the fields of animal behaviour, ecology and evolution, signal processing, machine learning, quantitative linguistics, and information theory, who gathered for a 2013 workshop entitled, ‘Analysing vocal sequences in animals’. Our goal is to present not just a review of the state of the art, but to propose a methodological framework that summarises what we suggest are the best practices for research in this field, across taxa and across disciplines. We also provide a tutorial‐style introduction to some of the most promising algorithmic approaches for analysing sequences. We divide our review into three sections: identifying the distinct units of an acoustic sequence, describing the different ways that information can be contained within a sequence, and analysing the structure of that sequence. Each of these sections is further subdivided to address the key questions and approaches in that area. We propose a uniform, systematic, and comprehensive approach to studying sequences, with the goal of clarifying research terms used in different fields, and facilitating collaboration and comparative studies. Allowing greater interdisciplinary collaboration will facilitate the investigation of many important questions in the evolution of communication and sociality.

Size assessment via a visual signal in snapping shrimp

Abstract Snapping shrimp are highly aggressive decapod crustaceans, with large, asymmetric chelae. Body size determines the outcome of both inter- and intrasexual interactions. Both the body and chela sizes of mated pairs are correlated, but the body size correlation is significantly stronger. In competitive interactions between individuals of the same sex, larger individuals usually win. Because the size of the major chela is a function of body size in both males and females, chela size could be used to assess body size early in interactions, before engaging in more high risk behaviors. To determine whether the major chela is used in size assessment, I presented shrimp with isolated chelae. Male snapping shrimp responded aggressively to isolated chelae when they were fixed open in a display posture, and the degree of aggressive response depended on the relative size of the chela. These data provide direct experimental evidence for the use of a visual signal in size assessment. Females, in contrast, responded aggressively to both the open and closed chela, and their responses did not depend on relative size. This sex difference in response may be due to differences in the value of certain resources, such as shelters, to males and females: females may be more willing to respond aggressively regardless of the apparent size of their opponent, in order to acquire more reliable information regarding size, motivation or fighting ability.

The evolution of bird song: Male and female response to song innovation in swamp sparrows

Abstract Closely related species of songbirds often show large differences in song syntax, suggesting that major innovations in syntax must sometimes arise and spread. Here we examine the response of male and female swamp sparrows,Melospiza georgiana , to an innovation in song syntax produced by males of this species. Young male swamp sparrows that have been exposed to tutor songs with experimentally increased trill rates reproduce these songs with periodic silent gaps (Podos 1996, Animal Behaviour,51, 1061–1070). This novel temporal pattern, termed ‘broken syntax’, has been demonstrated to transmit across generations (Podos et al. 1999, Animal Behaviour,58, 93–103). We show here that adult male swamp sparrows respond more strongly in territorial playback tests to songs with broken syntax than to heterospecific songs, and equally strongly to conspecific songs with normal and broken syntax. In tests using the solicitation display assay, adult female swamp sparrows respond more to broken syntax than to heterospecific songs, although they respond significantly less to conspecific songs with broken syntax than to those with normal syntax. We conclude that sexual selection by female choice is in this case conservative, acting against the spread of innovation. The conservative effect of female preferences must sometimes be overcome, however, to allow the evolution of the major structural differences in song observed between species.

The receiver-dependent cost of soft song: a signal of aggressive intent in songbirds

Social costs are one mechanism whereby reliability in signalling systems can be maintained. We measured the strength of aggressive response to territorial playback to ask whether the reliability of ‘soft song’, a strongly aggressive signal in the song sparrow, Melospiza melodia, is enforced by a social cost in the form of the receivers aggressive response. We also asked whether this cost is imposed by all receivers, or whether a differential response is found primarily or exclusively among the most aggressive subjects. We first measured the strength of each males aggressive response to playback on his territory, and then tested his responses to ‘warbled’ soft song and to broadcast song playbacks. While we found substantial variation in individual aggressiveness, nearly all males responded more strongly to warbled soft songs. Thus we provide evidence that warbled soft song imposes a social cost in the form of receiver retaliation, and provide a possible explanation for how the signals reliability is maintained. Questions about soft song remain, notably why selection should favour low amplitude in vocal signals of aggression in songbirds.

Why have long antennae? Exploring the function of antennal contact in snapping shrimp

Many organisms use antennae to gather tactile or chemical information from the environment. Crustaceans have two sets of antennae: short antennules (first antennae) and antennae posterior or lateral to them (second antennae) which are often much longer. While the function of crustacean antennae has received some attention, particularly the perception of dispersed chemical signals by the antennules, the role of direct antennal contact during social behavior is less well understood. We compared antennal contact in competitive and pairing interactions in two species of snapping shrimp: Alpheus angulosus and Alpheus heterochaelis. Conspecific interactions involve more antennal contact than heterospecific interactions. Alpheus heterochaelis perform more antennal contact than A. angulosus. Alpheus angulosus females perform more antennal contact in pairing than in competitions; however, A. heterochaelis females and males of both species use antennal contact equally in pairing and competitive interactions. Although antennal contact carries a risk of injury, such contact appears central to snapping shrimp social behavior.