Life-Stage Dependent Plasticity in the Auditory System of a Songbird Is Signal and Emitter-Specific.

Social animals flexibly use a variety of vocalizations to communicate in complex and dynamic environments. However, it remains unknown whether the auditory perception of different vocalizations changes according to the ecological context. By using miniature wireless devices to synchronously record vocal interactions and local neural activity in freely-behaving zebra finches in combination with playback experiments, we investigate whether the auditory processing of vocalizations changes across life-history stages. We show that during breeding, females (but not males) increase their estrogen levels and reply faster to their mates when interacting vocally. These changes are associated with an increase in the amplitude of the female's neural auditory responses. Furthermore, the changes in auditory response are not general, but specific to a subset of functionally distinct vocalizations and dependent on the emitter's identity. These results provide novel insights into auditory plasticity of communication systems, showing that the perception of specific signals can shift according to ecologically-determined physiological states.

Neurotelemetry Reveals Putative Predictive Activity in HVC during Call-Based Vocal Communications in Zebra Finches.

Premotor predictions facilitate vocal interactions. Here, we study such mechanisms in the forebrain nucleus HVC (proper name), a cortex-like sensorimotor area of songbirds, otherwise known for being essential for singing in zebra finches. To study the role of the HVC in calling interactions between male and female mates, we used wireless telemetric systems for simultaneous measurement of neuronal activity of male zebra finches and vocalizations of males and females that freely interact with each other. In a non-social context, male HVC neurons displayed stereotypic premotor activity in relation to active calling and showed auditory-evoked activity to hearing of played-back female calls. In a social context, HVC neurons displayed auditory-evoked activity to hearing of female calls only if that neuron showed activity preceding the upcoming female calls. We hypothesize that this activity preceding the auditory-evoked activity in the male HVC represents a neural correlate of behavioral anticipation, predictive activity that helps to coordinate vocal communication between social partners.SIGNIFICANCE STATEMENT Most social-living vertebrates produce large numbers of calls per day, and the calls have prominent roles in social interactions. Here, we show neuronal mechanisms that are active during call-based vocal communication of zebra finches, a highly social songbird species. HVC, a forebrain nucleus known for its importance in control of learned vocalizations of songbirds, displays predictive activity that may enable the male to adjust his own calling pattern to produce very fast sequences of male female call exchanges.

Cyclic bouts of extreme bradycardia counteract the high metabolism of frugivorous bats.

Active flight requires the ability to efficiently fuel bursts of costly locomotion while maximizing energy conservation during non-flying times. We took a multi-faceted approach to estimate how fruit-eating bats (Uroderma bilobatum) manage a high-energy lifestyle fueled primarily by fig juice. Miniaturized heart rate telemetry shows that they use a novel, cyclic, bradycardic state that reduces daily energetic expenditure by 10% and counteracts heart rates as high as 900 bpm during flight. Uroderma bilobatum support flight with some of the fastest metabolic incorporation rates and dynamic circulating cortisol in vertebrates. These bats will exchange fat reserves within 24 hr, meaning that they must survive on the food of the day and are at daily risk of starvation. Energetic flexibly in U. bilobatum highlights the fundamental role of ecological pressures on integrative energetic networks and the still poorly understood energetic strategies of animals in the tropics.

Individual recognition of opposite sex vocalizations in the zebra finch.

Individual vocal recognition plays an important role in the social lives of many vocally active species. In group-living songbirds the most common vocalizations during communal interactions are low-intensity, soft, unlearned calls. Being able to tell individuals apart solely from a short call would allow a sender to choose a specific group member to address, resulting in the possibility to form complex communication networks. However, little research has yet been carried out to discover whether soft calls contain individual identity. In this study, males and females of zebra finch pairs were tested with six vocalization types - four different soft calls, the distance call and the male song - to investigate whether they are able to distinguish individuals of the opposite sex. For both sexes, we provide the first evidence of individual vocal recognition for a zebra finch soft unlearned call. Moreover, while controlling for habituation and testing for repeatability of the findings, we quantify the effects of hitherto little studied variables such as partners' vocal exchange previous to the experiment, spectral content of playback calls and quality of the answers. We suggest that zebra finches can recognize individuals via soft vocalizations, therefore allowing complex directed communication within vocalizing flocks.

Vocal similarity in long-distance and short-distance vocalizations in raven pairs (Corvus corax) in captivity.

Vocal interactions in many birds are characterized by imitation or the matching of vocalizations whereby one individual makes its vocalizations more similar to those of a conspecific. This behaviour is aided by vocal learning, which allows birds to change the vocalizations already in their repertoires, or to add new ones. The majority of studies on vocal similarity have been focussing on the songs of birds rather than their calls, with evidence for vocal similarity in calls being rather scarce. Here, we investigated whether ravens make their calls acoustically similar to one another by analysing the extent to which short- and long-distance calls of their vocal repertoires exhibited vocal similarity. Our results showed that long-distance calls, but not short-distance calls, are highly similar between pair partners. This effect may be explained by the different functions underlying short- and long-distance communication in ravens, with vocal similarity possibly being scaffolded by specific social matrices such as pair-bonds and/or strong social relationships.

Vocal exchanges during pair formation and maintenance in the zebra finch (Taeniopygia guttata).

BACKGROUND: Pair compatibility affects the success of a pair; however, its causes and mechanisms are not fully understood. Vocal exchange may be very important for pair formation, coordinating pair activities, maintaining the pair bond and mate guarding. To investigate the role of vocal exchange in pair formation and pair maintenance, we explored whether new and established pairs of zebra finches differed in their calling relationships. We used individualised backpack microphones to examine the entire daily vocal emission of pairs, with parallel video recording of behaviour. RESULTS: We found that in non-breeding, isolated pairs, a specific type of call, the "stack call", was the most common. Furthermore, all pairs used the stack call for precisely timed antiphonal exchange. We confirmed a difference between new and established pairs in social behaviour, with the former spending less time in physical contact. Notably, we found that this was mirrored by a difference in calling behaviour: members of new pairs converged over time on a more symmetric calling relationship. Additionally, we observed different response rates to partners among individuals, but a repeatable relationship of answering within pairs, which may reflect different degrees of motivation to answer the partner. CONCLUSIONS: Our findings show that there is plasticity in calling behaviour and that it changes during pair formation, resulting in a coordinated stack call exchange with a similar number of answers between partners once the pair is established. It is possible that some of the calling relationship measurements that we present reflect pair compatibility.

Heart rate reveals torpor at high body temperatures in lowland tropical free-tailed bats.

Reduction in metabolic rate and body temperature is a common strategy for small endotherms to save energy. The daily reduction in metabolic rate and heterothermy, or torpor, is particularly pronounced in regions with a large variation in daily ambient temperature. This applies most strongly in temperate bat species (order Chiroptera), but it is less clear how tropical bats save energy if ambient temperatures remain high. However, many subtropical and tropical species use some daily heterothermy on cool days. We recorded the heart rate and the body temperature of free-ranging Pallas' mastiff bats (Molossus molossus) in Gamboa, Panamá, and showed that these individuals have low field metabolic rates across a wide range of body temperatures that conform to high ambient temperature. Importantly, low metabolic rates in controlled respirometry trials were best predicted by heart rate, and not body temperature. Molossus molossus enter torpor-like states characterized by low metabolic rate and heart rates at body temperatures of 32°C, and thermoconform across a range of temperatures. Flexible metabolic strategies may be far more common in tropical endotherms than currently known.

Natural melatonin fluctuation and its minimally invasive simulation in the zebra finch.

Melatonin is a key hormone in the regulation of circadian rhythms of vertebrates, including songbirds. Understanding diurnal melatonin fluctuations and being able to reverse or simulate natural melatonin levels are critical to investigating the influence of melatonin on various behaviors such as singing in birds. Here we give a detailed overview of natural fluctuations in plasma melatonin concentration throughout the night in the zebra finch. As shown in previous studies, we confirm that "lights off" initiates melatonin production at night in a natural situation. Notably, we find that melatonin levels return to daytime levels as early as two hours prior to the end of the dark-phase in some individuals and 30 min before "lights on" in all animals, suggesting that the presence of light in the morning is not essential for cessation of melatonin production in zebra finches. Thus, the duration of melatonin production seems not to be specified by the length of night and might therefore be less likely to directly couple circadian and annual rhythms. Additionally, we show that natural melatonin levels can be successfully simulated through a combination of light-treatment (daytime levels during subjective night) and the application of melatonin containing skin-cream (nighttime levels during subjective day). Moreover, natural levels and their fluctuation in the transition from day to night can be imitated, enabling the decoupling of the effects of melatonin, for example on neuronal activity, from sleep and circadian rhythmicity. Taken together, our high-resolution profile of natural melatonin levels and manipulation techniques open up new possibilities to answer various melatonin related questions in songbirds.

Patterns of call communication between group-housed zebra finches change during the breeding cycle.

Vocal signals such as calls play a crucial role for survival and successful reproduction, especially in group-living animals. However, call interactions and call dynamics within groups remain largely unexplored because their relation to relevant contexts or life-history stages could not be studied with individual-level resolution. Using on-bird microphone transmitters, we recorded the vocalisations of individual zebra finches (Taeniopygia guttata) behaving freely in social groups, while females and males previously unknown to each other passed through different stages of the breeding cycle. As birds formed pairs and shifted their reproductive status, their call repertoire composition changed. The recordings revealed that calls occurred non-randomly in fine-tuned vocal interactions and decreased within groups while pair-specific patterns emerged. Call-type combinations of vocal interactions changed within pairs and were associated with successful egg-laying, highlighting a potential fitness relevance of calling dynamics in communication systems.

Hatching late in the season requires flexibility in the timing of song learning.

Most songbirds learn their songs from adult tutors, who can be their father or other male conspecifics. However, the variables that control song learning in a natural social context are largely unknown. We investigated whether the time of hatching of male domesticated canaries has an impact on their song development and on the neuroendocrine parameters of the song control system. Average age difference between early- and late-hatched males was 50 days with a maximum of 90 days. Song activity of adult tutor males decreased significantly during the breeding season. While early-hatched males were exposed to tutor songs for on average the first 99 days, late-hatched peers heard adult song only during the first 48 days of life. Remarkably, although hatching late in the season negatively affected body condition, no differences between both groups of males were found in song characteristics either in autumn or in the following spring. Similarly, hatching date had no effect on song nucleus size and circulating testosterone levels. Our data suggest that late-hatched males must have undergone accelerated song development. Furthermore, the limited tutor song exposure did not affect adult song organization and song performance.

Effects of age, size, and mating history on sex role decision of a simultaneous hermaphrodite.

Contrasting with separate-sexed animals, simultaneous hermaphrodites display unique reproductive strategies as they are male and female at the same time. Simultaneous hermaphrodites that copulate unilaterally, for instance, make a decision to mate as a male or female. Previous studies have demonstrated that sex role preference in hermaphrodites is flexible and is controlled by several, often confounding, factors. We examined the relationship between sex role decisions and 3 life-history traits (age, size, and mating history) in the great pond snail, Lymnaea stagnalis. Based on our field observations, which indicate that adult individuals show overlapping generations and large variation in body size during the breeding season, we performed a sex role choice experiment in the laboratory. We found that young and small snails mate as males first. Both age and size significantly affected sex role decision, with age having a stronger effect. Furthermore, we tested whether L. stagnalis becomes reluctant to inseminate a mate after being inseminated because it is known that after insemination, male investment substantially reduces. Contrary to expectations, our results indicate that the receipt of seminal fluid does not seem to reduce male motivation. In sum, sex role decisions in L. stagnalis are largely determined by age and size but not by having received seminal fluid. This mating pattern, however, does not fully support the size-advantage model because large or old individuals did not perform better as females in our experiment. These results imply a conflicting mating interest, rather than harmonious agreement, between age- and size-different hermaphrodites.

Regulatory mechanisms of testosterone-stimulated song in the sensorimotor nucleus HVC of female songbirds.

BACKGROUND: In male birds, influence of the sex steroid hormone testosterone and its estrogenic metabolites on seasonal song behavior has been demonstrated for many species. In contrast, female song was only recently recognized to be widespread among songbird species, and to date, sex hormone effects on singing and brain regions controlling song development and production (song control nuclei) have been studied in females almost exclusively using domesticated canaries (Serinus canaria). However, domesticated female canaries hardly sing at all in normal circumstances and exhibit only very weak, if any, song seasonally under the natural photoperiod. By contrast, adult female European robins (Erithacus rubecula) routinely sing during the winter season, a time when they defend feeding territories and show elevated circulating testosterone levels. We therefore used wild female European robins captured in the fall to examine the effects of testosterone administration on song as well as on the anatomy and the transcriptome of the song control nucleus HVC (sic). The results obtained from female robins were compared to outcomes of a similar experiment done in female domesticated canaries. RESULTS: Testosterone treatment induced abundant song in female robins. Examination of HVC transcriptomes and histological analyses of song control nuclei showed testosterone-induced differentiation processes related to neuron growth and spacing, angiogenesis and neuron projection morphogenesis. Similar effects were found in female canaries treated with testosterone. In contrast, the expression of genes related to synaptic transmission was not enhanced in the HVC of testosterone treated female robins but was strongly up-regulated in female canaries. A comparison of the testosterone-stimulated transcriptomes indicated that brain-derived neurotrophic factor (BDNF) likely functions as a common mediator of the testosterone effects in HVC. CONCLUSIONS: Testosterone-induced singing of female robins correlated with cellular differentiation processes in the HVC that were partially similar to those seen in the HVC of testosterone-treated female canaries. Other modes of testosterone action, notably related to synaptic transmission, appeared to be regulated in a more species-specific manner in the female HVC. Divergent effects of testosterone on the HVC of different species might be related to differences between species in regulatory mechanisms of the singing behavior.

Zebra finch mates use their forebrain song system in unlearned call communication.

Unlearned calls are produced by all birds whereas learned songs are only found in three avian taxa, most notably in songbirds. The neural basis for song learning and production is formed by interconnected song nuclei: the song control system. In addition to song, zebra finches produce large numbers of soft, unlearned calls, among which "stack" calls are uttered frequently. To determine unequivocally the calls produced by each member of a group, we mounted miniature wireless microphones on each zebra finch. We find that group living paired males and females communicate using bilateral stack calling. To investigate the role of the song control system in call-based male female communication, we recorded the electrical activity in a premotor nucleus of the song control system in freely behaving male birds. The unique combination of acoustic monitoring together with wireless brain recording of individual zebra finches in groups shows that the neuronal activity of the song system correlates with the production of unlearned stack calls. The results suggest that the song system evolved from a brain circuit controlling simple unlearned calls to a system capable of producing acoustically rich, learned vocalizations.

Maximized song learning of juvenile male zebra finches following BDNF expression in the HVC.

During song learning, vocal patterns are matched to an auditory memory acquired from a tutor, a process involving sensorimotor feedback. Song sensorimotor learning and song production of birds is controlled by a set of interconnected brain nuclei, the song control system. In male zebra finches, the beginning of the sensorimotor phase of song learning parallels an increase of the brain-derived neurotrophic factor (BDNF) in just one part of the song control system, the forebrain nucleus HVC. We report here that transient BDNF-mRNA upregulation in the HVC results in a maximized copying of song syllables. Each treated bird shows motor learning to an extent similar to that of the selected best learners among untreated zebra finches. Because this result was not found following BDNF overexpression in the target areas of HVC within the song system, HVC-anchored mechanisms are limiting sensorimotor vocal learning.

Behavioural and physiological effects of population density on domesticated Zebra Finches (Taeniopygia guttata) held in aviaries.

Zebra Finches (Taeniopygia guttata) are highly social and monogamous birds that display relatively low levels of aggression and coordinate group life mainly by means of vocal communication. In the wild, small groups may congregate to larger flocks of up to 150-350 birds. Little is known, however, about possible effects of population density on development in captivity. Investigating density effects on physiology and behaviour might be helpful in identifying optimal group size, in order to optimise Zebra Finch wellbeing. A direct effect of population density on development and reproduction was found: birds in lower density conditions produced significantly more and larger (body mass, tarsus length) surviving offspring than birds in high density conditions. Furthermore, offspring in low density aviaries produced slightly longer song motifs and more different syllables than their tutors, whereas offspring in high density aviaries produced shorter motifs and a smaller or similar number of different syllables than their tutors. Aggression levels within the populations were low throughout the experiment, but the number of aggressive interactions was significantly higher in high density aviaries. Baseline corticosterone levels did not differ significantly between high- and low density aviaries for either adult or offspring birds. On day 15 post hatching, brood size and baseline corticosterone levels were positively correlated. On days 60 and 100 post hatching this correlation was no longer present. The results of this study prove that population density affects various aspects of Zebra Finch development, with birds living in low population density conditions having an advantage over those living under higher population density conditions.

Multisensory non-photoperiodic cue advances the onset of seasonal breeding in Island canaries (Serinus canaria).

In most temperate zone vertebrates, photoperiodic change plays the major role in the timing of seasonal breeding. However, direct environmental stimuli such as temperature, rainfall, or availability of food are thought to be important for fine-tuning breeding activities. Building on evidence from wild Island canaries (Serinus canaria), the authors had shown advancing effects of green vegetation on breeding under captive, short-day conditions. So far, the precise, sensory modalities of this stimulatory cue are unknown. Here the authors present new data that confirm advanced breeding activities in the presence of green vegetation and narrow its stimulatory components. They found that direct exposure of the birds to fresh green vegetation represented the strongest stimulus and advanced breeding by up to 2 months compared to controls. In contrast, access to artificial green vegetation, extracts from green vegetation, or olfactory components alone had no such effects. This is, to the best of the authors' knowledge, the first experiment that examines sensory components of an effective, supplementary, non-photoperiodic cue in a temperate zone species. The data suggest that in order to use non-photoperiodic information to time breeding, birds must be able to integrate and process multisensory stimuli. Single non-photoperiodic sensory cues are insufficient to affect the timing of seasonal breeding.

Male accessory gland protein reduces egg laying in a simultaneous hermaphrodite.

Seminal fluid is an important part of the ejaculate of internally fertilizing animals. This fluid contains substances that nourish and activate sperm for successful fertilization. Additionally, it contains components that influence female physiology to further enhance fertilization success of the sperm donor, possibly beyond the recipient's optimum. Although evidence for such substances abounds, few studies have unraveled their identities, and focus has been exclusively on separate-sex species. We present the first detailed study into the seminal fluid composition of a hermaphrodite (Lymnaea stagnalis). Eight novel peptides and proteins were identified from the seminal-fluid-producing prostate gland and tested for effects on oviposition, hatching and consumption. The gene for the protein found to suppress egg mass production, Ovipostatin, was sequenced, thereby providing the first fully-characterized seminal fluid substance in a simultaneous hermaphrodite. Thus, seminal fluid peptides and proteins have evolved and can play a crucial role in sexual selection even when the sexes are combined.

Coolidge effect in pond snails: male motivation in a simultaneous hermaphrodite.

BACKGROUND: The simultaneously hermaphroditic pond snail, Lymnaea stagnalis, can mate in the male and female role, but within one copulation only one sexual role is performed at a time. Previous work has shown that male motivation is determined by the availability of seminal fluid in the prostate gland, which is detected via a nervous connection by the brain area controlling male behaviour. Based on this knowledge, patterns of sexual role alternations within mating pairs can be explained. RESULTS: The data presented here reveal that these snails can donate and receive sperm several times within 24 hours, and that they have increased mating rates in larger groups (i.e. more mating opportunities). For mating pairs we show, by introducing novel mating partners after copulation, that animals do inseminate new partners, while they are no longer motivated to inseminate their original partners. CONCLUSION: Our findings provide the first direct evidence for higher motivation in a hermaphrodite to copulate when a new partner is encountered. This Coolidge effect seems to be attenuated when mucus trails are excluded, which suggests that a chemical or textural cue may be responsible for mediating this response to sperm competition.

A lightweight telemetry system for recording neuronal activity in freely behaving small animals.

A miniature lightweight radio telemetric device is described which is shown to be suitable for recording neuronal activity in freely behaving animals. Its size (12 x 5 x 8 mm) and weight (1.0-1.1 g with batteries, 0.4-0.5 g without) make the device particularly suitable for recording neuronal units in small animals such as mice or zebra finches. The device combines a high impedance preamplifier, RC-filters and an FM-transmitter. Using the device we recorded action potentials in field L of freely behaving zebra finches (12-17 g) through chronically implanted tungsten electrodes. In freely behaving birds we observed frequency dependent responses of field L units to auditory stimuli for periods of up to 7 days. We investigated the effect of the device on singing and locomotor activity of the zebra finches. Singing and locomotion were significantly affected on the first day after surgery. Both anesthesia and the presence of the transmitter contributed to the observed effect. After 1 day of recovery, singing activity returned to 99.6% and perch-hopping activity to 55.3% of the baseline levels. It is concluded that the device is well suited for recording spike trains from small animals while they behave freely and naturalistically.

Melatonin affects the temporal organization of the song of the zebra finch.

In birds and mammals, including humans, melatonin-binding sites are abundant in brain areas that have no known clock function. Although the role of such binding sites is still unclear, it is assumed that these sites link neural functions to circadian or circannual demands of neuroendocrine homeostasis and reproduction. To investigate a possible direct role of melatonin in motor control, we studied the song and neural song system of the zebra finch. Neurons of two sensory-motor areas of the descending song control circuit that are crucial for the organization of the song pattern, the HVC and RA, express the melatonin-1B receptor (Mel1B), while the hypoglossal motor neurons of the song circuit express melatonin-1C receptors (Mel1C). Application of melatonin to brain slices decreases the firing-rate of RA-neurons. Systemic administration of a Mel1B antagonist at the beginning of the night shortens the song and motif length and affects the song syllable lengths produced the next day. The temporal pattern of the song, however, does not undergo daily changes. Thus, melatonin is likely to affect a non-circadian motor pattern by local modulation of song control neurons and in consequence alters a sexual signal, the song of the zebra finch.