Development of species identification in ducklings: XI. Embryonic critical period for species-typical perception in the hatchling

In the embryonic stages prior to hatching, the contact-contentment call of the Peking duck (domesticated Anas platyrhynchos) is more highly variable (2–6 notes/s) than it is after hatching (4–6 notes/s). The embryos must be exposed to the normally wide range of repetition rates of their contact-contentment call (2, 4, 6 notes/s) if their preference for the species maternal call is to be normal at 24 h after hatching. Exposure of muted embryos to the higher (4, 6 notes/s) or lower (2, 4 notes/s) portions of the normal range was ineffective. Thus the normally highly variable nature of the embryo's vocalizations fits the requirements of its developing auditory system. An embryonic critical period was also demonstrated: when muted hatchlings were exposed to the rates typical of the postnatal period (4, 6 notes/s), or even the more widely variable rates of the embryonic period (2, 4, 6 notes/s), they failed to show a preference for the normal maternal call at 24 or 48 h after hatching. Thus the precise developmental linkage involves maturational stage as well as the representativeness of the stimulation.     

Development of heart rate responses to acoustic stimuli in Muscovy duck embryos

Heart rate (HR) of Muscovy duck embryos (Cairina moschata f. domestica) was continuously recorded from the 21st day of incubation (E21) until hatching (E35). During that period, embryos were exposed to different acoustic stimuli (species-specific maternal and duckling calls, music, rectangular and sine waves, white noise). Sudden HR changes occurred at the onset of acoustic stimulation (on-response), as well as spontaneously. From E27 onwards, the response rate was significantly higher than the rate of spontaneous HR changes. The on-response rate increased further until E30. Most responses were elicited by maternal calls and music, but rarely by duckling calls. On-responses could be classified into: HR increase (36.4%), HR decrease (37.9%) and an increase in instantaneous HR variability (23.2%). The increase in HR variability occurred only in response to sounds, but not spontaneously. HR increases were mainly observed when the baseline HR was lower than the long-term HR trend. On-response duration was no longer than 3 min in 90% of all observations. The hourly mean HR and standard deviation did not change, even during phonoperiods composed of several sound patterns and lasting several hours. We conclude that Muscovy duck embryos are able to perceive exogenous acoustic stimuli, and that the acousto-sensory-->cardiac axis is functional from E27.     

Increased yolk testosterone facilitates prenatal perceptual learning in Northern bobwhite quail (Colinus virginianus)

Prenatal learning plays an important role in the ontogeny of behavior and birds provide a useful model to explore whether and how prenatal exposure to hormones of maternal origin can influence prenatal learning and the development of behavior. In this study we assessed if prenatal exposure to yolk testosterone can influence auditory learning in embryos of Northern bobwhite quail (Colinus virginianus). We experimentally enhanced testosterone concentrations in bobwhite quail eggs prior to incubation. The embryos from these T-treated eggs as well as control embryos that had received the vehicle-only or were non-treated were exposed to an individual bobwhite hen's maternal call for 120 min over the course of the day prior to hatching. All chicks were tested at 24 h following hatching for their auditory preference between the familiar bobwhite maternal call versus an unfamiliar bobwhite maternal call. T-treated chicks spent significantly more time in proximity to the familiar call compared to the unfamiliar call and also showed shorter latencies to approach the familiar call than control birds. Increased emotional reactivity, i.e. propensity to express fear responses, was also found in T-treated chicks. Baseline heart rates recorded in a second group of T-treated embryos and control embryos did not differ, which suggests no effect of yolk testosterone on baseline arousal level. To our knowledge this is the first demonstration of the influence of prenatal exposure to testosterone on auditory learning.     

Maternal vocalizations of mallard ducks (Anas platyrhynchos)

While incubating, brooding and calling their young out of the nest, female mallard ducks (Anas platyrhynchos) utter a species-typical maternal vocalization that their young find highly attractive. To determine the characteristic acoustic features of these calls, we recorded the vocalizations of seven hens in the field. The pre-exodus and nest exodus calls of these hens were similar with respect to frequency modulation, presence of a low-frequency impulsive sound, note duration, and repetition rate. The exodus call differs from the pre-exodus call in having more notes per burst and more harmonics, with a corresponding upward shift in dominant frequency. Repetition rate and frequency modulation may be the critical acoustic features of the auditory basis of species identification in mallard ducklings.     

The role of frequency modulation in controlling the response of mallard ducklings (Anas platyrhynchos) to conspecific distress calls

Three experiments examined the response of mallard ducklings to conspecific distress calls. In experiment 1, a synthetic call was constructed from a single distress note, by recording it on a digital disc and then using a software routine to regularly repeat this stored note with the average period (267 ms) of the original call. Ducklings were then tested for their tendency to inhibit their own distress vocalizations in response to either this synthetic call, the original call, or a constant-frequency tone mimic; they showed a significantly stronger inhibitory response to the synthetic call than to the tone mimic, but an even stronger response to the original call. The former result indicated that the synthetic call was an effective stimulus, and suggested that some aspect of the frequency modulation found in distress notes is required to evoke the normal duckling inhibitory response. The latter result further suggested that the acoustic variability found in the original call, but not the synthetic call, may be of importance for controlling duckling behaviour. Experiment 2 demonstrated that several other minor differences between the synthetic call and original call (number of notes per call and the digital recording of the synthetic call) could not account for the difference in duckling response to these two calls. Finally, in experiment 3, two additional synthetic stimuli were constructed from the digitized note, by first excising either the initial (front-chop) or terminal (rear-chop) frequency modulation found in each distress note, then creating amplitude envelopes for these new notes similar to that of the unaltered note, and finally repeating these notes to form calls with the same note period as the original call. The ducklings tested with the synthetic normal-note call and synthetic front-chop call showed a significantly stronger inhibitory response than the ducklings tested with the synthetic rear-chop call and the tone mimic. These results indicate that the terminal descending frequency sweep is an important feature of distress notes for triggering the response of ducklings to conspecific distress calls.     

Development of brainstem auditory pathway in mallard duck embryos and hatchlings

Summary The development of the brainstem auditory evoked potential (BAEP) was studied in mallard duck (Anas platyrhynchos) embryos and hatchlings from 5–6 days before hatching through two days after hatching in response to tone pips of different frequencies. BAEPs showed a different time of onset and a different rate of development for low, middle, and high frequencies. Although auditory sensitivity in the mid-frequency range (1.0, 1.5, 2.0, and 3.0 kHz) appeared 1–2 days later than in the low-frequency range, development of the BAEPs in the mid-frequency range was almost complete by hatching. In contrast, the development of auditory sensitivity in the low- and high-frequency ranges continued to develop after hatching. Accelerated development of BAEPs to middle frequencies during the embryonic period and to high frequencies after hatching was correlated with the ducklings' exposure to their own mid-frequency and high-frequency vocalizations before and after hatching, respectively.Abbreviations BAEP brainstem auditory evoked potential - CM cochlear microphonic - CT contact-contentment call - DT distress call - EP evoked potential     

Prenatal learning in an Australian songbird: habituation and individual discrimination in superb fairy-wren embryos

Embryos were traditionally considered to possess limited learning abilities because of the immaturity of their developing brains. By contrast, neonates from diverse species show behaviours dependent on prior embryonic experience. Stimulus discrimination is a key component of learning and has been shown by a handful of studies in non-human embryos. Superb fairy-wren embryos (Malurus cyaneus) learn a vocal password that has been taught to them by the attending female during incubation. The fairy-wren embryos use the learned element as their begging call after hatching to solicit more parental feeding. In this study, we test whether superb fairy-wren embryos have the capacity to discriminate between acoustical stimuli and whether they show non-associative learning. We measured embryonic heart rate response using a habituation/dishabituation paradigm with eggs sourced from nests in the wild. Fairy-wren embryos lowered their heart rate in response to the broadcasts of conspecific versus heterospecific calls, and in response to the calls of novel conspecific individuals. Thus, fairy-wrens join humans as vocal-learning species with known prenatal learning and individual discrimination.     

Postnatal effects of incubation length in mallard and pheasant chicks

Eggs of mallard ducks ( Anas platyrhynchos ) and ring-necked pheasants ( Phasianus colchicus ) were incubated in clutches arranged to stimulate embryos to hatch earlier or later than normal. This manipulation of hatching time was achieved by combining eggs of different age in the same clutch. To ensure hatching synchrony, embryos communicate with each other during the last stage of incubation, resulting in either a delay or an acceleration of hatching. Embryos of both species that accelerated their hatching time suffered a higher mortality rate after hatching. Combining mortality with the proportion of hatchlings that suffered from leg deformities, impeding their movements, resulted in a cost also to pheasant chicks delaying their hatching. Chicks of both species accelerating hatching time had a lower minimum mass and a shorter tarsus length than control chicks, whereas chicks delaying hatching time either grew as well or slightly better than control chicks. Mallard chicks had better balance and mobility immediately after hatching the longer they stayed in the egg. This indicates that the period immediately before hatching, is an important period for muscular and organ maturity. Reducing this period results in costs affecting post-hatching survival. The strategy to assure synchronous hatching in mallards and pheasants probably reflect a trade-off between the negative effects of shifting the age at hatching away from normal and differences in predation risk during different stages of reproduction.     

Females that experience threat are better teachers

Superb fairy-wren (Malurus cyaneus) females use an incubation call to teach their embryos a vocal password to solicit parental feeding care after hatching. We previously showed that high call rate by the female was correlated with high call similarity in fairy-wren chicks, but not in cuckoo chicks, and that parent birds more often fed chicks with high call similarity. Hosts should be selected to increase their defence behaviour when the risk of brood parasitism is highest, such as when cuckoos are present in the area. Therefore, we experimentally test whether hosts increase call rate to embryos in the presence of a singing Horsfield''s bronze-cuckoo (Chalcites basalis). Female fairy-wrens increased incubation call rate when we experimentally broadcast cuckoo song near the nest. Embryos had higher call similarity when females had higher incubation call rate. We interpret the findings of increased call rate as increased teaching effort in response to a signal of threat.     

Maternal Vibration: An Important Cue for Embryo Hatching in a Subsocial Shield Bug

Hatching care has been reported for many taxonomic groups, from invertebrates to vertebrates. The sophisticated care that occurs around hatching time is expected to have an adaptive function supporting the feeble young. However, details of the characteristics of the adaptive function of hatching care remain unclear. This study investigated the hatching care of the subsocial shield bug, Parastrachia japonensis (Heteroptera: Parastrachiidae) to verify its function. Results show that the P. japonensis mothers vibrated the egg mass intermittently while maintaining an egg-guarding posture. Then embryos started to emerge from their shells synchronously. Unlike such behaviors of closely related species, this vibrating behavior was faint, but lasted more than 6 h. To investigate the effect of this behavior on hatching synchrony and hatching success, we observed the hatching pattern and the hatching rate in control, mother-removed, and two artificial vibration groups. Control broods experienced continuous guarding from the mother. Intermittent artificial vibration broods were exposed to vibrations that matched the temporal pattern of maternal vibration produced by a motor. They showed synchronous hatching patterns and high hatching rates. However, for mother-removed broods, which were isolated from the mother, and when we provided continuous artificial vibration that did not match the temporal pattern of the maternal vibration, embryo hatching was not only asynchronous: some embryos failed to emerge from their shells. These results lead us to infer that hatching care in P. japonensis has two functions: hatching regulation and hatching assistance. Nevertheless, several points of observational and circumstantial evidence clearly contraindicate hatching assistance. A reduction in the hatching rate might result from dependence on maternal hatching care as a strong cue in P. japonensis. We conclude that the hatching care of P. japonensis regulates the hatching pattern and serves as an important cue to induce embryo hatching.     

Complex Vocal Responses to Conspecific Calls in Whistling Frogs Eleutherodactylus johnstonei: Playback Experiments in the Field

We simulated the presence of an acoustic competitor by broadcasting conspecific playbacks to males of Johnstone's whistling frog, Eleutherodactylus johnstonei, in the field. We broadcast calls that differed in duration (short, typical, and long), dominant frequency (high, typical, and low), and period (short, typical, and long), and analyzed male vocal responses. We tested the hypothesis that males respond by escalating vocally when they are exposed to female‐attractive calls and by ignoring unattractive ones. At the population level, males responded to playbacks in ways that would potentially increase their attractiveness with regard to solo calling: males increased the duration, reduced the dominant frequency, and increased their calling effort (duty cycle), despite an increase in call period. The modification of call duration occurred only in response to playbacks of low‐frequency calls, long calls, and short‐period calls (selective response), while the modification of the dominant frequency was independent of the characteristic of the playback (fixed response). Contrary to the expected, males did not reduce the call period when they were exposed to attractive playbacks. At the ultimate level, the results suggest energy‐saving strategies. In addition, males seem to trade off call period for the avoidance of acoustic interference with attractive calls as calling effort was typically increased by increasing call duration but only rarely by reducing the call period. Interactive playbacks are necessary to better understand the calling strategies of males of E. johnstonei.     

Of Ducklings and Turing Machines: Interactive Playbacks Enhance Subsequent Responsiveness to Conspecific Calls

Distress calls of mallard ducklings consist of a highly stereotyped series of notes. When two ducklings are simultaneously separated from the brood, they characteristically call in alternation. Each bird inhibits its calls while the other is vocalizing, thus preventing masking and facilitating localization by the hen. I examined whether exposure of ducklings to calls presented in a naturalistic alternating pattern affected subsequent responsiveness to them. Individual ducklings were exposed to either 40 s or 80 s of computer-controlled distress call playbacks, using an algorithm that mimicked the behavior of another duckling. Calls were presented only when the subject was silent and ended as soon as it began to vocalize. Each of these ducklings was paired with a ‘yoked’ control. Birds in these control groups experienced exactly the same pattern of playbacks as the ‘interactive’ birds, but the stimuli had no consistent relationship to their own vocal behavior. When the same calls were played back 24 h later, in a fixed pattern that was independent of the ducklings' behavior, birds that had received 80 s of prior interactive exposure were significantly more responsive than both their yoked controls and birds receiving only 40 s of such playbacks. This result suggests that interactive vocal experience, characteristic of natural communication, affects the subsequent perceptual sensitivity of ducklings.     

Adult Richardson's Ground Squirrels (Spermophilus richardsonii) Ignore Rate Changes in Juvenile Alarm Calls: Age‐Differential Response Urgency Perception?

Juvenile Richardson's ground squirrels (RGS; Spermophilus richardsonii) communicate response urgency by modulating the rate of syllable production in repetitive alarm calls, although longer call bouts do not promote more pronounced or longer‐lasting (tonic) vigilance in juvenile call recipients. We exposed free‐living adult RGS to playbacks of alarm calls differing in rate and length to determine whether adult receivers respond to the same alarm parameters as juveniles. Adult squirrels did not respond differentially to differences in call rate or length, suggesting that adult RGS do not attend to call rate as do juveniles. This difference in response may be attributable to a developmental change in the perceptual mechanisms by which individuals extract information regarding response urgency, but could also be a product of adult receivers devaluing information encoded in alarm calls emitted by relatively inexperienced juvenile signalers.     

Distress call alternation in peking ducklings (Anas platyrhynchos)

Peking ducklings (Anas platyrhynchos) were found to alternate their distress calls with those of conspecifics. This alternation response consisted of two components: a strong tendency to suppress vocalizing during the conspecific's calls and a weaker tendency to increase vocalizing (above the level given by birds tested in auditory isolation) following those calls. Similar results were obtained whether pairs of ducklings were tested or whether individual ducklings were tested with tape-recorded calls. Further, visual contact with the conspecific, or a mirror image, although not necessary for eliciting an alternation response, did have a significant facilitative effect on this response. This latter effect may have been due to the general arousal-reducing properties of the visual stimulus. Finally, in contrast to the strong response of ducklings to conspecific distress calls, they displayed a much weaker tendency to alternate their calls with (a) clicking sounds and 7.0-kHz tones with the same temporal patterning and intensity as the taped distress calls, (b) 3.8-kHz tones that additionally matched the dominant frequency of the taped calls, and (c) synthetic distress calls that also approximated the frequency modulation of the taped calls. These results indicate that the distress call alternation response is not simply an artifact of a more general orienting response in which ducklings inhibit their own vocalizations whenever they hear any salient auditory stimulus. Rather, the response appears to be quite specifically triggered by the sound of the sibling calls. The present results stand in contrast to the common assertion that young birds largely ignore the distress calls of siblings. These results are, however, consistent with recent literature emphasizing the importance of sibling interactions in the analysis of mallard early social attachments.     

Does Gray Squirrel (Sciurus carolinensis) Response to Heterospecific Alarm Calls Depend on Familiarity or Acoustic Similarity?

In habitats in which multiple species are prey to the same predators, individuals can greatly benefit from recognizing information regarding predators that is provided by other species. Past studies have demonstrated that various mammals respond to familiar heterospecific alarm calls, but whether acoustic similarity to a familiar call can prompt a mammal's recognition of an unfamiliar call has yet to be shown. We presented alarm calls to free‐ranging eastern gray squirrels (Sciurus carolinensis) and recorded behavioral changes in vigilance and antipredatory response. Playbacks included alarm calls of a sympatric bird (American robin, Turdus migratorius), an allopatric bird with a call structure similar to that of the robin (common blackbird, Turdus merula), and an allopatric bird with a distinct call structure (New Holland honeyeater, Phylidonyris novaehollandiae). Squirrels responded significantly more frequently to squirrel alarm calls (positive control) than to robin song (negative control) or honeyeater calls. Squirrel response to robin and blackbird alarm calls was statistically similar to their response to squirrel alarm calls, indicating that squirrels responded to those alarm calls as if they provided information about the presence of predators. However, squirrel response to robin song was not statistically different from response to any of the other avian calls, including the robin and blackbird alarms, suggesting that squirrels neither respond to blackbird alarms as if they clearly signify danger, nor as if they clearly do not signify danger, perhaps reflecting some ambiguity in interpretation of the calls. These results suggest that squirrel responses to alarm calls are generally based on call familiarity, but that acoustic similarity of an unfamiliar allopatric call to a familiar call also can elicit antipredator behavior. The lack of response to honeyeater alarm calls also supports the hypothesis that call recognition by gray squirrels is dependent on familiarity, rather than simply detection of an acoustic feature common to alarm calls across a variety of avian species.     

Intraclutch Hatch Synchronization in Pheasants and Mallard Ducks

Synchronization of hatching within clutches of precocial bird species can be achieved either by acceleration or retardation, i.e. by shortening or prolonging the incubation period. The ability of mallard ( Anas platyrhynchos ) and ring-necked pheasant ( Phasianus colchicus ) embryos to accelerate or retard hatching was tested by incubating separate clutches, of which three eggs had 2 d longer or shorter incubation time than the others, and observing their individual time of pipping (breaking of the shell). Mallard embryos were able to delay hatching by on average 0.6 d (43% of the eggs delayed at least 1 d), but were better at acceleration (on average 1.3 d; 91% of the eggs accelerated more than 1 d). Conversely, pheasant embryos were only able to accelerate by 0.4 d (50% accelerated more than 1 d), but were better at delaying the hatching (1.2 days; 77% delayed more than 1 d). This difference between the species may depend on different degrees of relatedness within clutches in pheasants and mallards. It may also be an effect of the more developed sensory and neuromuscular systems in galliforms; a reduction of the incubation period would mean that the development of, for example, locomotion would be insufficient at hatching.     

Ontogenetic development of thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of the mallard embryo

Summary Developmental changes of thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of mallard embryos were studied by means of immunocytochemistry (PAP technique). The primary antibody was generated against synthetic TRH. Immunoreactive neurons were first detected in the hypothalamus of 14-day-old embryos. By day 20, increasing numbers of immunoreactive perikarya were observed in the paraventricular nucleus, anterior preoptic region and supraoptic region. Immunoreactive fiber projections were seen in the median eminence as early as embryonic day 20; they occurred also in some extrahypothalamic regions (lateral septum, accumbens nucleus). The number and staining intensity of the cell bodies increased up to hatching, and continued to increase during the first week after hatching.     

Development of spontaneous motility in chick embryos. Normal development and the activating effect of strychnine and picrotoxin.

The longitudinal development of spontaneous motility in chick embryos was studied by Kovach's method (Kovach 1970) from the 10th day of incubation up to hatching, in completely intact eggs. From the 10th to 12th day of incubation, very low amplitude movements of a burst character predominated in spontaneous motility. From the 13th day, both low and high amplitude movements could be distinguished. From the 18th day, high amplitude movements alternating with intervals of motor inactivity preponderated. This discontinuous motility, which was most pronounced on the 20th day of incubation, changed to periodic strong hatching movements. Reduction of spontaneous motility after the 17th day of incubation was not confirmed. Strychnine already activated spontaneous motility in 11-day embryos, but typical convulsions did not appear until the 15th incubation day. With picrotoxin, motility was likewise stimulated in 11-day embryos and paroxysmal activation did not occur until the 15th incubation day. In older embryos, convulsions were gradually succeeded by a continuous increase in spontaneous motility. The effect of picrotoxin had a much longer latent period than the effect of strychnine.     

Carryover effects of predation risk on postembryonic life-history stages in a freshwater shrimp

For organisms with complex life histories it is well known that risk experienced early in life, as embryos or larvae, may have effects throughout the life cycle. Although carryover effects have been well documented in invertebrates with different levels of parental care, there are few examples of predator-induced responses in externally brooded embryos. Here, we studied the effects of nonlethal predation risk throughout the embryonic development of newly spawned eggs carried by female shrimp on the timing of egg hatching, hatchling morphology, larval development and juvenile morphology. We also determined maternal body mass at the end of the embryonic period. Exposure to predation risk cues during embryonic development led to larger larvae which also had longer rostra but reached the juvenile stage sooner, at a smaller size and with shorter rostra. There was no difference in hatching timing, but changes in larval morphology and developmental timing showed that the embryos had perceived waterborne substances indicative of predation risk. In addition to carryover effects on larval and juvenile stages, predation threat provoked a decrease of body mass in mothers exposed to predator cues while brooding. Our results suggest that risk-exposed embryos were able to recognize the same infochemicals as their mothers, manifesting a response in the free-living larval stage. Thus, future studies assessing anti-predator phenotypes should include embryonic development, which seems to determine the morphology and developmental time of subsequent life-history stages according to perceived environmental conditions.     

The effect of light and darkness on hatching in the pomacentrid Abudefduf saxatilis

Synopsis Reports that pomacentrid embryos hatch after dusk are confirmed by photic manipulation of sergeant major eggs. Embryos placed in the dark for 20 minutes or longer prior to their normal hatching after sunset hatched, whereas controls held in light did not hatch. Percent of hatched embryos correlated with increasing exposure to darkness up to one hour after which no further improvement in hatching was observed. Embryos maintained in continuous light during their normal twilight hatching period did not hatch. Also, embryos exposed to 60 minutes of darkness, if interrupted by one minute of light every 10 minutes did not hatch. The percent hatch in dark treatments varied significantly between nests and, in some treatments, correlated negatively with the size of the egg clumps (number of eggs per clump) tested. To initiate hatching in the presence of light required intensities of 0.03 lux or less. These low intensities are not reached until about 20 minutes after sunset on the reef where the embryos occur. We conclude that hatching for some embryos occurs about 30 minutes after sunset but for most is not completed until at least one hour after sunset. Hatching therefore takes place at a time long after potential diurnal fish predators have refuged in the reef structure.