You are what you eat: diet-induced chemical crypsis in a coral-feeding reef fish

The vast majority of research into the mechanisms of camouflage has focused on forms that confound visual perception. However, many organisms primarily interact with their surroundings using chemosensory systems and may have evolved mechanisms to ‘blend in’ with chemical components of their habitat. One potential mechanism is ‘chemical crypsis'' via the sequestration of dietary elements, causing a consumer''s odour to chemically match that of its prey. Here, we test the potential for chemical crypsis in the coral-feeding filefish, Oxymonacanthus longirostris, by examining olfactory discrimination in obligate coral-dwelling crabs and a predatory cod. The crabs, which inhabit the corals consumed by O. longirostris, were used as a bioassay to determine the effect of coral diet on fish odour. Crabs preferred the odour of filefish fed their preferred coral over the odour of filefish fed a non-preferred coral, suggesting coral-specific dietary elements that influence odour are sequestered. Crabs also exhibited a similar preference for the odour of filefish fed their preferred coral and odour directly from that coral, suggesting a close chemical match. In behavioural trials, predatory cod were less attracted to filefish odour when presented alongside the coral it had been fed on, suggesting diet can reduce detectability. This is, we believe, the first evidence of diet-induced chemical crypsis in a vertebrate.     

Behavioural and electrophysiological responses of the phlebotomine sandfly Lutzomyia longipalpis (Diptera: Psychodidae) when exposed to canid host odour kairomones

Compounds from the odour-producing glands of the fox Vulpes vulpes were collected. This complex mixture of compounds was used to stimulate the ‘ascoid’ olfactory organs of female sandflies in single sensillum and gas chromatography-linked single sensillum recordings. Sixteen of these compounds were identified using gas chromatography-linked mass spectrometry and amounts present were determined. The compounds fell into four organic classes: ketones, carboxylic acids, alcohols and aldehydes. Specific neurones present in the ascoid sensillum that responded to each of these classes of compound were characterized. A bioassay chamber was developed that gave female sandflies the choice of two odour sources. Female sandflies were attracted upwind by fox odour and were trapped in closer proximity to the fox odour port than the control port. Synthetic compounds were recombined in appropriate quantities to mimic the fox odour. In this bioassay, the synthetic blend attracted sandflies upwind, and again they were caught closer to the test port than the control port. Furthermore, the synthetic fox odour induced an electrophysiological response from neurones in the ascoid sensillum that was very similar to that induced by natural fox odour. No synthetic compound alone induced the same behavioural response from sandflies as did whole fox odour. However, benzaldehyde, 4-hydroxy-4-methyl-2-pentanone and 4-methyl-2-pentanone alone did cause sandflies to fly upwind and to be caught closer to the test port than the control.     

A six-arm olfactometer permitting simultaneous observation of insect attraction and odour trapping

Abstract.  Behavioural assays to study insect attraction to specific odours are tedious, time consuming and often require large numbers of replications. Olfactometer and flight tunnel tests can usually only be conducted with one or two odour sources at a time. Moreover, chemical information on the odour sources has to be obtained in separate analytical studies. An olfactometer was developed in which six odours can be tested simultaneously for their relative attractiveness while during the assays, part of each test odour can be trapped for further analyses. The effectiveness of this six-arm olfactometer was tested by observing the responses of the solitary endoparasitoid Cotesia marginiventris (Cresson) to host-induced odours from young maize plants. For statistical analyses, we used log-linear models were adapted to account for overdispersion and possible positional biases. Female wasps responded extremely well in tests where they were offered a single odour source, as well as in tests with multiple choices. The responses of wasps released in groups were the same as those released individually and it was found that females did not attract or repel each other, but males preferred arms in which females had been released. Dose–response tests with varying numbers of plants or host larvae on plants revealed that the wasps responded in a dose-related manner, thus showing that the system is well suited to measure relative preference. The clear choices of the insects amongst six possibilities provided substantial statistical power. Gas chromatographic analyses of sampled air revealed clean and effective odour trapping, which largely facilitates the comparison of results from behavioural assays with the actual blends of volatiles that were emitted by the various odour sources. Advantages and disadvantages compared to other methods are discussed.     

Evidence of a rheotactic component in the odour search behaviour of freshwater eels

The detection of food odour by the freshwater eels, Anguilla australis and Anguilla dieffenbachii released a behavioural response to flow that resulted in direct upstream movement toward the odour source. Changes in various orientation parameters were observed as eels neared the source. Eels had substantially lower swimming velocities and considerably more variable heading and course angles close to the odour source (≤0·9 m) than further away (>0·9 m). Observed changes in orientation parameters were primarily due to changes in the behaviour of a searcher following odour loss. Cross-stream movements were initiated when the eel moved beyond the lateral margins of the odour plume. The behavioural switch from odour-conditioned rheotaxis to cross-stream casting following odour loss occurred more frequently close to the odour source where the plume was most narrow. Odour-conditioned rheotaxis enables the searcher to move quickly and efficiently toward the odour source without the need to extract directional information from a highly intermittent and complex chemical signal.     

Could maternal testosterone levels govern mammalian sex ratio deviations?

Although maternal dominance and good condition are frequently associated with raised offspring sex ratios in mammals, the key factor may be female testosterone, which not only underpins the behavioural indicators but could also provide a pathway to a possible proximate mechanism for sex determination. By taking into account the fact that female testosterone levels rise in response to environmental stressors, it is possible to re-interpret the findings of atypical sex ratios in mammals in a way that reconciles seemingly conflicting results and reveals instead what could be a coherent, adaptive system of sex allocation in mammals.     

A tangerine-scented social odour in a monogamous seabird

Social odours, conspecific chemical signals, have been demonstrated in every class of vertebrate except birds. The apparent absence is surprising, as every bird examined has a functional olfactory system and many produce odours. The crested auklet (Aethia cristatella), a monogamous seabird, exhibits a distinctive tangerine-like scent closely associated with courtship. Using T-maze experiments, we tested whether auklets preferred conspecific odours and whether they distinguished between different types of scent, two prerequisites of chemical communication. Crested auklets exhibited: (i) an attraction to conspecific feather odour; (ii) a preference for two chemical components of feather scent (cis-4-decenal and octanal), which we identified as seasonally elevated; and (iii) differential responses to odours, as indicated by a preference for auklet odour, an aversion to mammalian musk, but no significant response to banana essence (amyl acetate). Our results suggest that crested auklets detect plumage odour and preferentially orientate towards this stimulus. The striking and well-described courtship display that involves the scented neck region, the 'ruff sniff', provides a conspicuous behavioural mechanism for odour transmission and the potential for scent assessment during sexual selection. Although the importance and full social function of chemical signals are just beginning to be understood in birds, including crested auklets, social odours promise to reveal a largely unexplored and possibly widespread means of avian communication.     

Are olfactory cues involved in nest recognition in two social species of estrildid finches?

Reliably recognizing their own nest provides parents with a necessary skill to invest time and resources efficiently in raising their offspring and thereby maximising their own reproductive success. Studies investigating nest recognition in adult birds have focused mainly on visual cues of the nest or the nest site and acoustic cues of the nestlings. To determine whether adult songbirds also use olfaction for nest recognition, we investigated the use of olfactory nest cues for two estrildid finch species, zebra finches (Taeniopygia guttata) and Bengalese finches (Lonchura striata var. domestica) during the nestling and fledgling phase of their offspring. We found similar behavioural responses to nest odours in both songbird species. Females preferred the odour of their own nest over a control and avoided the foreign conspecific nest scent over a control during the nestling phase of their offspring, but when given the own odour and the foreign conspecific odour simultaneously we did not find a preference for the own nest odour. Males of both species did not show any preferences at all. The behavioural reaction to any nest odour decreased after fledging of the offspring. Our results show that only females show a behavioural response to olfactory nest cues, indicating that the use of olfactory cues for nest recognition seems to be sex-specific and dependent on the developmental stage of the offspring. Although estrildid finches are known to use visual and acoustic cues for nest recognition, the similar behavioural pattern of both species indicates that at least females gain additional information by olfactory nest cues during the nestling phase of their offspring. Thus olfactory cues might be important in general, even in situations in which visual and acoustic cues are known to be sufficient.     

Hygroreception in olfactometer studies

Abstract In olfactometric studies, a technique used to measure insect behavioural bias towards airborne chemicals, many of the factors that could affect the bioassay outcome other than the volatiles themselves are carefully controlled (e.g. lighting conditions, temperature, spatial position, physiological state of the insects, time of day when the bioassay is performed) but one factor, namely humidity, is overlooked frequently when studying responses to plant volatiles. Many insect species respond positively and negatively to changes in humidity and, during bioassays, differences arising from odour choices with different water vapour release rates might have confounding effects versus the original intent of the behavioural study. The present study aims to assess the effect of different substrates on the water vapour content of air in an olfactometer‐like set‐up. The results show that airflow humidity is affected by the odour source used in the olfactometer air flow. In addition, the response of the aphid parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiidae) is studied towards different humidities and plant material in an olfactometer. The present study suggests that humidity needs to be controlled in olfactometric studies.     

The response to the social environment reveals sex-dependent behavioural syndromes in the Bosca’s newt (<Emphasis Type="Italic">Lissotriton boscai</Emphasis>)

There is growing evidence of correlated behaviours across time, situations and/or contexts (behavioural syndromes) in animals, and their link with fitness makes these studies mandatory to understanding the species’ behavioural and evolutionary ecology. Whereas the role of the social environment on behavioural syndromes is receiving increasing attention, experimental studies testing whether environmental fluctuations govern sex-dependent behavioural syndromes are scarce. I performed an experiment to test for the existence of sex differences in activity syndromes through a changing social environment. Males and females of Bosca’s newt (Lissotriton boscai) were faced with three social situations perceived through chemical cues (own odour, no odour and same-sex conspecific odours) to measure their activity levels. Comparisons of the activity levels showed that both males and females discriminated the odourless stimuli from newt odour (either their own or the conspecific stimuli). Whereas activity levels were positively correlated between their own and the odourless stimuli in both sexes, the association between the odourless and conspecific stimuli was positive in males but decoupled in females. This is the first experimental evidence of sex differences in activity syndromes of wild-caught animals in response to social changes.     

Anti‐predator response of naïve and experienced common bully to chemical alarm cues

Histological analysis of the skin of common bully Gobiomorphus cotidianus , a New Zealand native eleotrid fish, revealed the presence of club cells in the epidermis. Epidermal club cells are frequently associated with the production of alarm substance (Schreckstoff). The behavioural responses of perch‐naïve and perch‐experienced common bullies to either conspecific skin extract or chemical cues from an introduced predator, perch Perca fluviatilis , were then examined. Both perch‐naïve and perch‐experienced common bullies exhibited a behavioural response when exposed to conspecific skin extract, indicating the probable presence of an alarm substance. In contrast, only perch‐experienced common bullies recognized and exhibited a subsequent behavioural response to the odour of perch. This study is the first to document the presence of epidermal club cells and a behavioural response to a conspecific chemical alarm signal for fishes in the Eleotridae. The results indicate that common bully can learn to recognize perch odour as a threat, and that this ability may be a result of previous predator labelling involving a conspecific alarm substance.     

Bioassay approaches to observing behavioural responses of adult female Cydia molesta to host plant odour

Abstract:  Three different olfactometers were evaluated in order to develop a bioassay procedure testing for the olfactory responses of Cydia molesta . Females were tested individually using a linear and a Y-tube olfactometer, and in groups using a dual-choice arena. Room temperature, relative humidity, light intensity, airflow, age of the moths and odour source were kept constant during experiments. The odour source tested was a green shoot of peach excised 10 min before experiments started. Cydia molesta females showed a significant response to this plant odour in all olfactometers. A number of qualitative aspects were found to be in favour of the dual choice arena as a tool for screening potentially attractive odour sources. It allowed for a differentiation of the response of mated and virgin females. Experimental conditions allowed the circadian rhythm of insects to be mimicked. Manipulation of individuals is reduced and flight is not precluded.     

The effect of lactic acid on odour-related host preference of yellow fever mosquitoes.

In a behavioural study we have investigated the role of lactic acid for the host preferences of yellow fever mosquitoes (Aedes aegypti) by comparing the attractiveness of rubbings from the hands of different human individuals and extracts obtained from skin rubbings from different mammals (Bos primigenius f. taurus, Capra aegagrus f. hircus, Felis silvestris f. catus and Homo sapiens). Certain human individuals were consistently more attractive to mosquitoes than others. Addition of lactic acid markedly increased the degree of attractiveness of formerly less attractive human odour samples and they were preferred over those which were originally the most attractive. There was almost no response to animal odour samples. In contrast to human samples, which contain a high amount of lactate, this compound could not be detected in samples from animals. When skin emanations from animals were combined with lactic acid, however, as many mosquitoes responded to odour samples of B. primigenius f. taurus and C. aegagrus f. hircus as did to human odours. All these data demonstrate that olfactory-based host preference of the anthropophilic mosquito A. aegypti is to a large extent due to differences in the amount of lactic acid in the odour samples.     

Behaviour of juvenile mud crabs Scylla serrata in aquaculture: Response to odours of moulting or injured crabs

Behaviour of juvenile mud crabs, Scylla serrata (70–90 mm carapace width, CW) were observed in response to odours of moulting and injured conspecifics and food (pilchard) under controlled flow conditions using bioassay technique. This study was undertaken to better understand the role that chemical cues have in mediating the attraction of cannibal crabs to moulting crabs in an aquaculture growout facility and thus aid in more successful production. In response to moult odour juvenile S. serrata spent 5.6 ± 1.9% of the time in locomotion but this did not differ significantly (P > 0.05) from that of control odours (seawater, 2.6 ± 1.2%), however a tactile response was observed for moult odour not seen in controls. Juveniles exposed to the odour of injured conspecifics (12.2 ± 2.3% of time in locomotion) and to food (22.7 ± 3.1%) differed significantly from the seawater control (P < 0.05). The possibility that the active agent in moult water is relatively dilute was considered, however varying concentrations of food and crushed conspecific odour failed to demonstrate concentration dependant behavioural results. Variation in the form of size (carapace width) and sex was revealed, so too was preliminary evidence of behavioural differences between crab hatchings. For example, larger crabs (around 80 mm CW) increased tactile investigation of the odour inlet pipe in response to the crab-based odours of moulting (3.5 ± 1.7% of the time) and injured conspecifics (2.6 ± 1.0%), though response to food remained constant over all sizes (10.0% of the time). In another experiment, larger female crabs maintained a high frequency of tactile response to the odour of injured conspecifics. In males, the response was attenuated in larger individuals. Despite no differences being found between moult odour and controls some evidence exists to suggest that a small proportion of crabs were responding to the moult odour and that this still has the potential to cause a dramatic cumulative reduction in growout survival. Variation in size and subsequent behaviour of individuals poses the question of whether behaviour contributes to growth rate and might be a focus of genetic selection for more uniform growth.     

Chemical kin label in seabirds

Chemical signals yield critical socio-ecological information in many animals, such as species, identity, social status or sex, but have been poorly investigated in birds. Recent results showed that chemical signals are used to recognize their nest and partner by some petrel seabirds whose olfactory anatomy is well developed and which possess a life-history propitious to olfactory-mediated behaviours. Here, we investigate whether blue petrels (Halobaena caerulea) produce some chemical labels potentially involved in kin recognition and inbreeding avoidance. To overcome methodological constraints of chemical analysis and field behavioural experiments, we used an indirect behavioural approach, based on mice olfactory abilities in discriminating odours. We showed that mice (i) can detect odour differences between individual petrels, (ii) perceive a high odour similarity between a chick and its parents, and (iii) perceive this similarity only before fledging but not during the nestling developmental stage. Our results confirm the existence of an individual olfactory signature in blue petrels and show for the first time, to our knowledge, that birds may exhibit an olfactory kin label, which may have strong implications for inbreeding avoidance.     

Dynamic representation of odours by oscillating neural assemblies

Stimulus evoked oscillatory synchronization of neural assemblies has been most clearly documented in the olfactory and visual systems. Recent results with the olfactory system of locusts show that information about odour identity is contained in spatial and temporal aspects of an oscillatory population response. This suggests that brain oscillations may reflect a common reference for messages encoded in time. Although stimulus-evoked oscillatory phenomena are reliable, their roles in perception, memory and pattern recognition remain to be demonstrated. Using honey bees, we demonstrated that odour encoding involves, as in locusts, the oscillatory synchronization of assemblies of neurons, and that this synchronization is, here also, selectively abolished by the GABA receptor antagonist picrotoxin. In collaboration with Dr Brian Smith's laboratory, we showed, using a behavioural learning paradigm, that picrotoxin-induced desynchronization impairs the discrimination of molecularly similar odourants, but not that of dissimilar odours. It appears, therefore, that oscillatory synchronization of neuronal assemblies is relevant, and essential for fine odour discrimination. Finally, experiments with locust mushroom body neurons, two synapses downstream from the antennal lobe, indicate that their responses to odours become less specific when antennal lobe neurons are desynchronized by picrotoxin injection. These results suggest that oscillatory synchronization and the kind of temporal encoding it affords provide an additional dimension by which the brain can segment spatially overlapping stimulus representations.     

Behavioural development of conspecific odour preferences in bank voles,Clethrionomys glareolus

Biological odours of conspecifics are known to have strong influences on behavioural interaction in bank voles Clethrionomys glareolus. This experiment tested two hypotheses. (1) Olfactory cues from familiar and unfamiliar mature opposite-sex conspecifics differ in their attractiveness to males and females, and their behavioural reactions change with age. (2) A genetically based mechanism is involved in female recognition of kin.In a two-choice preference test, prepubertal males and females were more attracted to familiar than to unfamiliar odours of opposite-sex conspecifics, as manifested by more time spent sniffing familiar voles. As the young reached sexual maturity they shifted their odour preferences. Mature males and females preferred the novel odour of unrelated opposite-sex conspecifics to that of relatives. The results of experiments testing the second hypothesis indicate that females use a genetically based mechanism to recognise their kin. Young and mature females were able to recognise the odour of their biological but socially unknown fathers, and showed the same pattern of behaviour as females in previous experiments.The possible biological functions of kin recognition in bank voles are discussed.     

Behavioural responses of sablefish, Anoplopoma fimbria, to bait odour

A behavioural bioassay was used to determine the response threshold to squid extract of sablefish, Anoplopoma fimbria, held at three different feeding regimens. Sablefish responded to the odour of bait by changing swimming activity and turning behaviour. The response threshold to bait odour was influenced by both the amount of food eaten and the duration of food deprivation. The total concentration of amino acids in the bait extract was assumed to determine the response threshold as chemical fractionation studies have shown that this class of compounds is essential for the stimulatory capacities of food extracts. When fed to satiation (9.4% wet body weight) and tested after one day of food deprivation, the mean response threshold to total dissolved free amino acids was 4.4 × 10^?8m (range=7.6 × 10^?8 to 3.6 × 10^?8m ). When fed at 1.6-2.3% wet body weight, the threshold sensitivity had increased to a mean value of 1.8 × 10^?10m (range=8.4 × 10^?10 to 7.0 × 10^?11m ) after one day of food deprivation; after four days of deprivation, the sensitivity had increased even further to a mean value of 1.4 × 10^?11m (range=1.6 × 10^?10 to 1.4 × 10^?12m ). It was also apparent that the intensity of behavioural responses to the bait odour increased with both stimulus concentration and duration of food deprivation. These results suggest that sablefish intensify their search for prey under increased feeding motivation. The active space of a bait source was estimated from the threshold values obtained. Depending on state of food deprivation, rate of chemical release from the bait and the current velocity, maximum lengths of active space within which sablefish would exhibit food searching responses vary from 10 m to several km. Stock assessment based on catch data from baited gear will need techniques that take into account those factors influencing active space for food searching.     

Avoidance of potentially harmful food cannot be socially transmitted between rats

The social transmission of food preferences(STFP) is a behavioural task of olfactory memory, in which an observer rat learns safe food odours from a demonstrator rat, and shows preference for this odour in a subsequent choice test. However, previous studies have failed to detect the transmission of information about food of potential danger and food aversion using STFP test. In this study, we tested how demonstrators' health affects the exchange of odour information and whether observers can learn danger information from an unhealthy demonstrator. As expected, the observer rat formed an odour preference after interacting with a demonstrator rat that had just eaten food containing a new odour, however, odour preference rather than aversion was also formed after interacting with a demonstrator rat injected with LiCl(used to induce gastric malaise). Furthermore, anaesthetized demonstrator rats and half-anaesthetized demonstrator rats, which showed obvious motor deficits suggesting an unhealthy state, also socially transmitted food preferences to observers. These results suggest that the social transmission of food preferences task is independent of a demonstrators' health, and that information about dangerous foods cannot be transmitted using this behavioural task.     

Detection of female mating status using chemical signals and cues

Males of many species choose their mate according to the female's reproductive status, and there is now increasing evidence that male fitness can depend on this discrimination. However, females will also aim to regulate their mating activity so as to maximize their own fitness. As such, both sexes may attempt to dictate the frequency and timing of female mating, reflecting the potentially different costs of female signaling to both sexes. Here, I review evidence that chemical cues and signals are used widely by males to discriminate between mated and unmated females, and explore the mechanisms by which female odour changes post‐mating. There is substantial empirical evidence that mated and unmated females differ in their chemical profile, and that this variation provides males with information on a female's mating status. Although there appears to be large variation among species regarding the mechanisms by which female odour is altered post‐mating, the transfer of male substances to females during or subsequent to copulation appear to play a major role. This transfer of substances by males may be part of their strategy to suppress reproduction by competing males, particularly in species where females mate more than once.     

Aim‐then‐shoot anemotaxis involved in the hopping approach of potato tuberworm moth Phthorimaea operculella toward a sex pheromone source

Male moths locate conspecific females by pheromone‐induced upwind flight maintained by detecting a visual flow, termed optomotor anemotaxis. Their behavioural pattern is characterized by an upwind surge in response to a pheromone stimulus and crosswind casting after odour loss, which is considered to be reset and restarted on receipt of another pheromone pulse. However, pheromone‐stimulated males of the potato tuberworm moth Phthorimaea operculella exhibit a series of short and straight intermittent flights, or hops, when moving upwind. It is unclear whether they navigate by employing the same behavioural pattern and wind detection mechanism as that used by flying moths. To analyze odour‐modulated anemotaxis in male potato tuberworm moths, a flat wind tunnel is constructed to give regular odour stimuli to an insect regardless of its location. Moths are subjected to pheromone pulses of different frequencies to test whether they show a behavioural pattern that is reset and restarted by a pheromone pulse. Moths on the ground are also subjected to crosswind shear to examine their detection of wind direction. Path analyses reveal that males surge upwind when they receive a pheromone pulse and exhibit casting by successive hops when they lose odour. This behavioural pattern appears to be similar to that of flying moths. When the direction of the airflow is switched orthogonally, males adjust their course angle accordingly when they are on the ground. It is suggested that, instead of optomotor anemotaxis, this ‘aim‐then‐shoot’ system aids the detection of wind direction, possibly by mechanosensory means.