Chemotactic bacteria are guided towards the source of a nutrient by local concentration gradients. That works on the microscopic scale, but at larger scales such local cues are unreliable pointers — for example, wind or water currents may disperse odours sought by foraging animals. Using statistical techniques, Vergassola et al. have developed a general search algorithm for movement strategies based on the detection of sporadic cues and partial information. The strategy, termed 'infotaxis' as it maximizes the expected rate of information gain, could find application in the design of 'sniffer' robots. A computational model of odour plume propagation and experimental data are used to devise a general search algorithm for movement strategies in chemotaxis, based on sporadic cues and partial information. The strategy is termed 'infotaxis' as it locally maximizes the expected rate of information gain. Chemotactic bacteria rely on local concentration gradients to guide them towards the source of a nutrient1. Such local cues pointing towards the location of the source are not always available at macroscopic scales because mixing in a flowing medium breaks up regions of high concentration into random and disconnected patches. Thus, animals sensing odours in air or water detect them only intermittently as patches sweep by on the wind or currents2,3,4,5,6. A macroscopic searcher must devise a strategy of movement based on sporadic cues and partial information. Here we propose a search algorithm, which we call ‘infotaxis’, designed to work under such conditions. Any search process can be thought of as acquisition of information on source location; for infotaxis, information plays a role similar to concentration in chemotaxis. The infotaxis strategy locally maximizes the expected rate of information gain. We demonstrate its efficiency using a computational model of odour plume propagation and experimental data on mixing flows7. Infotactic trajectories feature ‘zigzagging’ and ‘casting’ paths similar to those observed in the flight of moths8. The proposed search algorithm is relevant to the design of olfactory robots9,10,11, but the general idea of infotaxis can be applied more broadly in the context of searching with sparse information.

"Infotaxis" as a strategy for searching without gradients

Chemical signals mediate many of life's processes. For organisms that use these signals to orient and navigate in their environment, where and when these cues are encountered is crucial in determining behavioral responses. In air and water, fluid mechanics impinge directly upon the distribution of odorous molecules in time and space. Animals frequently employ behavioral mechanisms that allow them to take advantage of both chemical and fluid dynamic information in order to move toward the source. In turbulent plumes, where odor is patchily distributed, animals are exposed to a highly intermittent signal. The most detailed studies that have attempted to measure fluid dynamic conditions, odor plume structure, and resultant orientation behavior have involved moths, crabs, and lobsters. The behavioral mechanisms employed by these organisms are different but generally integrate some form of chemically modulated orientation (chemotaxis) with a visual or mechanical assessment of flow conditions in order to steer up-current or upwind (rheo- or anemo-taxis, respectively). Across-stream turns are another conspicuous feature of odor-modulated tracks of a variety of organisms in different fluid conditions. In some cases, turning is initiated by detection of the lateral edges of a well-defined plume (crabs), whereas in other animals turning appears to be steered according to an internally generated program modulated by odor contacts (moth counterturning). Other organisms such as birds and fish may use similar mechanisms, but the experimental data for these organisms is not yet as convincing. The behavioral strategies employed by a variety of animals result in orientation responses that are appropriate for the dispersed, intermittent plumes dictated by the fluid-mechanical conditions in the environments that these different macroscopic organisms inhabit.

Mechanisms of animal navigation in odor plumes

Insects locate many resources important to survival by tracking along wind-borne odor plumes to their source. It is well known that plumes are patchy distributions of high concentration packets of odor interspersed with clean air, not smooth Gaussian distributions of odor intensity. This realization has been crucial to our understanding of plume-tracking behavior, because insect locomotory movements and sensory processing typically take place in the range of tens to hundreds of milliseconds, permitting them to respond to the rapid changes in odor concentration they experience in plumes. Because odor plumes are not comprised of smooth concentration gradients, they cannot provide the directional information necessary to allow plume-tracking insects to steer toward the source. Many experiments have shown that, in the species examined, successful source location requires two sensory inputs: the presence of the attractive odor and the detection of the direction of the wind bearing that odor. All plume-tracking insects use the wind direction as the primary directional cue that enables them to steer their movements toward the odor source. Experimental manipulations of the presence and absence of the odor, and the presence, absence, or direction of the wind during plume tracking, have begun to resolve the relationship between these two sensory inputs and how they shape the maneuvers we observe. Experiments, especially those undertaken in the natural wind and odor environments of the organisms in question and those directed at understanding the neural processing that underlie plume tracking, promise to enhance our understanding of this remarkable behavior.

Navigational Strategies Used by Insects to Find Distant, Wind-Borne Sources of Odor

Robotic odor localization has become a prominent research area in recent years. It promises many valuable practical applications, and contributes to the knowledge of biological odor localization, which has in many cases been the source of inspiration. There have been a diversity of approaches, implemented in both simulated and practical experiments, with a wide variety of platforms, and in a number of environments. This article presents a survey of the existing methods, which have been organized into taxonomic classifications. This provides a framework in which to evaluate the methods, view how they relate to each other, and make qualitative comparisons. The methods are grouped at the highest level by environmental conditions, and then by the localization method which in most cases is closely associated with the type of sensors used.

Robot Odor Localization: A Taxonomy and Survey

SUMMARY Female mosquitoes are noted for their ability to use odours to locate a
host for a blood meal. Two sensory organs contribute to their sense of smell:
the maxillary palps, which measure the level of CO 2 , and the
antennae, which detect other host-released odours. To establish the relative
importance and interactions of CO 2 and other body emissions in
freely flying mosquitoes, we presented female yellow fever mosquitoes
 Aedes aegypti L. with broad plumes of human skin odour and
CO 2 at natural concentrations and dilutions thereof in a wind
tunnel. 3-D video-recorded flight tracks were reconstructed. Activation,
flight velocity, upwind turning and source finding waned quickly as skin
odours were diluted, whereas in the presence of CO 2 these
parameters remained unchanged over more than a 100-fold dilution from exhaled
concentrations. Although mosquitoes were behaviourally less sensitive to skin
odours than to CO 2 , their sensitivity to skin odours increased
transiently by at least fivefold immediately following a brief encounter with
a filament of CO 2 . This sensitization was reflected in flight
velocity, track angle, turning rate upon entering and exiting the broad odour
plume and, ultimately, in the source-finding rate. In Ae. aegypti ,
CO 2 thus functions as a `releaser9 for a higher sensitivity and
responsiveness to skin odours. The initially low responsiveness of mosquitoes
to skin odours, their high sensitivity to CO 2 , and the
sensitization of the olfactory circuitry by CO 2 are ecologically
relevant, because rapidly fluctuating CO 2 levels reliably signal a
potential host. Possible mechanisms of the instantaneous sensitization are
considered.

/pdf/carbon-dioxide-instantly-sensitizes-female-yellow-fever-1cagxjgxzj.pdf

Carbon dioxide instantly sensitizes female yellow fever mosquitoes to human skin odours.

This article presents the theoretical motivation, implementation approach, and example validation results for a computationally efficient plume simulation model, designed to replicate both the short-term time signature and long-term exposure statistics of a chemical plume evolving in a turbulent flow. Within the resulting plume, the odor concentration is intermittent with rapidly changing spatial gradient. The model includes a wind field defined over the region of interest that is continuous, but which varies with location and time in both magnitude and direction. The plume shape takes a time varying sinuous form that is determined by the integrated effect of the wind field. Simulated and field data are compared. The motivation for the development of such a simulation model was the desire to evaluate various strategies for tracing odor plumes to their source, under identical conditions. The performance of such strategies depends in part on the instantaneous response of target receptors; therefore, the sequence of events is of considerable consequence and individual exemplar plume realizations are required. Due to the high number of required simulations, computational efficiency was critically important.

Filament-based atmospheric dispersion model to achieve short time-scale structure of odor plumes

Wind-borne odour stimuli from a small point-source of pheromone are intermittent owing to the effects of atmospheric turbulence on the odour plume. The work reported here measures the characteristics of the intermittent stimulus in open fields and forests during typical daytime atmospheric conditions. To model the pheromone plume, negatively charged atmospheric ions were used as a tracer. They were released from an ion generator, and ion detectors measured the fluctuating flux of ions at positions up to 20 m downwind in the open field case and 10 m in the forest. In both the open field and in the forest, ion signals were highly intermittent, with a signal present only 20% of the time. Ion signals recorded in the forest consisted of bursts with gaps between them of at least three-fold greater duration than those from the open field. In both environments, bursts generally each comprised a series of 'spikers', on average three in the field and seven in the forest. To validate the use of ionized air plumes as models of pheromone plumes, the antennae of male Lymantria dispar (gypsy moth) were used as detectors to quantify the plume of synthetic (+)-disparlure emanating from a 2000 ng point source placed approximately equal to 10 cm from the ion source. A comparison of ion signals and EAGs (electronantennograms) suggests that the antennae respond to the main spikes within a burst, but no consistent relationship between the strength of the spikes and the magnitude of the EAG response was found. The average strengths of bursts in the ion detector signal decreased systematically as the distance from the ion generator to the ion detector increased. A similar trend, however, was not detected in the EAG response.

Spatial and temporal structures of pheromone plumes in fields and forests

The patterns of stimulus available to moths flying along pheromone plumes in a 3-m-long wind tunnel were characterized using a high frequency photoionization detector in conjunction with an inert tracer gas. Four contrasting flow regimes and source conditions were produced: odor released in pulses from a vertical and horizontal array of four sources, odor released continuously from a point source, and odor released continuously from a point source into an oscillatory wake. Although the four flow regimes produced plumes of intermittent and fluctuating concentration, there were considerable differences in the structure of the signal presented to the sensor. Pulses of tracer gas released at 10 Hz retained most of their longitudinal and lateral separation. The plume growing in the disturbed flow (`oscillatory'), was broader in its lateral extent than the plume growing in an undisturbed flow (`continuous'), and the concentrations in the former were lower at each downstream position. The signal recorded in the disturbed flow had higher intermittency, but the ratio between the peak concentration and the signal mean was lower than in the continuous plume. Time scales were typically longer in the tunnel than in a field setting, but length scales and the main features of intermittency and fluctuation were similar. Moths flying along plumes of pheromone in this and similar wind tunnels typically slow their velocity and narrow the lateral excursions of their flight track as they approach a pheromone source. Which features of the plumes measured in this study account for these behavioral reactions remains to be determined.

Measurement of Odor-Plume Structure in a Wind Tunnel Using a Photoionization Detector and a Tracer Gas

The pheromone-modulated upwind flight ofLymantria dispar males responding to different pheromone plume structures and visual stimuli designed to mimic trees was video recorded in a forest. Males flying upwind along pheromone plumes of similar structure generated tracks that were similar in appearance and quantitatively similar in almost all parameters measured, regardless of the experimentally manipulated visual stimuli associated with the pheromone source. Net velocities, ground speeds, and airspeeds of males flying in point-source plumes were slower than those of males flying in the wider, more diffuse plumes issuing from a cylindrical baffle. The mean track angle of males flying in plumes issuing from a point source was greater (oriented more across the wind) than that of males flying in plumes issuing from a transparent cylindrical baffle. Males flying in point-source plumes also turned more frequently and had narrower tracks overall than males responding to plumes from a cylindrical baffle. These data suggest thatL. dispar males orienting to pheromone sources (i.e., calling females) associated with visible vertical cylinders (i.e., trees) use predominantly olfactory cues to locate the source and that the structure of the pheromone plume markedly affects the flight orientation and the resultant track.

Effects of Pheromone Plume Structure and Visual Stimuli on the Pheromone-Modulated Upwind Flight of Male Gypsy Moths (Lymantria dispar) in a Forest (Lepidoptera: Lymantriidae)

Lymantria dispar L. males flying upwind in a pheromone plume in a forest were video-recorded at 2.5, 10 and 20 m from the source of pheromone. Males flew slower and steered more across the wind as they approached the source. In concert, their ground speed decreased and track angles increased. In contrast to these changes, their drift angles were fairly constant and the transverse component of image flow, above and/or below the moths eyes, showed almost no change. The inter-turn duration (time between sequential turns), a temporal aspect of the male flight manoeuvres, showed a consistent but relatively small increase as the distance from the source increased. The flight tracks narrowed as the males approached close (2.5 m) to the source. Because of unpredicted correlations between physical variables (i.e. temperature, wind velocity) and the distance from the source, we used principal components analysis to generate a set of completely independent variables. Greater than 90% of the variability in the data could be explained by four principal factors which corresponded well with known relationships in the flight manoeuvres. All four of these factors showed a significant regression against distance to the source. Although uncontrolled factors such as temperature and wind velocity may have contributed to changes in flight behaviour, recent data indicate that, in addition to concentration, certain temporal and spatial characteristics (i.e. burst period, burst return period) of plumes in wind vary systematically with distance from the source. We propose that L.dispar males might adjust their flight manoeuvres in response to these changes.

John Murlis

Papers

Filament-based atmospheric dispersion model to achieve short time-scale structure of odor plumes

Spatial and temporal structures of pheromone plumes in fields and forests

Measurement of Odor-Plume Structure in a Wind Tunnel Using a Photoionization Detector and a Tracer Gas

Effects of Pheromone Plume Structure and Visual Stimuli on the Pheromone-Modulated Upwind Flight of Male Gypsy Moths (Lymantria dispar) in a Forest (Lepidoptera: Lymantriidae)

Pheromone‐mediated upwind flight of male gypsy moths, Lymantria dispar, in a forest