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Olfactory Sense in Different Animals

Authors:
  • Kamdhenu University Gandhinagar

Abstract

The sense of olfaction in all the living things of world is unique and special. Through this sense animal can not only recognized the smell of food but also they communicate, interact, navigate, recognize and role are many more. Perception of odour can be separated of two system – olfactory system and trigeminal system. Chemoreceptors are located in the nasal cavity as well as in the oral cavity which can differentiate the flavor of the different food. Recently some amazing facts about the olfaction in marine animals and plants are studied. Due to this sense the importance of dog breeds are characterized and used for different purposed.
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Olfactory Sense in Different Animals
Padodara, R. J.1* and Ninan Jacob2
Department of Veterinary Physiology, Veterinary College, Junagadh Agricultural University, Junagadh-
362001, India
2Department of Veterinary Physiology, Rajiv Gandhi Institute of Veterinary Education and Research,
Puducherry-605001, India
*Corresponding author: rameshpadodara3@gmail.com
Abstract
The sense of olfaction in all the living things of world is unique and special. Through this sense animal
can not only recognized the smell of food but also they communicate, interact, navigate, recognize and
role are many more. Perception of odour can be separated of two system olfactory system and
trigeminal system. Chemoreceptors are located in the nasal cavity as well as in the oral cavity which can
differentiate the flavor of the different food. Recently some amazing facts about the olfaction in marine
animals and plants are studied. Due to this sense the importance of dog breeds are characterized and
used for different purposed.
Keywords: Animals, Chemoreceptor, Olfaction, Sense, Taste
Introduction
Animals receive information about the environment and communicate with each other through the sense
of Olfaction. The odours (or pheromones) emitted by the animal play an important role in insect and
animal reproduction behaviour (Rekwot et al., 2001), neonatemother interactions (Distel and Hudson,
1985) and in the detection of predators. In this review, we consider the role of odours in assisting animals
to find, recognize and discriminate between foods. Olfaction plays a major role in general animal
awareness. Sommerville and Broom (1998), defined it as ‘a state in which complex brain analysis is used
to process stimuli or constructs based on memory’. Size of the nasal epithelium is a good indicator of the
degree of an animal’s sense of smell because the number of olfactory receptor cells per unit surface area
is a constant. Longnosed mammals such as horses, cattle and sheep, olfactory senses are likely to be well
developed. Olfaction when linked to memory allows the animal to recollect the sensory characteristics of
the feed and also whether the metabolism of that feed is favourable to it. Thus animals exhibit ‘nutritional
wisdom’.
Voluminous literature is available on the olfaction and food preferences of insects. For example, moths
(MasanteRoca et al., 2002), mites (De Boer et al., 2005) and lacewing (Reddy, 2002) are known to
discriminate between volatile odors of various plants. Moths detect plant volatiles by projection neurons
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on the antennal lobe (Anton and Hansson, 1995; Greiner et al., 2002; MasanteRoca et al., 2002). Insects
locate plant through their odor and a strong interaction exists between specific plants and particular insect
species. Plant odours also play an important role in the location and selection of feedstuffs in mammals.
Anatomy of odor detection
The conscious perception of odor involves two separate systems i.e. The Olfactory system (consist of
paired nares, nasal cavity, respiratory epithelium, olfactory epithelium, olfactory nerves, olfactory bulb,
olfactory centre in the cerebrum) and Trigeminal system (consist of chemosensory trigeminal nerve
receptors which is spread in the nasal cavity and intranasal trigeminal nerve branches). The third structure
involved is the Vomeronasal organ (primarily concerned with pheromones). In mammals, it consists of
palatine duct located posterior to the 2nd incisor and connects nasal cavity to oral cavity. It is located in
hard palate between nasal and oral cavities and their neurons which travel to the lateral olfactory bulb and
to the limbic system structures and cortical centres also. The anatomy and location of the three structures
help in easy perception and sensation of variety of smell.
The physiology of odour detection
Odorants are volatile chemical compounds that are carried into the nasal cavity with inhaled air, dissolve
in the mucus membrane and come into contact with the olfactory epithelium. In some cases they are aided
by odorant binding proteins. The receptors are highly sensitive and act through a standard Gprotein
cascade, causing cation channels to open and action potentials to be fired. Olfactory neurones in the
olfactory epithelium project upwards through the cribriform plate to the ipsilateral olfactory bulb. This
region is one of the few places where new neurones are regenerated in the adult. The perception of gas
phase molecules involves the olfactory and trigeminal systems. The trigeminal system is responsible for
the perception of sensations such as irritation, stinging, burning, tickling, warm, cool and pain (Doty et
al., 1978; Doty and CommettoMuiz, 2003). Trigeminal perception occurs via free nerve endings found
in the nasal and oral cavities, with the nasal cavity being the more sensitive of the two (Silver and Finger,
1991).
Olfactory receptors
It is commonly believed that each neurone expresses one olfactory Gprotein coupled receptor
(Mombaerts, 2004). Odorant receptor (OR) genes comprise the largest gene family in the mammalian
genome. Mammalian ORs are disposed in clusters on virtually all chromosomes. They are encoded by the
largest multigene family (~1000 members) in the genome of mammals. Odor detection plays a significant
role in feed preferences of livestock.
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Figure 1: Microscopic view nasal cavity of mammals.
Species
OR genes
Reference
Humans
350 (560 pseudogenes)
Glusman et al. (2001)
Mice
1000 1300
Other Mammals
4000
Cow
2129
Niimura and Nei (2007)
Dog
1100
Macaque
606
Rat
1767
Mice
1391
Figure 2: Action pathway of odorant molecules
Different sensations of smell
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There may exist more than 50 different primary sensation of smell. The main among them are
musky, floral, camphoraceous, ethereal, pepperminty, pungent and putrid. In humans, it has been found
that they may be odor blind to one or more type of smell. However, odor blindness in animal is not
known.
Table No 1: Humans have seven primary odors that help them determine objects.
Odor
Camphoric
Musky
Floral
Pepperminty
Etheral
Pungent
Putrid
Olfaction in Cattle
Smell complements visual information and is responsible for social organisation in the group, recognition
of individual animals and helps to create a bond between the dam and the offspring. Olfactory
communication between animals and reproduction is mainly based on scents / pheromones released, but
has to be supported by other senses. Odours are detected by sensory cells (chemo-receptors) located in the
epithelium of the nostrils. However, cattle also possess a second olfactory organ: the Jacobson organ or
organum vomeronasale (vomeronasal organ), which is located in the mouth in the upper palate and is
more sensitive to pheromones than the mucus membrane of nose. Olfactory communication between
individuals is made via pheromones which are chemical molecules emitted by an animal that engender a
specific response in the animal that detects (perceives) them (Cheal, 1975). These molecules, present in
all animal secretions (perspiration, urine, faeces, oestrus and vaginal secretions), are of a varied chemical
nature but are mainly composed of aromatic alkenes (Phillips, 1993). Its use is closely linked to the
characteristic behaviour known as the ‘Flehmen behaviour’ whereby the animal lifts its nose with the
mouth slightly open, the upper lip curled up and the tongue lying flat to enable the air to pass into the
Jacobson organ. The two olfactory systems (mucous membrane of the nose and sinus and organum
vomeronasale) very likely have complementary functions but their chemical characterisation has not been
proven (Phillips, 1993). The sensitivity of these two organs varies according to the natural concentration
of the odour and its biological significance (Boissy et al., 1998). The fact that cattle have numerous
odoriferous glands confirms the use of odours in communication between individuals (Bouissou et al.,
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2001). Cattle perception of odour is, therefore, more acute than human perception (Albright and Arave,
1997).
Olfactory communication between cattle is made and recognised essentially via pheromones (Schloeth,
1956, Signoret et al., 1997). Thus, the presence of a stressed cattle or the odour of its urine will modify
the behavioural reactions of its fellow creatures (Boissy et al., 1998). One can also observe a slower
learning capacity in heifers when they are exposed to the odour of a stressed fellow animal. (Bouissou et
al., 2001). Thus, pheromones constitute a warning signal from the animal in danger to its fellows.
Smell also plays a role in the reproduction of cattle (Signoret et al., 1997). Vaginal secretions transmit
odorous molecules. The bull detects the scent of the female’s vaginal secretions (Klemm et al., 1987) and
sexual behaviour has been successfully stimulated by using the vaginal mucus of females in heat
(Albright and Arave, 1997).
The role of urine in stimulating sexual behaviour is recognised and corroborated by its chemical
composition: specific composites can be detected in the urine of cows that are in heat (Kurma et al.,
2000). The cow’s urine acts as a chemical signal to entice the male via pheromones, but its exact role is
not defined (Rekvot et al., 2001).
Similarly, perception of olfactory molecules influences sexual development. Thus, the presence of a male
accelerates puberty in heifers (Izard and Vandenbergh, 1982) and conversely, the presence of cows
stimulates testicular development and the production of testosterone in bulls (Katongole et al., 1971). The
Jacobson organ is essential for sexual behaviour stimulation (Klemm et al., 1987) and is often associated
with the Flehmen behaviour (Albright and Arave, 1997). In addition, removal of the olfactory bulb does
not impede the reproductive behaviour of cattle (Mansard and Bouissou, 1980).
Finally, cattle use odours along with colours and taste to identify and choose their food (Bailey et al.,
1996). The characteristics of food, odour included, condition the animal’s appetite (Baumont, 1996).
Thus, adding aromas to grass ensilage may increase the quantity consumed. Moreover, the odour of
manure has a lasting dissuasive effect on pasture; the animals refuse to graze in contaminated zones for a
month after (Dohi et al., 1991). The roles of maternal vision and olfaction in regulating the suckling-
mediated inhibition of LH secretion, expression of maternal selectivity, and lactational performance was
examined in anestrous beef cows by Griffith and Williams (1996).
Cattle perception of human scent
Cattle handlers observe that a stock breeder who regularly lets his animals sniff him gives the impression
of being ‘recognised’ by them. Conversely, they affirm that an outsider will be recognised by his ‘strange’
or even by his ‘parasitic’ scent, such as that of medication in the case of a veterinarian. However
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Rybarczyk et al. (2001, 2003), failed to prove that cows discriminate through smell. In their studies, only
one calf succeeded in identifying the ‘right’ handler if the human-stimuli presented for tests wore overalls
of the same colour. That is why they concluded that cattle principally use visual keys to differentiate
humans but that nevertheless, they can use other determining factors.
Olfaction in Dog
Olfaction, the act or process of smelling, is a dog’s primary special sense. A dog’s sense of smell is said
to be a thousand times more sensitive than that of humans. While the human brain is dominated by a large
visual cortex, the dog brain is dominated by an olfactory cortex. In fact, a dog has more than 220 million
olfactory receptors in its nose, while humans have only 5 million.
Dogs also possess vomeronasal organ (Jacobson’s organ) that also contains olfactory epithelium. It is
located above the roof of the mouth and behind the upper incisors. A dog’s nose which is normally cool
and moist helps in olfaction. Today, people use a dog’s keen sense of smell in many ways. Dogs are
trained for search and rescue missions, in the detection of narcotics and contraband agriculture products,
to respond to disasters worldwide, to detect drugs and to search for lost individuals, homicide victims and
forensic cadaver materials. They are trained to detect bombs so as to combat terrorist threats, stop the
illegal flow of narcotics, detect unreported currency and concealed humans. The most commonly used
breeds for the above purposes are Labrador retrievers, Golden retrievers, German shepherds, Belgian
Malinois, and many mixed breeds.
Beagles are used to detect agriculture contraband. They are also trained to detect prohibited fruits, plants,
and meats in baggage and vehicles of international travelers.
Medical tests have recently shown that specially trained dogs are capable of detecting certain types of
tumors, the beginning of a heart seizure and terminal stages of cancer in humans.
Table No 2: Scent-Detecting Cells in Humans and Dog Breeds (Stanley and Sarah, 2013)
Species
Number of Scent Receptors
Humans
5 million
Dachshund
125 million
Fox Terrier
147 million
Beagle
225 million
German Shepherd
225 million
Bloodhound
300 million
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Scent hounds as a group can smell one- to ten-million times more acutely than a human,
and Bloodhounds, which have the keenest sense of smell of any dogs, have noses ten- to one-hundred-
million times more sensitive than a human's. They were bred for the specific purpose of tracking humans,
and can detect a scent trail a few days old. The second-most-sensitive nose is possessed by the Basset
Hound, which was bred to track and hunt rabbits and other small animals.
Olfaction in Cat
A domestic cat's sense of smell is about fourteen times stronger than human beings. Cats have twice as
many receptors in the olfactory epithelium (i.e. smell-sensitive cells in their noses) than humans and
hence have a more acute sense of smell than humans. Cats also have a scent organ in the roof of their
mouths called the vomeronasal (or Jacobson's) organ. When a cat wrinkles its muzzle, lowers its chin, and
lets its tongue hang a bit, it is opening the passage to the vomeronasal. This is called gaping, "sneering",
"snake mouth", or "Flemming". Gaping is the equivalent of the Flehmen response in other animals, such
as dogs, horses and big cats.
Olfaction in Bear
Bears, such as the Silvertip Grizzly found in parts of North America, have a sense of smell seven times
stronger than that of the bloodhound. This is needed for locating food underground by using their
elongated claws, bears dig deep trenches in search of burrowing animals, nests, roots, bulbs and insects.
Bears can detect the scent of food from up to 18 miles away. Their immense size helps them to scavenge
new kills, driving away the predators (including packs of wolves and human hunters) in the process.
Olfaction in Primates
The sense of smell is poorly developed in the catarrhine primates (Catarrhini), and nonexistent
in cetaceans. This is compensated with a well-developed sense of taste. Some prosimians, such as
the Red-bellied Lemur have scent glands atop the head.
Olfaction in Fish
Most aquatic organisms have the olfactory cells mounted in a position where they will be exposed to
moving water, presumably to limit time lags imposed by diffusion of the chemical through the boundary
layer surrounding the sensory cell (remember that the boundary layer is smaller as water speed increases).
For the same reason, perhaps, the olfactory organs are usually anterior on the organism, where the
boundary layer is thinner, although many might argue that the anterior position simply allows the
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organism a better chance to smell what it's getting into. Vertebrates bear the olfactory cells in the nostrils,
usually with arrangements for moving water over them; the taste cells are located on the tongue. Fish, the
original aquatic vertebrates, have chemoreceptors scattered all over their bodies, but with particular
attention to sensory structures such as the lips or barbels. Invertebrates bear the olfactory structures on
various parts of the body, often tentacles or antennae near the head, but also on mouthparts, feeding
structures, legs, feet, tails, and so on.
In Fish, both olfaction and taste serve different ecological roles. The lateral olfactory system (dorsolateral
olfactory bulb glomeruli and lateral olfactory tract) and the external taste buds are probably specialized
for food search and amino acid discrimination. The medial olfactory system (basomedial olfactory bulb
glomeruli and medial olfactory tract) and the solitary chemosensory taste cells, however, may have their
roles in intra- and interspecific interactions (discriminating pheromones by olfaction, bile components by
both olfaction and taste). Whereas stimulation of the taste systems alone triggers reflexes, complex,
conditional or conditioned behaviours are only released when the olfactory system is intact. This points at
the importance of telencephalic and diencephalic integration of olfactory and taste inputs (Kotrschal,
2000).
Different fishes use their sense of smell for different purposes.
Salmon, to identify and return to their home stream waters; Catfish, to identify other individual catfish
and to maintain a social hierarchy; Many fishes, to identify mating partners or to alert to the presence of
food; Smooth dogfish (Mustelus canis), a species of Shark to maintain directional response (Parker,
1914). He provided a plausible explanation of how this was accomplished; he postulated that the two
separated nostrils have the ability to detect small differences in the concentration of odorous
materials enabling the shark to orient in the direction of equal stimulation and to head "upstream" to the
source. This tracking ability is well recognized by skin divers and fishermen who have involuntarily
attracted sharks by retaining speared fish or by discarding trash fish and offal from their boats.
It was believed that whales and dolphins lack the ability to smell. However, the finding of olfactory
hardware linking the brain and nose, and functional protein receptors required to smell showed that
whales have the ability to smell. Bowhead whales have a relatively large, developed olfactory bulb
similar to that in other animals with a developed sense of smell. Unlike most whales, bowheads have
separate nostrils, which suggest they may be able to sense the direction from where a particular smell is
coming (Walker, 2010).
Avian Olfaction
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The relatively small and simple avian olfactory bulb gave rise to the belief that birds had poor sense of
smell (Waldvogel, 1989). Recent research has emphasized the complexity and depth of the avian sense of
smell. Bang (1960), suggested that the sense of smell could be more developed in birds in certain
ecological niches. Birds with high olfactory ratios were typically ground-dweling carnivores, small New-
World Vultures, or marine birds like the kiwis, Turkey Vulture, tubenoses (Procellariiformes) (Bang and
Wenzel, 1985; Evans and Heiser, 2001). Bang's research initiated olfaction research in birds and
broadened the horizons of the understanding of how birds smell. Though birds seemingly would have
little use for smell; in the airy treetops odors disperse quickly and would be of minimal help in locating
obstacles, prey, enemies, or mates, yet the apparatus for detecting odors is present in the nasal passages of
all birds. Based on the relative size of the brain center used to process information on odors, physiologists
expect the sense of smell to be well developed in rails, cranes, grebes, and nightjars and less developed in
passerines, woodpeckers, pelicans, and parrots. By recording the electrical impulses transmitted through
the bird's olfactory nerves, physiologists have documented some of the substances that birds as diverse as
sparrows, chickens, pigeons, ducks, shearwaters, albatrosses, and vultures are able to smell.
Kiwis
Possibly have the best avian sense of smell. They are small, ancestral, flightless, nocturnal carnivores
found in the forests of New Zeland. Their olfactory lobe is ten times the size of other birds and nostrils
are located at the bill tip, rather than at the base of the bill (where they are found on all other birds). Kiwis
use their bill to probe for worms and bugs in the soil and are capable of locating food by smell alone,
although they also have a well-developed sense of hearing (Evans and Heiser, 2001; Gill, 1995 and
Wenzel, 1968)
Procellariformes
Tubenoses- albatrosses and petrels - pelagic (0pen ocean) birds have well-developed, tubular nostrils.
Tubenoses use olfactory cues to forage and also to return to their burrows over many kilometers. They are
attracted to fish oil and dimethyl sulfide which is volatile compounds released by dead fish and the
plankton that feed upon them. Smaller species, those that feed upon plankton, arrive first at most odor
sources, and appear to have the best-developed sense of smell. Procelliformes also are capable of using
smells to locate their burrows at night, locating individual nests by smell (Bonadonna et al., 2001; Evans
and Heiser, 2001; Verheyden and Jouventin, 1994).
New-World Vultures
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Different from Old-World Vultures (which are closely related to eagles), New-World Vultures
(Cathartidae) are more closely related to storks. Old-World Vultures have very little sense of smell and
rely mainly on their keen sense of sight to find carrion. Likewise, the larger New-World species are
primarily visual foragers. Smaller species (Turkey, Greater and Lesser Yellow-headed Vulture) are
capable of using olfactory cues to locate food and have the enlarged olfactory bulb to go with that ability.
Black Vultures and other larger species often use Turkey Vultures as cues to the location of a carrrion
source, clouding the issue of which species use olfactory cues. Pipeline engineers have used this ability as
well, injecting ethyl mercaptan (an odorant found in carrion) into pipes and following vultures to the leak
(Evans and Heiser, 2001; Gill, 1994; Kirk and Mossman, 1998; Smith and Paselk, 1986)
Orientation & Navigation in birds
Aside from the previous examples, many other species have been shown to use olfactory cues to forage,
to home and to migrate. Many of these species appear to be using small-scale olfactory cues for local
piloting, but homing pigeons and some migratory birds may be using olfaction for true navigation. While
it does not seem to be a primary navigational cue, odors are proving to be important cues in orientation
and migration. (Clark and Mason, 2000; Walraff and Andreae, 2000; Waldvogel, 1989).
Olfaction in Insects
In insects smells are sensed by olfactory sensory neurons in the chemosensory sensilla, which are present
in insect antenna, palps and tarsa, but also on other parts of the insect body. Odorants penetrate into the
cuticle pores of chemosensory sensilla and get in contact with insect Odorant-binding proteins (OBPs)
or chemosensory proteins (CSPs), before activating the sensory neurons.
Insects have been used as a model system to study olfaction. Insects use primarily their antennae for
detecting odors. Sensory neurons in the antenna generate odor-specific electrical signals called spikes in
response to binding of odors. The sensory neurons send this information via their axons to the antennal
lobe, where they synapse with other neurons in semidelineated (with membrane boundaries) structures
called glomeruli. The antennal lobes have two kinds of neurons, projection neurons (mostly excitatory)
and local neurons (inhibitory, and some excitatory). The projection neurons send their axon terminals
to mushroom bodies and the lateral horn (both of which are part of the protocerebrum of the insects).
Recordings from projection neurons show in some insects’ strong specialization and discrimination for
the odors presented (especially for the projection neurons of the macroglomeruli, a specialized complex
of glomeruli responsible for the pheromones detection).
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Interactions of Olfaction with other Senses
Olfaction and taste
Olfaction, taste and trigeminal receptors (also called Chemesthesis) together contribute to flavor. The
human tongue can distinguish only among five distinct qualities of taste, while the nose can distinguish
among hundreds of substances, even in minute quantities. It is during exhalation that the olfaction
contribution to flavor occurs, in contrast to that of proper smell, which occurs during
the inhalation phase. The neurons of the olfactory system are the only one of the human senses that
bypasses the thalamus and connects directly to the forebrain.
Toxic substances are noted for their taste and smell. Alkaloids which are poisonous are extremely bitter.
Its nature’s way of protecting an animal from eating poisonous plants. Cyanide gas gives an almond
smell. The presence of almond like smell in the stomach contents indicates cyanide poisoning. Bacteria
rich food emits noxious odors and has an unpleasant taste. The palatability of feed is determined by the
smell and taste. Unpalatable food makes the animal anorexic.
Olfaction and audition
Olfaction and sound information has been shown to converge in the olfactory tubercles of rodents. This
neural convergence is proposed to give rise to a percept termed smound. Whereas a flavor results from
interactions between smell and taste, a smound may result from interactions between smell and sound.
Olfcation in Plants
The tendrils of plants are especially sensitive to airborne volatile organic compounds. Parasites such
as dodder make use of this in locating their preferred hosts and locking on to them. The emission of
volatile compounds is detected when foliage is browsed by animals. Threatened plants are then able to
take defensive chemical measures, such as moving tannin compounds to their foliage.
Terms and conditions associated with olfaction
Loss of sense of smell is termed Anosmia, whereas Hyposmia indicate partial loss of sense of smell.
Anosmia induces anorexia in the cat, whereas Hyposmia induces hyperrexia (increased food intake) in the
same species. In dogs conditions like canine paarainfluenza, cushing’s disease, hypothyroidism, canine
distemper, nasal tumours and even diabetes mellitus can diminish the sense of smell.
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OIfactory foraging, although very rare among birds, is frequently found in members of the Procellariiformes; this finding is based on a small number of field studies using a standardized method (i.e. raft tests). Reactions of seven species previously tested under artificial conditions were tested again under natural feeding conditions (fish-oil slicks) to check validity. Concurrently, we compared the flight behavior of two groups of species (with and without olfactory capacities) when approaching an odor source. A large-scale experiment was then conducted in pelagic waters to test the reaction of a community of procellariiforms (15 species) to a food-related odor diffusing within a principal feeding area. We observed the same reactions (attraction or indifference) to oil slicks as to test rafts in all species evaluated. Results obtained with the standardized method thus hold under natural conditions. Species guided by oilaction approached the odor source by flying against the wind very dose to (< 1 m) the surface, whereas other species approached from a direction independent of wind direction and from a greater height (>6 m). Thus, specific searching behavior is associated with olfactory foraging and we found it to be closely related to direction, height, and speed of odor diffusion by wind. Reaction to the odor test varied according to families or subfamilies, some taxa showing consistent responses (attraction or indifference) to several experiments and some taxa showing conflicting reactions. We obtained some ev-idence that olfactory behavior may differ before and after locating odor sources, as well as vary according to oceanic zones (coastal vs. pelagic). We discuss the hypothesis that certain species rely mainly on visual cues, recognizing and following species that are tracking food-related odors. Finally, we propose some new ideas about the evolution of oilaction in birds.
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