Human and Animal Olfactory Capabilities Compared

Abstract

Humans are traditionally considered to have a poorly developed sense of smell that is clearly inferior to that of nonhuman animals. This view, however, is mainly based on an interpretation of neuroanatomical and recent genetic findings, and not on physiological or behavioral evidence. An increasing number of studies now suggest that the human sense of smell is much better than previously thought and that olfaction plays a significant role in regulating a wide variety of human behaviors. This chapter, therefore, aims at summarizing the current knowledge about human olfactory capabilities and compares them to those of animals.

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Human and A
675
PartD|32
32. Human and Animal
Olfactory Capabilities Compared
Matthias Laska
Humans are traditionally considered to have
a poorly developed sense of smell that is clearly
inferior to that of nonhuman animals. This view,
however, is mainly based on an interpretation
of neuroanatomical and recent genetic ndings,
and not on physiological or behavioral evidence.
An increasing number of studies now suggest that
thehumansenseofsmellismuchbetterthan
previously thought and that olfaction plays a sig-
nicant role in regulating a wide variety of human
behaviors. This chapter, therefore, aims at sum-
marizing the current knowledge about human
olfactory capabilities and compares them to those
of animals.
. Olfactory Sensitivity ............................. 
. Olfactory Discrimination Ability ............ 
. Qualitative Comparisons of Olfactory
Capabilities Between Species................ 
.. Gathering Information
About the Chemical Environment 
.. Foraging and Food Selection ...... 
.. Spatial Orientation .................... 
.. Social Communication ............... 
.. Reproduction............................ 
.. Learning and Memory ................ 
. General Conclusions............................. 
References................................................... 
Comparing olfactory capabilities between species is not
a trivial task. Several potentially confounding factors
have to be taken into consideration when trying to make
statements as to differences or similarities concerning
the efficiency of the sense of smell between species.
First, we find a high variability in results between
human studies. With regard to olfactory sensitivity, for
example, published mean threshold values for a given
odorant may vary up to 6 orders of magnitude [32.1].
This is probably, at least in part, due to different meth-
ods used to determine such thresholds. However, there
is a clear trend that more sophisticated psychophysi-
cal procedures employing signal detection methods and
rigorous stimulus control yield lower threshold values
than more simple procedures [32.2].
Second, the vast majority of psychophysical studies
on the human sense of smell only report mean values
of performance (usually plus a measure of variation),
but not the distribution of values or their range. This
is all the more problematic as measures of olfactory
performance are notorious for a high interindividual
variability. This is true for both studies on olfactory
sensitivity in which individual threshold values with
a given odorant have been reported to commonly vary
up to 3 orders of magnitude within a study popula-
tion [32.3], and for studies on olfactory discrimination
performance in which subjects have been found to vary
in their ability to distinguish between a given pair of
odorants from chance to perfect discrimination [32.4].
However, here too, more recent studies using state-of-
the-art psychophysical methods (pioneered by Cain and
coworkers) generally report a considerably lower range
between the most and least sensitive subjects within
a study population [32.5].
Third, it is inevitable to use different methods for
assessing olfactory performance with animals and hu-
mans, and also with different species of animals. This
may affect the comparability of results. However, dif-
ferences between methods employed with different
species of animals are usually necessary adaptations for
meeting the physiological, anatomical, and behavioral
needs and limitations of the study species to success-
fully cooperate in a behavioral test. (In other words,
even if it was possible to use the same method with dif-
ferent species, this would very likely put one species
at an advantage over another species, thus invalidating
any comparisons. Therefore, it is better to aim for op-
timizing a testing method for each study species.) It is
commonly agreed that operant conditioning procedures
are the gold standard among the methods employed
with animals for assessing sensory performance [32.6],
and therefore only studies based on such procedures
are considered in this chapter. Figures 32.1–32.5 il-
lustrate operant conditioning procedures for assessing

PartD|32
676 Part D Odorant Sensing and Physiological Eects
a) c)
b)
d)
Fig. 32.1a–d Olfactory conditioning
method used with spider monkeys.
(a)Portrait of a spider monkey
(Ateles geoffroyi). (b)The two-
choice apparatus used, viewed from
the animal’s side. It consists of
two manipulation boxes of which
one is baited with a Kellogg’s
honeyloop while the other is empty,
depending on the odorant applied on
the absorbent paper strip attached
to the box. (d)A spider monkey
smelling at one of the absorbent
paper strips bearing an odorant used
either as a rewarded stimulus or as
an unrewarded stimulus. (c)A spider
monkey indicating its decision
for one of the two simultaneously
presented odorants by opening the
corresponding manipulation box
(courtesy of M. Laska)
a) c)
b)
d)
Fig. 32.2a–d Olfactory conditioning
method used with squirrel monkeys.
(a)Portrait of a squirrel monkey
(Saimiri sciureus). (b)Eppendorf
cups equipped with absorbent paper
strips. The Eppendorf cups serve as
manipulation objects (artificial nuts)
that are either baited with a piece
of peanut or not, depending on the
odorant applied on the absorbent
paper strip. (d)Experimental setup.
An artificial nut tree is used to present
numerous artificial nuts,halfofthem
baited with a piece of peanut and
bearing an odorant used as rewarded
stimulus, and half of them empty
and bearing an odorant used as
unrewarded stimulus. (c)A squirrel
monkey inspecting an artificial nut
on a branch of the artificial nut tree
(courtesy of M. Laska)

Human and Animal Olfactory Capabilities Compared Human and Animal Olfactory Capabilities Compared 677
PartD|32
a) b)
d)
c)
Fig. 32.3a–d Olfactory conditioning
method used with Asian elephants
(Elephas maximus). (a)Elephant
sniffing at the left odor port. (b)Ele-
phant sniffing at the right odor port.
(d)Elephant indicating its decision
for one of the two simultaneously
presented odorants by placing her
trunk onto the grid on top of the
corresponding odor port. (c)Elephant
receiving a carrot as a food reward
after a correct choice (courtesy of
M. Laska)
a) b)
c) d)
OP2
OP1
V
C
SB
Fig. 32.4a–d Olfactory conditioning
method used with South African fur
seals. (a)Schematic drawing of the
experimental set up. C: container
bearing an odor stimulus; V: ventilator
for ingoing airflow; O: outlet for
outgoing airflow; SB: stimulus box;
OP1: odor port 1; OP2: odor port
2. (b)Portrait of a South African
fur seal (Arctocephalus pusillus).
(c)Simultaneous presentation of two
odor stimuli to a fur seal. (d)Afur
seal sniffing at one of the two odor
ports (courtesy of M. Laska)

PartD|32.1
678 Part D Odorant Sensing and Physiological Eects
S– odor
S+ odor
Odor
applied
Light barrier
Reward
a) b)
c) d)
Odor presentation
DecisionInitiation
Fig. 32.5a–d Olfactory condition-
ing procedure used with mice.
(a)Schematic drawing of the exper-
imental setup. (b)A mouse (Mus
musculus) in front of the odor port.
(d)A mouse poking its head into the
odor port. (c)A mouse licking at the
water spout after a correct choice
(courtesy of M. Laska)
olfactory performance in different species of mam-
mals.
Finally, animal studies usually only employ a low
number of individuals, sometimes only one or two an-
imals per species, thus making the use of mean values
arguable, and statements as to how representative the
findings are for the whole species difficult.
Despite all these difficulties, there are several good
reasons that make it worthwhile to compare olfactory
capabilities between humans and animals: first, such
comparisons allow us to study the neural and/or genetic
mechanisms underlying possible differences or simi-
larities in olfactory efficiency between species [32.7].
Second, between-species comparisons of olfactory per-
formance allow us to test hypotheses about the evo-
lution of sensory systems and the selective pressures
acting on them [32.8]. Finally, the integration of ani-
mal and human studies on olfactory performance may
help us to better understand medically relevant phe-
nomena such as aging processes or neurodegenerative
diseases, which are often accompanied by a loss of ol-
factory function [32.9,10].
32.1 Olfactory Sensitivity
The sensitivity of the sense of smell is usually as-
sessed by determining olfactory detection thresholds,
that is, the lowest concentration of a given odorant
that a human subject or an animal is able to de-
tect. Human olfactory detection thresholds have been
reported for a total of 3300 odorants [32.1]. In con-
trast, the total number of odorants tested in animals
is much lower. Table 32.1 summarizes the species
of mammals and the number of odorants for which
olfactory detection thresholds using operant condition-
ing procedures have been published. The total number
of species is 17 (please note that there exist about
5500 species of mammals), and the highest num-
ber of odorants tested with a given species is 81.
These 17 species represent 7 of the 29 orders of mam-
mals.
With the exception of four odorants tested with rats
(one explosive substance, and three explosive taggants,
that is, substances added to explosives to identify their
sources) and one odorant tested with mice (pyridazine),
all 138 odorants for which olfactory detection thresh-
olds have been determined with nonhuman mammals
have also been tested with human subjects, allowing for
direct comparisons of their performance.
In the following figures (as well as in all other
comparisons that follow), I compare the lowest mean
threshold values reported in human subjects to the
lowest individual threshold values reported in a given
animal species. The rationale for this is as follows:
1. Human studies only rarely report the range of
threshold values, but usually a mean value.

Human and Animal Olfactory Capabilities Compared 32.1 Olfactory Sensitivity 679
PartD|32.1
Tab l e 3 2.1 Animal species and number of odorants for which olfactory detection threshold values using operant condi-
tioning procedures have been published
No. Common name Scientific name Mammalian order Number of odorants tested
1Spider monkey Ateles geoffroyi Primates 81
2Mouse Mus musculus Rodentia 72
3Squirrel monkey Saimiri sciureus Primates 61
4Pigtail macaque Macaca nemestrina Primates 60
5Rat Rattus norvegicus Rodentia 45
6Short-tailed fruit bat Carollia perspicillata Chiroptera 18
7Dog Canis lupus familiaris Carnivora 15
8Common vampire bat Desmodus rotundus Chiroptera 15
9Common mouse-eared bat Myotis myotis Chiroptera 13
10 Sea otter Enhydra lutris Carnivora 7
11 Pig Sus scrofa domestica Artiodactyla 5
12 Hedgehog Erinaceus europaeus Insectivora 4
13 Great fruit-eating bat Artibeus lituratus Chiroptera 3
14 Pale spear-nosed bat Phyllostomus discolor Chiroptera 3
15 Common shrew Sorex araneus Insectivora 3
16 Rabbit Oryctolagus cuniculus Lagomorpha 1
17 Harbor seal Phoca vitulina Carnivora 1
Ethanoic
acid
n-Propanoic
acid
n-Pentanoic
acid
n-Heptanoic
acid
n-Butanoic
acid
n-Hexanoic
acid
n-Octanoic
acid
7
1
77
1
3
2
4
10
2
7
7
7
12
10
6
2
12
12
4
10
15
814
13
63
1
310
4
2
2
68
43
12 7
1
2
2
8
1
15
5
6
3
4
Threshold (log ppm)
Low sensitivityHigh sensitivity
0
–1
–2
–3
–4
–5
–6
–7
–8
–9
–10
15
Fig. 32.6 Comparison of the olfactory
detection threshold values (expressed as vapor
phase concentrations) of human subjects
for aliphatic n-carboxylic acids and those
of other mammalian species. Data points of
the human subjects (brown circles)represent
the lowest mean threshold values reported in
the literature, and data points of all animal
species (numbered circles)representthe
lowest threshold values of individual animals
reported in the literature. Numbers in circles
refer to the numbers assigned to each species
in Table 32.1 (human data: [32.1]; spider
monkey data: [32.11]; mouse data: [32.12];
squirrel monkey data: [32.13]; pigtail macaque
data: [32.11]; rat data: [32.14]; short-tailed
fruit bat data: [32.15]; dog data: [32.16,17];
common vampire bat data: [32.18]; sea otter
data: [32.19]; hedgehog data: [32.20]; great
fruit-eating bat data: [32.18]; pale spear-nosed
bat data: [32.18]; common shrew data: [32.21])
2. Animal studies usually only employ a low num-
ber of individuals (in some cases only one animal),
making the use of mean values arguable.
3. Comparing the threshold value of the very best in-
dividual animal with the mean threshold value of
a group of human subjects minimizes the risk of un-
intentionally favoring humans over animals.
Figure 32.6 compares human olfactory detection
threshold values for aliphatic n-carboxylic acids to
those from other mammalian species. With the notable
exception of the dog, human subjects have a higher
sensitivity, that is, lower olfactory detection thresh-
olds, than the majority of mammalian species tested
with this class of odorants. (Please note that all data
points for the dog, except the one for n-heptanoic
acid, are from a study by Neuhaus [32.16]whoem-
ployed only one animal. Later studies by other re-
searchers [32.17,57] employed several dogs as well
as more rigorous stimulus control, and, interestingly,

PartD|32.1
680 Part D Odorant Sensing and Physiological Eects
Ethanol
1-Propanol 1-Pentanol 1-Heptanol
1-Butanol 1-Hexanol 1-Octanol
11
2
8
1
9
3
54
6
5
43
1
2
4
3
1
533
5
4
1
2
5
41
34
5
8
8
1
3
45
8
11
Threshold (log ppm)
Low sensitivityHigh sensitivity
3
2
1
0
–1
–2
–3
–4
–5
–6
–7
99
2
Fig. 32.7 Comparison of the olfactory
detection threshold values (expressed as vapor
phase concentrations) of human subjects
for aliphatic alcohols and those of other
mammalian species. Data points of the
human subjects (brown circles)representthe
lowest mean threshold values reported in
the literature, and data points of all animal
species (numbered circles)representthe
lowest threshold values of individual animals
reported in the literature. Numbers in circles
refer to the numbers assigned to each species
in Table 32.1 (human data: [32.1]; spider
monkey data: [32.22]; mouse data: [32.23–25];
squirrel monkey data: [32.26]; pigtail macaque
data: [32.26]; rat data: [32.27,28]; short-tailed
fruit bat data: [32.15]; common vampire
bat data: [32.18]; common mouse-eared bat
data: [32.29]; pig data: [32.30,31])
Ethyl
acetate
n-Propyl
acetate
n-Pentyl
acetate
iso-Butyl
acetate
n-Butyl
acetate
n-Hexyl
acetate
iso-Pentyl
acetate
4
6
5
3
1
3
1
5
2
4
6
7
10
16
5
1
4
3
4
6
1
3
4
13
62
3
1
4
6
8
5
4
13
11
2
Threshold (log ppm)
Low sensitivityHigh sensitivity
2
1
0
–1
–2
–3
–4
–5
–6
–7
–8
Fig. 32.8 Comparison of the olfactory
detection threshold values (expressed as vapor
phase concentrations) of human subjects for
aliphatic acetic esters and those of other
mammalian species. Data points of the
human subjects (brown circles)representthe
lowest mean threshold values reported in
the literature, and data points of all animal
species (numbered circles)representthe
lowest threshold values of individual animals
reported in the literature. Numbers in circles
refer to the numbers assigned to each species
in Table 32.1 (human data: [32.1]; spider
monkey data: [32.32]; mouse data: [32.33–35];
squirrel monkey data: [32.36]; pigtail macaque
data: [32.36]; rat data: [32.37–39]; short-
tailed fruit bat data: [32.15]; dog data: [32.40];
common vampire bat data: [32.18]; sea
otter data: [32.19]; pig data: [32.31]; rabbit
data: [32.41])
obtained markedly higher threshold values with these
odorants compared to those reported by Neuhaus.) The
mouse, another mammal with a reputation for a keen
sense of smell, is more sensitive than humans with
only three of the seven n-carboxylic acids whereas hu-
mans outperform this rodent with four of these seven
odorants.
Figure 32.7 compares human olfactory detection
threshold values for aliphatic 1-alcohols to those from
other mammalian species. Here, too, human subjects
have a higher sensitivity, that is, lower olfactory detec-
tion thresholds, than most of the other species tested.
It is interesting to note that humans outperform the rat,
another mammal believed to have a highly developed
sense of smell, with all seven 1-alcohols. Similarly, hu-
mans generally have lower olfactory detection thresh-
olds than the bats and nonhuman primates tested with
these odorants. The pig, in contrast, is clearly more sen-
sitive than humans with the two alcohols tested in this
species.
Figure 32.8 compares human olfactory detection
threshold values for aliphatic acetic esters to those from

Human and Animal Olfactory Capabilities Compared 32.2 Olfactory Discrimination Ability 681
PartD|32.2
40 30 20
Animals
more sensitive
Humans
more sensitive
10
01
03
12
12
13
23
07
013
114
10 5
117
10 31
654
11 50
30 35
18 57
10 0 102030405060
Number of odorants
Spider monkey
Mouse
Squirrel monkey
Pigtail macaque
Rat
Short-tailed fruit bat
Dog
Common vampire bat
Common mouse-eared bat
Sea otter
Pig
Hedgehog
Great fruit-eating bat
Pale spear-nosed bat
Common shrew
Rabbit
Harbor seal
Fig. 32.9 Comparison of all olfactory detection
threshold values between human subjects and
animal species. Depicted are the number of odor-
ants for which either human subjects or a given
species of mammal are more sensitive (human
data: [32.1]; spider monkey data: [32.11,22,25,
32,42–49]; mouse data: [32.12,23,25,33–35,
42–44,50,51]; squirrel monkey data: [32.13,26,
36,45–48]; pigtail macaque data: [32.11,26,36,
45–48]; rat data: [32.14,27,28,37–39,45]; short-
tailed fruit bat data: [32.15]; dog data: [32.16,17,
40,52–54]; common vampire bat data: [32.18];
common mouse-eared bat data: [32.29]; sea otter
data: [32.19]; pig data: [32.30,31,55]; hedgehog
data: [32.20]; great fruit-eating bat data: [32.18];
pale spear-nosed bat data: [32.18]; common shrew
data: [32.21]; rabbit data: [32.41]; harbor seal
data: [32.56])
other mammalian species. With only two exceptions
(spider monkeys and squirrel monkeys with n-butyl ac-
etate), human subjects have lower olfactory detection
thresholds, that is, a higher sensitivity for these odorants
than all other mammal species tested, including dogs,
mice, and rats. With only few exceptions, humans are
also more sensitive for aliphatic acetic esters than spi-
der monkeys, squirrel monkeys, and pigtail macaques.
This is remarkable given that these nonhuman primates
are highly frugivorous suggesting that a high olfactory
sensitivity for fruit-associated odorants such as acetic
esters should be adaptive for these species. However,
both human and nonhuman primates generally outper-
form granivorous species, such as rats, insectivorous
species such as bats, and herbivorous species such as
the rabbit with this class of odorants.
Figure 32.9 summarizes all comparisons of olfac-
tory detection thresholds between human subjects and
other mammal species. Depicted are the number of
odorants for which either human subjects or a given
species of mammal are more sensitive. With the excep-
tion of the dog (and the harbor seal, which has been
tested with only one odorant), human subjects have
lower olfactory detection thresholds, that is, a higher
sensitivity with the majority of odorants tested so far
compared to all other mammal species tested so far.
This includes species traditionally considered to have
a highly developed sense of smell, such as mice, rats,
hedgehogs, shrews, pigs, and rabbits.
It is interesting to note that humans outperform
even the dog, often considered as the undisputed super-
nose of the animal kingdom, with 5 of the 15 odorants
tested with both species. The fact that these 5 odorants
comprise plant odor components such as ˇ-ionone and
n-pentyl acetate suggests that the behavioral relevance
of odorants rather than neuroanatomical or genetic fea-
tures may strongly affect a species’ olfactory sensitivity.
This idea is further supported by the fact that 7 of the
10 odorants for which the dog has been reported to be
more sensitive than humans comprise carboxylic acids
and thus typical componentsof the odor of prey species
of the dog.
Thus, based on these comparisons, and contrary to
traditional textbook wisdom, humans are not generally
inferior in their olfactory sensitivity compared to ani-
mals.
32.2 Olfactory Discrimination Ability
Olfactory discrimination can be defined as the abil-
ity to reliably respond differently to the successive
presentation of two nonidentical odorants. Thus, ol-
factory discrimination is usually assessed by deter-
mining the proportion of correct responses with re-
peated presentations of a given pair of odorants.
A statistical criterion can then be used to decide
whether a human subject or an animal is able to dis-
criminate between the two odorants in question or
not.
Human studies on olfactory discrimination ca-
pabilities usually either employ structurally related

PartD|32.2
682 Part D Odorant Sensing and Physiological Eects
Tab l e 3 2.2 Between-species comparison of olfactory discrimination performance with aliphatic odorants sharing the
same functional group but differing in carbon chain length
Human
subjects
Squirrel
monkeys
Asian
elephants
Fur seals CD-1 mice Honey bees
1-Alcohols +++++ øø+ø øø +ø++ +++ø++ ++++++ +++++
n-Aldehydes ++++ øø+ø++ øø+ø++ +++ø++ ++++++ ++++++
2-Ketones +++++ øø+ø++ øø+ø++ +++ø++ ++++++ ++++++
Acetic esters +++++ øø+ø++ øø+ø++ +++ø+++++++ øøøøøø
n-Carboxylic acids ++++++ +øø++ø øø+ø++ +++ø++ ++++++ øøøøøø
Success rate 25/30 13/15 15/15 24/25 30/30 17/18
A “+” symbol indicates that the group of human subjects or animals succeeded in discriminating a given aliphatic odor pair, a “”
symbol indicates failure to do so, and a ø symbol indicates that this odor pair was not tested. The six symbols in each table cell
refer to the discrimination of carbon chain lengths C4versus C5,C
4versus C6,C
4versus C7,C
5versus C6,C
5versus C7,andC
6
versus C7, respectively. Human data: [32.58–60]; squirrel monkey data: [32.59,61,62]; Asian elephant data: [32.63,64]; fur seal
data: [32.65]; mouse data: [32.66]; honeybee data: [32.67]
Tab l e 3 2.3 Between-species comparison of olfactory discrimination performance with aliphatic odorants sharing the
same carbon length but differing in functional group
Human subjects Squirrel monkeys CD-1 mice Honey bees
1-Alcohols versus n-aldehydes +++ +++ +++ +++
1-Alcohols versus 2-ketones +++ +++ +++ +++
1-Alcohols versus n-carboxylic acids +++ +++ +++ øøø
n-Aldehydes versus 2-ketones +++ +++ +++ +++
n-Aldehydes versus n-carboxylic acids +++ +++ +++ øøø
2-Ketones versus n-carboxylic acids +++ +++ +++ øøø
Success rate 18/18 18/18 18/18 9/9
A “+” symbol indicates that the group of human subjects or animals succeeded in discriminating a given aliphatic odor pair, a “”
symbol indicates failure to do so, and a ø symbol indicates that this odor pair was not tested. The three symbols in each table cell
refer to the discrimination of odorants that share chain lengths of either 4, or 6, or 8 carbon atoms, respectively. Human data: [32.68];
squirrel monkey data: [32.69]; mouse data: [32.66]; honeybee data: [32.67]
monomolecular odorants (to investigate correlations be-
tween structure and perceived quality of odorants), or
complex odor mixtures of commercial use such as fra-
grances (perfumes, body care products) or food odors
(wines, coffees, artificial flavors). In contrast, the vast
majority of animal studies assessing olfactory discrimi-
nation capabilities employ naturally occurring complex
odor mixtures that are behaviorally relevant for the
species under study such as conspecific body odors,
species-typical food odors, or predator odors. As a con-
sequence, there is only little overlap in the stimuli used
between human and animal studies on olfactory dis-
crimination. However, at least a few animal species
have also been studied for their ability to distinguish
between some of the structurally related monomolecu-
lar odorants tested with humans.
Tab le 32.2 compares the ability of humans and sev-
eral species of animals to discriminate between mem-
bers of homologous series of aliphatic odorants. These
are odorants sharing the same oxygen-containing func-
tional group (e.g., an alcohol- or an aldehyde group) but
differing in carbon chain length. With this set of odor-
ants, the proportion of successfully discriminated odor
pairs is slightly lower in humans compared to squirrel
monkeys, fur seals, honey bees, Asian elephants, and
mice. Nevertheless, humans succeeded with more than
80% of these odor pairs, which are both structurally and
qualitatively similar to each other.
Tab le 32.3 compares the ability of humans and
several species of animals to discriminate between
aliphatic odorants sharing thesame carbon chain length
but differing in functional group. With this set of odor-
ants, not only squirrel monkeys, mice, and honey bees,
but also human subjects successfully discriminated be-
tween all odor pairs tested.
Tab le 32.4 compares the ability of humans and
several species of animals to discriminate between
enantiomers. These are pairs of molecules with mirror-
image structures that exhibit identical chemical and
physical properties except for their optical activity, that
is, rotation of polarized light. They are particularly
useful for assessing how molecular structure is en-
coded by olfactory systems as perceptual differences
between enantiomers cannot be caused by differing

Human and Animal Olfactory Capabilities Compared 32.2 Olfactory Discrimination Ability 683
PartD|32.2
Tab l e 3 2.4 Between-species comparison of olfactory discrimination performance with enantiomers
Human
subjects
Squirrel
monkeys
Asian
elephants
Fur
seals
CD-1
mice
Honey
bees
Pigtail
macaques
SD/LE
rats
Carvone + + + + + + + +
Dihydrocarvone + + + + + +
Dihydrocarveol + + + + + +
Limonene ++++ + + +
˛-Pinene + + + + +
Isopulegol   ++ +
Menthol   + + + +
ˇ-Citronellol   + + + +
Rose oxide   ++
Fenchone + + + + +
Limonene oxide   + + + 
Camphor   ++
Success rate 5/12 6/12 12/12 8/12 11/11 5/8 5/6 3/3
A “+” symbol indicates that the group of animals or subjects succeeded in discriminating a given enantiomeric odor pair, and a “”
symbol indicates failure to do so. Human data: [32.70,71]; squirrel monkey data: [32.72,73]; Asian elephant data: [32.63]; fur seal
data: [32.74]; mouse data: [32.75]; honeybee data: [32.76]; pigtail macaque data: [32.73]; rat data: [32.77,78]
diffusion rates in the mucus covering the olfactory ep-
ithelium or differing air–mucus partition coefficients,
but must originate from chiral selectivity at the receptor
level [32.79].
Human subjects, as a group, are only able to dis-
criminate between 5 of the 12 enantiomeric odor pairs
tested and thus perform similar to squirrel monkeys,
which succeeded with 6 out of 12 pairs. Asian ele-
phants, mice, and rats, in contrast, succeeded with 12
out of 12, 11 out of 11, and 3 out of 3 of these odor pairs.
South African fur seals, pigtail macaques, and honey-
bees are able to discriminate between the majority, but
not all enantiomericodor pairs tested with these species
(Table 32.4).
Based on these comparisons, humans appear to
have a slightly inferior olfactory discrimination ability
compared to some species such as mice and Asian ele-
phants. However, their performance in discriminating
between structurally related monomolecular odorants
appears to be comparable to that of squirrel monkeys,
fur seals, and honeybees.
A special, and only rarely investigated, aspect of
olfactory discrimination performance is the ability to
distinguish between different concentrations of a given
odorant. The smallest concentration difference that
a nose can reliably detect is often referred to as just
noticeable difference (JND) and is usually expressed
as a so-called Weber fraction (according to Weber’s
law, which states that the JND between two stimuli is
proportional to the magnitude of the stimuli: I=ID
constant). A Weber fraction of 0:3, for example, in-
dicates that a stimulus has to be presented at a 30%
higher intensity, relative to a standard stimulus, in order
to be reliably perceived as stronger than the stan-
dard.
The human JND has been found to be odorant-
dependent and Weber fractions have been reported
to range from 0:09 for n-pentyl butyrate, 0:16 for
n-pentanol, 0:30 for pyridine, 0:35 for n-butanol, to
0:47 for phenylethanol [32.80–82]. The only study that
directly compared JNDs between species foundthe We-
ber fraction for n-pentyl acetate to be 0:041 in rats and
thus considerably lower than the 0:315 found with this
odorant in humans [32.83]. However, a later study re-
ported the rat’s Weber fraction for the same odorant to
be 0:28, and thus comparable to that of humans [32.84].
A possible explanation for the extreme paucity of
animal data on olfactory JNDs is their difficulty in
learning the concept that two different concentrations
of the same odorant should have different reward val-
ues. To understand this difficulty, one must know that
in operant conditioning procedures an animal learns
that an odorant A is rewarded and that an odorant B
(or a blank stimulus) is not rewarded. Once an ani-
mal has successfully learned the association between
odorant A and a reward, it will consider this odorant
as a rewarded stimulus irrespective of its concentration.
Biologically, this makes perfect sense as odors that are
behaviorally relevant for an animal hardly ever change
their significance as a function of concentration: a food
odor will always be attractive, whether detected at high
or low concentrations, and a predator odor will be al-
ways avoided, whether at high or low concentrations.
Thus, it takes an animal extensive training to overcome
this perseverance with regard to the learned reward
value of odorants.

PartD|32.3
684 Part D Odorant Sensing and Physiological Eects
32.3 Qualitative Comparisons of Olfactory Capabilities
Between Species
In addition to using quantifiable measures of olfac-
tory performance, such as sensitivity and discrimination
ability, one can also try to compare the sense of smell
between species using qualitative measures. In the sim-
plest case, this means to assess whether a given species
possesses a certain ability or not. To this end, it might be
useful to take a look at different behavioral contexts in
which the sense of smell is known to play a role for an-
imals and then to ask whether human subjects are able
to use their noses for the same purpose.
32.3.1 Gathering Information
About the Chemical Environment
The ability to gather information about the chemical en-
vironment is almost ubiquitous in the animal kingdom.
In most species of animals, the detection of chemi-
cal hazards, for example, leads to adaptive behavioral
responses intended to minimize further exposure. Hu-
mans are no exception to this rule and display adaptive
behaviors such as head turning, eye closure, apnea (sus-
pension of breathing), sneezing, and coughing when
exposed to harmful volatile chemicals. Although some
of these reflex-like responses involve the nasal trigem-
inal system, the significance of the olfactory system in
this context becomes apparent when considering anos-
mic subjects: humans without a functioning sense of
smell run a significantly higher risk of suffering from
gas poisoning [32.85] and of not detecting fire [32.86]
compared to healthy controls.
32.3.2 Foraging and Food Selection
The ability to find and select food using the sense
of smell is also widespread among animal species.
Human babies, similar to the offspring of other mam-
mals, already display adaptive behavioral responses
such as positive head turning and gaping mouth move-
ments when presented with the odor of their mother’s
breast or the odor of breast milk [32.87]. Although hu-
mans nowadays hardly ever need to use their noses
to forage, that is, to search for food, they do possess
the ability to follow a food odor scent trail [32.88].
The involvement of the sense of smell in human food
selection is more obvious: anosmic subjects run a sig-
nificantly higher risk of suffering from food poisoning
compared to healthy controls with an intact sense of
smell [32.89]. Similarly, the risk of suffering from
malnutrition is markedly higher in anosmic subjects
compared to normosmic control subjects [32.90]. Not
surprisingly, anosmic subjects – particularly those who
lost their sense of smell instantaneously, for exam-
ple, due to head trauma – often report a consider-
able loss of quality of life with regard to enjoying
food [32.91].
32.3.3 Spatial Orientation
Quite a number of animal species rely on their sense
of smell for spatial orientation. This is particularly true
for nocturnal and subterranean species. There are two
basic mechanisms that allow animals to find their way
through their habitat using their sense of smell: they
can either use existing landmarks that emit an odor,
or they can deposit scent marks themselves at certain
points in their habitat. Both types of odor sources serve
as navigational landmarks that can be used by animals
to recognize their position in space.
Although humans with an intact sense of vision
hardly ever need to use their noses for findingtheir way,
they do possess the ability to follow scent trails laid
by conspecifics and when blindfolded [32.88]. A re-
cent study suggests the existence of spatial information
processing in the human olfactory system and thus of
an implicit ability for directional smelling [32.92]. Fur-
ther, there is anecdotal evidence that blind persons use
olfactory landmarks as sensory cues for spatial orienta-
tion [32.93,94].
32.3.4 Social Communication
Many species of animals have been shown to use body-
borne odors for social communication. Such odors may
convey information about species, social group, sex,
age, reproductive status, health status, social rank, in-
dividual identity, genetic relatedness, dietary habit, and
emotional state of the odor donor.
Psychophysical studies have demonstrated that hu-
mans are able to correctly assign body odors to
sex [32.95] and to age classes [32.96]. Similarly, it has
been shown that humans are able to correctly assign
body odors to reproductive status [32.97,98]andto
health status [32.99,100].
Further, humans are able to distinguish between in-
dividual body odors [32.101] and between body odors
of kin and nonkin [32.102,103]. This includes mutual
recognition of the body odors between mothers and
their babies [32.104].
Humans are also able to distinguish between the
body odors of vegetarians and meat eaters [32.105]and

Human and Animal Olfactory Capabilities Compared 32.4 General Conclusions 685
PartD|32.4
thus they can sniff out the dietary habits of conspecifics.
Recent studies suggest that humans are also able to de-
tect emotions via body odors [32.106,107].
Dogs, mice, and rats are not only able to discrimi-
nate between the odors of individual conspecifics, but
also between the odors of individuals of other species,
for example, of individual humans. However, it has been
demonstrated that human subjects are also able to cor-
rectly identify the odor of their own pet dog among other
dog odor samples [32.108], to correctly assign body
odor samples to individual gorillas [32.109],andtocor-
rectly discriminate between the body odors of mouse
strains that only differ from each other in their major his-
tocompatibility complex (MHC) genes [32.110].
32.3.5 Reproduction
The ability to correctly identify the reproductive status
of potential mates using olfactory cues is widespread
among mammals. Similarly, mate choice has been
shown to be based on or at least influenced by ol-
factory cues in a number of mammal species. Human
males are able to distinguish between female body
odors from different phases of the estrous cycle and pre-
fer the body odor of females produced around the time
of ovulation [32.97,98,111]. Several lines of evidence
suggest that human mate choice may be influenced
by body odors and that, as is the case in mice, MHC
genes are involved in the formation of individual odor
signatures [32.112–114]. A recent study found that
congenitally anosmic men exhibit a strongly reduced
number of sexual relationships compared to norm-
osmic men, and that congenitally anosmic women feel
less secure about their romantic partner compared to
normosmic women [32.115]. Further, anosmia-related
depression has been shown to reduce sexual appetite in
humans [32.116].
32.3.6 Learning and Memory
Learning and memory are inevitably linked to sensory
input. Many species of animals rely on olfactory cues
for learning about their environment or about situa-
tional contexts as well as for building and retrieving
memories. Humans are no exception to this as they
are able to rapidly and robustly learn long-lasting as-
sociations between food odors and positive or negative
physiological consequences of ingestion [32.117]. Sim-
ilarly, humans are able to rapidly and robustly learn
appetitive and aversive associations between odors and
visual stimuli [32.118]. Several studies reported human
long-term memory for odors to be outstanding and su-
perior to that for other sensory modalities [32.119]. The
longest interval tested so far with humans for successful
odor recognition was 1 year [32.120], and for above-
chance level retention of odor–name associations even
9 years [32.121].
From these qualitative comparisons of olfactory ca-
pabilities, one must conclude that humans have at least
the basic ability of using their sense of smell in all be-
havioral contexts in which animals are known to use
their noses.
32.4 General Conclusions
The amount of published data on quantifiable measures
of olfactory performance, which allow for direct com-
parisons between humans and animals, is rather limited.
However, based on this limited set of data the following
conclusions can be drawn:
Human subjects have lower olfactory detection
thresholds, that is, a higher sensitivity with the major-
ity of odorants tested so far compared to most of the
mammal species tested so far. This includes species tra-
ditionally considered to have a highly developed sense
of smell such as mice, rats, hedgehogs, shrews, pigs,
and rabbits.
Human subjects appear to have a slightly inferior ol-
factory discrimination ability compared to species such
as mice and Asian elephants. However, their perfor-
mance in discriminating between structurally related
monomolecular odorants appears to be comparable to
that of species such as squirrel monkeys, fur seals, and
honeybees.
Qualitative comparisons of olfactory capabilities
suggest that human subjects have at least the basic
ability of using their sense of smell in all behavioral
contexts in which animals are known to use their noses.
This includes gathering of information about the chem-
ical environment, food selection, spatial orientation,
social communication, reproduction, as well as learn-
ing and memory.
Taken together, these findings suggestthat the human
sense of smell is not generally inferior compared to that
of animals and much better than traditionally thought.

PartD|32
686 Part D Odorant Sensing and Physiological Eects
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