Mammals can recognize a large variety of scents that give information about the environment,
conspecifics, and other species. The present research topic is focused on “scents
that matter,” i.e., scents that indicate stimuli which are crucial for the survival
of an organism. These can be positively related stimuli like the smell of familiar
conspecifics, mating partners, or food, but also negatively related stimuli like the
scent of potential predators, spoiled food, or territorial and aggressive conspecifics.
A prerequisite for this important role of scents in animals' lives is that they can
be well detected and recognized. During the last decades, our understanding of olfactory
perception has been largely improved, mainly inspired by the work of Linda Buck and
Richard Axel (e.g., Buck and Axel, 1991), which was awarded by the Nobel Prize in
2004. Many of the scents studied in this research topic are processed by the vomeronasal
system (e.g., Haga-Yamanaka et al.; Yu), but quite often the main olfactory system
is additionally involved (e.g., Rattazzi et al.). A lot of current research addresses
the questions about which molecules activate which olfactory receptors and which molecular
cascades are modulated by these receptors, or how the different olfactory receptors
and the two olfactory systems work together. In the current research topic the articles
of Ben-Shaul, Kelliher and Munger, Rattazzi et al. and Yu provide new perspectives
in this interesting field of research.
Besides the detection mechanisms of relevant scents, many studies are focusing on
the behavioral changes induced by these scents. Most of these studies are analyzing
scents signaling potential dangers. One reason for focusing on danger-signaling odors
may be that the behavioral effects of these scents are easier to be induced and measured.
In addition, it is widely believed that these scents are more critical for fostering
the survival of animals. Basically, such danger-signaling scents with aversive-like
effects are classified as (a) kairomones, which are emitted by another species such
as predators (e.g., Apfelbach et al.; Osada et al.) or (b) pheromones, that are emitted
by conspecifics such as alarm pheromones (e.g., Kobayashi et al.; Breitfeld et al.).
Both classes of scents warn about a potential threat, which is intended in the case
of pheromones, but unintended in the case of kairomones as they lead to a detriment
of the emitter (see Nielsen et al.). It is widely believed that predator odors and
alarm pheromones are innately recognized, as these stimuli are still effective in
laboratory animals that have lived many generations in the absence of predators (Apfelbach
et al.; Fendt et al., 2005).
In addition to the general impact of predator odors on the behavior of prey animals,
an interesting line of research is the identification of active components in these
scents. In the case of predator odors, several molecules have been identified so far:
trimethlythiazoline (Taugher et al.; Fortes-Marco et al.; summarized in Rosen et al.),
different pyrazines (Osada et al.), and pyridines (Brechbühl et al.), or 2-phenylethylamine
(Ferrero et al., 2011). In the present research topic, a number of studies demonstrating
that these compounds are able to induce a wide array of defensive responses in laboratory
rodents such as avoidance behavior (Wernecke and Fendt; Brechbühl et al.; Fortes-Marco
et al.), freezing (Taugher et al.; Fortes-Marco et al.), risk assessment behavior
(Breitfeld et al.), or an inhibition of appetitive-like behavior (Kobayashi et al.),
as well as physiological changes like a modulation of blood pressure (Brechbühl et
al.), or breathing (Taugher et al.). Although these single molecules have the advantage
that they can be better controlled in an experimental procedure (e.g., concentration),
the natural scents, i.e., blends, are usually more efficient in inducing behavioral
changes (summarized in Apfelbach et al.).
The neural mechanisms underlying the behavioral and physiological changes induced
by danger-signaling scents are meanwhile partly understood. In the current research
topic, studies are focused on brain sites like the bed nucleus of the stria terminalis
(Breitfeld et al.; Taugher et al.), the medial amygdala (Carvalho et al.), the periaqueductal
gray (Canteras et al.), and different subnuclei of the hypothalamus (Canteras et al.;
Kobayashi et al.). Interestingly, these brain sites are of minor or no importance
for learned fear whose neural basis is well understood (Fendt and Fanselow, 1999;
LeDoux, 2012), suggesting a clear neuronal differentiation between innate and learned
In fear learning, the danger-predicting property of a stimulus is learned by Pavlovian
associative learning. Of course, olfactory stimuli can be used for such associative
learning, either as unconditioned (Yuan et al.; Fortes-Marco et al.) or conditioned
stimuli (Ferry et al.; Yuan et al.). The latter means that a scent without emotional
valance can gain danger-predicting, i.e., fear-inducing, properties. Notably, even
if a stimulus from another sensory modality is used as a conditioned stimulus in such
a fear learning experiment, scents may still play some roles, since they are usually
part of the experimental context (e.g., conditioning box, experimenter) and may be
associated with the danger simultaneously. In fear learning, the lateral amygdala
is important for associating a discrete cue with a danger stimulus, whereas the hippocampus
plays an important role in contextual fear learning. Interestingly, novel work of
the present research topic demonstrated that different regions of the hippocampus
have different roles during contextual fear conditioning with odors (Yuan et al.).
In addition to the hippocampus, several cortical areas such as the entorhinal cortex
are involved in contextual fear learning (Ferry et al.).
So far, there is little research on the effects of danger signaling scents in humans.
However, the defensive behaviors induced by danger-predicting scents and the respective
physiological changes observed in animals are connected to anxiety in humans. Therefore,
one perspective is that a deeper understanding of the neuroanatomical and neuropharmacological
basis of odor-induced fear in animals may also help to find new treatment strategies
for anxiety disorders in humans.
As noted above, scents can also serve as positive stimuli. This is of specific interest
in the context of social behavior (Wöhr; Noack et al.; Fuzzo et al.) and foraging
(Kelliher and Munger). These aspects are also covered by several articles in this
special issue. It has been shown that one important function of these scents is to
help to recognize social partners (Ben-Shaul; Noack et al.). Thereby, they induce
and modulate a variety of behaviors, including ultrasonic calls which are typical
for pleasant situations (Wöhr). In the case of social buffering, the scent of a conspecific
is able to reduce fear (Fuzzo et al.). These two, quite different effects of social
scents are mediated by different subnuclei of the amygdala (Fuzzo et al.; Noack et
al.). Notably, there is also potential for translational research with “social scents.”
For example, a genetic mouse model of autism is less able to modulate ultrasonic vocalization
in response to familiar scents (Wöhr).
The present research topic nicely represents the different approaches used in “olfactory
research” of relevant scents. These approaches include cell biology, genetics, behavioral
pharmacology, neuroanatomy, as well as computational neuroscience. Scientists from
all these fields work effectively together to unravel the mechanisms of how scents
matter in humans and animals.
We are grateful to all contributors of this research topic. Eighty-five different
authors from 10 different countries contributed with research and review articles.
Furthermore, we thank the reviewers which helped us and the authors to create an interesting
and high-quality research topic.
We hope that you enjoy reading this research topic as much as we have enjoyed editing
MF wrote the first draft of the editorial, all authors revised the manuscript and
approved the final version of it.
Conflict of interest statement
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.