Olfaction

What are the 3 basics of Olfaction

• Chemosensation: One of the most ancient senses; detects volatile chemicals for survival and reproduction.

• Olfactory system: Directly linked to limbic system (emotion and memory), explaining smell-memory link.

• Proust Effect: Smells evoke vivid emotional memories better than other senses.

What are pheromones and what are the 2 types

Definition: Chemical signals between members of the same species that trigger behavioral or physiological changes.

Types:

• Releaser: Immediate behavioral effect.

• Primer: Long-term physiological changes.eg puberty

What are some of the functions of pheromones (9)

Functions:

• Mate choice & sexual selection

• Kin recognition

• Social bonding

• Menstrual synchrony

• Territorial marking

• Trail marking (e.g., ants)

• Deception (e.g., bolas spiders mimic female moths)

What are the 6 key animal studies

• Silkworm moths: Discovery of “bombykol” (first pheromone); effective at 1 molecule in 10 million.

• Trail pheromones: Used by ants to guide others to food.

• Dauer pheromone (C. elegans): Signals larvae to enter dormancy in poor conditions.

• Dominance pheromones: Seen in wolves through scent gland marking.

• Coolidge Effect: Male re-mating behavior triggered by novel female scent.

• Bruce Effect: Pregnancy block in mice upon exposure to unfamiliar male scent.

Silkworm moths Experiment

Key Findings

1. Bombykol is a volatile sex pheromone produced by female moths.

2. It is released into the air during mating periods to attract males.

3. Males have highly sensitive olfactory receptors on their antennae, specifically tuned to bombykol.

Trail Pheromones experiment

1. Purpose: Ants use trail pheromones to mark the path from a food source back to the nest, guiding other ants.

2. Secretion: Pheromones are released from glands as ants drag their abdomen along the ground.

3. Behavior:

• Other ants detect and follow the trail.

• More ants reinforce the trail if the food source is good, creating a positive feedback loop.

4. Persistence:

• Trails naturally fade if not reinforced.

• Some ants use a repellent pheromone to mark useless or dead-end trails.

5. Efficiency:

• Ants collectively find the shortest path to food through repeated reinforcement.

Dauer pheromone (C. elegans): experiment

1. C. elegans enters a special dormant stage called the dauer stage during harsh conditions (e.g. overcrowding or starvation).

2. The dauer pheromone is a chemical signal secreted by the worms that builds up as population density increases.

3. When larvae detect high pheromone levels (plus low food), they enter the dauer stage instead of growing normally.

4. Dauer worms are non-feeding, stress-resistant, and long-lived, helping them survive until conditions improve.

5. Once the environment improves, they exit the dauer stage and resume development.

Dominance pheromones: Seen in wolves through scent gland marking Experiment

Dominance Pheromones in Wolves – Explained

1. Scent Glands in Wolves:

• Wolves have multiple scent glands located at the base of the tail, paws (between toes), eyes, genitals, and skin.

• These glands secrete pheromones—chemical cues unique to each individual wolf.

2. Social Hierarchy Communication:

• Dominant wolves use their scent to mark pack members and maintain social structure.

• They often rub their bodies (especially flanks and head) against subordinate wolves, leaving their scent.

3. Function of the Behavior:

• This behavior helps reinforce dominance and signal pack membership.

• It keeps the social hierarchy stable without constant aggression.

4. Territory & Identity:

• Pheromones also help identify individual wolves and their rank, contributing to territory marking and pack cohesion.

Coolidge Effect experiment

1. Definition: Males show renewed sexual interest when introduced to a new receptive female, even after losing interest in the previous one.

2. Experiment:

• Male rats/mice mate with one female until sexually satiated.

• Remain uninterested in the same female.

• Reignite mating behavior when a new female is introduced.

3. Trigger:

• Response is driven by pheromonal and olfactory cues from novel females.

4. Purpose:

• Increases reproductive success and genetic diversity.

• Evolutionarily beneficial for maximizing mating opportunities

What are the 4 different phenomenal effects on female rodents

1. Coolidge Effect

• Re-mating occurs when a new mate is introduced.

• Driven by novel female pheromones stimulating male sexual interest again.

2. Vandenbergh Effect

• Male presence accelerates puberty in female rodent pups (as early as 40 days).

• Caused by male pheromones.

• Interestingly, isolation also speeds up puberty vs. being housed with other females (due to female-released inhibitory pheromones).

3. Lee-Boot Effect

• Females housed together in small groups without males show suppressed/reduced ovulation.

• Maintained by progesterone from the corpus luteum.

• Triggered by female pheromones in a male-free environment.

4. Whitten Effect

• In absence of males, females become anestrous (stop cycling).

• Introduction of a male or male scent causes females to synchronize their cycles within 3–4 days.

• Evolutionary benefit: synchronized births increase survival odds for offspring.

Bruce Effect experiment

1. What Is It?

The Bruce Effect is a phenomenon in rodents (especially mice) where a pregnant female spontaneously aborts her embryo when exposed to the scent of an unfamiliar male (not the father).

2. How It Happens:

• Female mice can detect male pheromones through the vomeronasal organ (VNO).

• If she encounters the scent of a strange male (different from the one who impregnated her), this triggers neuroendocrine signals that block implantation or cause early pregnancy loss.

• The effect typically occurs within the first 4 days of pregnancy (pre-implantation stage).

3. Why It Happens (Evolutionary Perspective):

• Prevents the female from investing resources in offspring likely to be killed by a new dominant male.

• Aborting allows her to re-mate with the new male, improving reproductive success under changed social conditions.

4. Who Discovered It?

• Named after Hilda Bruce, who first described it in 1959.

Key Points Summary

• Trigger: Scent of an unfamiliar male.

• Effect: Female terminates early pregnancy.

• Organ Involved: Vomeronasal organ (VNO).

• Function: Avoids wasted investment in offspring that might be at risk.

• Outcome: Female becomes fertile again within 3–6 days, often mating with the new male.

What is the olfactory systems

1. Main Olfactory System: Detects volatile odors.

2. Accessory Olfactory System:

• Includes Vomeronasal Organ (VNO) – detects non-volatile pheromones.

• Sends signals to amygdala and hypothalamus (influences behavior/hormones).

• In humans: thought to be vestigial or absorbed into main olfactory system.

What is MHC

• MHC genes code for special protein markers that attach to a cell and help the body recognize whether a cell belongs to the body or is an intruder.

• Different MHC molecules recognize different intruders.

• It is advantageous to a mother to select a mate that carries MHC genes different from her own.

• Female rodents learn the MHC identity of their relatives (via pheromones in urine) during development and prefer to mate with males who carry dissimilar MHC genes.

• Evolutionary drive

Pheromones – Odor Imprinting 2 types

1. Direct Odor Imprinting

• Definition: The animal directly learns the odor of another individual through personal exposure.

• Example:

• A rodent pup imprints on its mother’s scent, learning her specific odor through direct physical contact (e.g., nursing).

• Maternal bonding is formed through this early chemical exposure.

• Function: Enables recognition, bonding, and survival-related behaviors (like following the mother).

2. Indirect Odor Imprinting

• Definition: The animal learns odor cues associated with individuals or environments through contextual or associative learning, not necessarily through direct contact.

• Example:

• A female mouse can learn the MHC scent profile of her relatives by growing up around them—even without direct interaction.

• Later, she may prefer mates with dissimilar MHC to avoid inbreeding.

• Function: Influences mate choice, social memory, and kin recognition.

Feature	Direct Imprinting	Indirect Imprinting
Based on	Physical contact	Environmental/contextual exposure
Learned From	Individual (e.g., mother, pup)	Social group or shared environment
Examples	Maternal recognition, nipple preference	MHC-based mate selection, kin recognition
Timing	Often neonatal/early life	Can happen over a longer developmental windoW

Fun fact About the production of human pheromones

• Urine (dogs can detect bladder cancer from human urine: due to alterations in urinary odorants caused by MHC changes in oncotic cells)

What evidence is there for pheromones in humans (5)

Evidence:

• Menstrual synchrony: McClintock effect via axillary compounds.

• MHC preference: Women prefer the scent of men with dissimilar immune genes (MHC).

• T-shirt studies: Male sweat influences female ovulation timing.

• Brain imaging: Different brain areas activated in men vs. women by AND & EST compounds.

• Mother-infant recognition: Infants and mothers recognize each other by smell.

T shirt studies (1976 Russell and Wallace)

1. Odours collected from the axilla by a t-shirt worn over several days

2. 1976 Russell and Wallace showed that people can determine the sex of another person through their odour (volatile steroids)

3. Axillar extracts from men make ovulatory cycles of woman more regular (Like mice)

4. Increases release of luteinising hormone which causes ovulatioN

Pheromone responses in humans PET scan (4)

1. PET scan was used to study the regions of human brain activation in response to smell (Savic et al., Neuron 2001).

2. They found that males and females responded differently to two different odorants:

3. EST induced activity in dorsomedial and thalamic nucleus in men, but not in women (region regulating penile erections in primates)

4. AND induced activity in ventromedial hypothalamus in women, but not in men (region associated with copulatory functions in primates)

What was the female responses to male armpit odour

1. Androgene odorants are linked to perception of sexual attractiveness by woman

2. Exposure to male odour shortened time to the next LH peak

3. By self report, made woman less tense and more relaxed

Female body odor as a cue to ovulation? (4)

1. Women wore one t-shirt on three consecutive nights during their late follicular stage (ovulatory), and then a different t-shirt for three nights during their luteal phase (non-ovulatory)

2. Males sniffed the t-shirts and rated them on attractively and “sexiness”

3. T-shirts worn during the follicular phase were rated as being more pleasant and sexy than those worn during the luteal phase.

4. The males olfactory system is tuned to chemical cues released by the female that signal fertility

What is Menstrual synchrony

• In the late 1960’s, Martha McClintock, noticed that women on her dorm floor seemed to menstruate on the same schedule

• For her undergrad honours thesis, she showed that menstrual synchrony was more likely among roommates than among women who lived in the same dorm building.

• Female axillary extracts (‘driver’ female to which cohabiting woman synchronise)

Odours and kin recognition (3)

• Humans can recognise their own odour

• Odour of their own kin (babies can identify and are attracted to both axillary and breast odour of their own mother; mothers recognise odour of their own baby).

• MHC-based olfactory signalling

Can humans detect an MHC-specific odour?

• Women prefer t-shirts worn by men with dissimilar MHC genes.

– Similar MHC reminded them of brothers and fathers.

– Dissimilar MHC genes reminded them of past boyfriends.

• Couples with similar MHC genes report more fertility problems.

What are three fun facts

• Women are better at detecting odors than men.

• Breast odor can disrupt other women’s cycles.

• Sperm have olfactory receptors—chemotaxis may aid fertilization.

What are 2 Commercial & Cultural References

• Napoleon to Josephine: Historic example of scent-based attraction.

• Realm perfume: Marketed with synthetic human pheromones.