July 7, 2020
Written by: Greer Prettyman
Do you ever wonder why your puppy greets other dogs in the neighborhood with a thorough sniff of the butt? Do you ever wonder why humans don’t do this?
Beyond the obvious reason that this kind of behavior would be extremely socially taboo in humans, we also lack the sensory system that allows animals to gather information from this type of interaction. Many types of animals, including your pet dog, have a special sensory system called the accessory olfactory system (AOS) that is sensitive to social chemicals from other animals. These chemicals, also called pheromones and semiochemicals, help animals detect information about their social interaction partners, including their age, sex, and social status.
A Social Smell System
The main olfactory system allows animals and humans to sense chemical odorants in the air. In contrast, the accessory olfactory system is primarily sensitive to nonvolatile compounds that the main olfactory system cannot detect. For this reason, the AOS is only activated by direct contact with a chemical signal, making it more similar to our sense of taste that requires chemical compounds in food to make contact with the tongue directly1. The sensory organ of the AOS, the vomeronasal organ (VNO), is located in an animal’s sinus (Figure 1). Animals sense non-volatile compounds by getting close to other animals and sniffing to bring the chemicals into contact with the VNO.
Vomeronasal sensory neurons within the VNO are activated when an animal senses a social chemical. These neurons send signals to the accessory olfactory bulb (AOB), a small structure in the front of the brain that lies just behind the main olfactory bulb. Neurons in the AOB called mitral cells receive these signals and transmit them to other regions of the brain that regulate behaviors such as mating, fighting, and parenting. These regions include the bed nucleus of the stria terminalis (BNST), the amygdala, and the hypothalamus1. The social information received through the AOS helps animals to regulate their social behaviors.
The types of chemicals that carry social signals fall into several categories based on their chemical makeup and mechanisms of release. Levels of these chemicals differ in certain categories of animals, such males and females. The particular set of chemicals an animal releases may even serve as a type of fingerprint, allowing others to identify that particular animal by the unique set of signals it gives off.
Steroids are one class of chemicals that often carry social information and are sensed by the VNO. Some steroids found in urine of rodents provide information about the estrous cycle of females. Specific steroid compounds found in the urine of mature females but absent in the urine of juveniles and males serve as a signal that males use to select only mature female mates2.
Other types of chemicals in urine also carry social information, including a class called major urinary proteins (MUPs). One MUP called MUP20 or darcin (named for Mr. Darcy in Pride and Prejudice) is released only by males and is an attractant for females3. Males who are socially dominant, such as those who successfully beat other males in a competition for territory, express higher levels of darcin and are thus more attractive to females4. This effect can help to explain why dominant animals have better reproductive success than subordinate animals. Males who sense MUPs from other males typically respond with aggression and territorial behavior5. These urinary proteins activate the AOB which regulates downstream regions to facilitate fighting behaviors6.
Other neurons in the AOB are activated specifically during interactions with another animal’s face1. For example, exocrine gland-secreting peptides (ESPs) activate the AOS during facial interactions7. Particular members of the ESP family of peptides are expressed in different levels based on an animal’s age and sex and allow interacting animals to identify these traits and select appropriate behaviors. For example, ESP1 is expressed in adult male mice but not in females or juveniles. When a female mouse senses ESP1, she initiates mating behavior. However, when a male mouse senses ESP1, he initiates aggressive behavior toward this other male. Male mice that lack the receptor to sense ESP1 are less aggressive. Another ESP called ESP22 is expressed in high levels in juvenile animals and then levels drop with maturity. Sensing ESP22 inhibits males from initiating mating with juvenile animals8. The specific combinations of these ESPs allow animals to initiate appropriate behaviors and inhibit incorrect behaviors with their interaction partners.
Do humans have pheromones?
While rodents, dogs, and many animals have an AOS and use these signals for social communication, other animals lack this system, including birds, dolphins, many primates, and humans. Humans lack an AOB and have non-functional genes that would encode the VNO9. For this reason, a lot of claims about human pheromones, particularly when they are trying to sell you a product like a “pheromone perfume” that will help you attract a mate, are obviously pseudoscience. While there is evidence that we can gather information from socially-relevant odors, such as a partner’s sweat, using our olfactory systems, humans will never be able to do the type of social sensing that animals can do by using their VNOs.
- Holy, T. E. (2018). The Accessory Olfactory System: Innately Specialized or Microcosm of Mammalian Circuitry? Annual Review of Neuroscience, 41(1), 501–525.
- Fu, X., Yan, Y., Xu, P. S., Geerlof-Vidavsky, I., Chong, W., Gross, M. L., & Holy, T. E. (2015). A Molecular Code for Identity in the Vomeronasal System. Cell, 163(2), 313–323.
- Roberts, S. A., Simpson, D. M., Armstrong, S. D., Davidson, A. J., Robertson, D. H., McLean, L., … Hurst, J. L. (2010). Darcin: A male pheromone that stimulates female memory and sexual attraction to an individual male’s odour. BMC Biology, 8(1), 75.
- Thoß, M., Luzynski, K. C., Enk, V. M., Razzazi-Fazeli, E., Kwak, J., Ortner, I., & Penn, D. J. (2019). Regulation of volatile and non-volatile pheromone attractants depends upon male social status. Scientific Reports, 9(1), 1–14.
- Chamero, P., Leinders-Zufall, T., & Zufall, F. (2012). From genes to social communication: Molecular sensing by the vomeronasal organ. Trends in Neurosciences. 35(10), 597-606.
- Guo, X., Guo, H., Zhao, L., Zhang, Y. H., & Zhang, J. X. (2018). Two predominant MUPs, OBP3 and MUP13, are male pheromones in rats. Frontiers in Zoology, 15(1), 6.
- Kimoto, H., Haga, S., Sato, K., & Touhara, K. (2005). Sex-specific peptides from exocrine glands stimulate mouse vomeronasal sensory neurons. Nature, 437(7060), 898–901.
- Ferrero, D. M., Moeller, L. M., Osakada, T., Horio, N., Li, Q., Roy, D. S., … Liberles, S. D. (2013). A juvenile mouse pheromone inhibits sexual behaviour through the vomeronasal system. Nature, 502(7471), 368–371.
- Trotier, D. (2011). Vomeronasal organ and human pheromones. European Annals of Otorhinolaryngology, Head and Neck Diseases. Elsevier Masson SAS.
Cover image from Nick Fewings via Unsplash https://unsplash.com/photos/2qXNT7LMfpY
Figure 1 created with BioRender