A comment on my post about human pheromones got me thinking a bit more about the topic :-) Just how much do we know about these signalling chemicals & our ability to detect them?
Many animals use scent as a basis for communication. Many female moths release a sexual pheromone that males can detect from a considerable distance (using their featherlike antennae), flying up the 'odour stream' in the air as they seek a mate. Cats & dogs use urine sprays to mark territories & also to communicate about their social status - my old dog used to react quite differently to different scent marks, getting hugely excited by some & carefully backing off & walking around others. Bushbabies pee on their hands & then wipe urine on various surfaces (this, according to David Attenborough, is a black mark against otherwise adorable pets), while ring-tailed lemurs wipe pheromones from wrist glands onto their tails and then use these as scent wands during bouts of what are known as 'stink fights'.
But primates also rely heavily on other senses, vision in particular, and it's been fairly widely assumed that pheromones play only a very minor role (if any) in communication among humans and among other great apes. (Bear in mind here that communication doesn't have to be conscious, in the sense of making deliberate signals. You can tell a lot about an individual by paying attention to their body language, and as that research on 'fear' pheromones showed, these involuntary signals can have a significant impact on the behaviour of others.)
That has changed in recent years. For example, we know that vertebrates use vomeronasal receptors, concentrated in a vomeronasal organ (VNO - also known as Jacobson's organ), to detect pheromones. Primates have a small VNO, and this structure is vestigial in humans, apes, and 'Old World' monkeys, which would have contributed to the general feeling that we don't detect or respond to pheromones. However, several of the genes that express vomeronasal receptor proteins have been identified, and some work's been done on examining the evolution of one class of these genes (called the V1RL 1 genes) (Mundy & Cook, 2003). This research found evidence that in some primate groups V1RL 1 has been selected for, noting that this provides indirect support for the hypothesis that V1RL genes have a function in species-specific pheromone detection in primates. Humans express V1RL 1 mRNA (in our nasal epithelium, rather than the VNO), although we don't know whether a working receptor molecule is actually produced.
And I've also just read a review article (Grammer, Fink & Neave 2005) that goes over the available evidence for use of odours in general, and pheromones in particular, in our own species. Scent does appear to have a role in human reproductive behaviour, whether or not we're actually aware of it. What about pheromones, & that vestigial VNO? In addition to Mundy & Cook's work, Grammer & his colleagues write about patients with Kallman's syndrome, which occurs due to the underdevelopment of the olfactory bulb in the embryo and minimal [hormone] secretions from the hypothalamus. Individuals have underdeveloped gonads, lack secondary sexual characteristics, are anosmic [unable to smell], and preliminary research indicates that they show no response to pheromones (Grammer et al. 2005: 136). That is, lack of response to pheromones in these patients suggests that individuals with a 'normal' olfactory bulb & associated VNO do detect pheromones, even though we may not be aware of it.
What about response? Earlier studies effects of pheromones on human reproductive behaviour did suggest that pheromones had an influence, but many of these were flawed - not least because they relied on 'self-reporting' by the experimental subjects, which means that there was no objective record of results. But Grammer et al. note that more recent work does indicate that pheromones play an active role in reproductive behaviour, in terms of changing the way we perceive & respond to others. (Fertile ground for research by perfume manufacturers!) And of course, that 'scent of fear' research shows another way that pheromones can affect those detecting them.
Still more evidence - if any was required! - of our membership of the animal kingdom.
K.Grammer, B.Fink & N.Neave (2005) Human pheromones and sexual attraction. European Journal of Obstetrics & Gynaecology & Reproductive Biology 118: 135-142
N.J.Mundy & S.Cook (2003) Positive selection during the diversification of Class I vomeronasal receptor-like (V1RL) genes, putative pheromone receptor genes, in human and primate evolution. Molecular Biology & Evolution 20(11): 1805-1810. doi: 10.1093/molbev.msg192