We all know that a dog’s sense of smell is incredible and far superior to our own, with over 300 million scent receptors in its nose compared to our paltry 6 million. Which is why dogs have been trained and employed to help us in countless ways for as long as anyone can remember. To offer just a few examples: bloodhounds track criminals, medical detection dogs predict the onset of seizures and cancer, police dogs find trafficked animals and illegal contraband, canine ‘surveyors’ identify dry rot and gun dogs flush game.
Yet despite the many ways in which we are able employ a dog’s sense of smell, we still know very little about how scent communication in animals actually works. This is because it’s hard to detect, it’s highly sophisticated and, as our own reliance on scent communication is under researched and often misunderstood, we find it difficult to understand.
Scent is an area of communication in the animal world (used by everything from insects to fish and from reptiles to birds) that is absolutely critical for sending messages between and within species – it’s a bit like the very first natural Internet. Together with other forms of communication such as vision, hearing, taste and touch, it is a way of learning about the place we live in, and whom we share it with. It is remarkably versatile, too, serving to help in the search for the essentials of life (food and mates, for example) as well as to perform more complex tasks such as synchronising and repressing reproduction, to signal aggression and family bonding.
Any form of communication requires a sender and receiver and the objective is to create a response. The most obvious form of this is seen in dogs where a male dog will ‘mark’ another dogs’ scent mark. The dog’s long muzzle facilitates the separation of breath and smell at the same time, a kind of radar, monitoring the environment constantly. They have a specialist part of the brain dedicated to decoding smells. A dog can sieve out smells in the same way that whales sieve krill from the sea.
The ‘computer’ that makes sense of these signals is the Jacobsen organ, which some scientists consider to be so powerful that they refer to it as ‘a second brain’. Responses to signals are often seen as specific behaviours and assist to tease out the information from a scent. The most obvious of which you will have seen in horses as they curl their top lips – the Flehmen response, which draws the scent through the Jacobsen organ.
One of the reasons for communicating via scent is to change another individual’s behaviour. This might be a physical change, to bring a female into season for example, or for navigation, to find mates or to avoid enemies. A single scent mark can contain a huge amount of information, it’s almost like a regular social media post, telling the recipient everything you want to know, much that you don’t and allowing the recipient to make deductions. The sort of information a mark transmit could include: reproductive status, diet, recency of visit, whether alone or with another and so forth. The recipient not only receives information but is also able to make deductions: the marker is bigger than me, eating better food than me, healthier than me, maybe even having more fun than me… not, in some respects, unlike the effect of social media
A unique quality of scent is that it is deposited and remains in the environment. In other words, it allows species to communicate without both having to be in the same place at the same time. In this way, it can can help to avoid conflicts and allow animals to assess whether there are benefits to meeting each other. Scents are frequently combined with visual reference points (think lamp posts), and may be accompanied by visual marks such as scratches from claws. Have you ever seen your dog scuffing the ground with their back feet? Sites to mark may be chosen for specific qualities. This ensures the signal reaches the intended recipient; surface texture, height, local environmental conditions and their effects on surface bacteria are also essential to the message.
The timing of mark is key to the message too: scent mark chemistry changes over time and therefore the message and meaning changes too. For example – scent is used by ring tailed lemurs to waft threats at competitors in a ‘stink fight’ which backs up aggressive posturing face to face but scent left on clothing. As many dog lovers know, a well-worn item of clothing, can be a huge reassurance to dogs suffering from separation anxiety, although the effect wears off over time. in short, the traditional view of territorial scent marking – that animals scent mark as a ‘keep out’ warning to others – is an over simplification.
The mix of messages left in the environment are like an over-crowded noticeboard of varying importance to the receiver, all with best before and sell by dates on them, the messages can change depending on who receives them, how they receive them (what detection equipment they have), when they receive them (the environmental conditions where they are deposited) and how important that message is to them at the time. Depositing scent in the environment and at specific locations lends itself neatly to navigation, the gradient of scent that wafts from a scent mark means that again other animals can use this to pinpoint, with great accuracy, the scents’ origin and navigate towards or away from it. This is obvious in fish migrations and recent research has even shown that birds have a sense of smell and use this for navigational purposes. Trails of scent are left all over the environment and can take the form of expensively produced glandular secretions such as musk, to urine and faeces, a physiologically cheap but effective alternative.
However, it’s not always the intended recipient that can take advantage of these signals. It’s also known that predators can eavesdrop on scent mark conversations too, to locate prey and maximize their chances of a kill, for example kestrels are able to ‘see’ the ultraviolet signals of rodent urine trails and are able to locate well-used areas. At the ultimate extreme, reproduction can be curtailed completely in the case of mole rats whose lifestyles are akin to a beehive with just one queen able to reproduce at any one time.
So, as you can see, the world of animal smells is a vast and complex one … a world-wide natural web. Next time your faithful friend stops suddenly fixated on sniffing a blade of grass, they are just checking their Instagram feed and being nosey!
By Dr Tracey Rich
Dr Tracey Rich is a zoologist and wildlife photographer. Her website is www.traceyrich.com
