Unnatural behaviour? What do dogs rolling in worms say about nature?
“The message is not so much that the worms will inherit the Earth, but that all things play a role in nature, even the lowly worm.” Gary Larson (1950- )
There’s nothing like a good roll in worms!
With the spring thaw and ensuing rain showers, the earthworms have got about their business again. Actively burrowing through the soil, they have created little mounds of detritus at each wormhole. When the ground has become overly saturated with water, they depart the safety of their subterranean routes and take an overland journey. This is when they are most susceptible to a dog in the mood for a roll.
In fact, once the ground gets damp, earthworms are susceptible to canine intent, whether they are above ground or not. The finely honed canine sense of smell readily sniffs out the characteristic earthy odour of worms and their abodes. Once they have been identified, the rolling begins.
If anything, dogs seem as keen on the little piles of detritus the earthworms leave outside their holes as the worms themselves. These little piles, or casts, are worm faeces. They are the product of the great efforts of the earthworm in creating an underground feast hall.
Earthworms produce casts as a by-product of their activity as soil engineers. Earthworms are like mobile, vegetarian digestive tracts. They travel above and through the soil collecting small pieces of organic matter and consuming it. Dead and rotting leaves, as well as plant matter embedded within the soil, all figure in the earthworm diet.
Three major types of earthworm function to convert organic matter into humus, the organic layer in soil that cannot be broken down further – the layer that provides sustenance for a diverse array of microbes and plants.
Non-borrowing earthworms live at the interface between leaf litter and the soil proper, consuming decomposing organic matter. In striking contrast to the non-burrowing earthworms, deep burrowing earthworms come to the surface periodically to bring leaf litter down to their lair, far beneath the ground. In between these two extremes are earthworms that reside in the top 10-30 cm of soil, the so-called topsoil or subsoil-dwelling earthworms that create elaborate subterranean tunnels and consume organic matter within the soil as they do so.
The earthworms that dwell in the topsoil are crucial to soil conditioning. Not only do they break down organic matter to enrich the soil with nutrients, but they aerate the soil when they create their burrows. Earthworms have been likened to little air pistons – pushing air through the soil. This soil aeration encourages the growth of microbes that support a variety of other organisms, and promotes access to organic matter and minerals by plant roots. The burrows also increase the number of channels for water flow through the soil to sustain the plants above. In the process, the worms also fertilise the soil through the production of their casts.
To dogs, worm casts are a delight – a canine equivalent of ambergris, the most sought-after perfumers ingredient one can imagine. Dogs will seek out casts and roll in them. They will also use the casts as indicators of a worm beneath, and excavate the hole until the worm is found. The worm is then merely chewed, not swallowed, as the real satisfaction is to be found in rolling in its mashed-up guts.
Why would a dog do this? What enjoyment is to be found in rolling in worms?
The prevailing hypothesis is that dogs roll in worms, as well as faeces and animal carcasses, to mask their scent. By rolling in these aromatic materials, dogs conceal their “dogginess” – they no longer smell like a dog. This is certainly true.
This still leaves the question, “Why would dogs desire to mask their smell?”
It has been suggested that scent masking is a throwback to a time prior to when dogs were domesticated. The hypothesis is that proto-dogs masked their scent as a means by which to evade detection by prey animals. This has some sense to it, but is there really any evidence to support it?
Today, wild canine species use their own scent as a part of a communication system that conveys social order, mating status, and relatedness. Masking one’s scent with that of another organism seems like it might be something that could subvert this means of communication. In keeping with this, there is no evidence to suggest that wild canine species prepare themselves for hunting by rolling in foul substances to mask their odour.
What’s more, it is difficult to understand how a scent-masking trait would be retained over the 12000-year history of dog domestication – a history characterised by intensive selection of individuals with desired traits. If anything, it is most likely that a behavioural trait that promoted smelling foul would be unfavourable as humans selected dogs with which to live their lives. This is especially true if rolling in worms, faeces, or a carcass had the potential to undermine human health – which it almost certainly could.
Finally, if scent masking was a motivator, why are dogs choosy in terms of the materials in which they roll? In fact, on the basis of casual observation, almost all dogs seem to favour worms significantly more over faeces or even a dead animal. And this is not because they receive positive reinforcement to roll in worms over the others – all seem to invoke consternation and certainly no rewards from their human companions.
All told, why would an animal so divorced from anything resembling a natural existence roll in worms?
Taken together, the evidence suggests that scent-masking is not a robust hypothesis to explain dogs rolling in worms. Given the delight that dogs seem to derive from it, a utilitarian purpose seems counter-intuitive as well. It really does seem that there is something pleasurable derived from rolling in worm. What’s more, this pleasure seems completely self-motivated. Why?
Here’s an alternative hypothesis…
Something associated with the earthworm, including the worm itself, but more likely a resident microbe or parasite, invites dog rolling. That is, the earthworms, or an organism such as a microbe or parasite that lives in or on the worm, manipulates the dog. They produce a scent that stimulates canine pleasure. So pleasurable is this odour that dogs roll in it.
Why would worms, or the organisms they host do this?
Well, soil bound earthworms, and the organisms that associate with them, are not so mobile. Their existence depends on sustained organic matter availability, and is constrained by the depth and area available to the soil. Given that worms and their passengers are not able to travel great distances, their existence is continuously at risk within this constrained environment. Crucially, there is also a greater risk of inbreeding and narrowing of the gene pool if mating also takes place within a very restrictively defined environment.
Dogs provide a means for organisms associated with the worms to disperse more widely, to ensure greater opportunities for existence for the next generation. Microbes and parasite that are expelled in the casts are in a great position to be picked up and moved. Consequently, earthworms with their associated organisms, that create a “roll on me” odour are likely to be more widely distributed, more successful then their less aromatic kin.
Of course, this is merely a hypothesis. But it is easily testable.
The reason the hypothesis is so easily testable is because the entire situation described here is already like a grand experiment taking place in North America. Both dogs and the earthworms in question are not native species there. In fact, of the 182 species of earthworm known in North America, almost a third (60 species) are introduced, including some of the most predominant.
This unusual situation provides conditions well suited to hypothesis testing. One could examine the distribution patterns of invasive worms and their associated microbes and parasites in relation to their attractiveness to dogs. Together, these pieces of information would provide a level of support or refutation of the hypothesis. They would provide insights into the role of the interplay between different organisms in species distributions and invasions. They would provide insights into the role of behavioural modification of a third-party species in species dispersion.
Recently, Adam Welz wrote a brilliant article about the relationship between companion animals, particularly dogs and cats, and nature. As he wrote, “Your & my pets are, bluntly speaking, a danger to the planet”. This sobering look at companion animals made the case that companion animals are not only divorced from nature but an actual harm to it. Welz brought into question the connectivity that companion animals provide with nature.
On one level, it is impossible to disagree with Welz, the negative impact of companion animals on the environment cannot be disputed.
Nevertheless, there is something to be said about how companion animals provide us with a window on how other organisms, largely divorced from the concerns of human existence, go about perceiving and interacting with each other. In this way, they do provide an insight into a world that is not human dominated. A world that, distant as it is from “wildness”, is still more connected to nature than much of the technology-driven, consumer-oriented human culture.
Perhaps that connection, tenuous as it is, provides a tangible link back to nature. Even if it only serves as an entry point, it is an entry point. The trick now is to use that point of entry as the start to a more substantive path back to understanding the natural world.
Even though neither dogs nor earthworms are “natural” here, watching the canine delight of rolling in worms provides some level of connectivity to an existence that is one giant step closer to nature than the cars whizzing by meters away.
Images: All photographs by Malcolm M. Campbell.
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