Some animals pee on trees, and others rub their anal secretions on mounds of dirt. Some animals send Facebook messages to all the ex-girlfriends of their new boyfriends. No matter what the species, it probably engages in some form of territoriality. But as I've noted before, "studies of animal territoriality are particularly hard to conduct, because territorial behavior exists across multiple levels of analysis, from the individual animal, to groups, to entire populations." That empirical investigations of territoriality are hard to conduct is one thing, but how much harder to then determine the mechanism by which territorial behaviors such as scent-marking operate! Sure, they're meant as olfactory signals that say "get offa my lawn!"

One leading hypothesis says that scent marking is meant as a deterrence to potential intruders, acting as an invisible "fence" around an individual's territory. This is known, appropriately, as the scent-fence hypothesis. One important alternative hypothesis is known as the scent-matching hypothesis. In this version, scent marks allow potential intruders to match the signals (scents) with the signalers (territory owners), so that intruders know who their potential rival is. Scent marks alone can't keep out intruders, after all. But a smart critter will know who he has a decent chance of beating in a fight over territory, and who to avoid messing with. So the invading party will assess the situation, and in order to do that, he must be able to match a scent with the individual who matches that scent.

One 1990 study found that invading mice were less willing to fight with resident mice when the odor matched the resident than when it did not. That study was done in a highly artificial laboratory setting, though, which makes it hard to generalize to natural ecosystems.

It is to address this issue that we now turn to the North American beaver, Castor canadensis.

Each beaver family colonizes one or more small, year-round ponds, and are highly territorial. In order to mark their territory, they construct mounds of dirt at the boundaries and mark them with castoreum (similar to urine) or with secretions from their anal glands.

Biologists Lixing Sun and Dietland Muller-Schwarze of the State University of New York reasoned that the scent-matching hypothesis predicts that beavers would:

  • (1) mark a place that can be easily detected by intruders,
  • (2) mark itself with the same materials used in territorial marking,
  • (3) make itself available for scent matching by intruders, and
  • (4) remove or replace alien scent materials

The thing is these behaviors are also consistent with the scent-fence hypothesis. Therefore, the researchers decided to assess the responses of territory owners to unfamiliar scents over the course of six days, but without the actual presence of the invading beaver.

It may seem like a trivial distinction, but each hypothesis leads to distinct behavioral predictions. If the scent-matching hypothesis is correct, then resident beavers should maintain or decrease their behaviors towards unfamiliar scents over time, given that no intruder is actually encountered. That is, the resident individuals should habituate to the unfamiliar scent. On the other hand, if the scent-fence hypothesis is correct, then resident anti-intruder behavior should increase over time, as the scent alone is enough to be considered a challenge to his or her own territory.

So Sun and Muller-Schwarze (and, presumably, a horde of unfortunate research assistants) trapped some beavers, collected some castoreum and anal gland secretions, and released the critters back into the wild. Then, they went to the territories of other beaver families, constructed dirt mounds in those territories, and hid scented corks within them. And then, they waited to see what would happen for six days, replacing the corks each day to keep the scents fresh.

The researchers coded each type of behavioral response to the unfamiliar scent: general response (e.g. arousal), "pawing," "removing," "flattening," or "obliterating" the dirt mound, and "touching," "digging out," "removing," or "chewing" on the cork. They also noted whether the resident beavers constructed new scent mounds near the foreign ones.

In this image, left is the response to anal gland secretions, and right is response to castoreum. In the legend, ESM is "experimental scent mound" and BSM is "beaver scent mound." Click to enlarge.

With respect to the experimental scent mounds baited with anal gland secretions, the response intensity decreased slightly over time, but not in a statistically significant way. That means that the response was essentially stable over time. For the mounds baited with castoreum, the story was basically the same: for no recorded behaviors did response intensity increase over time - indeed, they all maintained the same level of response or decreased over time. Why would there be decreases for castoreum but not for anal gland secretions? The researchers reason that these slight differences may be due to the functional difference between castoreum and anal gland secretions: anal gland secretions are involved in kin recognition, while castoreum is more explicitly related to territoriality.

Taken together, these results support the scent-matching hypothesis: the importance of the scent alone, without the corresponding beaver decreases over time. This also suggests that when a beaver is deciding whether to invade a new territory, the presence of a scent alone is not a deterrent. Instead, they ought to determine which beaver belongs to which scent, and then decide if they could win a fight over the territory. Indeed, the researchers write, "Intruders do not have complete knowledge about the likely outcome unless they meet the signalling territory owner." It is especially important for an invading beaver to size up his or her opponent, since resident beavers have more to lose, and are therefore more apt to escalate conflicts.

And beaver fights could get nasty. I would never want to cross the wrong beaver.

Sun L, & MÜller-schwarze D (1998). Beaver response to recurrent alien scents: scent fence or scent match? Animal behaviour, 55 (6), 1529-36 PMID: 9641998

For more on territory behavior:

Defending Your Territory: Be Smelly, Be Fast

Is Peeing on the Wall Just For The Dogs?

It Pays To Have a Bigger Brain

Header photo: Flickr/Dano Hart. Beaver photo: Flickr/Minette Layne.

With thanks to Scicurious for the headline.