Sunday 20 November 2022

The Importance of Blue Bullshit

Monkeys and apes in general, and humans in particular, have a relatively poor sense of smell but the same is far from true of most other mammal species. For such animals, scent can be an important method of communication and this is often done through scent-marking, leaving long-term messages that can be understood by other passing members of your species.

A common way to do this is by rubbing specially adapted scent glands onto objects - these glands are often on the feet or the sides of face, which is why many animals will rub their heads against things to mark their territory. An alternative is to either urinate or defecate in a particular location, the natural aroma of the excreta often being aided by anal scent glands. We are all familiar with a dog's need to mark its territory in this way and it's hardly alone in this respect.

This can provide all sorts of useful information to other animals from the same species, such as territorial ownership, social dominance, willingness to mate, and so forth. When the method used to deposit this information is defecation rather than urinating, the result is termed a latrine or midden - which, in the biological sense, are basically alternative words for "dungheap". Animals that use this method of scent-marking include rhinoceroses, lemurs, and antelopes. (And we can but wonder what human cities would be like had we evolved from a species that communicated in this way).

Some species that use latrines deposit them near the centre of their territory. Meerkats do this, for instance, as do klipspringers (a type of antelope). In this case, the latrine probably helps to maintain social cohesion among the animals living in the area, although it can also serve as a warning to intruders who manage to get that far. 

Perhaps more commonly, latrines are placed at the outer edges of territory. This may simply be any location that happens to be on the relevant boundary, but more likely, it's going to be in a place where it's most likely to be spotted, whether because it's somewhere highly visible, or because it's in the middle of an obvious trail down which animals are likely to be passing. 

Male mammals are most likely to do this sort of thing, since they are usually the ones who compete the most for mates, and are eager to show off their dominant status to others. In white rhinos, for example, dominant males deposit large latrines near the centre of their territory, while lower-ranked ones deposit them around the periphery, perhaps as a challenge or to better advertise their presence to females. It's often the case that the more rivals are likely to be around, the more latrines the resident males will deposit.

The practice is not restricted to males, however. That's partly because some females can be territorial, too, if often only to protect quality food sources rather than to fight one another. But it can also be a signal of fidelity, as seen in dik-diks, a monogamous species of antelope where females deposit their droppings together with those of their partner, presumably indicating their unavailability to solitary males. In other species, it might be the exact opposite, with single females advertising for a mate, as seen, for example, in ocelots.

A few years ago, as part of my series looking at the world's various bovine species, I discussed the latrine habits of the nilgai (Boselaphus tragecamelus). Sometimes known as "blue bulls" because of the coat colour of the males, these are cow-like animals native to the Indian subcontinent. They are "bovine" in the taxonomic sense, in that they are more closely related to cows than to, say, gazelles, but they are more commonly described as "antelopes", since they aren't quite as closely related as buffalo, yak, and so on are and they do have a number of differences from the more truly bovine bovines.

I mentioned that nilgai leave particularly large and obvious latrines, that can be up to 3 metres (9 feet) across, and that they regularly revisit them to keep them topped up. Clearly, they're important to the nilgai, not least because whatever they're for, it's worth the risk of predators locating the animals by their presence. The problem is that nilgai don't seem especially territorial, which would imply that, at least outside of the breeding season, it's not obvious what that purpose might be. But, on the other hand, it isn't as if nilgai social behaviour in general, let alone their use of latrines specifically, has been extensively studied. So we honestly could just have missed something.

While nilgai are native to India, Nepal, and Pakistan, in the early 20th century they were introduced to the southern US as farm animals. Some of them escaped, and they are now found free-range across southern Texas and across the border in northeast Mexico. Indeed, in the 1990s, the Texas Parks and Wildlife Department estimated that over 36,000 of the animals roamed across the southern reaches of their state and there's no evidence of a rapid decline since then.

This gave American researchers the chance to evaluate the behaviour of the animals if not in their native habitat, but at least somewhere approximating it that was relatively close to hand. Over three years between 2017 and 2019 they drove through three ranches with a combined area of just under 19,000 hectares (73 square miles) looking out for nilgai latrines, mostly along roads but also in some off-road locations. They examined their size, location, and freshness and placed cameras at some of them to determine the age and sex of the animals using them and what time of day they did so. This provided what is surely the most detailed analysis of nilgai dung and its deposition so far published in the scientific literature.

Because nobody said zoology was likely to be fragrant.

While nilgai in India have been reported to produce latrines only in forested areas, that was not the case in the Texas study, possibly due to the nature of the local vegetation. Instead, they seemed to focus on areas that were particularly visible, such as road junctions. There were far more latrines than nilgai, so each individual had to be using several sites, and they sometimes defecated in other areas, too, indicating that the latrines did have a particular function.

The camera traps revealed that the latrines were left mainly by adult males. Younger males would often take a sniff, but only rarely added to the pile, and, like subordinate rhinos, do so on the edge of the pile, while dominant males would stand right on top of it and position their rear end as close to the centre as they could. 

On the other hand, females would sometimes wander past the latrines without paying any attention at all. On the roughly 10% of occasions that females did contribute, however, the next male to pass by would often become excited, exhibiting the flehmen response - a sort of lip curl seen in many mammals that allows them to maximise their chance of detecting pheromones. Or, if another nilgai was nearby at the time, they would try to either fight or mate with them, as the case might be.

What this shows is male nilgai, at least, are likely more territorial than previously supposed. Older males are showing dominance, warning off younger rivals. At least one of the dungheaps was used by two different males, however, so they're unlikely to be maintaining exclusive territories, perhaps just needing to know who else is in the neighbourhood. That females are usually disinterested implies that there isn't much information of use to them; they probably aren't territorial and find that any males will make themselves known without having to leave a message. And, when they do add to the piles, it's because they're ready to mate and know that a suitable male will be along shortly and come looking for them.

In a sense, this isn't too surprising: it's what most other antelopes use latrines for. But this had not previously been clear with nilgai, with some of the few previous studies on the matter outright contradictory - perhaps because they didn't have the benefit of the camera traps to determine exactly what was going on. But, just from examining where and how they happen to poo, we now know more about nilgai than we used to.

[Photo by Jon Connell, from Wikimedia Commons.]

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