Binturongs smell like popcorn. Or popcorn smells like binturongs. I guess it depends on your perspective. Either way, when I stopped by the enclosure of the large, blue-grey bearcat at the San Diego Zoo last month, the warm, buttery scent was unmistakable. What I heard celebrity zookeeper Jack Hanna say on television for so many years was true – the big viverrid smelled like a movie theater lobby.

Binturongs, a cousin of civets and found in the rainforests of Asia from Nepal to Java, aren’t the only mammals with perplexingly familiar scents. Before I started wondering about butter-scented binturongs, my attention was drawn to the pee of maned wolves. The urine of these stilt-legged canids is redolent of marijuana. The reason why has to do with organic compounds called pyrazines which are often used for communication in both plants and animals – in milkweed and maned wolves alike, pyrazines create long-lasting, smelly “Get lost!” signals. After I caught a whiff of the captive binturongs, I wondered if something similar might be behind their unique odor.

Finding a precise answer has been difficult. Everyone knows that binturongs smell buttery, but the relatively small literature on their scent primarily deals with behavior rather than chemistry. While not the first to identify where the scent emanated from, in 1945 zoologist H. Elizabeth Story described the binturong’s anal glands as the major odor-producing centers in the almost-euphemistically-titled “The External Genitalia and Perfume Gland in Arctictis binturong.” (Names can make all the difference. The reaction to “anal glad” is “EWWW!”, while “perfume gland” sounds fairly pleasant, as if the animal exuded a scent concocted by Calvin Klein.)

A diagram of binturong genitals and scent glands. The dashed line in the male (a, b) and female (c) represents the location of the "perfume gland." From Story, 1945.

Story’s paper was an effort to set naturalists straight on binturong anal glands. Three decades earlier, British zoologist Reginald Innes Pocock described and illustrated the genitals and scent glands of the bearcat, but, when Story looked at several dead animals donated to the Chicago Natural History Museum by the Chicago Zoological Society, she found that “[Pocok’s] drawings, admittedly somewhat diagrammatic, prove to be extremely misleading.” Likewise, other previous studies had suggested that the position of the anal glands shifted as the animals grew up. Story did not find evidence of any change in placement among the specimens in her collection, and she lamented “The problems arising from the confused state of the literature on the civets.” Was there no respect for the anatomy of viverrid rumps?

In both male and female binturongs, Story found, the “perfume gland” was only visible as a swelling in the vicinity of the genitals. In males, the gland was a U-shaped pad which sat between the penis and scrotum, and in females the gland was divided into two halves – which looked like a pair of parentheses – underlying either side of the vulva. And, in both sexes, the ducts of these glands led to openings on either side of the anus. That’s where the popcorn scent is coming from.

Devra Kleiman described how binturongs went about applying this scent in a 1974 paper based on observations of a captive animals at the National Zoo. The keepers wanted some baby binturongs, but couldn’t figure out how to get their animals to show anything more than a passing interest in each other. The paper came out of observations of the binturongs after one female had been given pregnant mare’s serum gonadotropin – a substance extracted from the blood of gravid horses used to stimulate the ovaries of other animals – in an effort to pique her interest in her potential mate.

Kleiman pointed out that binturongs left scent behind while climbing and sitting, but the mammals also engaged in deliberate scent-marking behavior. Whether on the ground, upside down, or hanging vertically, the binturongs scuffed their feet on the surface they were about to mark and then dragged their anal gland over the surface. (Kleiman called the inverted behavior “an upside-down perineal drag,” which also sounds like a suitable alternate description for an excruciatingly boring event. “Hey, did you catch the latest Republican debate?” “Ugh, it was such an upside-down perineal drag.”) All of this is advertising. You might leave a business card; a binturong drags their ass over whatever it is they want to carry their message.

Based on Kleiman’s observations, binturongs seemed to take almost any excuse to scent mark. That pole not as stinky as it used to be? Scent mark. Feeding time? Scent mark. An unfamiliar human comes by? Scent mark. Smell is essential to these animals, and that’s especially true during the mating process.

Chris Wemmer and James Murtaugh described how binturongs get ready for business time in a 1981 paper, also based on observations of captive animals. The zoologists collated data about binturong reproduction from 47 zoos around the world and observed captive animals over several years at Front Royal, Virginia’s Conservation and Research Center. While males and the more dominant female bearcats usually avoided each other most of the time, when females came into estrus the males started tailing them. Both sexes called to each other with peculiar, “single blowing sound.”

During this time, the enclosures of binturongs must smell particularly buttery. Wemmer and Murtaugh reported that females in heat typically “secreted a profuse mucus” from their vulva, “which, together with perineal gland secretions, presumably was responsible for a strong odor resembling cooked popcorn.” We can’t read what these scent signals mean – thank heavens for that – but, to a male binturong, the odors carry essential information about the female’s reproductive cycle. Males nuzzle right up to the pungent orifices and sniff to their heart’s content while the female binturongs let out a “coarse purring sound” labeled “geckering.” The Act itself often required acrobatic skull. Most of the copulations, Wemmer and Murtaugh reported, happened off the ground, and females often wrapped their prehensile tails around the midsections of their mates.

(For what happens later, when the little baby binturongs come along, see today’s Friday Weird Science by Scicurious. When it comes to teat ownership, those bitty bearcats are vicious.)

Sadly, though, I’m not any closer to understanding why binturongs smell like popcorn in the first place. Is the smell created by a chemical compound, or something else? I don’t know. Hopefully someone with the means and know-how will be able to sniff out the cause. Just remember, should you catch a waft of popcorn at the zoo, you may be catching a hint of binturong on the air.

[Many thanks to Scicurious for encouraging me to venture further afield from paleo to write today's post.]

References:

Kleiman, D. 1974. Scent marking in the binturong: Arctictis binturong. Journal of Mammalogy. 55 (1), 224-227

Story, H. 1945. The external genitalia and perfume gland in Arctictis binturong. Journal of Mammalogy. 26 (1), 64-66

Wemmer, C., and Murtaugh, J. 1981. Copulatory behavior and reproduction in the binturong, Arctictis binturong. Journal of Mammalogy. 62 (2), 342-352