Sense of smell differs among animals, with some having keener abilities than others, according to a new study whose authors name the top 10 animals with the most genes devoted to smelling.

The study, published in the journal Genome Research, highlights how an animal's genetic makeup can underlie its ability to distinguish different odors.

Guinea pigs, with 796 olfactory receptor (OR) genes, landed in the No. 10 spot, according to lead author Yoshihito Niimura of the University of Tokyo's Department of Applied Biological Chemistry, and colleagues Atsushi Matsui and Kazushige Touhara.

Humans surprisingly didn't rank too shabbily, winding up with a number 13 placement (396 OR genes) just behind tree shrews (563) and rabbits (768).

“In general, primates have fewer numbers of OR genes than other non-primate mammals, possibly because primates have a well-developed visual sense," Niimura told Discovery News. “We humans actually have a larger repertoire of OR genes than some other primates."

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Dogs, with 811 OR genes, rank No. 9.

“Dogs are generally thought to be a good smeller, but they do not have a particularly large number of OR genes," Niimura said.

Niimura added, “Carnivores may not need to distinguish among many different types of odors, but they may be very sensitive to the odors that they can discern."

A cat, therefore, might be able to smell stinky cat food seemingly from miles away, but would be less able to distinguish various types of different odors.

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Representing amphibians on the list is the western clawed frog (824 OR), which is a medium-sized, aquatic frog living in the West African rainforest belt. It inhabits slow-moving streams, as well as pools and temporary ponds.

This particular species has been widely studied and is considered to be a model organism for genetics. It could be that other frogs are keen smellers too, but simply haven't been studied as much.

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There is a big jump from the western clawed frog to horses, which have 1,066 OR.

“In general, mammals are good at smelling compared with other vertebrates (animals with a backbone)," Niimura said. “Mammals were originally nocturnal, mouse-like small creatures that survived in the era of dinosaurs. Therefore, it seems that the importance of olfaction was already high at that time."

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Speaking of mice, they rank No. 6 and possess 1,130 OR. They have a wide, varied diet, so it's understandable that these animals -- although tiny -- have a mighty sense of smell.

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The reptile representative in the Top 10 is the Chinese softshell turtle with its 1,137 OR. As its name suggests, it lives in China, but also in many other parts of the world. In Hawaii, it's even found in marshes and drainage ditches. It's a very nosey-looking animal that spends much of its time trying to detect food in brackish water at night. The turtle's diet includes remains of fish, crustaceans, mollusks, insects, seeds of marsh plants and more.

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Cows have 1,186 OR and landed in the No. 4 position on the list. The powerful nose of a cow is said to be able to detect odors from 5–6 miles away. They can also hear both low and high frequency sounds beyond the human sense of hearing.

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Opossums -- notorious foragers often seen sniffing around gardens -- have 1,188 OR, earning them the No. 3 spot. Another opossum claim to fame is that they have an incredibly robust immune system, which allows them to ward off viruses and venom from biting snakes.

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Rats beat mice in this particular ranking, due to their 1,207 OR. Rodents in general are excellent sniffers, facilitating their wide and flexible diet. Cats can sniff out mice and rats, but they are pickier eaters that are unable to detect as many odors as rats can.

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The undisputed king of animal sniffers is the African elephant with its 1,948 OR. Prior studies support this finding, and have determined that African elephants can distinguish odor molecules with extremely subtle structural differences that humans and other primates completely miss.

As for why elephants might evolve such a strong sniffing ability, Niimura explained: “Because their body is so large, they have evolved a dexterous long trunk, which functions like a hand. It can grasp foods or other things. Therefore, they always use olfaction when they search the outer world, maybe driving a superior sense of smell."

Niimura added that “more analysis is necessary to see when and why the elephant ancestor acquired such a large OR gene repertoire."