Olfaction simply refers to the sense of smell. The chemosensory/olfactory system is the most ancient sensory system that we know of. Nevertheless, it is perhaps the least-studied, inciting much less scientific interest than vision or hearing, for example. There are many possible reasons why this disparity exists. One of the most compelling reasons I’ve heard is simply because humans have relatively poor olfactory abilities. Humans are primarily visual and auditory creatures, every other sense comes after. For other animals however, this may not be the case. While dogs have fine vision (despite being red-green colorblind) and exceptional hearing, they are also fantastic smellers. For many fish that inhabit murky waters, their sense of smell is how they perceive their world. Even for fish that are visual predators, like salmon, they still need to use their world-class noses to find their home stream where they will eventually reproduce and die.
The study of olfaction in birds has a long and confusing history. Until the 20th century, it was believed that birds did not have a sense of smell at all. Classic studies conducted in the 1820s by John James Audubon on the Turkey Vulture (Cathartes aura) are some of the first known experiments on avian olfaction. Surprisingly, Audubon’s conclusion was that these vultures have no sense of smell. We now know this not to be true. In fact, a Turkey Vulture’s nose is so attuned to methyl mercaptan – a compound found in decaying flesh as well as crude oil extract – that early 20th century engineers would use the presence of these birds to detect leaks in oil pipelines well before humans could.
While it is true that many birds are primarily visual animals and may have poor senses of smell, we don’t have very much data on the subject. We do know, however, that new world vultures, kiwis (the flightless birds from New Zealand, not the fruit), and tube-nosed (a.k.a. procellariiform) seabirds lead the pack, having the best avian senses of smell. Not only that, but these birds have some of the most sensitive olfactory systems known in the entire animal kingdom. My lab studies olfaction using procellariiform seabirds as model organisms.
After two decades of research, Gabrielle Nevitt and her collaborators have discovered some amazing things about these “noses with wings”. It had been well known for decades that procellariiform seabirds had an excellent sense of smell from anatomical (Bang 1966) and behavioral studies (Grubb 1972, 1973), but Nevitt was able to show how specific food-related odors attract different species of seabirds, depending on what they prefer to eat. Her results have shown that seabird species feeding on primary consumers (e.g. krill, copepods, etc.) are often attracted to the phytoplankton-derived odorant dimethyl sulfide (DMS; Nevitt et al. 1995), while seabird species foraging higher on the food chain (e.g. predatory fish and squid) tend to be attracted to krill-related odors, such as tri-methyl amine and tri-methyl pyrazine (Nevitt 1999, Nevitt et al. 2004). Furthermore, it was discovered that these birds can detect specific airborne odors in parts per billion concentrations or lower, which is at least three orders of magnitude (1000x) more sensitive than our noses are for the same odors!
In addition to finding food, these birds have also been shown to use their sense of smell in the contexts of homing and individual recognition. In what many may come as a shock to some, many of the smaller species of procellariiforms actually nest underground in earthen burrows that can be up to a meter long. It’s incredible to imagine these little birds (most of the burrow-nesting seabirds are smaller than a crow) excavating their own burrows! For these burrow-nesting procellariiforms, they are often active at night, returning or departing from their colonies in pitch blackness.
It has now been shown that burrow-nesting Blue Petrels (Halobaena caerulea), Common (Pelecanoides urinatrix) and South-Georgian Diving Petrels (P. georgicus), as well as Thin-billed (Pachyptila belcheri) and Antarctic Prions (P. desolata) use smell to locate their burrows in complete darkness (Bonadonna et al. 2003a, Bonadonna et al. 2003b, Bonadonna et al. 2003c). In another simple, yet elegant, experiment Bonadonna and Nevitt (2004) demonstrated that Antarctic Prions were able to recognize and distinguish their personal odor from the odor of their partner as well as a randomly selected prion. It was found that these birds actually preferred the scent of their partner’s odor to their own, and of the three different odors, they were least interested in the odor of a randomly-selected conspecific.
As always in science, answers only lead to more questions. The lab I work in is currently involved in a multi-year project studying Leach's Storm-Petrels (Oceanodroma leucorhoa) to determine if these diminutive, burrow-nesting seabirds select mates by scent with a particular MHC, a complex cluster of ultra-variable genes involved in immune function. Many more questions related to olfaction in wild animals can and should be addressed; there’s a whole world of discovery out there, who nose what one may find next!
Matthew Savoca holds a PhD in Ecology from the University of California, Davis. His research interests include sensory behavioral ecology, marine conservation biology, and seabird ecology.