I’ve recently had the chance to review the literature regarding olfaction in birds and to my surprise found that there is little research done on the olfactory functions (e.g. olfaction threshold) and their relations to the olfactory bulbs. The main reason I got into this was primarily for the claim that T. rex had an acute sense of smell because of its enlarged olfactory bulbs. Now the latter part of this statement is obviously true. According to Brochu (2000), the olfactory bulb is 1.5 times as wide as the cerebral region of the endocast in T. rex. Following Bang and Cobb’s (1968) simple method, the greatest diameter of the olfactory bulb is about 41% of the greatest diameter (in this case the longest length) of the total brain. That’s higher than the largest proportion of olfactory bulb in modern birds according to Bang and Cobb (1968), which is as follows:
37.0% - Snow Petrel
33.0% - Wilson’s petrel
30.0% - Wedge-tailed Shearwater
30.0% - Greater Shearwater
29.5% - Dove Prion
29.0% - Black-footed Albatross
29.0% - California Shearwater
28.7% - Turkey Vulture
27.5% - Cape Pigeon
27.0% - Fulmar
Notice that the Turkey Vulture is seventh place within the seabirds. The Turkey Vulture has repeatedly been brought up (especially by Jack Horner) to suggest a similar mode of life (i.e. scavenging) for T. rex because of the enlarged olfactory bulbs in both taxa. Well, it seems that seabirds generally have enlarged olfactory bulbs as well, a higher proportion at that, and that the odd one here is the Turkey Vulture. Surely, you cannot speculate a mode of life of an extinct animal based on one modern example that doesn’t follow the general pattern – that is, that in general, seabirds have the highest proportion of olfactory bulb to the whole brain and not scavenging vultures where Turkey Vulture is the exception – the only other vulture in Bang and Cobb’s (1968) dataset is the Black Vulture with 17%; within the range of raptors and comparable to Woodcock, Belted Kingfisher, and Adélie Penguin.
So we know that T. rex had enlarged olfactory bulbs unheard of in modern birds, but that still does not answer the fundamental question of “what does that tell us about function”.
As far as I’ve read so far, there doesn’t seem to be any studies actually conducted to test for any correlations between olfactory sensitivity and relative size of olfactory bulbs. As Bang and Cobb (1968) nicely put it, “the significance of such measurements […i.e. olfactory bulb] rests on the general assumption that increase in size of a part of the brain indicates increase in function”. More recent work (Smith and Paselk 1986, McKeegan et al. 2002, 2005) that test the olfaction threshold, i.e. the olfactory sensitivity, in birds only cite Bang and Cobb’s (1968) work for the relative sizes of the olfactory bulbs but never test if there is a relationship between the two – function and size. This “general assumption” mentioned in Bang and Cobb back in 1968 still seems to hold true today as well and has remained untested for the last 40 years. That is, of course, if I’m missing some key reference.
In any case, olfaction threshold in birds are shown to be much higher than those of mammals, i.e. birds have a worse sense of smell than mammals do. Clark (1991) cite Wenzel and Sieck (1972) to suggest that birds have an olfaction threshold range of 0.01 to 0.5 ppm, while McKeegan et al. (2002) report thresholds of 1 and 2.5 ppm for ammonia and hydrogen sulphide respectively in hens, Gallus domesticus. Contrast to these results, human detection threshold for ammonia ranges from 0.0005 to 0.37 ppm and much lower threshold for hydrogen sulphide at 1×10E-7 to 0.0002 ppm (McKeegan et al. 2002).
In the Turkey Vulture, Smith and Paselk (1986) report an olfaction threshold of 1×10E-6 M (or molar, mol/L) for butanoic acid and ethanethiol, and 1×10E-5 M for trimethylalanine; three odorants generally associated with animal decomposition. Now, if my calculations are correct, then Turkey Vulture has olfaction thresholds of about 0.09, 0.06, and 0.6 ppm for the above odorants respectively, or more simply 0.06 to 0.6 ppm depending on the substance. In their odour dispersion model, Smith and Paselk (1986) predict that in order for a Turkey Vulture to detect these odorants at approximately 61 m altitude and 183 m downwind - an observed distance for carrion detection in wild Turkey Vultures - it would have to have olfaction thresholds of roughly 1×10E-12 to 1×10E-13 M for ethanethiol. If you were to trust their results, then clearly, the Turkey Vulture does not have a high enough olfactory sensitivity to detect carrion on smell alone – perhaps they also rely on other senses such as sight (watching out for activities of other scavengers), or hearing (sounds of scavenging insects). Further, since the absolute quantity and rate of odorant emission depends on the mass of the carcass, and since Turkey Vultures prefer to feed on small-bodied carrion that would be expected to emit relatively low levels of odorant, detection of odorants from these carrions cannot be expected unless their thresholds were lower. On the other hand, it could be that their experiments did not detect to the full extent the Turkey Vulture’s olfaction threshold. Or perhaps their dispersion model is wrong and odorants can disperse farther without being diluted as much.
This seems rather odd, that the Turkey Vulture with one of the highest proportions of olfactory bulbs don’t seem to have as keen a sense of smell than one would expect. Perhaps the olfactory bulbs process some other information associated with smell such as filtering out background smell. So they’d be sensitive to smell but in a way to sniff something out from other smells.
In any case, olfaction thresholds are probably more directly associated with the surface area of the olfactory epithelium, and at this point, we don’t seem to have the necessary information to relate it to the relative sizes of the olfactory bulbs.
Bang, B. G., and S. Cobb. 1968. Size of Olfactory Bulb in 108 Species of Birds. Auk 85(1):55-61.
Brochu, C. A. 2000. A digitally-rendered endocast for Tyrannosaurus rex. Journal of Vertebrate Paleontology 20(1):1-6.
Clark, L. 1991. Odor Detection Thresholds in Tree Swallows and Cedar Waxwings. Auk 108(1):177-180.
McKeegan, D. E. F., T. G. M. Demmers, C. M. Wathes, R. B. Jones, and M. J. Gentle. 2002. Stimulus-response functions of single avian olfactory bulb neurones. Brain Research 953(1-2):101-111.
McKeegan, D. E. F., F. S. Smith, T. G. M. Demmers, C. M. Wathes, and R. B. Jones. 2005. Behavioral correlates of olfactory and trigeminal gaseous stimulation in chickens, Gallus domesticus. Physiology & Behavior 84(5):761-768.
Smith, S. A., and R. A. Paselk. 1986. Olfactory Sensitivity of the Turkey Vulture (Cathartes aura) to Three Carrion-Associated Odorants. Auk 103(3):586-592