Page 100 - The Welfare of Cattle

sense and sensIbILItY 77

(Harkema, 1991; Reznik, 1990). Due to this distinction in nasal complexity, humans can breathe
both nasally and oronasally, while other mammals, such as cattle, are obligate nose breathers
(Harkema, Carey, & Wagner, 2006; Proctor & Chang, 1983). As air travels into the cow’s nostrils, it
passes through the nasal vestibule and into the main nasal chamber. The distance from the nostrils
to the nasopharynx is proportional to the length of the animal’s snout. Additionally, the surface area
of the nasal epithelium varies between species influenced by size of the animal and the specializa-
tion (respiratory vs olfactory) which varies by species’ olfactory ability. In macrosmatic species
like cattle, deep in the nasal cavity is a space called the olfactory recess filled with convoluted
scroll-like bone structures called turbinates lined by the well-vascularized mucosal tissue (Eiting,
Smith, Perot, & Dumont, 2014; Moore, 1981). The complexity of the olfactory recess is thought
to be related to olfactory ability, and is a structure essentially absent in humans (Smith, Eiting, &
Bhatnagar, 2015) and highly developed in canines (Craven et al., 2007). In cattle and other ungulate
species, this area is a double scroll-like structure resembling a “T” in cross-section with double ends
coiled (Reznik, 1990). The function of this olfactory recess is to sequester air so that the odor-laden
air can circulate throughout the extended surface area increasing the chances that odors will be
identified (Yang, Scherer, & Mozell, 2007).
The percentage of nasal airway that is lined with olfactory epithelium (OE) is species specific
(Harkema, Carey, & Wagner, 2006), microsmatic species having less surface area covered by OE.
To provide perspective, the OE in rats covers 50% of the nasal cavity and the OE in humans covers
3% (Gross, Swenberg, Fields, & Popp, 1982; Weiss, 1988); there is a paucity of information detail-
ing these comparisons in cattle but they are likely closer to rats than humans. There are millions
of olfactory sensory cells (often called olfactory sensory neurons) within the OE, the total number
variable by species and directly proportional to the surface area of the OE. The olfactory sen-
sory cells extend above the epithelial surface and have cilia that increase the surface area for the
reception of odorant molecules; the actual olfactory receptors (ORs) are in the cilial membranes
(Harkema, Carey, & Wagner, 2006). Each OR cell extends through the basal lamina of the OE and
joins with other fibers forming olfactory nerves that extend across the cribiform plate out of the
nasal cavity forming the outer nerve layer of the olfactory bulb of the brain.
Multiple different types of OR cells will respond to the same odorant but one odor will elicit the
greatest response from one type of receptor cell (Møller, 2003). The brain interprets the impulses
from many different receptors to provide a distinct sensation of one specific odor (Firestein, Picco, &
Menini, 1993). The threshold of perception of a certain odor varies with the odor itself and with
the species, and even with the individual (Møller, 2003). The number of odors that an individual
can recognize is determined by training/learning and experience in addition to inherited ability.
Additionally, the surrounding environment, such as the speed and direction of the wind, tempera-
ture and humidity, may change the strength of a scent.
In addition to the OE in the nasal cavity, the vomeronasal organ (VNO; secondary olfactory
system) also contains ORs. The VNO is situated bilaterally at the base of the nasal septum and its
function is greatly determined by species. This sensory organ is responsible for detecting phero-
mones which are, for certain species like cattle, very significant in intraspecies communication and
reproduction. Cattle (and other ungulates and felids) will exhibit the Flehmen response which is the
curling of the upper lip exposing the front teeth to assist facilitating the transfer of pheromones into
the VNO. The significant differences in size of the VNO receptor gene family between species are
considered to be the largest variation in size in all mammalian gene families (Grus, Shi, Zhang, &
Zhang, 2005). Additionally, there is a relationship between the size of VNO receptor gene family
and the complexity and size of the VNO morphology of a species suggesting that the latter may be
indicative of the sophistication of species pheromone communication (Grus, Shi, Zhang, & Zhang,
2005). Rats and mice have a relatively large VNO functional gene family, cows having fewer but
humans having an extremely small gene family in comparison Grus, Shi, Zhang, & Zhang, 2005;
Shi, Bielawski, Yang, & Zhang, 2005; Zhang & Webb, 2003).