Sense of sight

The pig's eyes are deeply placed and may appear small, especially in over-conditioned adults. The eye has a diameter of approximately 24 mm. The overall ocular power of the pig's eye has been calculated as 78 diopters (Coile and O'Keefe, Reference Coile and O'Keefe1988). Pigs have a wide angle of vision (310°, Prince, Reference Prince and Swenson1977), thus, a panoramic view of their surroundings. The radiation wavelengths to which pigs are maximally sensitive are slightly lower than those for humans. Within the range of 465–680 nm, pigs can distinguish wavelength differences as small as 20 nm (Signoret et al., Reference Signoret, Baldwin, Fraser, Hafez and Hafez1975). The retina contains a substantial population of two types of cones with sensitivity peaks at about 439 mm (indigo) and 556 mm (green–yellow) in the spectrum of visible light (Neitz and Jacobs, Reference Neitz and Jacobs1989). The pig is thus believed to have dichromatic color vision.

Pigs generally prefer lighted areas as opposed to darkness. For example, Tanida et al. (Reference Tanida, Miura, Tanaka and Yoshimoto1996) found that piglets tended to move toward more brightly illuminated areas and recommended even, diffuse lighting to encourage movement into poorly illuminated areas. Likewise, Grandin (Reference Grandin1982) noted that pigs tended to move toward more brightly lighted areas and suggested shining light into the interior space, rather than into the pigs’ ‘eyes, to direct and encourage their movement, e.g. into the unlighted interior of a truck to facilitate loading.

Pigs do respond to color, e.g. the change in the color of the handlers' uniform (Hemsworth, Reference Hemsworth and Grandin2007), but the presence of a particular photo pigment is a weak indicator of the information actually supplied by activation of this pigment. Tanida et al. (Reference Tanida, Senda, Suzuki, Tanaka and Yoshimoto1991) concluded that pigs can identify ‘blue’, but suggested that pigs discriminated blue from other colors on the basis of hue rather than brightness and may be red–green color blind. In a study evaluating responses to water dispenser color, Stelios et al. (Reference Stelios, Kostas and Georgios2006) reported that newborn pigs were not attracted to the green water dispenser and their behavioral reaction to red and blue was gender-driven. Females preferred the blue dispenser, visiting it more times than the males, while males preferred the red dispenser. Edge et al. (Reference Edge, Varley and Rowlinson2004) found no significant effect of trough color (yellow, red, and black) on feed intake, average daily gain or feed conversion. Overall, it may be concluded that, although pigs have some ability to discriminate among colors, there is little information on the spectrum visible to pigs and even less on the impact of color on pig behavior.

Sense of taste

Pigs have taste papillae on the tongue, epiglottis and soft palate epithelia (Montavon and Lindstrand, Reference Montavon and Lindstrand1991). Each papillus contains high numbers of garlic clove-shaped taste buds, with each taste bud composed of groups of 50 to 120 sensory cells, each of which projects a number of microvilli to the mucus layer of the tongue (Roura and Tedó, Reference Roura, Tedó, Torrallardona and Roura2009). Taste buds are composed of at least four different types of cells; three taste-type cells (I, II and III) and one basal-type cell believed to be a progenitor of the other three. Type I cells are sour sensing, type II are sweet, umami and bitter sensing and type III cells play an intermediate signaling role between the true taste cells (types I and II) and the sensory neurons.

Kumar and Bate (Reference Kumar and Bate2004) described five types of taste papillae in pigs: filiform, fungiform, foliate, vallate and conical. Filiform papillae have either sharp or blunt apical tips. Large numbers of densely packed sharp-tipped papillae are distributed throughout the tongue, except in the mid-raised area of the tongue, which is mainly covered with blunt tip papillae. Large, circular, fungiform papillae located on the lateral side of the tongue contain surface taste pores, but fungiform papillae on the dorsal surface of the tongue lack taste pores. Foliate papillae are located principally on the caudal third and lateral part of the tongue and are composed of four to five leaves and additional pseudopapillae. Two vallate papillae, found in the caudal third and dorsal area of the tongue, have well-defined vallum and pseudopapillae on their surface. Finally, two types of conical papillae are present at the root of the tongue directed caudally in a shingle-like arrangement. The first type of conical papillae has a broad base and a tapering pedicle-like apex, whereas the second type of conical papillae is cone shaped and displays a blunt apex.

All papillae, except for the filiform, have taste buds on their surface. In particular, the fungiform and vallate papillae are involved in taste (Ojima, Reference Ojima2001), whereas the filiform papillae and conical papillae play a role in the mechanical transport of food and liquid toward the pharynx (Ojima et al., Reference Ojima, Takeda, Matsumoto and Nakanishi1997). Taste information from the fungi-form taste buds is conveyed in the chord tympani nerve and from the foliate and vallate taste buds through the glossopharyngeal nerve (Hellekant and Danilova, Reference Hellekant and Danilova1999).

Food preference, among other factors, reflects the taste sensation that results from flavor–taste papilla interaction. Recordings of impulses in these nerves have been used to assess a species' ability to taste (Hellekant and Danilova, Reference Hellekant and Danilova1999). In humans, the sense of taste differentiates between the taste qualities of sour, salty, sweet and bitter. It is not possible to know whether the gustatory sensations of animals are equivalent to the tastes humans' experience. More likely, each animal species has its own sensory spectrum. Furthermore, individuals within the species have preferences – even cloned pigs respond differently to taste (Archer et al., Reference Archer, Friend, Piedrahita, Nevill and Walker2003).

A number of studies have examined porcine flavor preferences. McLaughlin et al. (Reference McLaughlin, Baile, Buckholtz and Freeman1983) investigated flavor additives in 248 trials conducted to determine preferences among 129 flavors and found that sweet, meaty and cheese were the most preferred flavors. Pigs rejected foods that taste bitter to humans (Nelson and Sanregret, Reference Nelson and Sanregret1997). Plant extracts and pharmaceutical compounds produced aversion behaviors in pigs, some of which were found to be dose dependent. Many studies have reported a preference for a sweet taste (Jacela et al., Reference Jacela, DeRouchey, Tokach, Goodband, Nelssen, Renter and Dritz2010) and, for this reason, sweet flavoring agents have been used for creating interest in solid food in weanling pigs (Forbes, Reference Forbes1995). Kare et al. (Reference Kare, Pond and Campbell1965) examined the taste preferences of young pigs to sucrose, glucose and lactose using concentrations ranging from 5 to 40 g l⁻¹ and found that pigs preferred all three sugars to water, but sucrose was more attractive than glucose or lactose. Kennedy and Baldwin (Reference Kennedy and Baldwin1972) examined the taste preferences of pigs for nutritive and non-nutritive sweeteners over a range of concentrations and found that young pigs preferred sucrose solutions at concentrations >10.3 g l⁻¹. Pigs also preferred water containing >1.8 g l⁻¹of glucose, although the consumption volume was lower than for sucrose, suggesting that sucrose was the more strongly preferred sugar. Saccharin was also preferred at concentrations from 1 to 2 g l⁻¹compared to non-supplemented water, but higher concentrations (>18 g l⁻¹) decreased the preference substantially, perhaps due to the bitter taste associated with high concentrations of saccharin. Not all non-nutritive sweeteners are attractive to pigs. Nofre et al. (Reference Nofre, Glaser, Tinti and Wanner2002) investigated the responses of pigs to 60 artificial sweetener compounds and found that 35 compounds were attractive to pigs, but less so in pigs than in humans. Lugduname and carrelame, the two most potent sweeteners in humans, were also the most readily accepted by pigs.

Sense of smell

Certainly the most celebrated anatomical feature of the pig is the snout, the movable disc at the tip of the muzzle (Sack, Reference Sack1982). The snout incorporates the central part of the upper lip and is supported by the rostral bone – a structure unique to the pig (Dyce et al., Reference Dyce, Sack, Wensing and Dyce2010). Olfactory cells in the nasal epithelium convert incoming odor molecules into electrical signals, which are then transmitted via the olfactory nerve to the brain. In addition, the vomeronasal organ, located in the upper air passages, contains receptors that access the central nervous system through the accessory olfactory bulb.

The sense of smell develops early in pigs and is important for survival because piglets are very mobile from birth. They can and need to follow chemical cues learned early in life, e.g. the odor of their mother. Pigs use a wide range of olfactory cues in their natural behavior; e.g. piglets use olfactory cues for recognizing their dam and teat position; older pigs use olfactory information as the predominant basis for individual recognition (Curtis et al., Reference Curtis, Edwards, Gonyou, Pond and Mersmann2001).

Pigs have a highly sensitive sense of smell. Sows, for example, were able to identify which of otherwise identical cards they had previously touched. That is, they were able to detect their odor on the cards after several hours and even after the cards were washed (Signoret et al., Reference Signoret, Baldwin, Fraser, Hafez and Hafez1975). Feral pigs use their sense of smell in order to find food, detect potential predators or prey and mark territory. Odors related to eating are also important and can be used to increase the palatability of feeds. Thus, the smells of cod-liver oil, petrolatum, rancid fish meal and fish oil, vegetable oils or sour chicken were attractive when presented in baits for feral hogs (Fletcher et al., Reference Fletcher, Creekmore, Smith and Nettles1990).

The effect of odor on pig behavior or stress has not been clearly established. Morrow-Tesch and McGlone (Reference Morrow-Tesch and McGlone1990) demonstrated that piglets preferred sow fecal odors and were not attracted to novel odors (e.g. the scent of orange and banana). In young or adult pigs, odor preference or aversion varied depending on the individual (Archer et al., Reference Archer, Friend, Piedrahita, Nevill and Walker2003). Krebs (Reference Krebs2007) evaluated the effect of aroma conditioning (‘aroma therapy’) on aggressive behavior in groups of weaned piglets by treating rooms with either maternal pheromone or amyl acetate, a scent similar to bananas and apples. Pigs exposed to either maternal pheromone or amyl acetate spent less time standing and more time lying down compared to control groups, but none of the treatments reduced serum cortisol levels. On the other hand, McGlone and Anderson (Reference McGlone and Anderson2002) demonstrated that the synthetic maternal pheromone decreased aggressive behavior and improved weight gain and feed efficiency. Non-maternal pheromones, such as androstenone and urine from actively fighting pigs also reduced aggression when sprayed in pens of newly weaned pigs (McGlone and Morrow Reference McGlone and Morrow1988). In contrast, non-pheromonal compounds were not as efficient at reducing aggressive behavior as pheromones and had no effect on performance (Friend et al., Reference Friend, Knabe and Tanksley1983).

Sense of hearing

Pigs' ears are oval with a fairly wide base attached to the sides of the high caudal part of the head. The ears have a pointed tip and hang down over the face in some domestic breeds, but stand erect in others and in the wild S. scrofa (Dyce et al., Reference Dyce, Sack, Wensing and Dyce2010). Pigs’ hearing range is similar to that of humans, but with a shift toward ultrasound. The frequency range for reasonable detection varies between 42 Hz and 40.5 kHz, with a region of best sensitivity from 250 Hz to 16 kHz (Heffner and Heffner, Reference Heffner and Heffner1990). Spatial discrimination of the source of a sound depends on the difference in time it takes for the sound to reach each ear. Pigs have a sound localization threshold of approximately 4.6°. This is above the average sound localization threshold for mammals (12°), but below the threshold of 1.3° in humans (Heffner and Heffner, Reference Heffner and Heffner1989). Pigs show an aversion to sudden loud noise, i.e. pigs tested in an open-field situation showed increased heart rate and retreated in response to loud noise (Talling et al., Reference Talling, Waran, Wathes and Lines1996). This response was stronger for a frequency of 8 kHz than for 500 Hz and for an intensity of 97 dB than for 85 dB, although habituation occurred relatively quickly (Talling et al., Reference Talling, Waran, Wathes and Lines1996).

The effect of music on behavior and/or welfare is largely under-investigated in pigs and the experiments conducted to date have produced conflicting results. One study in weaned piglets found that music had no effect on piglet vocalization during stressful events, e.g. castration and weaning (Cloutier et al., Reference Cloutier, Weary and Fraser2000). However, de Jonge et al. (Reference de Jonge, Boleij, Baars, Dudink and Spruijt2008) reported that music could facilitate play behavior in piglets post weaning if music had been presented pre-weaning as a contextual cue associated with access to a playroom. They also suggested that music elicited play behavior – a positive welfare indicator because play behavior occurs when major stressors are absent and the primary needs have been fulfilled.

Pig behavior, welfare and health issues

For both feral and domestic swine, stress is a routine part of life and occurs in the context of social dynamics (feeding, mating, aggressive interactions) and coping with the environment (heat, cold and novel environmental situations). Hessing et al. (Reference Hessing, Hagelsø, van Beek, Wiepkema, Schouten and Krukow1993) suggested that individual pig responses to stress can be quantified using the ‘back test’. In the back test, a pig is manually restrained on its back for one minute, during which time its escape behavior is recorded. Pigs that perform many escape attempts are considered ‘proactive’, whereas pigs that do not are referred to as ‘reactive’. According to Hessing et al. (Reference Hessing, Hagelsø, van Beek, Wiepkema, Schouten and Krukow1993), proactive pigs are also more aggressive in social interactions. When faced with non-social challenges, e.g. exposure to novel situations or objects, proactive individuals more quickly approach novel stimuli, but show more superficial exploratory behavior. In contrast, reactive individuals show higher initial levels of passive avoidance, but spend a longer time exploring after they gain familiarity. Thus, pigs show similar reactions to challenging events over time and across related situations, but demonstrate large variations in behavior among individuals (Lawrence et al., Reference Lawrence, Terlouw and Illius1991; Forkman et al., Reference Forkman, Furuhaug and Jensen1995; Spoolder et al., Reference Spoolder, Burbidge, Lawrence, Simmins and Edwards1996).

In principle, welfare issues arise in pig production when there is a mismatch between a pig's instincts and its environment. That is, behavioral impulses may be expressed inappropriately when instinctual behavior is thwarted. Feral pigs are normally active during the day and spend 75% of their active time in foraging-related activities, including rooting, grazing and exploring with their snout (Stolba and Wood-Gush, Reference Stolba and Wood-Gush1984). Domestic pigs in commercial production, while routinely supplied with the basic necessities (food, water and shelter), continue to express exploratory behaviors and preferentially select environments (pens) with novel objects to investigate (Van Putten and Dammers Reference Van Putten and Dammers1976; Wood-Gush and Vestergaard Reference Wood-Gush and Vestergaard1991). The typical porcine response to a novel object is chewing, but it is not certain whether chewing reflects feeding motivation, exploratory motivation or a combination of both (Day et al., Reference Day, Kyriazakis and Lawrence1995). This uncertainty is reflected in the literature, i.e. some authors conclude that chewing behavior is mediated by feeding motivation (Fraser, Reference Fraser1987) and others that chewing behavior reflects exploratory behavior (Van Putten and Dammers, Reference Van Putten and Dammers1976). Day et al. (Reference Day, Kyriazakis and Lawrence1996) proposed that the initial exploratory chewing of a novel substrate may lead to nutritional feedback, which could modify subsequent foraging behavior.

The effect of environmental enrichment on the behavior, performance and welfare of growing pigs has been extensively researched. In general, these studies investigated the preferences of the pigs for a variety of objects and attempted to measure their effects on aggression, performance and productivity. Docking et al. (Reference Docking, Van de Weerd, Day and Edwards2008) noted that the age of the pigs should be taken into account when selecting enrichment objects because the pattern of use varies by pig age. That is, grower pigs (13 weeks) displayed a shorter latency to approach the novel objects compared to sucking piglets (3 weeks) and weaned pigs (5 weeks), while sucking piglets used the objects to a much lesser extent than either the weaned or grower pigs.

Studies of enrichment for pigs housed in indoor systems have focused on several types of ‘toys’, including objects such as tires, chains, rubber hoses or dog toys (Apple and Craig, Reference Apple and Craig1992; Pearce and Paterson, Reference Pearce and Paterson1993; Blackshaw et al., Reference Blackshaw, Thomas and Lee1997; Hill et al., Reference Hill, McGlone, Fullwood and Miller1998). A number of studies have focused on the inherent properties of enrichment objects, e.g. ingestible, destructible, chewable and odorous (Feddes and Fraser, Reference Feddes and Fraser1994; Van de Weerd et al., Reference Van de Weerd, Docking, Day, Avery and Edwards2003), and their presentation to the animals, e.g. suspended as opposed to being on the floor (Blackshaw et al., Reference Blackshaw, Thomas and Lee1997). Feddes and Fraser (Reference Feddes and Fraser1994) concluded from their study of the stimulus features of non-nutritive chewing, that pigs used objects (cotton cords or rubber strips) more if they could alter the object by chewing. Zonderland et al. (Reference Zonderland, Vermeer, Vereijken and Spoolder2001) tested several characteristics of four materials (rope, wood, chain and metal pipe) in a more systematic fashion and concluded that the most preferred objects combined both ‘flexibility’ and ‘destructibility’, whereas the orientation of the object (‘horizontal’ or ‘vertical’) had no effect on preference. The novelty of an object is important for initiating exploration (Gifford et al., Reference Gifford, Clotier and Newberry2007) and has been reported to be intrinsically rewarding to pigs (Wood-Gush and Vestergaard, Reference Wood-Gush and Vestergaard1991). However, loss of ‘novelty’ occurs rapidly as pigs become habituated to objects (Apple and Craig, Reference Apple and Craig1992; Van de Weerd et al., Reference Van de Weerd, Docking, Day, Avery and Edwards2003). Although there are less data regarding the question of how long a pig can remember a particular object, one way to renew novelty is by replacing familiar objects with new objects (Gifford et al., Reference Gifford, Clotier and Newberry2007).

Pigs’ natural tendency to chew objects in their environment can be misdirected at other pigs. Van Putten (Reference Van Putten1969) reported that ear and tails are the easiest targets, but ear-chewing is more likely to provoke an attack than tail biting. According to Van Putten (Reference Van Putten1969), the bitten animal reacts by waving its tail vigorously, but this further attracts bites by other pen-mates. Tail biting is a behavior and welfare problem that has been attributed to a complex of factors including crowding, lack of bedding, poor ventilation, uncomfortable temperatures, disease (Fraser, Reference Fraser1987), diet, e.g. low fiber, inadequate or poor-quality protein, excessive dietary energy and deficiencies or imbalances of minerals (Gadd, Reference Gadd1967; Ewbank, Reference Ewbank1973). Van Putten (Reference Van Putten1969) argues that tail biting is actually misdirected behavior derived from quiet, low-intensity, chewing and rooting on pen-mates. Fraser (Reference Fraser1987) developed a cotton rope tail model (rope soaked in pig blood) to measure pigs’ attraction to blood and its effect on their tendency to chew. Pigs showed a preference for the blood-flavored rope against the untreated rope, but the model was not balanced for color and, therefore, it was unclear whether the color of blood or the flavor of blood attracted pigs to chew. Jankevicius and Widowski (Reference Jankevicius and Widowski2003) studied tail models balanced for color and found that, regardless of color, pigs preferred to chew on tail models soaked in blood. This suggested that pigs used olfactory or taste cues, not color, to discriminate among choices.

Various strategies have been used for countering tail biting, including providing materials to allow natural biting, rooting and play behavior. Blackshaw et al. (Reference Blackshaw, Thomas and Lee1997) evaluated fixed toys hanging from the ceiling and free toys on the pen floor and found that neither affected relative growth rate, but both affected pig behavior by reducing aggressive behavior (tail biting, fighting, etc.). On the other hand, some studies report that the presence of enrichment objects can lead to agonistic behaviors (Van de Weerd et al., Reference Van de Weerd, Docking, Day, Breuer and Edwards2006; Day et al., Reference Day, van de Weerd and Edwards2008), perhaps resulting from increased competition because of spatially limited access to a particular object (Docking et al., Reference Docking, Van de Weerd, Day and Edwards2008).

Enrichment strategies, such as providing objects suitable for chewing and rooting, may present a stimulus or route for eliciting and reinforcing exploratory activities involving the snout and mouth (Van de Weerd et al., Reference Van de Weerd, Docking, Day, Avery and Edwards2003) and result in a reduction in adverse behaviors, such as tail-biting and belly nosing (Fraser et al., Reference Fraser, Phillips, Thompson and Tennessen1991; Peterson et al., Reference Peterson, Simonsen and Lawson1995). For example, providing growing pigs with straw is considered to reduce harmful social behavior, such as ear and tail biting, and to increase behavior directed toward the straw bedding (de Jong et al., Reference de Jong, Ekkel, van de Burgwal, Lambooij, Korte, Ruis, Koolhaas and Blokhuis1998). An experiment by Beattie et al. (Reference Beattie, Sneddon, Walker, Collins and Boon1993) showed that growing pigs housed indoors and given a substrate in which to root increased the amount of time spent exploring and decreased the time spent in inactivity or in behaviors directed toward their pen-mates, such as chewing ear and tails. Further work showed that, at 18°C, peat, mushroom compost, and sawdust were the preferred substrates, with sand, wood bark and straw preferred only to concrete flooring. Although there was also a variation of 16% in moisture content among the preferred substrates, Beattie et al. (Reference Beattie, Sneddon, Walker, Collins and Boon1993) concluded that particle size or texture played a greater role in determining pigs’ preference than moisture content. However, temperature can also affect pig bedding preferences, i.e. Fraser (Reference Fraser1985) demonstrated that pigs only showed a preference for straw bedding over concrete at low temperatures and concluded that straw was beneficial in the role of thermoregulation, but not necessarily the preferred choice for substrate-directed behavior.

Pigs’ natural tendency to chew novel objects in the environment can also be exploited for animal health purposes. For example Prickett et al. (Reference Prickett, Kim, Simer, Yoon and Zimmerman2008a, Reference Prickett, Simer, Christopher-Hennings, Yoon, Evans and Zimmermanb) demonstrated that oral fluids collected by allowing pigs to chew on cotton rope could serve as diagnostic samples to monitor for porcine reproductive and respiratory syndrome virus, porcine circovirus type-2 and probably other infections of swine. Kittawornrat et al. (Reference Kittawornrat, Prickett, Chittick, Wang, Engle, Johnson, Patnayak, Schwartz, Whitney, Olsen, Schwartz and Zimmerman2010) demonstrated that this approach could be applied to individually penned boars. These reports suggested that normal pig behavior could be exploited for the surveillance of a variety of infectious diseases.

In free-ranging feral swine, pigs’ natural inclination to chew novel objects can be exploited for the oral delivery of pharmaceuticals (Kavanaugh and Linhart, Reference Kavanaugh and Linhart2000) and biologics (Ballesteros et al., Reference Ballesteros, Gortázar, Canales, Vicente, Lasagna, Gamarra, Carrasco-García and Fuente2009). Fletcher et al. (Reference Fletcher, Creekmore, Smith and Nettles1990) demonstrated the feasibility of feral swine vaccination via oral baits using tetracycline as a biomarker to estimate coverage. Ballesteros et al. (Reference Ballesteros, Gortázar, Canales, Vicente, Lasagna, Gamarra, Carrasco-García and Fuente2009) proved the effectiveness of bait for the oral delivery of vaccine to 2- to 4-month-old wild boar by demonstrating serum antibody titers and recombinant Escherichia coli in the feces after consumption of bait containing a live recombinant E. coli biomarker.

Flavor formulation of baits is important for delivery to the targeted population. Fletcher et al. (Reference Fletcher, Creekmore, Smith and Nettles1990) reported 95% bait consumption by feral swine using fish meal-based bait against 63% bait consumption using bovine-flavored bait (Mitchell, Reference Mitchell1998) and 31–72% consumption of kangaroo-based bait (Fleming et al., Reference Fleming, Choquenot and Mason2000). Ballesteros et al. (Reference Ballesteros, Gortázar, Canales, Vicente, Lasagna, Gamarra, Carrasco-García and Fuente2009) prepared bait using a matrix of wild boar feed, wheat flour, paraffin, saccharose and cinnamon-truffle powder attractant with polyethylene capsules of vaccine within the matrix. One of the problems with this self-selected method of vaccine delivery is that baits can be attractive to non-target species. Campbell et al. (Reference Campbell, Lapidge and Long2006) reported that 22% of the bait was consumed by feral swine, but 51% was consumed by raccoons (Procyon lotor), and 20% by collared peccaries (Tayassu tajacu). Thus, bait flavorings need to be highly specific for feral swine in order to avoid consumption by non-target species.

Vaccine formulation is an important consideration in oral baits. Vaccine stability and suitability for oral delivery is a primary consideration, but effect on non-target species must also be taken into account. For example, gene-deleted pseudorabies (Aujeszky's disease) vaccines can replicate in susceptible species, e.g. racoons (Weigel et al., Reference Weigel, Hahn and Scherba2003), resulting in death and/or the potential for transmission to other wild or domestic animals. Likewise, fertility-control products or vaccines targeting gonadotropin-releasing hormone (GnRH), a hormone common to wild and domestic animals, may impair reproduction if consumed by non-target animal species (Campbell et al., Reference Campbell, Lapidge and Long2006).

Regardless of the current technical challenges, feral swine are increasing their range and population numbers in many parts of the world, including North America (Witmer et al., Reference Witmer, Sanders and Taft2003; Ruiz-Fons et al., Reference Ruiz-Fons, Segalés and Gortázar2007). Highly destructive to the environment in large numbers, this population also serves as a reservoir for a variety of pathogens of veterinary and public health importance (Meng et al., Reference Meng, Lindsay and Sriranganathan2009). Therefore, control of feral swine numbers and/or prevention of disease transmission by these animals will mandate additional work on the efficient and effective delivery of vaccines and contraceptives via oral baits.