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Dissertation, University of Illinois, 1989
Isolated postweaning rearing conditions will increase the reactivity and excitability of rodents (Korn and Moyer, 1968; Valzelli, 1973; Riittenen et al., 1986). In the Korn and Moyer (1968) experiments on adult rats, four weeks of isolation in standard laboratory cages was less detrimental in these respects than fourteen weeks. Isolation heightened emotionality; isolated rats reacted violently when they were picked up.
Hyperexcitability has been documented in dogs kept in a sensory-restricted environment. Pairs of puppies residing in barren kennels become unusually aroused and excited when exposed to something new (Melzack, 1954), These puppies could hear and smell dogs in adjacent kennels, and the kennels were illuminated continuously so the puppies in a pair could see and interact with each other. Almost a year after being returned to a residential environment with a human family, the subjects were still hyperexcitable (Melzack, 1954) and electroencephalographic patterns from their brains indicated extreme arousal (Helzack and Burns, 1965).
Animals living in a barren or restricted environment will seek stimulation. During a four-hour experiment, rhesus monkeys in an opaque cage worked hard to discriminate between blue and yellow cards to operantly open a window for a brief glimpse into the rest of the laboratory (Butler, 1953). Rats residing in a barren environment will bar press more than those in an enriched environment (Ehrlich, 1959). The restricted environment consisted of five rats in a 60 x 30 x 23-cm wire cage. Enriched environment rats were raised in groups of ten in a two-tiered environment consisting of a 120 x 76 x 60-cm cage filled with playthings such as corks, tunnels, ramps, and platforms. During testing, each rat was placed individually in a Skinner box. Reinforcement for bar pressing consisted of either clicks or turning on a light.
Pigs residing in crates which were just wide enough to allow turning around at a flared end, turned around 11 to 12 times per day (McFarlane et al., 1988). Even animals which did not have to turn around to gain access to feed or water nevertheless did turn around about the same number times as did those which had to do so. This is presumptive evidence of the pig's need for a certain level of physical activity.
Pigs in intensive housing systems spend long periods sleeping, punctuated by brief periods of intense excited activity, as when a door slams or a person walks into the room. This behavior is similar to that of Melzack's puppies; in the restricted environment, they became very excited when minor changes were made in their cages (Melzack, 1954). Stallions kept in stalls without exercise were harder to handle than those given daily exercise (Dinger and Noiles, 1986).
Pigs reared in indoor pens with minimal contact with people were more excitable and difficult to load into a trailer than those reared outside with frequent contact with people (Warriss et al., 1983). Stolba and Wood-Gush (1980) found that pigs in barren pens with concrete floors reacted more strongly to and played longer with a tire than did those in straw-bedded pens.
Wood-Gush and Beilharz (1983) found that pigs in cages spent less time lying when their environment was enriched with a trough filled with dirt. Maybe pigs in more barren pens sleep longer to reduce the arousal level of an overly aroused nervous system. Zentall and Zentall (1983) suggested that autistic children withdraw from stimulation to prevent further arousal of an already overly aroused nervous system.
The experiments reviewed in this paper support the hypothesis put forth by Walsh and Cummins (1975) that animals living in an enriched environment usually are less excitable than animals in barren surroundings. Restricting sensory input makes the nervous system of both humans and animals more sensitive and more reactive to external stimulation. Schultz (1965) stated: "When stimulus variation is restricted, central regulation of threshold sensitivities will function to lower sensory thresholds. Thus, the organism becomes increasingly sensitized to stimulation in an attempt to restore balance." Walsh and Cummins (1975) concluded that animals in an enriched environment are subject to greater arousal during rearing and therefore are less likely to exhibit over arousal in a novel or highly stimulating environment.
Sensitization to external stimuli occurs within the central nervous system. Placing a small cup on a person's forearm to block tactile stimulation for one week increased tactile sensitivity on the opposite (unshielded) forearm (Aftanas and Zubek, 1964). This effect was quite persistent, as increased sensitivity was still present three days after the cup had been removed.
The sensitizing effect of restricted sensory input also can occur across sensory modalities. The brain needs sensory input to maintain normal reactivity levels. Zubek et al. (1964a) found that a person who is blindfolded or wears translucent goggles for one week has increased tactile, pain, and heat sensitivities, respectively. Even partial deprivation of visual input will sensitize the skin (Zubek et al., 1964b). Trimming the whiskers of baby rats causes the areas of the brain that receive sensory input from the whiskers to become more excitable (Simons and Land, 1987), and this effect persisted. Receptive fields were still enlarged three months after the whiskers had regrown.
Other indicators of increased central nervous system arousal are the effects of depressant and convulsive drugs, respectively, on animals residing in different environments. Rats living singly in small cages were compared to those in groups, and the isolated rats had higher thresholds for anesthetics. Juraska et al. (1983) administered both depressant and convulsive drugs to rats isolated in small laboratory cages (isolated condition, IC) or living in a group of 12 in an enriched environment (enriched condition, EC) which consisted of a large enclosure with many different toys. The IC rats injected with sodium pentobarbital took longer to lose the righting reflex. In a second experiment, when rats reared in IC and EC, respectively, were given the convulsant drug, pentylenetetrazol, the IC rats went into light flash-induced convulsions at lower doses.
Bombarding an animal with excessive stimulation can cause detrimental signs similar to those caused by restricted stimulation. In the IC/overstimulated condition, mice were subjected involuntarily to intense sound, light, and vibration, whereas those in the EC were allowed to initiate and control their interaction with toys. Both IC and IC/overstimulated mice were more irritable than were EC mice, and both forced under stimulation and forced overstimulation increased irritability.
Dantzer and Mormede (1983) and Dantzer (1986) suggested that repetitive stereotypic behavior can serve a de-arousal function. For example, food-deprived pigs will engage in repeated chain-pulling in between food deliveries which occurred every four minutes, and pigs which had access to the chain had lower plasma corticosteroid levels. Stereotypes also occur in pigs living in barren environments which provide low levels of sensory stimulation (Ewbank, 1978; Markowitz, 1982). Therefore, stereotypes may occur in both under stimulating and overstimulating environments. In under stimulating environments, of course, they probably serve to increase sensory input (Ewbank, 1978; Harkowitz, 1982). Young children deprived of normal hugging will engage in excessive masturbation, which stopped when tactile contact with parents was increased (McGray, 1978).
Even though animals living in a barren environment may attempt to reduce central nervous system arousal level, the central nervous system nevertheless may remain in an abnormally aroused state. Perhaps some of the environments that have been imposed experimentally deprived the animals beyond their physiological ability to cope. For example, extremely barren environments would never be encountered in nature.
Nervous systems are endowed with a certain amount of plasticity so the animals can deal with changing environmental demands. The ability to adjust the relative sensitivity of sensory systems to different environments would be useful to an animal for survival in the wild. Simons and Land (1987) concluded that sensory input affects the development of the brain's somatosensory cortex.
More recent research results indicate that new synapses may be formed by neural activity induced by the enriched environment (Greenough et al., 1985). Researchers also have compared IC and EC with (social condition, SC), the latter consisting of two animals living together in a 22 x 25 x 30-cm laboratory cage. Floeter and Greenough (1979) found no differences in dendritic branching in the cerebellums of SC and IC monkeys (Macaca fascicularis). The EC monkeys had the most dendritic branching.
In rats, SC animals had intermediate levels of dendritic branching in the visual cortex (Volkmar and Greenough, 1972). The IC rats had the least branching, the EC rats the most. Weanling rats in EC also had heavier brains than did those in IC (Bennett et al., 1964; Diamond et al., 1964). Rats in EC also had a thicker cortex and additional glial cells (Bennett et al., 1964; Diamond et al., 1964; Diamond, et al., 1966). In these experiments, the control rats were housed in trios in 32 x 32 x 20-cm laboratory cages, and all rats were in their respective environments for at least 29 days.
Very short (four-day) periods of differential housing affected cortical depth in the dorsal-medial part of the brain's occipital cortex (Diamond et al., 1976). Cortical depth differences in weanling rats were induced mainly by environmental impoverishment, whereas in adults they were induced mainly by enrichment.
Neural hypertrophy does not necessarily mean that an animal is in an environment that is higher quality overall. Research indicates that neural hyptertrophy can sometimes be detrimental. Rats exposed to continuous lighting had greater spine density in the visual cortex (Parnavelas et al., 1973), but their retinas were damaged (Bennett et al., 1972, 0'Steen, 1970). Stimulation of the hippocampus with an implanted electrode induced axonal growth and reorganization of synapses, but these new connections increased excitability and seizures developed (Sutala et al., 1988)
Pigs living in indoor pens with minimal contact with people were more excitable and difficult to load than those residing outside with frequent contact with people (Warris et al., 1983). Pigs from 1.2 x 1.2-m relatively barren pens also were slower to approach a novel object or human strangers compared to pigs that had had access to toys and frequent contact with people (Grandin et al., 1983).
Pigs which are either balky or excitable are more difficult to transport and handle at the slaughter plant. Those that at reluctant or refuse to move are more likely to be subjected to excessive electric-prodding. Electric prod-induced excitement is detrimental to pork quality (Barton-Gade, 1985; Grandin 1986). Repeated electric-prodding also will increase bloodsplashing (hemorrhage) in the pork carcass (Calkins et al., 1980). Further, it is detrimental to the pig's welfare; heart rate increases progressively with successive prods, and if it is continued, the pig's heart rate will reach dangerously high levels and death can result (van Putten and Elshof, 1978; Mayes and Jesse, 1980). A pig will stop and lie down when its heart rate is near the lethal point (Hayes and Jesse, 1980).
Choice tests have been used to determine animals' preferences for type of flooring, social groupmates, and mate (Hughes and Black, 1973; Hughes, 1976; Dawkins; 1982; Farmer and Christison, 1982; McGlone and Morrow, 1987); to test sheep's preferences for type of handling facilities (Hitchcock and Hutson, 1979; Hutson, 1981) and restraint methods (Grandin et al., 1986; Rushen, 1986); and to determine pigs' preferred angle and flooring surface for ramps (Phillips et al., 1988).
When choice tests are used, it is essential that they be controlled adequately to prevent previous experiences from influencing the choices. For example, previous experience with different pastures can alter pasture preference in sheep (Arnold and Maller, 1917), and the environment in which they were reared can affect caged hens' subsequent flooring and social preferences (Hughes, 1976).
Another potential problem with preference tests is that something that is preferred on a short-term basis may have detrimental long-term effects on the animal's well-being (Hughes and Black, 1973; Duncan, 1978; van Rooijen, 1982) For example, animals may prefer one type of flooring during a two-hour preference test, but the preferred flooring may injure the animals' feet after several months.
Piglets usually prefer plastic-coated expanded-metal floor over concrete (Farmer and Christison, 1982). In practice, plastic-coated expanded-metal flooring is excellent for farrowing crates and nursery pens but causes problems when used for a finishing floor. Casual observations indicated that pigs finished on either flattened or plastic-coated expanded metal had excessive hoof growth, and they balked and were difficult to drive into a high-speed slaughter line (Grandin, 1988).
Preference tests are one useful method for determining practical objects that producers could use to enrich a pig's environment. Unlike flooring, feed, or housing, object choices are unlikely to have adverse long-term effects on the pigs' health or performance. There is a need for information on the pig's short- and long-term preferences for means of environmental enrichment so that recommendations can be made to producers.
Fagen (1984) described play as follows: "Play means behavioral performance that emphasizes skills for interacting with the physical and social environments and that occur under circumstances under which the function of the exercised skills can not possibly be achieved." Play occurs in many farm animals, including cattle, sheep, pigs and horses (Brownlee, 1954, 1984; Tyler, 1972; Schoen et al., 1978; Sachs and Harris, 1978; van Putten, 1978; Dobao et al., 1984-85; Crowell-Davis et al., 1987), as well as a relative of the domestic pig, the collared peccary (Byers and Beckoff, 1981; Byers, 1984). It is interesting that in the collared peccary both juveniles and adults play, because ordinarily in ungulates only juveniles and subadult males play (Byers, 1984).
Object play, including "tug-of-war", has been observed in dogs, cats, horses, and jackals (van Lawick and Goodall, 1971; West, 1977; Fagen, 1981; Martin, 1984). Captive bushdogs and foxes play with sticks (Biben, 1982). A captive rhinoceros will root a ball and other objects (Inhelder, 1978; Hediger, 1968). Piglets also engage in object play (Gundlach, 1988; Fraedrich, 1974). Weanling pigs tugged on cloth strips in a manner similar to that of dogs playing "tug-of-war" (Grandin et al., 1983). Foals will pick up sticks and carry them around and toss them in the air (Crowell-Davis et al., 1987).
Fighting among newly weaned piglets generally has little adverse effect on the pigs' long-term performance, but it can be detrimental to welfare. McGlone and Curtis (1985) found that fighting and injuries could be reduced by providing newly weaned pigs small hides where they could place their head and shoulders. Anecdotal reports suggest that providing pigs toys during times of social mixing also will reduce fighting.
Straw bedding did not reduce fighting in growing pigs fed ad libitum, but it had a tendency to reduce fighting in fasted pigs (Kelley et al., 1980) Another factor which will influence aggression is type of housing. Pigs in indoor pens were more aggressive than those residing outdoors (Meese and Ewbank, 1973). Crowding tends to increase pig aggression (Bryant and Ewbank, 1972; Kelley et al., 1980; Randolph et al., 1981), but it does not increase all kinds of aggression in a uniform manner. Reducing floor space reduces aggression which occurs away from the feeder, but it has no clear effect on aggression at the feeder (Ewbank and Bryant, 1972). Restricting lying space increased aggressive encounters when animals were standing but had little effect on aggression in other areas (Ewbank and Bryant, 1972). More information is needed about the effects of environmental richness on aggression in pigs.
The environmental enrichment methods used in all of these experiments were simple, practical procedures that swine producers could use. Results of these experiments may help answer questions as to whether relatively simple environmental enrichment procedures will improve the productivity and welfare of pigs residing in intensive production systems.
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