stimulus (CS) (food and bell, respectively, in the case of Pavlov’s dogs) if the interpretation of the results is to be unequivocal. While there is not space, nor is it necessary to go into experimental detail in every case given, the methods of Arsenos and Kyriazakis (1999) are given in detail below in the section on
‘Continuum from Preference to Aversion’ as an example of good methodology.
to the animal’s knowledge, but not necessarily awareness, of the consequences of eating particular foods. This information is integrated with awareness of foods gained by the special senses and committed to memory to serve the animal when it next has to make decisions about how much, or what, food to eat. Of particular importance in the hindbrain is the area postrema(AP), which possesses chemoreceptors. Destruction of the AP prevents the development of conditioned taste aversions (CTA) to a variety of toxic agents.
Awareness of digestive and metabolic sensations
Once a bite of food has been swallowed there are few conscious sensations of food passing through the digestive tract and being processed, unless the food contains toxins or induces excessive distension. Nevertheless, information concerning both physical and chemical changes in various parts of the digestive tract is transmitted to the central nervous system (CNS) via the autonomic nervous system and in the circulating blood. There is no compelling reason why learning or cognitive abilities require awareness (Nicol, 1996).
For example, ruminant animals have mechano- and chemoreceptors in the rumen and reticulum with afferent pathways in the vagus nerve (see Chapter 3). There is a clear relationship between the degree of stimulation of a family of receptors by inflation of a balloon, or infusion of salts of volatile fatty acids into the rumen, and the increase in firing rate in vagal fibres leading to the hindbrain, particularly the nucleus of the solitary tract. This, in turn, is correlated with the depression in food intake. Thus, the CNS is informed, normally subconsciously, of the consequences of eating in terms of physical and chemical effects on the digestive tract, and uses this information in how it responds when it again becomes aware of the food, i.e. what food it chooses to eat and how much it eats.
The liver is the first organ to have a reasonably complete picture of the results of eating a particular food, in terms of supply of metabolites. Evidence for neural transmission of metabolic information from liver to CNS is extensive, and it is clear that the brain uses the degree to which substrates are being oxidized as part of the complex control of food choice and intake (see Chapter 4). A major function of the liver is to stabilize the fluctuating supply of nutrients arriving from the digestive tract as a result of eating discrete meals into a more even supply of nutrients to the rest of the body, particularly the CNS. Thus, the brain is informed by the liver of the balance between the supply of nutrients from the gut and the demand for nutrients by the rest of the body.
Measuring motivation – operant conditioning
Rewards of food are powerful, unconditioned stimuli in an operant-conditioning situation, i.e. one in which the animal has to respond (e.g. by pressing a button) in order to elicit a reward (technically known as a reinforcement, e.g. the
delivery of a portion of food). Animals quickly learn to respond, and the bigger the reinforcement the more quickly is the association learned.
When animals are trained to press a bar to obtain food and the number of presses required to initiate a meal is varied, increasing the effort required to start a meal causes a reduction in the number of meals, but an increased meal size so as to maintain total intake and body weight (see Chapter 2). This shows that feeding is not dependent simply on short-term changes in the supply of metabolites and must involve the interaction of numerous factors.
Once animals are accustomed to working for their food they like to do so;
ostriches trained to respond for food prefer to peck at a button for food than to obtain it ‘free’. When a bag was accidentally dropped in the pen, they ate only a little of the spilled food and then returned to the operant dispenser.
Poultry
In laying hens, daily food intake stayed the same up to a fixed ratio of reinforcements:responses of 160 (FR160) (Fig. 2.6), but the total time spent feeding and meal frequency were negatively related to FR, while inter-meal interval, meal size and rate of eating within meals were positively related to FR.
It appeared that randomness in meal taking declined at higher FRs, and FR20 seems the most appropriate to use in experiments in which it is desired to get birds to eat only when they feel hungry.
Pre- and postprandial correlations are generally higher when animals have to expend some energy to get food. As the cost rose from 1 to 5000 pecks to gain access, so the number of meals fell from about 20 to 1/day, and meal size rose from being very small to > 200 g (Kaufman and Collier, 1983). Daily food intake remained approximately constant up to about 500 pecks/meal, but rate of eating increased more or less continuously with FR. Unfortunately, there was confounding between the FR imposed and the age/weight of birds, so the observed reduction in daily intake at higher ratios might just have been due to a slowing of growth as birds were about 3 kg at this stage, at which point they were growing only slowly.
When hens trained to push a weighted door to get food were deprived of food for 12 h, they took significantly longer to gain access to food by applying the required force ⫻ time than when deprived for 43 h, due to longer pauses between attempts to open the door. It is not necessary to construct special equipment in order to quantify animals’ motivation to eat. For example, Petherick et al. (1992) trained hens to run down a 14.4 m alley for food. The speed with which they ran was significantly increased by deprivation of ⭓ 6 h (0.29, 0.62, 0.65, 0.57 m/sec for deprivation periods of 0, 6, 12 or 18 h, respectively).
The speed with which chicks approached food was also used by Hannah (2001) in studies of motivation to eat foods made moderately (M) or highly (H) imbalanced by dilution with wheat, colouring and quinine being added as cues and compared with unadulterated food (C) given a different colour (but no quinine). During the first phase of the experiment, from 17–32 days of age, they were fasted for 1 h and given the test food for 2 h daily; for the rest of the time untreated food was available ad libitum.On days 26 and 33 each bird’s
motivation was tested individually by recording the time taken to attack the test food to which it had been trained.
During the second period of the experiment, the training and testing procedures continued but the test food was either kept the same or changed to a moderately toxic food, as shown in Fig. 6.1, in order to study the duration and extent of the adjustment in motivation. The results show very clearly the reduction in motivation (increase in ‘attack’ time) of the chicks given toxic test food compared with those given the control food. By 4 days after the test food having being changed there were clear changes in motivation, which had stabilized some 10 days after the change. This demonstrates very clearly the learning and relearning involved, the latter taking only a few days in these young chicks.
In this and many other similar studies, including operant conditioning, birds have been fasted for short periods in order to increase their motivation to eat. This creates an abnormal situation, mildly stressful, and adds significantly to the time taken to perform the experiments. It has been shown that maggots are highly attractive to hens, who show great motivation to eat them even without prior fasting, and it has been suggested that they could be used as test foods for poultry (Bruceet al., 2003).
Pigs
Sows, fed at a commercial level of 2.3 kg/day, were trained to lift a lever to obtain reinforcements of 2.7 g of food on a 10:1 FR. After 1 h of responding, the FR was raised to 20, and so on. Extinction (failure to respond sufficiently to obtain a reinforcement) occurred at ratios which varied from 70:1 to 430:1 for different
0 100 200 300 400 500
16 21 26 31 36 41 46 51
Day of age Low attack
motivation
High attack motivation
Period 2 – conditioning Period 1 – training
Attack time (s)
Fig. 6.1. Attack times for each treatment group: 䡲HH,
♦
CC,䡩HM,▫CM. C, control test food; M, moderately; H, highly toxic test food. Vertical dotted line shows where the foods were changed (see text for details) (from Hannah, 2001).sows. For high FRs, it was estimated that the energy cost of responding was greater than the benefit gained from the food. Sows trained to lift a lever to obtain straw, twigs, nothing or a portion of food responded for food, most frequently (Hutson, 1992).
Specific nutritional deficits or general nutrient restriction will increase hunger and the motivation of the animal to ingest food (Lawrenceet al., 1993).
Jensen et al.(1993) found that individually housed, protein-deficient, growing pigs increased their general activity relative to those animals offered a diet excessive in protein or to those offered a choice between the two foods. Food- restricted (ad libitum ⳮ 20%) pigs spend less time lying than pigs fed ad libitum. They also rooted the straw substrate significantly more.
In group-housed situations, pigs may redirect their behaviour towards penmates as alternative foraging stimuli and hence increase the chance of injuries through excessive abnormal behaviour. Pigs with an inadequate dietary protein supply tend to have an enhanced attraction for blood. It is not known whether the inadequate protein nutrition predisposed the animal’s attraction specifically for blood or would have been similar for other protein-rich substances. Thus it is proposed that, if animals are allowed a choice of balanced foods such that they can select a diet to meet their requirements, they can satisfy their motivation to ingest food.
Sheep
Sheep can quite easily be trained to respond for food (Kilgouret al., 1990).