It is known that when an unpleasant metabolic experience occurs around the time a novel food has been eaten, this food becomes aversive. For example, injections of lithium chloride, known to induce nauseous feelings, when paired with red food, induce aversion for that colour by chickens.
There are several unusual features concerning the conditioned aversion to food paired with abdominal discomfort: (i) the aversion can develop after a single trial; (ii) it can occur even though there may be a long delay (several hours) between the unconditioned stimulus (US) and conditioned stimulus (CS), making this learning different in character from Pavlovian association, in which US and CS must be presented almost simultaneously; and (iii) tastes are much more effective than visual or aural cues, at least in mammals. Even animals under the influence of an anaesthetic when a painful stimulus (US) is given learn to associate it with the food available when they are regaining consciousness (CS). It is important for food to be novel, with a strong taste and/or odour, and these characteristics are often associated with high-protein foods.
As an example of the methodology used in studies of the development of conditioned aversions, research with CCK and coloured foods for chicks is presented. CCK, produced in the duodenum and proposed as a satiety hormone (see Chapter 4), has been shown to condition an aversion to coloured food. CCK given intraperitoneally (i.p.) is known to depress food intake, and the aim of studies by Covasa and Forbes (1994b) was to determine whether this reduction was due to a pleasantly satiating effect or an unpleasantly nauseating effect of CCK.
On days 1, 4 and 7, one-half of the birds were injected with 100 g CCK i.p., sufficient to depress intake by about 20% over the next 2 h; for 2 h after the injection the normal food was replaced with food of the same composition, but coloured green for one-quarter of the birds and red for the other quarter.
Learning about Food: Conditioned Preferences and Aversions 123
The other half were injected with saline and, again, one-quarter given green food and one-quarter red for the next 2 h. On days 2, 5 and 8, each bird was given the alternate injection with the alternate colour and on days 3, 6 and 9 were made mildly hungry by fasting for 1 h and then individually given a choice between green and red food.
The colour approached was noted and, as shown in Fig. 6.2, they became progressively averse to the colour of food associated with CCK – it is clear that the effects of the hormone are unpleasant and that the birds had become conditioned to believe that their discomfort was due to food of a particular colour – the identity of the colour is not important. For the next 11 days, no injections were made but the preference test was repeated from time to time – the aversion was lost within a few days. Then, from day 26, injections were paired with the opposite colours to those used in the first phase of the experiment and aversion rapidly developed, again followed by return to no preference within a few days of stopping the injections. Thus, colour preference can be easily manipulated and the same is true for the association between food flavour and the consequences of eating, which is more readily evident in mammals.
It is known that the intake depression caused by giving CCK i.p. can be prevented by section of the vagal nerves (i.e. CCK stimulates abdominal receptors that transmit information to the CNS via the vagal pathway, Chapter 4) and, when the above experiment was repeated with chickens vagotomized at the level of the proventriculus, no significant colour preferences or aversions became established.
0 20 40 60 80 100
0 10 20 30 40
Days
Chicks eating CCK-paired coloured food (%)
Fig. 6.2. Percentage of chicks favouring the colour of food associated with CCK injection;
dotted line, 50% (see text for explanation) (from Covasa and Forbes, 1994b).
Unnatural toxins
Lithium chloride (LiCl) has been widely used as US in studies of conditioned aversions. It has approximately the same effects in various species on a per- body weight basis and is effective when given either enterally or parenterally.
Poultry
Hayne et al. (1996) studied aversion learning and retention in chicks whose drinking water was coloured blue and adulterated with LiCl for a period of 24 h.
The chicks self-administered large and often lethal doses of the LiCl solution but subsequently avoided blue water during two-bottle preference tests administered 3–7 days, but not 14 days, after exposure. Surprisingly, neophobia alone was insufficient to prevent non-deprived chicks from ingesting large quantities of a toxin during their initial encounter with it. On the other hand, those that survived had learned to avoid the colour associated with toxic consequences although this had been forgotten 1–2 weeks later.
Pigs
It appears that no studies with LiCl have been made in pigs!
Cattle
Pfister (2000) used LiCl to induce aversion to toxic pine needles, and the work of Cibils et al. (2004) is another example in which the ability to associate stimulation of the external skin-defence system with the internal gut-defence system was tested. Young cattle made averse to high-quality foods by means of electric shocks were not put off from eating these foods when they were in
‘safe’ locations, whereas they avoided foods rendered aversive by means of LiCl irrespective of location. It can be concluded that non-food features of the environment are not used as CS for food aversions, a conclusion also reached by J. Catterall and J.M. Forbes (unpublished observations), who paired solid floor (preferred) or wire mesh (avoided) with different coloured foods and found no aversion to the mesh-paired colour of food.
Sheep
The rumen, by storing food for several hours, may act as a buffer between eating a meal and experiencing its metabolic consequences and thereby reduce the chance of a toxin being associated with its originating food. Alternatively, the rumen may assist learning by prolonging the time over which a toxin from a particular meal is available to the host animal. Presumably because of their large size and high cost of carrying out research with cattle, most of the studies on ruminants have been carried out with sheep.
Sheep find LiCl, injected or in the food, to be unpleasant and the degree of learned taste aversion varies closely with the dose. Strong aversion was shown with injection of 150 mg/kg, similar to the dose causing strong aversion in rats.
Learning about Food: Conditioned Preferences and Aversions 125
Feeding neophobia was also increased in proportion to the last dose of LiCl used. When sheep and goats were given 2% LiCl in food, they ate a maximum of around 39 mg/kg body weight for sheep and 27 mg/kg for goats, levels not much higher than those causing mild discomfort in humans.
Lambs made averse to a novel food by means of LiCl persisted in avoiding this novel food, even when not adulterated, but gradually this aversion wore off (Burritt and Provenza, 1996). However, when at a later date they were given the novel food together with another novel food adulterated with LiCl, they subsequently avoided the original novel food at first, demonstrating a memory for the effects of LiCl. In a second experiment, lambs were offered a novel food for either 28, 14, 7 or 1 day, followed by a single exposure to LiCl. The longer the lambs had experienced this food, and become convinced that it was safe, the less persistent the aversion.
Sheep can clearly detect different concentrations of a flavour and prefer a lower concentration of a flavour associated with the unpleasant consequences of LiCl administration. Lambs offered food with a medium concentration of LiCl after being offered a high-LiCl food increased their intake, while those previously on a low-LiCl food decreased their intake when given the medium- LiCl food; the influence of either US (LiCl) or CS (flavour) on preferences and intake is, therefore, relative rather than absolute.
Naturally occurring toxins
Oxalic acid is a mild toxin found in some materials used as animal feed, such as the leaves of many root crops. Conditioned taste aversions have been induced in sheep that persisted for up to 60 days after the last dose of oxalic acid (Kyriazakis et al., 1997). The rate of development and severity of the aversion were dependent on the rate of administration of oxalic acid, developing after as little as one exposure to a very high dose but requiring many exposures to low doses (Kyriazakiset al., 1998). These findings show that it is not only to powerful unnatural toxins that aversions are developed, but to substances that are found in the diet and are only mildly toxic at natural concentrations.
Excess of nutrients
Not only do substances that are overtly toxic act as US in conditioned aversion, but even essential nutrients can depress intake and preference when their supply to the animal exceed the animal’s needs. Sheep prefer a straw of a flavour associated with the provision of NaCl until their requirements are met, then they find this same flavour aversive; a similar situation applies to sulphur.
These and other appetites are described and discussed in Chapters 8 and 13.
There is a large and significant literature on learned preferences and aversions to the protein:energy ratio of foods, and this is discussed below in the section on ‘Continuum from Preference to Aversion’ and in Chapters 7 and 8.
Distension of the digestive tract
We have seen above that animals prefer foods, or flavours associated with foods, that provide adequate amounts of nutrients. Foods high in fibre and therefore occupying a lot of space in the stomach might be expected to generate aversion, as it has been shown that gut distension in rats conditions an aversion to the paired taste (Bardos, 2001). In pigs, feeding motivation is inversely proportional to the bulkiness (measured as water-holding capacity) of iso- energetic meals (Dayet al., 1996b).
In the only relevant study in ruminants, non-lactating cows had a balloon in the rumen inflated with 12 l of water for 2 h, immediately after which each was given 500 g of either unflavoured or citrus-flavoured concentrate food (Klaiss and Forbes, 1999). This was repeated on the following 2 days and for a further 3 days when the treatments (± balloon, ± flavour) were reversed. The following day, a preference test was administered in which 500 g of flavoured and 500 g of unflavoured concentrate were given at different ends of the food trough and the times at which each animal changed from one food to the other were noted. For the next 6 days, the training was continued by alternating the treatments (± balloon), still associated with the same flavour as before, on alternate days, followed by another preference test.
No significant preferences were observed, for which the authors gave three possible reasons: (i) the cows could not differentiate between the flavoured and unflavoured foods. This is highly unlikely, as the citrus flavour was easily recognized by humans and the concentration used was greater than that of a similar citrus flavour that was clearly distinguished by cows; and (ii) the distension of the rumen caused neither pleasant nor unpleasant sensations.
This seems unlikely because such distension depresses intake of forage and the balloon was added to a rumen that was already fairly full; or (iii) filling the balloon just before, rather than just after, giving the flavoured/unflavoured concentrate. While animals associate unpleasant stimuli applied from a few seconds to several hours aftereating a distinctively flavoured food, application of the CS (in this case balloon inflation) beforeexposure to the US (in this case the flavoured food) might not condition the association. The reason for inflating the balloon before the concentrate meal was to avoid the disturbance associated with inflation coinciding with the critical time just after exposure to the US – the eating of flavoured or unflavoured food.