Various ways of analysing meal pattern data are discussed by Panksepp (1978).
This section covers some of the more commonly used methods.
Univariate analyses
Methods such as t-test and analysis of variance have usually been used to compare means of parameters such as daily intake, meal size and rate of eating.
However, because many feeding characteristics, such as meal duration, meal size and inter-meal interval, are inter-correlated, the use of univariate tests for each variable is likely to yield some false significant differences. It is, therefore, often advisable to use multivariate methods of analysis. Where intake has been measured at several intervals during and after an experimental treatment, the intakes during each time interval should be analysed rather than cumulative intakes (Fitts, 2006).
Multivariate analyses
These methods include multiple analysis of variance, discriminant analysis and multiple regression analysis, in which more than one meal-related variable can be included. This is a complex area of statistical analysis (see Geiselmanet al., 1980, for examples).
Sometimes, to reduce the huge amount of data collected by automatic systems, calculations are made of mean meal size and inter-meal interval.
When done for whole days this disguises the fact that there is a circadian rhythm of feeding behaviour, so the day can be divided into periods of, say, 4 h and means calculated for each period (Forbeset al., 1989).
Figure 2.7 shows the number of meals, meal size and rate of eating by sheep during the six 4-h periods of the day during late pregnancy, lactation and after weaning. The number of meals/4 h declined from the time when fresh food was offered, as did meal size. However, rate of eating did not differ between different periods of the day. The increase in total food intake between pregnancy and lactation was due to an increase in meal numbers, while the slight fall in intake after weaning was due to fewer meals of larger size than in lactation, eaten at a faster rate.
1000 600 500 400 200 0
Food intake (g/4 h) No. of meals (/4 h)
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
Period of day Physiological status Pregnant Lactating Weaned
Pregnant Lactating Weaned
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 Pregnant Lactating Weaned Pregnant Lactating Weaned
(a) (b)
(d) (c)
10 8 6 4 2 0
Rate of eating (g/min)
40 30 20 10 0
Meal size (g)
150 120 90 60 30 0
Fig. 2.7. (a) Food intake, (b) meal numbers, (c) meal size and (d) rate of eating by pregnant, lactating and weaned ewes during different periods of the day (from Forbes et al., 1989).
Feeding Behaviour 23
Forbes et al. (1989) confirmed that feeding behaviour of ewes was markedly affected by physiological state using discriminant function analysis, which showed that there was very little overlap between pregnancy, lactation and weaning in the characteristics of eating as described by total daily intake, rate of eating, meal weight, number of meals, satiety ratio and time spent eating.
Division of the day into arbitrary periods is an artificial procedure, and methods that adopt a more continuous approach have been developed.
Deswysen et al. (1989) used Fourier analysis to extract rhythms from meal patterns recorded from heifers offered maize silage. There were large and consistent cycles with phases of 24, 12 and 8 h; the authors speculate about the physiological significance of these rhythms, but only the 24-h cycle is clearly explicable as it is related to the diurnal light cycle and the daily offering of fresh food.
Barrio et al. (2000) noted that individual water buffalo had different feeding patterns but that any one animal had a similar pattern on consecutive days. They used cluster analysis to characterize these patterns and saw that the first meal after fresh food (hay) had been offered was longer and heavier than subsequent meals.
Hunger and satiety ratios
Close correlation between the size of meals and the length of the preceding inter-meal intervals implies that there is a mechanism that determines when feeding should stop, i.e. that satiety mechanisms predominate. If, on the other hand, there is a significant correlation between meal size and post-meal interval, this implies that there is a mechanism for determining when feeding should start, i.e. a hunger mechanism. The hunger ratio has usually been found to be more important than the satiety ratio in chickens, as in rats. It has been suggested that using a fixed schedule operant situation gives more ‘normal’
feeding patterns with more reliable hunger ratios. In cattle, Metz (1975) found a positive correlation between meal size and the length of the pre-meal interval, as did Baile (1975) with sheep offered a 0.6 concentrate:0.4 forage mixture, i.e. the hunger ratio was thought to be more useful than the satiety ratio.
Note, however, that statistically significant ratios between meal weight and prior or subsequent intervals might be a consequence of using inappropriate inter-meal intervals. If arbitrarily defined inter-meal criteria are used that are shorter than the ‘true’ biologically defined interval (as in the two examples cited immediately above), there will be many small meals separated by short intervals, which will give closer correlations between meal size and pre- or postprandial interval than if the more biologically meaningful interval is used. It is important, therefore, that a method with biological integrity be used for the calculation of inter-meal intervals if false conclusions are not to be drawn from data on feeding behaviour.
In Tolkamp’s observations, lengths and weights of meals by cows were distributed according to negative exponentials, suggesting that the termination of
meals was largely random and not tightly controlled by the animal reaching a threshold level of satiety. Although statistically significant, pre- and postprandial correlations were associated with very small proportions of the variation in meal size (Tolkamp et al., 2002). Preprandial correlations, although low (r ≈ 0.12) were about four times higher than postprandial correlations. This is a sign that feeding behaviour is determined more by satiety than by hunger mechanisms, and means that cows manage their daily intake by means other than inter-meal interval, i.e. feeding behaviour is flexible but, the longer the time over which meals are summed, the more stable and predictable is intake. Therefore, detailed studies of feeding behaviour are of little direct help in improving our understanding of what physiological mechanisms underlie the control of voluntary food intake on a longer timescale.