In many respects the kitten's development is remarkably well ordered. Within limits the systems that generate the beautifully integrated behaviour of an adult cat seemingly have a goal-directed character to them and are resilient to both internal and external disturbances. Most cats eventually become reasonably competent predators, for example, almost irrespective of the type of experiences they have as young kittens.
In reaching an understanding of these sorts of effects, one useful principle is the system theory concept of 'equifinality'. In an open system, such as a living organism, the same steady state at the end of development may be reached from different starting conditions and by different developmental routes. In behavioural terms, this principle suggests that the same skill might be achieved as the result of quite different developmental histories.
The cat's predatory skills provide a particularly good example of the same set of behaviour patterns developing via different routes. Individuals differ considerably in their predatory behaviour during early development--particularly during the second and third months. This variation lies not so much in the basic predatory motor patterns, which virtually all individuals express, but in their integration, in the assessment of whether a prey can be caught, and in choosing the appropriate tactics. Despite this individual variation among young cats, however, most eventually become competent predators, albeit with different preferences and specialisations for particular types of prey. At the crude level of overall predatory competence, much of the early individual variation in predatory skill disappears by adulthood. Some measures of predatory skills made before three months of age are not related to those made at six months, because individuals which were poor predators as kittens have usually caught up by the time they are fully grown.
These fascinating and almost uncanny aspects of development make sense in the light of the very different kinds of early experience that can enhance predatory skills. Adult predatory skills are improved by experience with prey when young, by watching the mother dealing with prey when young and, possibly, by the effects of competition between littermates in the presence of prey. Kittens that have never killed a rat, for example, can become ratkillers merely by watching another cat kill a rat (Kuo, 1930). In addition, experience of prey when adult may also improve adult skills, which means that adults which have lacked early experience with prey can, to some extent, catch up later in ontogeny.
The main point here is that a given set of adult behaviour patterns--in this case predatory behaviour--is affected by several different types of experience. Lack of one type of experience--say, experience of dealing with prey when young--may be compensated for by other forms of experience, such as watching the mother deal with prey when young, or experience with prey when adult. Thus, a given developmental outcome--competence as a predator--might be attained via many different types of developmental history. In functional terms, this type of process would clearly be of benefit to the individual, in that it allows the same type of behaviour to develop in a variable environment where individuals might have quite different types of early experience.
Of course, other processes may lead to apparently similar results. The effects of trauma or injury may disappear as the result of normal repair mechanisms. Where certain types of experience exert a facilitatory effect on development, it is also possible that considerable individual variation early in life will have disappeared by adulthood. In this case, though, the same developmental end-point is reached via the same developmental route, but at different rates. For example, exposing kittens to a cool environment during the first few days after birth hastens the development of temperature regulation. At two weeks of age, therefore, individuals may differ considerably as a result of differences in their exposure to low temperatures, but by four weeks of age they no longer differ.
Alternative routes in development may also lead to different outcomes for adaptive reasons. In the domestic cat, weaning is a gradual process during which the mother progressively reduces the rate at which she gives care and resources (notably milk) to her offspring. Under favourable laboratory conditions, weaning commences at about four weeks after birth and is largely completed by seven weeks.
Weaning represents a period of major transition for young mammals, marking a change from complete dependence on parental care to partial or complete independence. This transition, which is shown most obviously by the change in food source, involves a whole range of behavioural and physiological changes on the part of both mother and offspring. If, as is likely for a variety of reasons, the time of weaning may vary according to factors such as maternal food supply, then the developing offspring must be able to adapt by altering its behaviour accordingly.
Evidence that kittens may alter their development in response to changes in weaning time comes from two sources. Tan and Counsilman, ('The influence of weaning on prey-catching behaviour in kittens,' in Zeitschrift für Tierpsychologie, vol. 70, pp. 148-64, 1985) looked at the development of predatory behaviour in kittens which had experienced early, normal or late weaning. Early weaning was simulated by gradual separation from the mother starting at four weeks, while late-weaned kittens were left with their mothers but were denied access to solid food until the ninth week. Tan and Counsilman found that early-weaned kittens developed predatory behaviour sooner than normally-weaned kittens and were more likely to become mouse-killers. Conversely, late weaning was associated with delayed development of predatory behaviour and a reduced propensity to kill mice, although these effects might have been to due to non-specific debilitating effects of delayed weaning. In general, Tan and Counsilman's results fit with the notion that the development of predatory behaviour is linked in an adaptive way to the time of weaning: in other words, that it develops when it is needed.
A series of studies has shown that the development of play behaviour is markedly influenced by the time of weaning. Under normal laboratory conditions, kittens' play behaviour undergoes a number of major changes towards the end of the second month, most notably by showing a large increase in the frequency of object play (Barrett, P. & Bateson, P., 'The development of play in cats,' in Behaviour, vol. 66, pp. 106-20, 1978). This change in play coincides with the end of the weaning period, suggesting that the change from social to object play occurs in response to the kitten's increasing independence from the social environment of the nest.
To test this hypothesis, early weaning--or, more specifically, a reduction in maternal care--was simulated in a variety of different ways: by gradual separation from the mother starting at five weeks; by interrupting the maternal milk supply with the lactation-blocking drug bromocriptine starting at four weeks or five weeks; or by slightly reducing the mothers' food supply. In all cases, the experimental manipulation led to an increase in the frequency of certain types of play. A higher rate of play after early weaning may mark a conditional response by the kitten to enforced early independence, by boosting the benefits of play before complete independence.
