Anita. Behav., 1985, 33, 502-510 The ontogeny of locomotor play behaviour in the domestic cat PAUL MARTIN & PATRICK BATESON Sub-department of Animal Behaviour, University of Cambridge, Madingley, Cambridge CB3 8AA, England Abstract. A descriptive account is given of the development of locomotor play in the domestic cat (Felis catus) under laboratory conditions. The subjects were seven families, each family consisting of a mother and her two kittens and living in its own indoor pen. Each family was separately presented with a multi-level climbing frame for 30 rain once every 3 days, between 36 and 60 days after birth (nine presentations in total). Kittens spent a substantial proportion of the observation session climbing, sitting, standing and walking on the frame, especially after 48 days of age. Kittens sometimes lost balance and fell off the frame, particularly when performing more difficult motor acts. Standing and walking on the upper levels of the frame were first seen at 48 days of age and increased in incidence thereafter. Mothers sometimes climbed on to the frame and there was an overall positive association between mothers' and kittens' use of the frame. There were large and consistent behavioural differences between families. The climbing frame provides a potentially useful tool for assessing the development of certain locomotor skills under conditions where the subject's behaviour is spontaneous. Young domestic cats (Felis catus) exhibit a diverse array of motor patterns of unknown biological function which are commonly lumped together under the term 'play' (reviewed by Martin 1984a). These undergo marked changes in form, frequency and inter-relationship during the course of onto- geny (West 1974; Barrett & Bateson 1978; Caro 1981a, b) and provide a rich source of material for those interested in the processes of behavioural development (Bateson 1981). The ontogeny of social play and object play in the cat have been extensively described (e.g. West 1974, 1979; Barrett & Bateson 1978; Baerends-van Roon & Baerends 1979; Moelk 1979; Caro 1981a, b), but so far no study has looked at the ontogeny of locomotor play in this species. This paper presents an account of the development of locomotor play in kittens under laboratory conditions. Play is a broad and often inconsistently used term that is applied to a wide variety of behaviour patterns. Its definition (if, that is, one is stated) may include both structural and functional criteria (Bekoff & Byers 1981; Fagen 1981). Clearly, any account of play is necessarily influenced by the definition of the term, whether or not this is made explicit. In this paper, a modification of the definition suggested by Bekoff & Byers (1981) is adopted and 'play' is used in the sense of motor activity that appears to have no obvious short-term benefits, in which motor patterns from other functional contexts may be used in modified forms and altered temporal sequencing. Locomotor play, as distinct from social or object play, is playful activity that carries the individual about its en- vironment, but is neither specifically directed at other individuals, nor involves manipulating inani- mate objects. METHODS Subjects, Housing and Care The subjects were seven litters of domestic kittens from seven different mothers. Each litter was culled to one male and one female kitten within a few days after birth. The kittens were born and reared in a laboratory colony and were housed with their mothers from birth in large indoor enclosures. All mothers had previously given birth to at least three other litters. Each family (mother and two kittens) was housed in its own trapezoid-shaped indoor pen of floor area 8-8 m 2. The short side of the pen was 1.1 m wide and had a door in it. Set into the door was a darkened sheet of Perspex which provided an effective one-way screen. The long side of the pen had a translucent window (area 4.1 m 2) in it and was 4.0 m long. The radiating walls were 3.8 m long and the pen varied in height from 2.1 m to 2.4 m. In addition to receiving daylight, each pen was artifi- cially lit for 14 h each day (0600-2000 hours). Temperature fluctuated to some extent with exter- nal temperature, but heating ensured that it did not 502
Transcript
Anita. Behav., 1985, 33, 502-510
The ontogeny of locomotor play behaviour in the domestic cat
PAUL M A R T I N & P A T R I C K BATESON Sub-department of Animal Behaviour, University of Cambridge, Madingley, Cambridge CB3 8AA, England
Abstract. A descriptive account is given of the development of locomotor play in the domestic cat (Felis catus) under laboratory conditions. The subjects were seven families, each family consisting of a mother and her two kittens and living in its own indoor pen. Each family was separately presented with a multi-level climbing frame for 30 rain once every 3 days, between 36 and 60 days after birth (nine presentations in total). Kittens spent a substantial proportion of the observation session climbing, sitting, standing and walking on the frame, especially after 48 days of age. Kittens sometimes lost balance and fell off the frame, particularly when performing more difficult motor acts. Standing and walking on the upper levels of the frame were first seen at 48 days of age and increased in incidence thereafter. Mothers sometimes climbed on to the frame and there was an overall positive association between mothers' and kittens' use of the frame. There were large and consistent behavioural differences between families. The climbing frame provides a potentially useful tool for assessing the development of certain locomotor skills under conditions where the subject's behaviour is spontaneous.
Young domestic cats (Felis catus) exhibit a diverse array of motor patterns of unknown biological function which are commonly lumped together under the term 'play' (reviewed by Martin 1984a). These undergo marked changes in form, frequency and inter-relationship during the course of onto- geny (West 1974; Barrett & Bateson 1978; Caro 1981a, b) and provide a rich source of material for those interested in the processes of behavioural development (Bateson 1981). The ontogeny of social play and object play in the cat have been extensively described (e.g. West 1974, 1979; Barrett & Bateson 1978; Baerends-van Roon & Baerends 1979; Moelk 1979; Caro 1981a, b), but so far no study has looked at the ontogeny of locomotor play in this species. This paper presents an account of the development of locomotor play in kittens under laboratory conditions.
Play is a broad and often inconsistently used term that is applied to a wide variety of behaviour patterns. Its definition (if, that is, one is stated) may include both structural and functional criteria (Bekoff & Byers 1981; Fagen 1981). Clearly, any account of play is necessarily influenced by the definition of the term, whether or not this is made explicit. In this paper, a modification of the definition suggested by Bekoff & Byers (1981) is adopted and 'play' is used in the sense of motor activity that appears to have no obvious short-term benefits, in which motor patterns from other functional contexts may be used in modified forms and altered temporal sequencing. Locomotor play,
as distinct from social or object play, is playful activity that carries the individual about its en- vironment, but is neither specifically directed at other individuals, nor involves manipulating inani- mate objects.
M E T H O D S
Subjects, Housing and Care
The subjects were seven litters of domestic kittens from seven different mothers. Each litter was culled to one male and one female kitten within a few days after birth. The kittens were born and reared in a laboratory colony and were housed with their mothers from birth in large indoor enclosures. All mothers had previously given birth to at least three other litters.
Each family (mother and two kittens) was housed in its own trapezoid-shaped indoor pen of floor area 8-8 m 2. The short side of the pen was 1.1 m wide and had a door in it. Set into the door was a darkened sheet of Perspex which provided an effective one-way screen. The long side of the pen had a translucent window (area 4.1 m 2) in it and was 4.0 m long. The radiating walls were 3.8 m long and the pen varied in height from 2.1 m to 2.4 m. In addition to receiving daylight, each pen was artifi- cially lit for 14 h each day (0600-2000 hours). Temperature fluctuated to some extent with exter- nal temperature, but heating ensured that it did not
502
Mart in & Bateson: Locomotor p lay in cats 503
drop below 12~ Each pen contained a wooden nest box, a litter tray, two food bowls and a water bowl. In addition, there were two wooden shelves: a small one (0.70 m x 0.24 m) which was 0.84 m above the ground; and a longer one (2.7 m x 0.26 m) which was 0"65 m above the ground. By the time they were 5-6 weeks old, kittens could reach the lower shelf by climbing on top of the nest box, but no kitten was able to reach the higher shelf during the course of the study.
Every morning, between 0830 and 1000 hours, each family was provided with 0-80 kg of Whiskas Supermeat moist cat food. Dry Purina cat chow and water were available ad libitum. Kittens were weighed and inspected daily. The cats in one pen were not able to see those in other pens, although a limited degree of olfactory and auditory contact was possible. All pens were similar and each family lived in its own pen throughout the study. The cats were used to and apparently undisturbed by the presence of humans. As part of a separate experi- mental study (Martin & Bateson 1985) each mother received a single subcutaneous injection of saline solution (control injection) on days 28, 30 and 33 post partum. The mean litter-mean weight of the kittens rose linearly with age, f rom 0-39_-t-0.04 (sD) kg at 21 days, to 0.89___0.10 (sD) kg at 60 days of age.
Observation Procedure
For each family, nine 30-min observation ses- sions were held, one during each 3-day period between 36 and 60 days after the kittens were born (i.e. on days 36, 39, 4 2 , . . . 60 post partum, each + 1 day). Sessions took place in the afternoon (1330 1530 hours).
At the beginning of a session, the food and water bowls were removed from a pen and a climbing frame was brought in and placed in a central position on the floor. The climbing frame was a multi-level wooden structure with several horizon- tal cross-bars at different heights above the ground (Fig. 1). The climbing frame was never left in any of the pens other than during the nine observation sessions.
Behavioural observations were made from out- side the pen, using the one-way screen. Each 30-min session was divided into 60 30-s blocks, denoted by an audio 'bleeper'. The mother and both kittens were all watched continuously throughout the 30-min session and behavioural observations were
, I , I i II~
I I ,
Figure 1. Drawing of the wooden climbing frame that was used to assess locomotor play, with two kittens shown for scale. The overall external dimensions were 91 x 91 x 91 cm. On one side of the frame there were six horizontal cross-bars, each 2.5 cm in width, at heights ranging from 14.5 cm to 91 cm above the ground. To score Stand (High) or Walk (High) a kitten had to reach the fourth cross-bar from the ground (height 60.5 cm) or higher. (Drawn from a photograph.)
scored on a check sheet. Some of the locomotor behaviour patterns (defined below) were infrequent or intermittent, and often had poorly distinguish- able beginning- and end-points. Fo r these cate- gories, instantaneous time-sampling or continuous recording methods were inappropriate and one- zero time-sampling was employed. Since this method does not generally provide true estimates of absolute frequencies or durations (Tyler 1979), these scores are given as Hansen frequencies; that is, the percentage of sample intervals i n which the behaviour pattern occurred. Vocalizations could be heard through stereo headphones connected to two microphones in the pen. All behavioural records were made by the same observer (PM).
Behavioural Categories
The following categories were recorded using one-zero time-sampling; i.e. scoring as occurrence or non-occurrence within each 30-s block.
On Frame: the kitten has at least one paw in contact with the frame and all of its body is off the ground.
504 Animal Behaviour, 33, 2
Climb: the kitten climbs from one horizontal cross-bar to another (higher or lower) cross- bar.
Stand (Low): the kitten stands still with all four paws on one of the lower three cross-bars.
Stand (High): the kitten stands still with all four paws on one of the upper three cross-bars.
Walk (Low): the kitten walks along one of the lower three cross-bars, taking at least two steps.
Walk (High): the kitten walks along one of the upper three cross-bars, taking at least two steps.
Mother on Frame: the mother has at least one paw in contact with the flame and all of her body is off the ground.
All of the above scores (Hansen frequencies) were expressed as the percentage of 30-s blocks in which the behaviour pattern occurred during each session. The following categories were scored using continuous recording, i.e. each occurrence was recorded.
Fall: the kitten obviously loses its balance and falls off the climbing flame on to the floor.
Fall-Recover: the kitten obviously loses its balance and totters, but immediately regains balance and maintains its position on the climbing frame.
Both of the above scores were expressed as the total number of occurrences per 30-min session for each kitten. The following two categories were derived from the latter two.
Lose Balance: the total number of times during the 30-min session that the kitten lost balance, corrected for time spent on the frame; given by 100 x (Fall + Fall-Recover)/On Frame.
Recover Balance: the number of times the kitten lost balance but immediately recovered balance, expressed as a percentage of the total number of times that it lost balance during a 30-min session; given by 100xFatl-Recover/ (Fall + Fall-Recover).
Finally, two general categories were recorded: Inactive: the time (out of 30 min) spent by the
kitten lying down and apparently resting or asleep, expressed as a percentage of total time. This was recorded on two cumulative digital stop-watches, one for each kitten.
Kitten Vocalization: any distinct vocalization made by a kitten, expressed as the total number for both kittens during a 30-min session.
All measures of kittens' behaviour, with the exception of Kitten Vocalization, were scored separately for each kitten.
Analysis of Results
The two kittens in a litter clearly could not be treated as though they were independent for statis- tical purposes and litter-mean values (N= 7) are referred to throughout (Abbey & Howard 1973). Scores were analysed using a two-way analysis of variance procedure, with the effect of differences between families (Subjects effect) as one factor (seven levels) and the effect of kittens' age as the other factor (nine levels). Scores expressed as percentages were transformed using the angular (arcsine-square root) transformation, while the square root transformation was used for scores expressed as the total number of occurrences (Sokal & Rohlf 1969). Procedures for non-para- metric tests were taken from Siegel (1956). Spear- man rank correlation coefficients (N= 7) and Ken- dall Coefficients of Concordance (N=7, k=9) were corrected for ties. For those categories where scores did not vary systematically with the kittens' age, the individual scores for each subject (mother or litter) could be averaged across all nine ages. The mean of these seven mean scores is referred to as the 'overall mean score' for that category. To obtain an 'overall ranking' of the seven subjects for a particu- lar category, the sum of ranks for all nine sessions was calculated for each subject, and these sums of ranks were then rank-ordered (Siegel 1956). Scores for Lose Balance and Recover Balance were calcu- lated as ratios and so had several missing values, while Fall and Fall-Recover had a high proportion of zero scores and showed extreme departure from normality even after transformation. Accordingly, scores for these four categories were not analysed using analysis of variance.
RESULTS
Kittens' Behaviour
Most kittens investigated the climbing frame during the first session, at 36 days of age, and occasionally jumped or climbed on to one of the lower cross-bars, but generally they spent little time on the frame at this age. The mean score for On Frame increased markedly over subsequent ses- sions and stayed at about 40-50% from 48 days of age onwards (Fig. 2). Older kittens usually climbed on to the frame almost as soon as it was brought into the pen and often played on or around it for a substantial proportion of the 30-rain session. Social
Mart in & Bateson: Locomotor p lay in cats 505
% 6o
4 0
2 0
ON FRAME
L i i i i i = = i
36 39 42 45 48 51 54 57 60
AGE IN DAYS
Figure 2. Changes with age for the On Frame score, expressed as the percentage of 30-s blocks in which the behaviour pattern occurred. Ages refer to the middle day of a 3-day block. The graph shows mean litter-mean scores (N= 7). Error bars denote+ 1 SEM.
% 2s CLIMB
2 0
10
, , - - , , , i J ~ p
36 39 42 45 48 51 54 57 60
AGE IN DAYS
Figure 3. Changes with age for the Climb score, expressed as the percentage of 30-s blocks in which the behaviour pattern occurred. Ages refer to the middle day of a 3-day block. The graph shows mean litter-mean scores (N= 7). Error bars denote 4- 1 SEM.
play was also frequently observed, though not recorded in this study, in which the two kittens would chase each other around the frame, often alternating roles and hopping briefly on to or over the lower cross-bars. On several occasions the climbing frame acted as a 'prop' for bouts of play fighting, with the two kittens pawing and batting at each other through the gaps between the cross- bars.
A few kittens (five out of 14) attempted to climb to a higher cross-bar during the first session, but at this age they appeared unable to climb beyond the lowest levels of the frame. The mean litter-mean age at which Climb was first recorded was 40 4-4 (SD) days, although individual kittens differed widely (range: 36-48 days). The mean Climb score rose over the first five sessions and stayed at around 15% from 48 days of age onwards (Fig. 3).
Until kittens were 48 clays old, all activities on the climbing frame were restricted to the lower three levels, even though some younger kittens did attempt to climb to the top. Instances of Stand (Low) were first seen, on average, at 42_+ 5 (SD) days, but the range for individual kittens (36-51 days) was again considerable. Stand (Low) subse- quently increased in incidence with age (Fig. 4a). A single example of Walk (Low) was exhibited by one 36-day old kitten. However, the group-mean score did not reach a substantial level (> 1%) until kittens were 45 days old (Fig. 4b) and one kitten never showed this behaviour pattern at all.
Walking and standing on the upper three levels of the climbing frame (>60.5 cm above the
ground) were first seen at 48 days. At this age both kittens from one litter climbed to the top of the frame and exhibited the first instances of Stand (High) and Walk (High). The next two kittens to achieve this (females from two different litters) did so at 51 days of age. In total, only six kittens (from three litters) ever exhibited Stand (High) and, of these, only five ever exhibited Walk (High) at any age. Other kittens did sometimes climb to the top of the frame during later sessions, but they either sat there, jumped off, or immediately climbed down again. The mean scores for Stand (High) and Walk (High) both rose progressively from 48 days of age onwards (Fig. 4c, d).
Kittens were usually active in some way during most of each observation session. The mean per- centage of time spent resting or asleep (Inactive) fluctuated between 2% and 23%, but showed no clear age-related trend. The overall mean score for Inactive was 11 4-8 (SD) %. Similarly, the kittens' vocalization rates were generally low, highly vari- able, and apparently unrelated to age. The overall mean score for Kitten Vocalization was 7 + 4 (SD) per 30-min session.
The indications of strong age-relatedness for the locomotor play measures were confirmed by the two-way ANOVAs. The kittens' age was a signifi- cant source of variation for On Frame (P < 0.001), Climb (P < 0.001), Stand (Low) (P < 0.01), Stand (High) (P < 0.05) and Walk (Low) (P < 0.05). The effect of age on Walk (High) only bordered on significance (P < 0-10), presumably due in part to the infrequency and variability of this behaviour
506 Animal Behaviour, 33, 2
( a ) S t a n d ( L o w ) ( c ) S t a n d
25 I 25
% T %
~ 0 15 "IS
l o _
= i i i i r _ i i i i i i
36 39 42 45 48 51 64 57 60 36 39
( b ) W a l k ( L o w ) t ) W a l k
o I E - ----" i i i i I i i 1 i
36 39 42 45 48 51 54 57 60 36 39
AGE IN DAYS
( H i g h )
J ( H i g h )
412 I i I I I I 45 48 51 54 57 60
Figure 4. Changes with age for (a) Stand (Low); (b) Walk (Low); (c) Stand (High); and (d) Walk (High) scores. Ages refer to the middle day of a 3-day block. The scores are expressed as the percentage of 30-s blocks in which the behaviour pattern occurred. Mean litter-mean scores (N=7) are shown. Error bars denote__+ 1 SEM. Note that the y-axis scales differ.
pattern. Neither Inactive nor Kitten Vocalization showed a significant effect due to age (P > 0.10).
The impression of large differences in behaviour between litters was also borne out by analysis of variance. The effect of differences between families (Subjects effect) was a significant source of varia- tion for On Frame (P<0.01), Climb (P<0.001), Stand (Low) (P < 0-001), Stand (High) (P < 0.001), Walk (Low) (P<0-01), Walk (High) (P<0-001) and Kitten Vocalization (P< 0.05). There was no significant Subjects effect for Inactive, however (P > 0-10). To assess the degree of consistency over time in the rank order of the seven litters, the Kendall Coefficient of Concordance (IV) was calcu- lated for the On Frame and Climb scores (Siegel 1956). The coefficient had highly significant values, both for On Frame (W=0.39, S=887, P<0.01) and for Climb (W=0.40, S=898, P<0.01), con- firming that the rank order of litters tended to be the same at different ages.
In the course of clambering over and upon the frame, kittens sometimes lost balance and occa- sionally fell off completely. Not surprisingly, this tended to happen mostly when kittens were execut- ing apparently difficult motor acts, such as walking along, or standing still on, one of the narrow
(2.5-cm wide) cross-bars. The only three litters ever to exhibit Stand (High) and Walk (High) also had the three highest total scores for both Lose Balance and Fall. Thus, there was a strong association between the total number of Falls and the overall ranking for both Stand (High) (rs=0'90, N=7, P<0'01, one-tailed) and Walk (High) (rs=0'82, N=7, P<0.05, one-tailed). However, the kittens that fell most often were not necessarily those that merely spent most time overall on the frame (correlation between overall rankings for Fall and On Frame: r~ = 0-33, N=7 , P > 0.10, one-tailed). In other words, falls tended to occur mostly when kittens were attempting to perform the more demanding motor patterns.
Scores for Lose Balance, Recover Balance and Fall all fluctuated considerably from session to session, but did not appear to vary systematically with age. The overall mean score for Lose balance was 1'40_+ 1-47 (SD) per 30 min, while the overall mean for Fall was 0.09 • 0.11 (SD) per 30 min. The overall mean score for Recover Balance was 89 • 11 (SD) ~. In other words, on most occasions when kittens lost their balance, they recovered balance immediately and did not fall off. During the whole study, only 11 instances of Fall were
Martin & Bateson." Locomotor play in cats 507
observed. There were no consistent sex differences in behaviour and male and female litter-mates were virtually indistinguishable statistically on every measure.
Mothers' Behaviour
Mothers sometimes climbed on to the frame and occasionally sat there for a few minutes, but generally they exhibited far less interest in the climbing frame than their kittens. There was considerable inter- and intra-individual variation in how much time mothers spent on or near the frame and this seemed to bear no systematic relation to the age of their kittens. The mean Mother on Frame score varied, from session to session, between 3% and 15%, with an overall mean of 8 _+ 10 (SD) %. A two-way analysis of variance for Mother on Frame scores confirmed that the mothers differed significantly (P<0.01) and that there was no significant effect due to the kittens' age (P> 0,10). The rank order for Mother on Frame was moderately consistent across time (Kendall's W=0.25, S=493, P<0.10).
If a, mother did jump on to the climbing frame at all during an observation session (and all mothers did this in at least three of the nine sessions) it tended to be during the first few minutes. Thus, during the early sessions when the kittens were young and spent little time on the frame, the mother was often the first to climb on to the frame. This observation raised the possibility that a mother's behaviour might encourage her kittens to use the frame. It was certainly the observer's subjective impression that young kittens sometimes climbed on to the frame specifically in response to the mother's presence there. The suggestion of a general positive relationship between mothers' and kittens' use of the frame was lent some credence by the finding that there was a positive correlation between the kittens' overall ranking for On Frame and the corresponding Mother on Frame overall ranking (rs=0"714, N=7, P<0.05, one-tailed). However, the correlations between kittens' and mothers' scores at each particular age were gener- ally lower (Table I). During the first three sessions there was a moderate positive correlation between the On Frame and Mother on Frame scores (median rs=0-63) and at 42 days the correlation was statistically significant. In the later sessions, however, the correlation was small or even negative (median rs for 45 days onwards=0.04). It is
Table I. Spearman rank correlations (rs) between kittens' On Frame scores and the corresponding Mother on Frame scores at each of nine kitten ages
perhaps noteworthy that the three mothers who ranked first, second and third overall for Mother on Frame were also the mothers of the only three litters ever to exhibit Stand (High) and Walk (High).
D I S C U S S I O N
When young domestic cats were given free access to a multi-level climbing frame for 30-min periods, they generally spent a considerable proportion of this time climbing, sitting, standing and walking on it, despite sometimes falling off. This repeated playful performance of apparently difficult motor acts is reminiscent of Simpson's (1976) description of the locomotor 'projects' executed by young rhesus monkeys. Admittedly, the debate about what 'really' constitutes play can be sterile and unproductive (Chalmers 1980; Fagen 1981). Nonetheless, the kittens were spontaneously per- forming motor acts that appeared to a human observer to be without immediate benefits, in- volved motor patterns from other functional con- texts and seemed to be rewarding. In this sense they were playing (Bekoff & Byers 1981). Some of the more demanding components of the kittens' beha- viour, such as climbing to the top of the frame and walking or standing on one of the narrow cross- bars, depended critically on their age, and the level of performance varied considerably between dif- lent litters. Perhaps four aspects of the results merit further comment.
The most obvious, yet perhaps the most easily overlooked, characteristic of the kittens' behaviour
508 Animal Behaviour, 33, 2
was its spontaneity. Kittens played on the climbing frame in the absence of any additional reward. Older individuals climbed on to the frame almost as soon as it was brought into their pen and persisted in playing on it, despite occasionally losing their balance or falling offcompletely. It could be argued that the kittens were 'play-deprived', in that they normally lacked the opportunities for the particu- lar types of locomotor play allowed by the climbing frame. However, the kittens could climb on the shelves and fittings in their home pen (see Methods) and had unrestricted opportunities for running and jumping during social play. The kittens would probably have spent a much smaller proportion of the observation time playing on the frame if it had been available ad libitum, rather than being pre- sented for only 30 min once every 3 days. Under the laboratory conditions of this study, weanling-age kittens devote on average only about 9~ of total time (per 24 h) to play (Martin 1984b).
Walking and standing on the highest cross-bars appeared to be particularly difficult: only three litters ever did either and these were also the litters that lost balance and fell most often. This suggests, incidentally, that the climbing frame might offer a useful method for assessing locomotor skill and development under conditions where the subject's actions are entirely self-motivated. The highest cross-bar was 91 cm above the ground, with only an unyielding concrete floor to break any fall, yet three different kittens were seen to fall from the frame twice within a single session and climb back on again. Falling from the upper levels of the frame seemed, somewhat surprisingly, only to wind kittens slightly. At worst, kittens who fell appeared slightly dazed for a few seconds and no detectable injuries ever resulted from falls. Kittens sometimes climbed to the top of the frame and then appeared to be stuck there, crying in distress and apparently unable to climb down again. They eventually jumped or climbed down, usually after consider- able hesitation, yet often they immediately climbed back on to the frame. In conclusion, whilst it may be true that 'subjectivity lies closest to the surface' when speculating about the proximate causation of play (Fagen 1981, page 44), it is difficult, and probably unnecessary, to avoid applying anthro- pomorphic terms such as 'spontaneous', 'reward- ing' and 'enjoyable' to the behaviour of these young animals (see also Bekoff & Byers 1981; Humphreys & Einon 1981).
A second issue is the dependence of play beha-
viour on more general features of sensory and motor development. Clearly, the specific nature of play behaviour observed at any stage in ontogeny is to some extent dependent on the changing capabili- ties of the developing organism. This point is especially relevant in the case of semi-altricial young such as kittens, which are born with sensory and motor capacities very different from those of adults (Rosenblatt 1976; see Hinde 1982). How- ever, the extent to which general features of sensory and motor development shape specific aspects of play remains uncertain (Bekoff& Byers 1981). For example, the kittens in this study did not walk or stand on the upper levels of the frame until they were at least 48 days old. One interpretation of this might be that, as kittens developed specific skills as a result of performing the easier components of locomotor play, they tried out progressively more difficult motor patterns in order to perfect these skills and to develop new ones. A more parsi- monious explanation might be that 48 days was the earliest age at which kittens were physically capable of playing in this way. One way of addressing this issue experimentally would be to vary either the age at which the frame was first presented, or the total amount of time the kittens were allowed to play on the frame.
As a broad generalization, a kitten's basic sen- sory and motor apparatus does not develop until about 4 weeks after birth, and considerable im- provement in and integration between the various faculties continue throughout the second month of life (Baerends-van Roon & Baerends 1979; Villab- lanca & Olmstead 1979). For example, rudimen- tary walking first appears at about 3 weeks, but kittens cannot walk any great distance from the nest until they are about 4 weeks old (Windle 1930; Rosenblatt & Schneirla 1962). Adult-like patterns of walking and running are present by 6-7 weeks, but some complex locomotor acts may not success- fully be executed until 10-11 weeks after birth (Villablanca & Olmstead 1979). Thus, the first appearance of some locomotor play patterns as late as the eighth week of life may have resulted from a simple physical inability to perform them any earlier. However, 'maturation' (that is, purely age-related change) was confounded with exper- ience, as in all longitudinal studies. It is quite likely that if we had varied the age at which the kittens were first introduced to the frame, the relationship between age and time spent on the frame would have been affected.
Martin & Bateson." Locomotor play in cats 509
A third feature of the results was the presence of marked and consistent differences in behaviour between litters. Analysis of variance showed that litters were significantly different from one another on all behavioural measures except the general index of inactivity, and the rank ordering of the litters for both On Frame and Climb were consis- tent across time. The overall mean scores for On Frame differed by a factor of more than two between the highest- and the lowest-ranking litters (52% versus 20%) and the ages at which various motor patterns first appeared also varied consider- ably. Only six of the 14 kittens (from three litters) ever showed the Walk (High) or Stand (High) patterns. Significant differences between litters in kittens' social play and object play have been reported previously (Barrett & Bateson 1978; Bate- son & Young 1981). The ontogenetic origins of behavioural differences such as these between individuals or litters, and their functional signifi- cance (if any), are as yet largely unknown and present an important challenge to those studying behavioural development (see Slater 1981).
Finally, there was some evidence for a positive influence of maternal behaviour on the kittens' use of the frame. The kittens of mothers who, overall, spent more time on the frame themselves tended to rank higher overall for the On Frame score. Furthermore, the three mothers who ranked high- est overall for Mother on Frame were the same mothers whose kittens were the only ones ever to perform the apparently difficult Stand (High) and Walk (High) motor patterns. Thus, it seems plaus- ible that one of the factors leading to the beha- vioural differences between litters might have been the differences in behaviour between mothers (although causality cannot, of course, be inferred from correlation). Correlations between kittens' and mothers' scores at particular ages were gener- ally low, however, and there was effectively no positive relationship between the two after 42 days of age (Table I). Thus, any short-term effect of maternal behaviour on the kittens, as assessed by a correlation at a particular age, seems to have been limited to the earliest sessions (36-42 days of age). Whether the proposed maternal influence on play is dependent on the immediate presence of the mother could be established by comparing kittens' play in the presence and absence of their mother. Overall levels of object and social play tend to be suppressed when the mother is removed (Barrett & Bateson 1978), but a relatively enduring maternal
influence on kittens' play would be indicated if the rank ordering of the litters was the same both in the presence and absence of the mother. Under the laboratory conditions of this study, weaning occurs during the period from 4 to 7 weeks after the kittens are born (Martin 1982). Thus, short-term maternal effects on kittens' behaviour might be expected to be more pronounced before 7 weeks of age, as was apparently the case here. The behaviour of mother cats is known to influence the predatory behaviour of their kittens (Caro 1980).
In conclusion, this paper has described the development of one form of locomotor play in kittens and a novel method for assessing this under laboratory conditions. Kittens will spontaneously play on a multi-level climbing frame, despite the real risk of falling off, and differ considerably in their performance on the frame. In the following paper (Martin & Bateson 1985) we give the results of an experiment that looked at the effects of intervening in the mother-offspring relationship on the development of kittens' locomotor play, using the same method.
A C K N O W L E D G M E N T S
This work was carried out while PM held a Medical Research Council studentship and was written up while PM was a Harkness Fellow at Stanford University. We are grateful to Paul Heavens and Les Barden for their assistance. Marc Bekoff, Tim Caro and Michael Simpson kindly commented on an earlier version of the manuscript.
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(Received 11 July 1983; revised 13 February 1984; MS. number: 2416)
