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The effect of playback volume and duration on the response of sows to piglet distress calls
AppliedAnimal Behaviour Science, 37 ( 1 9 9 3 ) 3 1 - 3 7
31
0 1 6 8 - 1 5 9 1 / 9 3 / $ 0 6 . 0 0 © 1993 - Elsevier Science Publishers B.V. All rights reserved
The effect of playback volume and duration on the response of sows to piglet distress calls G.D. Hutson *'a E.O. Price b, L.G.
Dickenson
a
aSchool of Agriculture and Forestry, University of Melbourne, Parkville, Vic. 3052, Australia bDepartment of Animal Science, University of CaliJbrnia, Davis, CA 95616. USA (Accepted 12 January 1993)
Abstract
The effect of variation in playback volume and duration on the responses of recently farrowed sows to a recorded piglet squeal was evaluated in two experiments. In Experiment 1 the squeal was played back at soft (76-77 dB), normal (84-86 dB) or loud (92-95 dB) volumes as the sow was kneeling prior to lying down. Sows took longer to lie down in response to loud playback than normal or soft playback, and were more likely to sit or stand in response to loud playbacks than normal or soft playbacks. In Experiment 2 the recorded squeal was played back at normal volume for 30, 60 or 120 s, together with a control of no playback. Sows took longer Io lie down in response to sound playback compared with no playback, but there were no significant differences between the three durations. Similarly, sows were more likely to sit or stand in response to sound playback than no sound playback, but there were no differences between the three durations of playback. These results indicate that the important acoustic properties of a piglet distress call are its sudden onset and high intensity, and that prolonged calling by a piglet is unlikely to cause a sow to stand. Keywords: Pig; Auditory system; Parental behaviour
Introduction In a study of maternal responsiveness in sows Hutson et al. ( 1991 ) found that recently farrowed sows did not respond to visual and tactile stimuli associated with a model piglet, but did respond to an auditory stimulus. Sows had higher response scores and shorter lying durations after playback of a piglet squeal than playback of control sounds. These results suggest that sows will ignore tactile and visual cues from piglets in danger of being crushed as they lie down, but will respond to auditory cues. In another study Hutson et al. (1992) found that sow parity (number of previous litters carried by the sow, including the current pregnancy) did not influenc7 this response. Responsiveness to piglet distress calling appears to be an innate response of female pigs, which is not influenced by previous exposure to piglet squeals or previous experience of maternal behaviour. *Corresponding author.
32
G.D. Hutson et al. / AppL Anim. Behav. Sci. 37 (1993) 31-37
Responsiveness to a distress call can also be conditioned. Friend et al. (1989) conducted trials with a sound-activated device which administered an electric shock to a sow when activated by the squeal of a distressed piglet. One piglet was saved in each of five litters by this device, although more piglets may have been saved by the increased responsiveness and caution displayed by the sows. Sows not fitted with the device rarely responded by getting off a squealing piglet, even though most of these piglets gave a strong squeal. These studies indicate that there is wide variation between individual sows in responsiveness to a squealing piglet. The response tends to be all or nothing, with a sow either sitting or standing in response to a squeal, or ignoring it. Cronin and Cropley ( 1991 ) found similar variation and reported that seven out of 12 sows (58%) responded to squeal playback. Hutson et al. (1991) also found that about 60% of sows could be classified as auditory responders and suggested that variation in responsiveness to squeal playback may reflect underlying genetic variability. However, before embarking on a large study of the genetic c o m p o n e n t of maternal responsiveness it is important to assess whether any properties of the stimulus itself could account for the variation in individual responsiveness. In this study we investigated the effect of stimulus intensity on sow responsiveness in two experiments. In the first experiment we tested the effect of playback volume (loudness) on the responsiveness of sows to playback of a recorded piglet squeal. In the second experiment we investigated the effect of playback duration.
Animals, materials and methods Twenty-four Large White X Landrace sows from the University of Melbourne Pig Centre herd, Mt. Derrimut, were used for the tests. The parity of the sows ranged from one to seven with a median of two. The sows were housed in 0.6 m × 2.1 m horizontally barred farrowing crates, in sheds accommodating either eight or 16 sows. The crate flooring was 55% concrete and 45% plastic slats at the rear in the eight-sow shed and 61% concrete and 39% steel mesh in the 16-sow shed. Response to playback was tested following normal feeding at 08:30, 10:30 and 16:00 h on Day 2 post-partum. A Blaupunkt HiFi speaker (Blaupunkt-werke, Hildesheim, Germany) was placed at the rear of the crate of the test sow and all the sows in the shed were then fed in the usual order. The experimenter sat at the rear of the crate and when the sow was kneeling on both forelegs, prior to lying down, the experimenter played back a tape recording of a squealing piglet for 30 s using a Uher 4400 tape recorder (Uher, Germany) and Toa P.A. amplifier (Toa Electric, Takarazuka, Japan). The test was terminated after 10 min of observation, timed from the start of playback. The recording was made from a 6-h-old piglet removed from a sow and litter, held gently but firmly between two hands, and squeezed.
G.D. Hutson et al. / Appl. Anita. Behav. Sci. 37(1993) 31-37
33
Tests of the effect of playback volume were done as a 3 X 3 Latin square replicated four times (n = 12 sows). The volume control on the amplifier was calibrated prior to the experiment using a BriJel and Kjaer (Naerum, Denmark) sound level meter. The playback volume could be either soft (76-77 dB), normal (80-86 dB) or loud (92-95 dB), measured at the front of the farrowing crate. The normal range of 80-86 dB was equivalent to the sound levels measured at a distance of 2 m during the recording of the piglet squeal. Tests of the effect of playback duration were done as a 4 X 4 Latin square replicated three times ( n = 12 sows ). Playback durations were 30, 60 and 120 s, together with a control of no playback. The squeal recording was duplicated and then successively repeated for the longer playbacks. The tests were done following feeding at 08:30, 10:30 and 16:00 h on Day 2 post-partum and 08:30 h on Day 3 post-partum. The normal playback volume was used in these tests. The responses of the sow to playback were recorded on videotape. The videotapes were analyzed, recording the following variables: latency to lying was measured from when the sow was kneeling on both forelegs until she had lain down; duration of lying in the 600-s period following playback; total number of pre-lying rooting movements; number of pre-lying looks, sniffs, grunts and contacts with piglets; response to playback, scored as described in Table 1. The data were analyzed by a Latin square analysis of variance (Steel and Torrie, 1960), with sows as rows and test order as columns. Significant treatment effects were further partitioned into orthogonal single degree of freed o m contrasts. Response score data were assumed to reflect an underlying continuous distribution. Duration data were transformed to c o m m o n logarithms before analysis and are reported as geometric means with 95% confidence intervals.
Table 1 Classification of sow response scores to playback of a piglet distress call Score
Behaviour
0
No observable reaction, no change in transition from kneeling to lying Sow kneeling, grunts, while facing forwards Sow kneeling, looks at speaker (head deflection > 15 ° ) Sow kneeling, looks at speaker and grunts Sow sits Sow stands Sow stands and grunts while facing forwards Sow stands and looks at speaker Sow stands and looks and grunts at speaker
34
G.D. Hutson et al. / Appl. Anita. Behav. Sci. 37 (1993) 31-37
Results
Experiment I Sows showed a significant response to playback volume. They took longer to lie down in response to loud playback than to the soft or normal playback levels ( P < 0.05, Fig. 1 (a)). Sows also had significantly higher response scores towards loud playbacks than normal or soft playback levels ( P < 0.05, Fig. 1 (b)). Sows tended to sit or stand in response to loud playbacks, compared with normal playbacks, where the sow usually continued to kneel. Eight of 12 sows responded to loud playbacks by sitting or standing. The difference between normal and soft playback response scores was also significant ( P < 0.05 ). Playback volume did not affect lying duration or pre-lying rooting behaviour and behaviour towards piglets (all P > 0.05 ). There was significant variation between sows in the number of looks (range 0-11, P < 0.05 ) and the number of sniffs (range 0-13, P<0.001 ) at piglets in the 5 min preceding lying and in the number of occurrences of rooting behaviour (range 0-33, P < 0.01 ) before lying. Individual variation in latency to lying just failed to be significant (range 4-271 s, P = 0.06 ). Test order did not influence the sows' responses.
Experiment 2 The playback treatment had a significant effect on latency to lying, lying duration and response scores (Fig. 2). Sows took longer to lie down in response to sound playback compared with the control of no sound playback, but there were no significant differences between the three durations (P> 0.05, Fig. 2 (a)). Lying duration was significantly affected by the playback treat(a)
(b) 6 b
T
100 1 80
5 o)
o°
to
o c
6O c o)
4 3 2
40
1
20
76
77 Playback
84 - 86 volume
92 - 95 (dB)
76 - 77
84 - 86
Playback
volume
92 - 9 5 (dB)
Fig. 1. Responses of sows to playback volume. (a) Geometric means of latency to lying. Vertical bars indicate the upper 95% confidence limit. (b) Mean response scores. Vertical bars indicate standard errors. Different letters represent significant differences ( P < 0.05) between means.
35
G.D. Hutson et al. / Appl. Anim. Behav. Sci. 37 (1993) 31-37
(a)
(b)
120 800 100 " •~.,
600
80 "
g
o 60 7o
400
40" ]_~
200
20
0
30
60
120
Playback duration (s)
0
30
60
Playback d u r a t i o n
120 (s)
(c)
g
4
co 3 c 2 rr
1
0
30
60
Playback d u r a t i o n
120 (s)
Fig. 2. Responses of sows to playback duration. Geometric means of (a) latency to lying and (b) lying duration. Vertical bars indicate the upper 95% confidence limit. (c) Mean response scores. Vertical bars indicate standard errors. Different letters represent significant differences (a,b: P< 0.05; x,y: P<0.001 ) between means. ment ( P < 0.05, Fig. 2 ( b ) ) , but the variability in the data was such that statistical contrasts between the control and playback treatments (P-- 0.14 ) and within the playback treatments ( P = 0.06 ) were not significant. Sows had significantly higher response scores to sound playback than no sound playback ( P < 0.001, Fig. 2 ( c ) ) , but there were no differences between the three durations o f playback ( P > 0.05 ). Playback duration did not influence pre-lying rooting behaviour and behaviour towards piglets (all P > 0.05 ). There was significant variation between sows in the number of looks (range 0-11, P < 0 . 0 1 ), the number of sniffs (range 0-4, P < 0 . 0 5 ) and the number of contacts with piglets (range 0 - 1 8 , P < 0.01 ) in the 5 min preceding lying and in the number of occurrences of rooting behaviour (range 0 - 1 3 3 , P < 0 . 0 0 1 ) before lying. There was also significant individual variation in latency to lying (range 3-601 s, P < 0 . 0 1 ), lying duration (range 0 - 5 9 7 s,
36
G.D. Hutson et al. / AppL Anim. Behav. Sci. 37(1993) 31-37
P < 0.05 ) and response scores (range 0-8, P < 0.01 ). Test order did not influence the sows' responses. Discussion The results of these experiments suggest that the property of a piglet squeal which has the greatest effect on sow responsiveness is sound intensity. Playback volume influenced both the attentiveness of the sow towards the squeal and her lying behaviour. Attentiveness towards the squeal increased as loudness increased, but only influenced lying behaviour at the loudest playback volume. In contrast, there was no difference in attentiveness towards squeals of different durations, although sows were more attentive to the squeals than the silent control. Similarly, the different durations did not influence the time taken to lie down, although sows took longer to lie down after squeal playback than no-sound playback. Thus, the important properties of a piglet squeal, in terms of eliciting sow responsiveness, are its abrupt acoustic onset and loudness, rather than its duration. Kiley ( 1972 ) has reported that the amplitude of pig vocalizations gradually increases from the common grunt to the repeated scream. The threshold for evocation of piglet squeals is very low and the eliciting stimulus is non-specific. At an age of about 6 months the squeal turns into a scream and is elicited by any fearful or painful situation, such as being squashed by another pig, isolated or forcibly held. The stimulus properties of sudden onset and high contrast are typical of sounds which must function as distress calls. These stimuli typically elicit an immediate and maximal response in a potentially life-threatening situation. Thus for a distress call to be effective the sow must quickly respond, by sitting or standing. If a sow does not immediately respond, prolonged calling by the piglet is unlikely to cause the sow to stand. These results confirm that the common producer practice of immediately responding to a squealing piglet rather than relying on the sow to respond is a good management practice. The abrupt onset of the call is a characteristic which Marler (1960) has described as facilitating the precise localization of the source of sound by comparison of differences in the time of arrival of the sound at each ear. The enhanced responsiveness of sows observed at loud playback levels resembles the responses to a 'supernormal' stimulus first described by Tinbergen (1951 ). For example, an oyster-catcher will attempt to incubate a giant egg in preference to a normal-sized egg. Further experimentation is necessary to determine whether sows will respond in a similar manner to non-biological alarm sounds, or whether other properties of a distress squeal can influence responsiveness. The identity of the squealing piglet seems unlikely to be an important variable, because the sow has few opportunities to hear distress squeals from all her piglets and
G.D. Hutson et at./AppL Anirn. Behav. Sci. 37(1993) 31-37
37
learn their identity within the first few days o f birth. The time elapsed since the previous nursing period could be an i m p o r t a n t variable. The probability of a sow lying down to suckle increases in direct proportion to the time elapsed since the last nursing period. A sow m a y be less willing to sit or stand in response to distress vocalizations as this time interval increases. Insufficient animals were studied in these experiments to provide further i n f o r m a t i o n on the source o f individual variation in responsiveness. However, eight out o f 12 (66.7%) animals responded to the loud playback which is o f the same order as seven out o f 12 (58%) reported by Cronin and Cropley ( 1991 ) and 60% by H u t s o n et al. ( 1991 ). The failure o f four out of 12 animals to respond to the loud playback confirms that variation in stimulus characteristics alone cannot account for the variation in individual responsiveness, and that this variation might reflect underlying genetic variability.
Acknowledgements This study was funded by a grant from the Australian Pig Research and Development Corporation and University o f Melbourne research funds. We would like to t h a n k M. Price for her assistance.
References Cronin, G.M. and Cropley, J.A., 1991. The effect of piglet stimuli on the posture changing behaviour of recently farrowed sows. Appl. Anim. Behav. Sci., 30:167-172. Friend, T.H., O'Connor, L.A., Knabe, D.A. and Dellmeier, G.R., 1989. Preliminary trials of a sound-activated device to reduce crushing of piglets by sows. Appl. Anim. Behav. Sci. 24: 23-29. Hutson, G.D., Wilkinson, J.L. and Luxford, B.G., 1991. The response of lactating sows to tactile, visual and auditory stimuli associated with a model piglet. Appl. Anita. Behav. Sci. 32: 129-137. Hutson, G.D., Argent, M.F., Dickenson, L.G. and Luxford, B.G., 1992. The influenceof parity and time since parturition on responsiveness of sows to a piglet distress call. Appl. Anita. Behav. Sci. 34: 303-313. Kiley, M., 1972. The vocalizations of ungulates, their causation and function. Z. Tierpsychol. 31: 171-222. Marler, P., 1960. Bird songs and mate selection. In: W.E. Lanyon and W.N. Tavolga (Editors), Animal Sounds and Communication. American Institute Biological Sciences, Washington, DC, pp. 348-367. Steel, R.G.D. and Torrie, J.H., 1960. Principles and Procedures in Statistics. McGraw-Hill, New York, NY, 481 pp. Tinbergen, N., 1951. The Study of Instinct. Clarendon Press, Oxford, 228 pp.
31
0 1 6 8 - 1 5 9 1 / 9 3 / $ 0 6 . 0 0 © 1993 - Elsevier Science Publishers B.V. All rights reserved
The effect of playback volume and duration on the response of sows to piglet distress calls G.D. Hutson *'a E.O. Price b, L.G.
Dickenson
a
aSchool of Agriculture and Forestry, University of Melbourne, Parkville, Vic. 3052, Australia bDepartment of Animal Science, University of CaliJbrnia, Davis, CA 95616. USA (Accepted 12 January 1993)
Abstract
The effect of variation in playback volume and duration on the responses of recently farrowed sows to a recorded piglet squeal was evaluated in two experiments. In Experiment 1 the squeal was played back at soft (76-77 dB), normal (84-86 dB) or loud (92-95 dB) volumes as the sow was kneeling prior to lying down. Sows took longer to lie down in response to loud playback than normal or soft playback, and were more likely to sit or stand in response to loud playbacks than normal or soft playbacks. In Experiment 2 the recorded squeal was played back at normal volume for 30, 60 or 120 s, together with a control of no playback. Sows took longer Io lie down in response to sound playback compared with no playback, but there were no significant differences between the three durations. Similarly, sows were more likely to sit or stand in response to sound playback than no sound playback, but there were no differences between the three durations of playback. These results indicate that the important acoustic properties of a piglet distress call are its sudden onset and high intensity, and that prolonged calling by a piglet is unlikely to cause a sow to stand. Keywords: Pig; Auditory system; Parental behaviour
Introduction In a study of maternal responsiveness in sows Hutson et al. ( 1991 ) found that recently farrowed sows did not respond to visual and tactile stimuli associated with a model piglet, but did respond to an auditory stimulus. Sows had higher response scores and shorter lying durations after playback of a piglet squeal than playback of control sounds. These results suggest that sows will ignore tactile and visual cues from piglets in danger of being crushed as they lie down, but will respond to auditory cues. In another study Hutson et al. (1992) found that sow parity (number of previous litters carried by the sow, including the current pregnancy) did not influenc7 this response. Responsiveness to piglet distress calling appears to be an innate response of female pigs, which is not influenced by previous exposure to piglet squeals or previous experience of maternal behaviour. *Corresponding author.
32
G.D. Hutson et al. / AppL Anim. Behav. Sci. 37 (1993) 31-37
Responsiveness to a distress call can also be conditioned. Friend et al. (1989) conducted trials with a sound-activated device which administered an electric shock to a sow when activated by the squeal of a distressed piglet. One piglet was saved in each of five litters by this device, although more piglets may have been saved by the increased responsiveness and caution displayed by the sows. Sows not fitted with the device rarely responded by getting off a squealing piglet, even though most of these piglets gave a strong squeal. These studies indicate that there is wide variation between individual sows in responsiveness to a squealing piglet. The response tends to be all or nothing, with a sow either sitting or standing in response to a squeal, or ignoring it. Cronin and Cropley ( 1991 ) found similar variation and reported that seven out of 12 sows (58%) responded to squeal playback. Hutson et al. (1991) also found that about 60% of sows could be classified as auditory responders and suggested that variation in responsiveness to squeal playback may reflect underlying genetic variability. However, before embarking on a large study of the genetic c o m p o n e n t of maternal responsiveness it is important to assess whether any properties of the stimulus itself could account for the variation in individual responsiveness. In this study we investigated the effect of stimulus intensity on sow responsiveness in two experiments. In the first experiment we tested the effect of playback volume (loudness) on the responsiveness of sows to playback of a recorded piglet squeal. In the second experiment we investigated the effect of playback duration.
Animals, materials and methods Twenty-four Large White X Landrace sows from the University of Melbourne Pig Centre herd, Mt. Derrimut, were used for the tests. The parity of the sows ranged from one to seven with a median of two. The sows were housed in 0.6 m × 2.1 m horizontally barred farrowing crates, in sheds accommodating either eight or 16 sows. The crate flooring was 55% concrete and 45% plastic slats at the rear in the eight-sow shed and 61% concrete and 39% steel mesh in the 16-sow shed. Response to playback was tested following normal feeding at 08:30, 10:30 and 16:00 h on Day 2 post-partum. A Blaupunkt HiFi speaker (Blaupunkt-werke, Hildesheim, Germany) was placed at the rear of the crate of the test sow and all the sows in the shed were then fed in the usual order. The experimenter sat at the rear of the crate and when the sow was kneeling on both forelegs, prior to lying down, the experimenter played back a tape recording of a squealing piglet for 30 s using a Uher 4400 tape recorder (Uher, Germany) and Toa P.A. amplifier (Toa Electric, Takarazuka, Japan). The test was terminated after 10 min of observation, timed from the start of playback. The recording was made from a 6-h-old piglet removed from a sow and litter, held gently but firmly between two hands, and squeezed.
G.D. Hutson et al. / Appl. Anita. Behav. Sci. 37(1993) 31-37
33
Tests of the effect of playback volume were done as a 3 X 3 Latin square replicated four times (n = 12 sows). The volume control on the amplifier was calibrated prior to the experiment using a BriJel and Kjaer (Naerum, Denmark) sound level meter. The playback volume could be either soft (76-77 dB), normal (80-86 dB) or loud (92-95 dB), measured at the front of the farrowing crate. The normal range of 80-86 dB was equivalent to the sound levels measured at a distance of 2 m during the recording of the piglet squeal. Tests of the effect of playback duration were done as a 4 X 4 Latin square replicated three times ( n = 12 sows ). Playback durations were 30, 60 and 120 s, together with a control of no playback. The squeal recording was duplicated and then successively repeated for the longer playbacks. The tests were done following feeding at 08:30, 10:30 and 16:00 h on Day 2 post-partum and 08:30 h on Day 3 post-partum. The normal playback volume was used in these tests. The responses of the sow to playback were recorded on videotape. The videotapes were analyzed, recording the following variables: latency to lying was measured from when the sow was kneeling on both forelegs until she had lain down; duration of lying in the 600-s period following playback; total number of pre-lying rooting movements; number of pre-lying looks, sniffs, grunts and contacts with piglets; response to playback, scored as described in Table 1. The data were analyzed by a Latin square analysis of variance (Steel and Torrie, 1960), with sows as rows and test order as columns. Significant treatment effects were further partitioned into orthogonal single degree of freed o m contrasts. Response score data were assumed to reflect an underlying continuous distribution. Duration data were transformed to c o m m o n logarithms before analysis and are reported as geometric means with 95% confidence intervals.
Table 1 Classification of sow response scores to playback of a piglet distress call Score
Behaviour
0
No observable reaction, no change in transition from kneeling to lying Sow kneeling, grunts, while facing forwards Sow kneeling, looks at speaker (head deflection > 15 ° ) Sow kneeling, looks at speaker and grunts Sow sits Sow stands Sow stands and grunts while facing forwards Sow stands and looks at speaker Sow stands and looks and grunts at speaker
34
G.D. Hutson et al. / Appl. Anita. Behav. Sci. 37 (1993) 31-37
Results
Experiment I Sows showed a significant response to playback volume. They took longer to lie down in response to loud playback than to the soft or normal playback levels ( P < 0.05, Fig. 1 (a)). Sows also had significantly higher response scores towards loud playbacks than normal or soft playback levels ( P < 0.05, Fig. 1 (b)). Sows tended to sit or stand in response to loud playbacks, compared with normal playbacks, where the sow usually continued to kneel. Eight of 12 sows responded to loud playbacks by sitting or standing. The difference between normal and soft playback response scores was also significant ( P < 0.05 ). Playback volume did not affect lying duration or pre-lying rooting behaviour and behaviour towards piglets (all P > 0.05 ). There was significant variation between sows in the number of looks (range 0-11, P < 0.05 ) and the number of sniffs (range 0-13, P<0.001 ) at piglets in the 5 min preceding lying and in the number of occurrences of rooting behaviour (range 0-33, P < 0.01 ) before lying. Individual variation in latency to lying just failed to be significant (range 4-271 s, P = 0.06 ). Test order did not influence the sows' responses.
Experiment 2 The playback treatment had a significant effect on latency to lying, lying duration and response scores (Fig. 2). Sows took longer to lie down in response to sound playback compared with the control of no sound playback, but there were no significant differences between the three durations (P> 0.05, Fig. 2 (a)). Lying duration was significantly affected by the playback treat(a)
(b) 6 b
T
100 1 80
5 o)
o°
to
o c
6O c o)
4 3 2
40
1
20
76
77 Playback
84 - 86 volume
92 - 95 (dB)
76 - 77
84 - 86
Playback
volume
92 - 9 5 (dB)
Fig. 1. Responses of sows to playback volume. (a) Geometric means of latency to lying. Vertical bars indicate the upper 95% confidence limit. (b) Mean response scores. Vertical bars indicate standard errors. Different letters represent significant differences ( P < 0.05) between means.
35
G.D. Hutson et al. / Appl. Anim. Behav. Sci. 37 (1993) 31-37
(a)
(b)
120 800 100 " •~.,
600
80 "
g
o 60 7o
400
40" ]_~
200
20
0
30
60
120
Playback duration (s)
0
30
60
Playback d u r a t i o n
120 (s)
(c)
g
4
co 3 c 2 rr
1
0
30
60
Playback d u r a t i o n
120 (s)
Fig. 2. Responses of sows to playback duration. Geometric means of (a) latency to lying and (b) lying duration. Vertical bars indicate the upper 95% confidence limit. (c) Mean response scores. Vertical bars indicate standard errors. Different letters represent significant differences (a,b: P< 0.05; x,y: P<0.001 ) between means. ment ( P < 0.05, Fig. 2 ( b ) ) , but the variability in the data was such that statistical contrasts between the control and playback treatments (P-- 0.14 ) and within the playback treatments ( P = 0.06 ) were not significant. Sows had significantly higher response scores to sound playback than no sound playback ( P < 0.001, Fig. 2 ( c ) ) , but there were no differences between the three durations o f playback ( P > 0.05 ). Playback duration did not influence pre-lying rooting behaviour and behaviour towards piglets (all P > 0.05 ). There was significant variation between sows in the number of looks (range 0-11, P < 0 . 0 1 ), the number of sniffs (range 0-4, P < 0 . 0 5 ) and the number of contacts with piglets (range 0 - 1 8 , P < 0.01 ) in the 5 min preceding lying and in the number of occurrences of rooting behaviour (range 0 - 1 3 3 , P < 0 . 0 0 1 ) before lying. There was also significant individual variation in latency to lying (range 3-601 s, P < 0 . 0 1 ), lying duration (range 0 - 5 9 7 s,
36
G.D. Hutson et al. / AppL Anim. Behav. Sci. 37(1993) 31-37
P < 0.05 ) and response scores (range 0-8, P < 0.01 ). Test order did not influence the sows' responses. Discussion The results of these experiments suggest that the property of a piglet squeal which has the greatest effect on sow responsiveness is sound intensity. Playback volume influenced both the attentiveness of the sow towards the squeal and her lying behaviour. Attentiveness towards the squeal increased as loudness increased, but only influenced lying behaviour at the loudest playback volume. In contrast, there was no difference in attentiveness towards squeals of different durations, although sows were more attentive to the squeals than the silent control. Similarly, the different durations did not influence the time taken to lie down, although sows took longer to lie down after squeal playback than no-sound playback. Thus, the important properties of a piglet squeal, in terms of eliciting sow responsiveness, are its abrupt acoustic onset and loudness, rather than its duration. Kiley ( 1972 ) has reported that the amplitude of pig vocalizations gradually increases from the common grunt to the repeated scream. The threshold for evocation of piglet squeals is very low and the eliciting stimulus is non-specific. At an age of about 6 months the squeal turns into a scream and is elicited by any fearful or painful situation, such as being squashed by another pig, isolated or forcibly held. The stimulus properties of sudden onset and high contrast are typical of sounds which must function as distress calls. These stimuli typically elicit an immediate and maximal response in a potentially life-threatening situation. Thus for a distress call to be effective the sow must quickly respond, by sitting or standing. If a sow does not immediately respond, prolonged calling by the piglet is unlikely to cause the sow to stand. These results confirm that the common producer practice of immediately responding to a squealing piglet rather than relying on the sow to respond is a good management practice. The abrupt onset of the call is a characteristic which Marler (1960) has described as facilitating the precise localization of the source of sound by comparison of differences in the time of arrival of the sound at each ear. The enhanced responsiveness of sows observed at loud playback levels resembles the responses to a 'supernormal' stimulus first described by Tinbergen (1951 ). For example, an oyster-catcher will attempt to incubate a giant egg in preference to a normal-sized egg. Further experimentation is necessary to determine whether sows will respond in a similar manner to non-biological alarm sounds, or whether other properties of a distress squeal can influence responsiveness. The identity of the squealing piglet seems unlikely to be an important variable, because the sow has few opportunities to hear distress squeals from all her piglets and
G.D. Hutson et at./AppL Anirn. Behav. Sci. 37(1993) 31-37
37
learn their identity within the first few days o f birth. The time elapsed since the previous nursing period could be an i m p o r t a n t variable. The probability of a sow lying down to suckle increases in direct proportion to the time elapsed since the last nursing period. A sow m a y be less willing to sit or stand in response to distress vocalizations as this time interval increases. Insufficient animals were studied in these experiments to provide further i n f o r m a t i o n on the source o f individual variation in responsiveness. However, eight out o f 12 (66.7%) animals responded to the loud playback which is o f the same order as seven out o f 12 (58%) reported by Cronin and Cropley ( 1991 ) and 60% by H u t s o n et al. ( 1991 ). The failure o f four out of 12 animals to respond to the loud playback confirms that variation in stimulus characteristics alone cannot account for the variation in individual responsiveness, and that this variation might reflect underlying genetic variability.
Acknowledgements This study was funded by a grant from the Australian Pig Research and Development Corporation and University o f Melbourne research funds. We would like to t h a n k M. Price for her assistance.
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