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Population lateralization in zebra finch courtship: a re-assessment
Anim. Behav., 1991,41,900-901
Population lateralization in zebra finch courtship: a re-assessment CAREL TEN CATE Zoological Laboratory, University o f Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands
(Received 12 October 1990; acceptance 14 October 1990; MS. number: sc-600) Lateral orientation in displays is an intriguing and largely neglected area for ethological research, which, I agree with Workman & Andrew (1991), is worthy of further study. This having been said, the specific issue at stake is the interpretation of experiments reported by my colleagues and I (ten Cate et al. 1990): are these relevant for the question of whether zebra finch, Taeniopygia guttata, males preferentially use the right eye during the 'static phase' of courtship? Workman & Andrew's conclusion seems to be that they are not, leading them to state that their own findings (Workman & Andrew 1986), demonstrating the presence of a significant tendency for zebra finch males to orient themselves with the right flank towards females, 'remains to be tested'. I feel no need to challenge this statement, as I also believe this issue may need further examination. However, the question of whether the experiments reported earlier (ten Cate et al. 1990) extend our insight into the orientational biases in zebra finches needs some re-assessment. In our experiment (ten Cate et al. 1990), we found no asymmetry in orientation at either the population or the individual level, but several birds showed a locomotor bias to hop either clockwise or anticlockwise through a ring-shaped corridor. Workman & Andrew discuss both findings and I will respond to their main comments. First, I discuss the possible reasons why we did not find an orientational bias. We acknowledged that this might have arisen from our design. We assumed that the use of established pairs by Workman & Andrew and of unknown stimulus birds of the opposite sex, separated from the experimental birds by a wire-mesh screen in our experiments would be unlikely to result in a different outcome. This seemed plausible at the time, as Workman & Andrew (1986) did claim the occurrence of a lateral orientation during courtship without limitations such as the specific context of an established pair. Their present commentary suggests that they now propose such a limitation. As there is no evidence suggesting that zebra finch 0003-3472/91/050900+ 02 $03.00/0
males court familiar females in a different way from unfamiliar females, or that a 'static phase' in courtship is absent when stimulus birds are behind a wire screen, there seems to be no strong reason to accept this criticism. Of more substance is the objection that our measurements (orientation when the experimental birds crossed a perch in front of, or between, cages containing stimulus birds) may not reflect the preferred orientation of males during the 'static phase' of courtship. This objection is a valid one, as acknowledged in our paper. The assumption underlying our method was that if zebra finches oriented themselves preferentially with a specific flank at any stage during courtship, this would be reflected in a higher chance of observing the bird more frequently with that flank towards the stimulus bird than with the other. This effect will, of course, be diluted because the bias may be absent at other stages during courtship or when the birds are not courting. Workman & Andrew (1991) suggest that courtship in our experiment may have been limited and that birds could have given much of their visual attention to other objects. Had this been the case, then orientational biases during specific, brief phases of courtship may have gone undetected as a result of dilution. To this objection it can be said that both male and female zebra finches, isolated for a few months and then exposed to conspecifics of the opposite sex for short periods such as the 20 min we used in our experiment show all signs of being sexually aroused for a substantial part of these periods. Of the 14 males used, 13 showed directed singing in each of the eight tests and the remaining male sang in seven tests. Song bouts were usually preceded and/or followed by periods during which a courtship posture was adopted and during which males visited and observed (!) females. When, in addition to the earlier analysis (ten Cate et al. 1990), male orientation scores are calculated for periods of 1-5 min before and after songbouts, thus focusing on a period during which static phases were most likely
9 1991 The Association for the Study of Animal Behaviour 900
Short Communications to have occurred, again no indication of a lateral bias is found. On average 49.5% of orientation scores were with the (expected) right flank towards females, a by no means significant deviation from a 50% chance level. These data cannot refute the criticism that it is an indirect way to gather evidence for the orientation during the static phase, but provide no support for the presence of an orientational bias during courtship. Our second finding was the presence of a bias in the direction of locomotion through the corridor. In contrast to what Workman & Andrew seemed to have understood, the data for males and females originate from two separate experimental groups; males were tested with females in the cages along the corridor and vice versa. The bias was measured by using the same data as for calculating the preferred lateral orientation, but ordered in a different way. When a test with stimulus birds on the outside of the ring showed more scores for an orientation with the left flank than with the right one towards stimulus birds, this indicates that the bird tended to move around clockwise. Similarly, a clockwise movement is inferred from experimental birds that oriented themselves with the right flank towards stimulus birds on the inside of the corridor. Thus all tests could be scored for preferred direction of movement and as the stimulus birds were on the inside or on the outside of the corridor for four tests each, the method controls for biases due to preferred orientation during courtship. Should a rotational bias be
901
present at the population level, then it is clear that experiments aimed at assessing the presence of a lateral orientation based on eye use, also at the population level, need to be controlled for this, as we did (ten Cate et al. 1990), but Workman & Andrew (1986) did not. At the population level, females showed a tendency to move clockwise (goodness-of-fit test, Gp=3.13, d f = l , P<0-08); also, the majority of males moved around clockwise. Rather then interpreting this finding as an indication for absence of a rotational bias, it seems another issue calling for further examination. To conclude, with Workman & Andrew (1991) I hope that the interest in the lateral orientation of birds will not be stifled; we (ten Cate et al. 1990) never intended such an effect. However, I hope to have put the balance straight with respect to what can or cannot be concluded from the present findings.
REFERENCES ten Cate, C., Baauw, A., Ballintijn, M. & van der Horst, I. 1990. Lateralization of orientation in sexually active zebra finches: eye use asymmetry or locomotor bias? Anita. Behav., 39, 992-994. Workman, L. & Andrew, R. J. 1986. Asymmetries of eye use in birds. Anita. Behav., 34, 158~1584. Workman, L. & Andrew, R. 1991. Population lateralization in zebra finch courtship: an unresolved issue? Anita. Behav., 41,545-546.
Population lateralization in zebra finch courtship: a re-assessment CAREL TEN CATE Zoological Laboratory, University o f Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands
(Received 12 October 1990; acceptance 14 October 1990; MS. number: sc-600) Lateral orientation in displays is an intriguing and largely neglected area for ethological research, which, I agree with Workman & Andrew (1991), is worthy of further study. This having been said, the specific issue at stake is the interpretation of experiments reported by my colleagues and I (ten Cate et al. 1990): are these relevant for the question of whether zebra finch, Taeniopygia guttata, males preferentially use the right eye during the 'static phase' of courtship? Workman & Andrew's conclusion seems to be that they are not, leading them to state that their own findings (Workman & Andrew 1986), demonstrating the presence of a significant tendency for zebra finch males to orient themselves with the right flank towards females, 'remains to be tested'. I feel no need to challenge this statement, as I also believe this issue may need further examination. However, the question of whether the experiments reported earlier (ten Cate et al. 1990) extend our insight into the orientational biases in zebra finches needs some re-assessment. In our experiment (ten Cate et al. 1990), we found no asymmetry in orientation at either the population or the individual level, but several birds showed a locomotor bias to hop either clockwise or anticlockwise through a ring-shaped corridor. Workman & Andrew discuss both findings and I will respond to their main comments. First, I discuss the possible reasons why we did not find an orientational bias. We acknowledged that this might have arisen from our design. We assumed that the use of established pairs by Workman & Andrew and of unknown stimulus birds of the opposite sex, separated from the experimental birds by a wire-mesh screen in our experiments would be unlikely to result in a different outcome. This seemed plausible at the time, as Workman & Andrew (1986) did claim the occurrence of a lateral orientation during courtship without limitations such as the specific context of an established pair. Their present commentary suggests that they now propose such a limitation. As there is no evidence suggesting that zebra finch 0003-3472/91/050900+ 02 $03.00/0
males court familiar females in a different way from unfamiliar females, or that a 'static phase' in courtship is absent when stimulus birds are behind a wire screen, there seems to be no strong reason to accept this criticism. Of more substance is the objection that our measurements (orientation when the experimental birds crossed a perch in front of, or between, cages containing stimulus birds) may not reflect the preferred orientation of males during the 'static phase' of courtship. This objection is a valid one, as acknowledged in our paper. The assumption underlying our method was that if zebra finches oriented themselves preferentially with a specific flank at any stage during courtship, this would be reflected in a higher chance of observing the bird more frequently with that flank towards the stimulus bird than with the other. This effect will, of course, be diluted because the bias may be absent at other stages during courtship or when the birds are not courting. Workman & Andrew (1991) suggest that courtship in our experiment may have been limited and that birds could have given much of their visual attention to other objects. Had this been the case, then orientational biases during specific, brief phases of courtship may have gone undetected as a result of dilution. To this objection it can be said that both male and female zebra finches, isolated for a few months and then exposed to conspecifics of the opposite sex for short periods such as the 20 min we used in our experiment show all signs of being sexually aroused for a substantial part of these periods. Of the 14 males used, 13 showed directed singing in each of the eight tests and the remaining male sang in seven tests. Song bouts were usually preceded and/or followed by periods during which a courtship posture was adopted and during which males visited and observed (!) females. When, in addition to the earlier analysis (ten Cate et al. 1990), male orientation scores are calculated for periods of 1-5 min before and after songbouts, thus focusing on a period during which static phases were most likely
9 1991 The Association for the Study of Animal Behaviour 900
Short Communications to have occurred, again no indication of a lateral bias is found. On average 49.5% of orientation scores were with the (expected) right flank towards females, a by no means significant deviation from a 50% chance level. These data cannot refute the criticism that it is an indirect way to gather evidence for the orientation during the static phase, but provide no support for the presence of an orientational bias during courtship. Our second finding was the presence of a bias in the direction of locomotion through the corridor. In contrast to what Workman & Andrew seemed to have understood, the data for males and females originate from two separate experimental groups; males were tested with females in the cages along the corridor and vice versa. The bias was measured by using the same data as for calculating the preferred lateral orientation, but ordered in a different way. When a test with stimulus birds on the outside of the ring showed more scores for an orientation with the left flank than with the right one towards stimulus birds, this indicates that the bird tended to move around clockwise. Similarly, a clockwise movement is inferred from experimental birds that oriented themselves with the right flank towards stimulus birds on the inside of the corridor. Thus all tests could be scored for preferred direction of movement and as the stimulus birds were on the inside or on the outside of the corridor for four tests each, the method controls for biases due to preferred orientation during courtship. Should a rotational bias be
901
present at the population level, then it is clear that experiments aimed at assessing the presence of a lateral orientation based on eye use, also at the population level, need to be controlled for this, as we did (ten Cate et al. 1990), but Workman & Andrew (1986) did not. At the population level, females showed a tendency to move clockwise (goodness-of-fit test, Gp=3.13, d f = l , P<0-08); also, the majority of males moved around clockwise. Rather then interpreting this finding as an indication for absence of a rotational bias, it seems another issue calling for further examination. To conclude, with Workman & Andrew (1991) I hope that the interest in the lateral orientation of birds will not be stifled; we (ten Cate et al. 1990) never intended such an effect. However, I hope to have put the balance straight with respect to what can or cannot be concluded from the present findings.
REFERENCES ten Cate, C., Baauw, A., Ballintijn, M. & van der Horst, I. 1990. Lateralization of orientation in sexually active zebra finches: eye use asymmetry or locomotor bias? Anita. Behav., 39, 992-994. Workman, L. & Andrew, R. J. 1986. Asymmetries of eye use in birds. Anita. Behav., 34, 158~1584. Workman, L. & Andrew, R. 1991. Population lateralization in zebra finch courtship: an unresolved issue? Anita. Behav., 41,545-546.