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Behavioural laterality in the shrimp-eating cichlid fish Neolamprologus fasciatus in Lake Tanganyika
ANIMAL BEHAVIOUR, 2008, 75, 1359e1366 doi:10.1016/j.anbehav.2007.09.008
Available online at www.sciencedirect.com
Behavioural laterality in the shrimp-eating cichlid fish Neolamprologus fasciatus in Lake Tanganyika YU I CH I TA KEUC HI & MI CH I O H ORI
Department of Zoology, Graduate School of Science, Kyoto University (Received 23 April 2007; initial acceptance 1 September 2007; final acceptance 13 September 2007; published online 8 November 2007; MS. number: 9371)
Behavioural laterality has been observed in various vertebrates, including fish, but its significance is little known. This study investigated behavioural laterality, corresponding to morphological asymmetry, in individuals of the cichlid Neolamprologus fasciatus as they hunted shrimp. This species shows lateralized hunting; when aiming at prey, individuals bend with either the left or the right side of the body abutting a rock. In field observations, the numbers of leftward and rightward hunts were recorded during 1-h periods for each of 44 individuals. The frequency distribution of the proportion of rightward hunting was bimodal, and approximately one-third of the observed individuals showed a significant leftward or rightward bias. The degree of behavioural laterality of each fish was associated with that of morphological asymmetry of the mouth but not with any of four bilateral characters (outer teeth in upper jaw, gill rakers, upper lateral line scales and lower lateral line scales); ‘lefties’ (‘righties’) showed more rightward (leftward) hunting. Furthermore, it was suggested that hunts corresponding to each individual’s mouth laterality achieved higher hunting success than did reverse hunts. Antisymmetrical hunting behaviour in a population may affect predation efficiency on the basis of frequency-dependent selection. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Keywords: antisymmetry; behavioural laterality; cichlid; fluctuating asymmetry; morphological asymmetry; mouth laterality; Neolamprologus fasciatus; predatoreprey interaction
Behavioural laterality occurs at either the population or the individual level (Denenberg 1981; Lehman 1981). The population level refers to the majority of individuals in a population being lateralized in one direction (right or left) more significantly than in the other direction, whereas the individual level refers to each individual having either a left or right bias with the population being composed of both lateral types. Behavioural laterality within populations has been explained by brain lateralization (reviewed by Vallortigara & Rogers 2005). Even if a brain must be lateralized to function efficiently (e.g. how zebrafish at first view strange objects with the right frontal field; Miklosi et al. 1997), which side of the brain is used to conduct a specific function in each individual may be irrelevant. Thus, there appears to be no reason for behavioural laterality to be biased towards one side through evolutionary processes (Rogers 1989). In contrast,
Correspondence: Y. Takeuchi, Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan (email: [email protected]). 0003e 3472/08/$34.00/0
it is unlikely that polymorphic behavioural laterality within populations can be interpreted within the context of brain lateralization. Lateral polymorphisms may be maintained by another mechanism. Traditional thinking suggests that behavioural laterality in individuals may reflect morphological traits that show fluctuating asymmetry (FA) in individuals (e.g. Collins et al. 1993). FA, consisting of random deviations from lateral symmetry, occurs in various taxa including fish (Downhower et al. 1990; Marques et al. 2005). FA is generally considered to indicate instabilities experienced during development (Mather 1953; Soule 1982). However, Cantalupo et al. (1996) and Bisazza et al. (1997) found no relationship between behavioural laterality and FA in fish; no factor affecting behavioural laterality was identified. Therefore, the significance of behavioural lateralization in fish individuals remains unclear. Scale-eating cichlid fish, Perissodus spp., in Lake Tanganyika have a genetically determined dimorphism of laterally asymmetric bodies (Liem & Stewart 1976; Hori 1991, 1993; Hori et al. 2007). They show skewed mouths
1359 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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ANIMAL BEHAVIOUR, 75, 4
opening either rightward (‘lefty’) or leftward (‘righty’) owing to asymmetrical joints of the lower jaws to the suspensorium (note that Hori (1991, 1993) designated lefties and righties as ‘right-handed’ and ‘left-handed’, respectively, at that time). This phenomenon is termed lateral dimorphism of the mouth. This morphological asymmetry correlates with hunting behaviour; lefties always tear off scales from only the left flank of prey fish, and righties tear off scales from the right flank (Hori 1993). The correlation between morphological asymmetry and behavioural laterality of Perissodus can be regarded an adaptation for efficient removal of the prey’s scales. The relative abundances of the two phenotypes in a population are maintained nearly equal, because each phenotype appears to have an advantage when it is rare (Hori 1993; Takahashi & Hori 1994). Recently, several studies have shown that some fish including Tanganyikan cichlids investigated in this regard have the same kind of mouth laterality as do scale-eaters (e.g. Telmatochromis temporalis (Mboko et al. 1998); Neolamprologus moorii (Hori et al. 2007); Rhinogobius flumineus (Seki et al. 2000)). Hori (2000) found that stomach contents of piscivorous cichlids in the lake usually were composed mainly of prey of the opposite laterality in natural conditions. In simulations, Nakajima et al. (2004) showed that, when predators exploit mainly prey of the opposite laterality (called ‘cross predation’), the condition drives alternation of the fitness advantage between two phenotypes, leading to frequency-dependent selection that maintains the dimorphism. Because each phenotype is expected to have a directional advantage for predation and/or escape behaviour based on the mouth laterality, laterality may have an important function in predatoreprey interactions. The purpose of this study was to investigate behavioural laterality of hunting in the shrimp-eating Tanganyikan cichlid Neolamprologus fasciatus. We measured morphological differences between the left and the right sides of individuals after observing their hunting behaviour to test whether lateralized hunting behaviour correlates with their morphological asymmetry. Our results show that their laterally biased hunting behaviour is associated with the mouth laterality and suggest that the association of these traits increases their hunting success.
(ca. 300 m2) several tens of centimetres above the rocky substrate and show characteristic hunting behaviour (Hori 1983). As the fish finds a shrimp or a small fish hiding at the foot or on the vertical surface of a rock, the fish stalks the prey from behind (Fig. 1). Reaching the upper side of the rock, the fish bends its body with either the left or the right side abutting against the rock and stands on its head while aiming at the prey. The fish gradually bends its body laterally and then darts at the prey.
Behavioural Observations Observations and sample collections were made using scuba equipment. The behavioural observations were made on 44 subadults of N. fasciatus (X SD ¼ 78:2 9:7 mm in SL). Each fish was tracked for 1 h, and its hunting behaviours were recorded with regard to which side of its body abutted the rock. The hunting behaviours were categorized as ‘leftward hunting’ or ‘rightward hunting’, depending on which body side abutted the rock. When the fish darted at the prey, we noted whether the hunt succeeded or failed, judging from the chewing action of the
(a)
(b)
METHODS
Study Site and Organism The field study was conducted at Kasenga Point near Mpulungu, Zambia, at the southern end of Lake Tanganyika (8 430 S, 31 080 E), from September to November 2005. This area consisted of rocky shores that extended for several kilometres. Neolamprologus fasciatus is a mid-sized (maximum standard length [SL] ¼ ca. 130 mm) carnivorous cichlid found in shallow water (3e10 m in depth) in rocky areas. The females and subadults (approximately 50e100 mm SL) feed mainly on shrimp (Atyidae, Limnocaridina latipes), whereas larger males feed mainly on small fish (Hori 1983; Yuma et al. 1998). The fish forage solitarily over a large area
Figure 1. Hunting behaviour of Neolamprologus fasciatus. (a) Hunting sequence: searching, approaching, aiming and darting. (b) Posture of aiming. The fish bends with the left side of its body abutting a rock and then darts at the prey (leftward hunting).
TAKEUCHI & HORI: BEHAVIOURAL LATERALITY IN CICHLID
mouth or lack thereof, respectively. After observation, the fish were caught using a portable short gill net and put in a container. Because of difficulty of preparing the large amount of an anaesthetic agent in the field, these fish removed from the container were anesthetized by placement into an ice-water bath (15 9 11 cm), which was kept in the freezer (20 C), for more than 20 min. Whether or not the ice-water bath works as effectively as an anaesthetic, we confirmed that the fish placed in the ice-water bath immediately quit their respiration (within <1 min). Subsequently, these fish were fixed in 10% formalin. Although the best way to guarantee brain death is to crush the head of the fish, we could not do so in this study because we needed to measure the cranial and external body morphology in detail. All of the morphological measurements were performed on the fixed samples. Thus, these fish were treated as well as possible. These operations were performed in accordance with the Regulation on Animal Experimentation at Kyoto University. Behavioural data on the proportion of rightward hunting were analysed for each individual, as follows: ½no: of rightward hunting=ðno: of rightward hunting þ no: of leftward huntingÞ: Significant departures from a random level (0.5) were examined by a one-sample Wilcoxon signed-ranks test to examine the tendency of behavioural laterality within populations. ShapiroeWilk tests and binomial tests were performed to examine deviance of the individuals as a group from a normal distribution and the tendency of each individual’s behaviours to differ from a frequency of 0.5, respectively.
Lateral Differences in Morphological Traits We examined the differences between the morphological traits on the left and those on the right sides of individuals, which were expected to relate to asymmetrical hunting behaviour. In scale-eaters, lateral dimorphism of the mouth is associated with hunting behaviour (Hori 1993). This dimorphism is defined by the direction of the skewed mouth opening (Hori 1991, 1993). In lefties, the left side of the head more or less faces the front, whereas, in righties, the right side of the head more or less faces frontward (Nakajima et al. 2004). The mouth of N. fasciatus opens widely in either direction, although the directional deviation of the mouth opening is difficult to quantify. In this study, following Hori et al. (2007), the mouth laterality of N. fasciatus was quantified as the difference in height between the right and the left mandible posterior ends (MPEs; the distance between the bottom of the suspensoriad articulation facet of the anguloarticular and the ventral end of the retroarticular process). MPE height was measured to 0.001-mm accuracy using a digital microscope (VHX-100, Keyence Ltd., Osaka, Japan) at 40 magnification. The mandibles were independently positioned on the microscope for each of three replicate measurements. Because measuring the distance between any
two points on a three-dimensional object is prone to yield some extreme values, the median value rather than the mean value was used for analysis. Measurements were conducted without reference to hunting behaviour data. The asymmetry index (AI) of the traits was quantified as RL 100; ðR þ LÞ=2 where R and L are the measured values of the right side and left side, respectively (Hori et al. 2007). Fish that showed large AI were those whose mouth opened more towards the direction with the smaller MPE. The mouth laterality of each individual was categorized by the index, such that an AI < 0 was considered lefty and an AI > 0 was righty. The laterality of individuals defined by this method accords with mouth laterality defined on the basis of the direction of the skewed mouth opening used in previous studies of other fish (Hori 1991, 1993; Mboko et al. 1998; Seki et al. 2000). Following Snoeks (2004), six bilateral meristic characters were also examined to determine basic morphological asymmetry: (1) outer teeth (OT) in the upper jaw, (2) gill rakers (GR), which are present on the first ceratobranchial arch, (3) pectoral fin rays (PFR), (4) upper lateral line scales (ULS), (5) lower lateral line scales (LLS), and (6) longitudinal line scales (LS). These were counted under a binocular microscope. The asymmetry index of these traits was calculated as the right side value minus the left side value (R L).
Statistics Palmer & Strobeck (1986) categorized three types of morphological asymmetries as follows: FA, in which traits have a normal distribution, with a L R difference whose mean is zero; directional asymmetry (DA), for which traits the mean is expected to be significantly different from zero; and antisymmetry (AS), in which traits are distinguished by a bimodal distribution in L R differences. To examine the distribution of AI of the morphological traits, we performed the following analyses. DA was tested using a one-sample Wilcoxon signed-ranks test for deviation from 0. Kurtosis (b2) was calculated, and an AnscombeeGlynn kurtosis test (Anscombe & Glynn 1983) was performed. Kurtosis is equal to 3 in a normal distribution, >3 in a leptokurtic distribution and <3 in a platykurtic distribution. We regarded kurtosis of <3 as an indicator of AS, following Palmer (1994). This method is commonly used, although it does not positively test whether traits show AS. Because the height of MPE was a metric variable, the deviance from the normal distribution was tested using a ShapiroeWilk test. Because the six meristic characters were discrete variables, a chi-square goodness-of-fit test was used. To assess the morphological factors affecting the degree of behavioural laterality of each individual, a generalized linear model (GLM) analysis was performed. The dependent variable was defined as the bias of hunting behaviour as
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ANIMAL BEHAVIOUR, 75, 4
arcsin
ðno:of rightwardhuntingno:of leftwardhuntingÞ ; ðno:of rightwardhuntingþno:of leftwardhuntingÞ
and the independent variables were the degrees of AI for the selected morphological traits. To assess the effect of the relationship between mouth laterality and asymmetrical hunting behaviour on the success ratio of hunting, a generalized linear mixed-model (GLMM) analysis was performed. Individuals with low foraging motivation (total number of darts at prey 4 in 1 h) were omitted from the analysis. We designed a GLMM with hunting success (hit or miss) as the dependent variable and the following as the independent variables: mouth laterality (lefty or righty), lateral type of hunting behaviour (leftward or rightward), combination of mouth laterality and lateral type of hunting behaviour as the fixed effect and individual as a random effect. Furthermore, we calculated the weighted mean of the success ratio of hunting of each combination, which was the value-weighted average of the ratios where the weight was proportionate to the number of darts at prey. The success ratio of hunting was compared between the combinations of the mouth laterality and the lateral type of hunting behaviour. The AnscombeeGlynn kurtosis test and chi-square goodness-of-fit test were performed using R statistical package. Other statistical analyses were performed using JMP version 5 (SAS Institute Inc., Cary, NC, U.S.A.).
RESULTS
Hunting Behaviour The average number of hunting behaviours observed per hour (X SD) was 31.3 9.5 (N ¼ 44). There was no significant bias in behavioural laterality across the whole population (i.e. no overall tendency to deviate towards either left or right for all individuals observed; X SD ¼ 0:505 0:193; one-sample Wilcoxon signedranks test: T ¼ 24.50, P ¼ 0.763). We analysed whether the distribution of the proportion of rightward hunting for each individual followed a normal distribution (Fig. 2). The distribution differed significantly from normal (ShapiroeWilk test: W ¼ 0.942, P ¼ 0.041) and was bimodal. Individual fish tended to show a bias for either leftward or rightward hunting; 14 of 44 fish had a significant bias (five fish showed left bias and nine fish right bias; binomial test: P < 0.05); when assuming P < 0.10, 22 of 44 fish showed bias (nine fish showed left bias and 13 fish right bias). A 1000-times simulation in which 44 fish each hunted randomly 31 times indicated that the average number of individuals that showed significant bias (binominal test: P < 0.05) was 1.3 fish (or 3.1 fish, if P < 0.10). Therefore, the observed values are higher than those expected from a population in which every fish behaves in an unbiased way relative to a normal distribution.
6
Number of individuals
1362
4
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Proportion of rightward hunting Figure 2. Frequency distribution of the proportion of rightward hunting in a field population of N. fasciatus. The dashed line indicates a normal curve fitted to the data. The solid line indicates the smoothing curve estimated by using Kernel smoothing (Kanel standard deviation ¼ 0.081).
Morphological Asymmetry The mean AI for each bilateral character was approximately 0 (Table 1). The frequency distribution of the AI of the height of MPE strongly deviated from normal (ShapiroeWilk test: W ¼ 0.879, P < 0.001), and its kurtosis was <3, indicating a clearly bimodal distribution (Fig. 3). Therefore, the heights of MPE were regarded as indicating AS. In PFR and LS, the variations in the difference between right and left sides were extremely small, and a chi-square goodness-of-fit test could not be performed. Thus, these two characters were excluded from subsequent analysis because they were inappropriate for examining the relationship of morphological asymmetry and behavioural laterality. The other four meristic traits (OT, GR, ULS and LLS) were regarded as showing FA: the mean values (R L) were approximately 0 and the distributions were normal, following the definition of Palmer (1994). No significant relationship existed among the degrees of asymmetry for MPE, OT, GR, ULS and LLS (Spearman rank correlation: MPE versus OT: P ¼ 0.966; MPE versus GR: P ¼ 0.913; MPE versus ULS: P ¼ 0.642; MPE versus LLS: P ¼ 0.990; OT versus GR: P ¼ 0.594; OT versus ULS: P ¼ 0.623; OT versus LLS: P ¼ 0.837; GR versus ULS: P ¼ 0.863; GR versus LLS: P ¼ 0.102; ULS versus LLS: P ¼ 0.663). The mouths of all sampled fish opened to either the left side or the right side; no fish had a mouth that opened straight forward. Because the AI of the height of MPE of 26 fish was negative and that of the other 18 fish was positive, the former and the latter were termed lefty and righty, respectively. In lefties (righties), the left (right) side of the head was more or less larger than the right (left) side, with the mouth opening rightward (leftward).
TAKEUCHI & HORI: BEHAVIOURAL LATERALITY IN CICHLID
Table 1. Statistics of the frequency distribution of the asymmetry index (AI) in selected morphological traits in N. fasciatus (N ¼ 44) Normal distribution test
Trait
AI: meanSD
Wilcoxon signed-ranks test (P)
MPE OT GR PFR ULS LLS LS
0.844.95 0.252.26 0.050.89 0.000.22 0.321.54 0.272.63 0.110.62
0.469 0.490 0.770 1.000 0.198 0.643 0.332
Kurtosis (b2)
AnscombeeGlynn kurtosis test (P)
ShapiroeWilk test (P)
Chi-square goodness-of-fit test (P)
1.56 2.79 2.64 22.00 4.52 2.97 2.61
<0.001 0.940 0.832 <0.001 0.042 0.703 0.787
<0.001 d d d d d d
d 0.988 0.815 NA 0.718 0.166 NA
A Wilcoxon signed-ranks test shows whether each mean significantly differs from the symmetric point (0). MPE, the height of mandible posterior end; the distance between bottom of the suspensoriad articulation facet of the anguloarticular and ventral end of the retroarticular process; the other traits are the number of OT, outer teeth in upper jaw; GR, gill rakers; PFR, pectoral fin rays; ULS, upper lateral line scales; LLS, lower lateral line scales; LS, longitudinal line scales. NA indicates that the test could not be performed for the trait.
Factors Affecting Behavioural Laterality We assessed whether the degree of behavioural laterality was affected by five morphological traits (MPE, OT, GR, ULS and LLS) using GLM analysis (Table 2). The GLM for the degree of behavioural laterality was significant (c25 ¼ 2:55, P < 0.001). Only the height of MPE had a significant effect. The fish with higher degrees of mouth laterality showed higher degrees of behavioural laterality. Thus the lateral difference in the height of MPE (i.e. mouth laterality) is a suitable predictor of behavioural laterality. The proportion (X SE) of rightward hunting by lefties was 0.57 0.04, whereas that of righties was 0.41 0.04. The proportion of rightward hunting by lefties was significantly higher than random (one-sample Wilcoxon signed-ranks test: P ¼ 0.041), and that of righties
was significantly lower than random (P ¼ 0.036). Thus, lefties bent mainly with the right side of their body abutting the rock, whereas righties tended to bend mainly the left side.
Factor Affecting Hunting Success The hunting success of righties was significantly higher than that of lefties on average, but the hunting success was different for each lateral type of hunting behaviour (Table 3). The success ratio of hunting of righties that adopted leftward hunting was significantly higher than both that of lefties that adopted leftward hunting and that of righties that adopted rightward hunting(GLMM: lefty leftward: coefficient ¼ 0.653, z ¼ 2.13, P ¼ 0.033; lefty rightward: coefficient ¼ 0.377, z ¼ 1.31, P ¼ 0.190; righty rightward: coefficient ¼ 0.704, z ¼ 2.07, P ¼ 0.038; Fig. 4).
Number of individuals
6
DISCUSSION Morphological traits in animals can show FA, DA or AS (van Valen 1962; Palmer & Strobeck 1986). The mouth laterality in N. fasciatus was considered to show AS, rather than FA or DA, because the frequency of AI in the mouths (MPE) of this species showed a bimodal distribution, with no symmetric individuals (Fig. 3). The deviation from symmetry of the mouth of N. fasciatus is at a level comparable to that of other fish of various feeding habits in
4
2 Table 2. Result of a GLM analysis on the degree of asymmetry index of hunting on the lateral difference of five morphological traits Variable
0
−10 −8
−6
−4
−2
0
2
4
6
8
10
Asymmetry index in height of MPE Figure 3. Frequency distribution of the AI of the height of the mandible posterior end (MPE) of mouths of N. fasciatus whose hunting was observed. The dashed line indicates the normal curve fitted to the data.
MPE OT GR ULS LLS
b
SE
t
P
0.04 0.03 0.05 0.04 0.04
0.01 0.02 0.06 0.03 0.02
3.86 1.17 0.75 1.12 1.83
<0.001 0.248 0.456 0.269 0.076
b indicates estimate of regression coefficient.
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ANIMAL BEHAVIOUR, 75, 4
Table 3. Result of GLMM analysis on the hunting success of N. fasciatus on the effects of the morphological and behavioural variables Variable
Coefficient
Mouth laterality (righty) Lateral type of hunting behaviour (rightward) Laterality (righty) Hunting (rightward)
0.710 0.303 1.001
SE
z
0.305 0.268
P
2.33 0.020 1.13 0.259
0.432 2.33 0.021
previous studies on asymmetry (Hori 1991; Mboko et al. 1998; Seki et al. 2000; Hori et al. 2007). The number of N. fasciatus individuals that performed significantly lateralized hunts in the population was larger than that expected in a population performing random hunting, and the proportion of rightward hunts of each individual in the population showed a bimodal distribution. The mouth-opening direction was consistent with the bias in direction of hunting behaviour. Behavioural laterality has often been interpreted as arising from FA (e.g. Collins et al. 1993). According to the FA hypothesis, an individual is unable to undergo symmetrical development and thus shows some behavioural laterality (Leary & Allendorf 1989; Bisazza et al. 1998). However, our results show that the behavioural laterality has no relation to the four traits showing FA (OT, GR, ULS and LLS) (Table 2). Behavioural laterality was obviously correlated to mouth laterality. Lefties and righties showed significantly more rightward and leftward hunting than random, respectively. The lateral dimorphism of mouths in fish seems to be determined by a Mendelian system of one locus-two alleles, with the lefty dominant over the righty and the dominant gene acting as either lethal or incompatible in homozygotes (Hori 1993; Seki et al. 2000; Hori et al. 2007). Thus, the bias in hunting behaviour may be consistently retained during the lifetime of each
0.6 Success ratio of hunting
1364
a 0.5
0.4
b
b
ab
0.3
0.2 leftward
rightward
Lefties
leftward
rightward
Righties
Figure 4. Comparisons of weighted means of the success ratio of hunting for lateral types of hunting behaviour and mouth laterality of N. fasciatus. The weighted mean ratio is the value-weighted average of the ratios where the weight is proportionate to the number of darts at prey. Error bars indicate 95% confidence intervals. Different letters above the bars represent significant differences of pairwise comparisons in the GLMM (P < 0.05).
individual. Behaviourally lateralized animals have enhanced skill performance and faster reaction times than nonlateralized animals, and thus their fitness should increase (Rogers 2000). Righties had more hunting success than lefties. The difference between phenotypes might be attributable to the brain lateralization. Pigeons tend to process a visual object using left hemisphere dominance (they use the right-eye field), and the discrimination performance of right-eye-dominant birds was superior to that of left¨ ntu ¨ rku ¨ n et al. 2000). Some reeye-dominant birds (Gu searchers have suggested that behavioural laterality arises from asymmetries of brain function, but cerebral asymmetry in fish remains an open question. Adding to this is that the explanation for the difference of hunting success from the brain lateralization has a logical defect from the point of view of population dynamics. If the higher hunting success of N. fasciatus in the study arose from brain lateralization and righties always had a hunting advantage over the lefties, righties should have been a majority in a population and the lefties might have perished sooner or later. But, in fact, the number of righties was less than that of lefties in 2005 (lefty:righty ¼ 26:18), and the proportion appears to have changed year by year during the past 10 years (Y. Takeuchi & M. Hori, personal observation). Alternatively, the consistent behavioural laterality may result from ‘dominance’ and the higher hunting success in righties may be a temporal phenomenon during a dynamic balancing between the two phenotypes. Seki et al. (2000) suggested that the mouth laterality correlated with some functional lateral differentiation, such as dominance in sensory abilities and/or in locomotion of one side of the body over the other. Consequently, lateralized behaviour corresponding to the dominance of each individual may increase that individual’s fitness. Furthermore, the presence and the ratio of two types of dominance in a population seem to affect their hunting efficiency (Hori 1993). In predatoreprey interactions, any attack from an unpredictable direction may lead to an increase in hunting success. If most individuals attack from a predetermined direction, their behaviour will become more predictable for prey animals, and as a result, their hunting success will be lowered. When both phenotypes co-occur, fish with the less frequent phenotype may enjoy a higher hunting success by confounding the vigilance of prey. In scale-eating cichlids, the minor morphs of mouth laterality could enjoy a higher hunting success than the dominant morphs and then become dominant in number after 1 or 2 years (Hori 1993). Our results show that, when righties of N. fasciatus abutted leftwards, their hunting success increased compared to rightward hunting, but when lefties abutted rightwards, their hunting success did not increase compared to leftward hunting (Fig. 4). Because the lefties were dominant in number in 2005, they might have had a disadvantage in hunting under frequency-dependent selection. This hypothesis needs to be tested with regard to whether lefties have a higher hunting success using rightward hunting when they are rare. Another factor that may be involved in behavioural antisymmetry is predation avoidance (Ghirlanda & Vallortigara 2004). Social fish (e.g. school-forming poeciliids
TAKEUCHI & HORI: BEHAVIOURAL LATERALITY IN CICHLID
such as Girardinus falcatus) tend to show behavioural laterality within populations, whereas solitary fish show two preferential directions of turning in a population (Rogers 1989; Bisazza et al. 2000; Vallortigara & Bisazza 2002). Because N. fasciatus rarely swims in groups, even in the young stage after independence, this fish is considered solitary, and young N. fasciatus are exposed to heavy predation pressure from many piscivorous cichlids (Hori 1983; Hori et al. 1993). When a predator attacks N. fasciatus, if individuals escape to either the left or the right, the predator may have difficulty in predicting the direction of escape compared to a situation in which individuals escape to only one side. Thus, predation avoidance may also be in play. About half of the observed N. fasciatus individuals showed a marginally significant lateral bias in hunting behaviour (P < 0.10), whereas the remaining half did not show any detectable bias. It is difficult to determine whether individuals with little bias were ‘ambidextrous’ under natural conditions. Behavioural tests should be designed to determine whether the individuals choose to hunt leftwards and rightwards equally. On the whole, the hunting behaviour of N. fasciatus under natural conditions showed laterality that was retained as AS (at the so-called individual level; however, this nomenclature may be troublesome when we discuss the meaning of laterality in a population, because the proportion of left- or right-biased individuals may change periodically (Hori 1993); we therefore refrain from using this term). The behavioural laterality was associated with mouth laterality and appeared to increase the hunting success of each individual. This phenomenon seems to be that of ‘behavioural dominance’. The behavioural dominance retained in a population may play a role in predatoreprey interactions in which an advantage for foraging or predator avoidance for each individual would depend on the ratio of the two morphs. Future studies should be focused on the dynamics of the laterality between animals of the two trophic levels: the focal species and their prey (shrimp) or enemies (fish predators).
Acknowledgments We thank the staff of the Lake Tanganyika Research Unit, Department of Research and Specialist Services, Republic of Zambia for providing facilities for field research and M. Kohda, H. Ochi, and other members of the Japanese Tanganyikan Research Team for their cooperation, encouragement and discussion during the field work. We are grateful to K. Watanabe, T. Takahashi, T. Sota, M. Yasugi, S. Tobo, Y. Takami, M. Sasabe and other members of the Laboratory of Animal Ecology, Graduate School of Science, Kyoto University for their valuable and critical comments on an early draft. Early versions of the manuscript were greatly improved by the comments of the editor and the three anonymous referees. This study was partly supported by the Grant for the Biodiversity Research of the 21st Century COE (A14), grants-in-aid of Scientific Research on Priority Areas (14087203), and the Global Center of Excellence Program ‘‘Formation of
a Strategic Base for Biodiversity and Evolutionary Research: from Genome to Ecosystem’’, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
References Anscombe, F. J. & Glynn, W. J. 1983. Distribution of kurtosis statistic b2 for normal statistics. Biometrika, 70, 227e234. Bisazza, A., Cantalupo, C. & Vallortigara, G. 1997. Lateral asymmetries during escape behavior in species of teleost fish (Jenynsia lineata). Physiology and Behavior, 61, 31e35. Bisazza, A., Rogers, L. J. & Vallortigara, G. 1998. The origins of cerebral asymmetry: a review of behavioural and brain lateralization in fishes, reptiles and amphibians. Neuroscience and Biobehavioral Reviews, 22, 411e426. Bisazza, A., Cantalupo, C., Capocchiano, M. & Vallortigara, G. 2000. Population lateralization and social behaviour: a study with 16 species of fish. Laterality, 5, 269e284. Cantalupo, C., Bisazza, A. & Vallortigara, G. 1996. Lateralization of displays during aggressive and courtship behaviour in the siamese fighting fish (Betta splendens). Physiology and Behavior, 60, 249e252. Collins, R. L., Sargent, E. E. & Neumann, P. E. 1993. Genetic and behavioral tests of the McManus hypothesis relating response to selection for lateralization of handedness in mice to degree of heterozygosity. Behavior Genetics, 23, 413e421. Denenberg, V. H. 1981. Hemispheric laterality in animals and the effects of early experience. Behavioral and Brain Sciences, 4, 1e21. Downhower, J. F., Blumer, L. S., Lejeune, P., Gaudin, P., Marconato, A. & Bisazza, A. 1990. Otholith asymmetry in Cottus bairdi and C. gobio. Polish Archives of Hydrobiology, 37, 209e220. Ghirlanda, S. & Vallortigara, G. 2004. The evolution of brain lateralization: a game-theoretical analysis of population structure. Proceedings of the Royal Society of London, Series B, 271, 853e857. ¨ ntu ¨ rku ¨ n, O., Diekamp, B., Manns, M., Nottelmann, F., Prior, Gu H., Schwarz, A. & Skiba, M. 2000. Asymmetry pays: visual lateralization improves discrimination success in pigeons. Current Biology, 10, 1079e1081. Hori, M. 1983. Feeding ecology of thirteen species of Lamprologus (Teleostei; Cichlidae) coexisting at a rocky shore of Lake Tanganyika. Physiology and Ecology Japan, 20, 129e149. Hori, M. 1991. Feeding relationships among cichlid fishes in Lake Tanganyika: effects of intra- and interspecific variations of feeding behavior on their coexistence. Ecology International Bulletin, 19, 89e101. Hori, M. 1993. Frequency-dependent natural selection in the handedness of scale-eating cichlid fish. Science, 260, 216e219. Hori, M. 2000. Gunshuu no tayousei to anteika-kikou (The diversities and stabilizing mechanism of communities). In: Gunshuu-seitaigaku no genzai (Current Community Ecology) (Ed. by H. Sato, T. Yamamoto & H. Yasuda), pp. 257e283. Kyoto: Kyoto University Press. Hori, M., Gashagaza, M. M., Nshombo, M. & Kawanabe, H. 1993. Littoral fish communities in Lake Tanganyika: irreplaceable diversity supported by intricate interactions among species. Conservation Biology, 7, 657e666. Hori, M., Ochi, H. & Kohda, M. 2007. Inheritance pattern of lateral dimorphism in two cichlids, a scale eater Perissodus microlepis and the herbivore Neolamprologus moorii, in Lake Tanganyika. Zoological Science, 24, 486e492. Leary, R. F. & Allendorf, F. W. 1989. Fluctuating asymmetry as an indicator of stress: implication for conservation biology. Trends in Ecology & Evolution, 4, 214e217.
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Lehman, R. A. 1981. Lateralized asymmetry of behavior in animals at the population and individual level. Behavioral and Brain Sciences, 4, 28. Liem, K. F. & Stewart, D. J. 1976. Evolution of the scale eating cichlid fishes of Lake Tanganyika: a generic revision with a description of a new species. Bulletin of the Museum of Comparative Zoology, 147, 319e350. Marques, J. F., Costa, J. L. & Cabral, H. N. 2005. Variation in bilateral asymmetry of the Lusitanian toadfish along the Portuguese coast. Journal of Applied Ichthyology, 21, 205e209. Mather, K. 1953. Genetical control of stability in development. Heredity, 7, 297e336. Mboko, S. K., Kohda, M. & Hori, M. 1998. Asymmetry of mouthopening a small herbivorous cichlid fish Telmatochromis temporalis in Lake Tanganyika. Zoological Science, 15, 405e408. Miklosi, A., Andrew, R. J. & Savage, H. 1997. Behavioural lateralisation of the tetrapod type in the zebrafish (Brachydanio rerio). Physiology and Behavior, 63, 127e135. Nakajima, M., Matsuda, H. & Hori, M. 2004. Persistence and fluctuating of lateral dimorphism in fishes. American Naturalist, 163, 692e698. Palmer, A. R. 1994. Fluctuating asymmetry analysis: a primer. In: Developmental Instability: Its Origins and Evolutionary Implications (Ed. by T. A. Markow), pp. 335e364. Netherlands: Kluwer Academic. Palmer, A. R. & Strobeck, C. 1986. Fluctuating asymmetry: measurement, analysis, patterns. Annual Review of Ecology and Systematics, 17, 391e421.
Rogers, L. J. 1989. Laterality in animals. International Journal of Comparative Psychology, 3, 5e25. Rogers, L. J. 2000. Evolution of hemispheric specialization: advantages and disadvantages. Brain and Language, 73, 236e253. Seki, S., Kohda, M. & Hori, M. 2000. Asymmetry of mouth morph of a freshwater goby, Rhinogobius flumineus. Zoological Science, 17, 1321e1325. Snoeks, J. 2004. The Cichlid Diversity of Lake Malawi/Nyasa/Niassa: Identification, Distribution and Taxonomy. Cichlid Press. Soule, M. E. 1982. Allometric variation: 1. The theory and some consequences. American Naturalist, 120, 751e764. Takahashi, S. & Hori, M. 1994. Unstable evolutionarily stable strategy and oscillation: a model of lateral asymmetry in scale-eating cichlids. American Naturalist, 144, 1001e1020. van Valen, L. 1962. A study of fluctuating asymmetry. Evolution, 16, 125e142. Vallortigara, G. & Bisazza, A. 2002. How ancient is brain lateralization? In: Comparative Vertebrate Lateralization (Ed. by L. J. Rogers & R. J. Andrew), pp. 9e69. Cambridge: Cambridge University Press. Vallortigara, G. & Rogers, L. 2005. Survival with an asymmetrical brain: advantages and disadvantages of cerebral lateralization. Behavioral and Brain Sciences, 28, 575e633. Yuma, M., Narita, T., Hori, M. & Kondo, T. 1998. Food resources of shrimp-eating cichlid fishes in Lake Tanganyika. Environmental Biology of Fishes, 52, 371e378.
Available online at www.sciencedirect.com
Behavioural laterality in the shrimp-eating cichlid fish Neolamprologus fasciatus in Lake Tanganyika YU I CH I TA KEUC HI & MI CH I O H ORI
Department of Zoology, Graduate School of Science, Kyoto University (Received 23 April 2007; initial acceptance 1 September 2007; final acceptance 13 September 2007; published online 8 November 2007; MS. number: 9371)
Behavioural laterality has been observed in various vertebrates, including fish, but its significance is little known. This study investigated behavioural laterality, corresponding to morphological asymmetry, in individuals of the cichlid Neolamprologus fasciatus as they hunted shrimp. This species shows lateralized hunting; when aiming at prey, individuals bend with either the left or the right side of the body abutting a rock. In field observations, the numbers of leftward and rightward hunts were recorded during 1-h periods for each of 44 individuals. The frequency distribution of the proportion of rightward hunting was bimodal, and approximately one-third of the observed individuals showed a significant leftward or rightward bias. The degree of behavioural laterality of each fish was associated with that of morphological asymmetry of the mouth but not with any of four bilateral characters (outer teeth in upper jaw, gill rakers, upper lateral line scales and lower lateral line scales); ‘lefties’ (‘righties’) showed more rightward (leftward) hunting. Furthermore, it was suggested that hunts corresponding to each individual’s mouth laterality achieved higher hunting success than did reverse hunts. Antisymmetrical hunting behaviour in a population may affect predation efficiency on the basis of frequency-dependent selection. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Keywords: antisymmetry; behavioural laterality; cichlid; fluctuating asymmetry; morphological asymmetry; mouth laterality; Neolamprologus fasciatus; predatoreprey interaction
Behavioural laterality occurs at either the population or the individual level (Denenberg 1981; Lehman 1981). The population level refers to the majority of individuals in a population being lateralized in one direction (right or left) more significantly than in the other direction, whereas the individual level refers to each individual having either a left or right bias with the population being composed of both lateral types. Behavioural laterality within populations has been explained by brain lateralization (reviewed by Vallortigara & Rogers 2005). Even if a brain must be lateralized to function efficiently (e.g. how zebrafish at first view strange objects with the right frontal field; Miklosi et al. 1997), which side of the brain is used to conduct a specific function in each individual may be irrelevant. Thus, there appears to be no reason for behavioural laterality to be biased towards one side through evolutionary processes (Rogers 1989). In contrast,
Correspondence: Y. Takeuchi, Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan (email: [email protected]). 0003e 3472/08/$34.00/0
it is unlikely that polymorphic behavioural laterality within populations can be interpreted within the context of brain lateralization. Lateral polymorphisms may be maintained by another mechanism. Traditional thinking suggests that behavioural laterality in individuals may reflect morphological traits that show fluctuating asymmetry (FA) in individuals (e.g. Collins et al. 1993). FA, consisting of random deviations from lateral symmetry, occurs in various taxa including fish (Downhower et al. 1990; Marques et al. 2005). FA is generally considered to indicate instabilities experienced during development (Mather 1953; Soule 1982). However, Cantalupo et al. (1996) and Bisazza et al. (1997) found no relationship between behavioural laterality and FA in fish; no factor affecting behavioural laterality was identified. Therefore, the significance of behavioural lateralization in fish individuals remains unclear. Scale-eating cichlid fish, Perissodus spp., in Lake Tanganyika have a genetically determined dimorphism of laterally asymmetric bodies (Liem & Stewart 1976; Hori 1991, 1993; Hori et al. 2007). They show skewed mouths
1359 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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ANIMAL BEHAVIOUR, 75, 4
opening either rightward (‘lefty’) or leftward (‘righty’) owing to asymmetrical joints of the lower jaws to the suspensorium (note that Hori (1991, 1993) designated lefties and righties as ‘right-handed’ and ‘left-handed’, respectively, at that time). This phenomenon is termed lateral dimorphism of the mouth. This morphological asymmetry correlates with hunting behaviour; lefties always tear off scales from only the left flank of prey fish, and righties tear off scales from the right flank (Hori 1993). The correlation between morphological asymmetry and behavioural laterality of Perissodus can be regarded an adaptation for efficient removal of the prey’s scales. The relative abundances of the two phenotypes in a population are maintained nearly equal, because each phenotype appears to have an advantage when it is rare (Hori 1993; Takahashi & Hori 1994). Recently, several studies have shown that some fish including Tanganyikan cichlids investigated in this regard have the same kind of mouth laterality as do scale-eaters (e.g. Telmatochromis temporalis (Mboko et al. 1998); Neolamprologus moorii (Hori et al. 2007); Rhinogobius flumineus (Seki et al. 2000)). Hori (2000) found that stomach contents of piscivorous cichlids in the lake usually were composed mainly of prey of the opposite laterality in natural conditions. In simulations, Nakajima et al. (2004) showed that, when predators exploit mainly prey of the opposite laterality (called ‘cross predation’), the condition drives alternation of the fitness advantage between two phenotypes, leading to frequency-dependent selection that maintains the dimorphism. Because each phenotype is expected to have a directional advantage for predation and/or escape behaviour based on the mouth laterality, laterality may have an important function in predatoreprey interactions. The purpose of this study was to investigate behavioural laterality of hunting in the shrimp-eating Tanganyikan cichlid Neolamprologus fasciatus. We measured morphological differences between the left and the right sides of individuals after observing their hunting behaviour to test whether lateralized hunting behaviour correlates with their morphological asymmetry. Our results show that their laterally biased hunting behaviour is associated with the mouth laterality and suggest that the association of these traits increases their hunting success.
(ca. 300 m2) several tens of centimetres above the rocky substrate and show characteristic hunting behaviour (Hori 1983). As the fish finds a shrimp or a small fish hiding at the foot or on the vertical surface of a rock, the fish stalks the prey from behind (Fig. 1). Reaching the upper side of the rock, the fish bends its body with either the left or the right side abutting against the rock and stands on its head while aiming at the prey. The fish gradually bends its body laterally and then darts at the prey.
Behavioural Observations Observations and sample collections were made using scuba equipment. The behavioural observations were made on 44 subadults of N. fasciatus (X SD ¼ 78:2 9:7 mm in SL). Each fish was tracked for 1 h, and its hunting behaviours were recorded with regard to which side of its body abutted the rock. The hunting behaviours were categorized as ‘leftward hunting’ or ‘rightward hunting’, depending on which body side abutted the rock. When the fish darted at the prey, we noted whether the hunt succeeded or failed, judging from the chewing action of the
(a)
(b)
METHODS
Study Site and Organism The field study was conducted at Kasenga Point near Mpulungu, Zambia, at the southern end of Lake Tanganyika (8 430 S, 31 080 E), from September to November 2005. This area consisted of rocky shores that extended for several kilometres. Neolamprologus fasciatus is a mid-sized (maximum standard length [SL] ¼ ca. 130 mm) carnivorous cichlid found in shallow water (3e10 m in depth) in rocky areas. The females and subadults (approximately 50e100 mm SL) feed mainly on shrimp (Atyidae, Limnocaridina latipes), whereas larger males feed mainly on small fish (Hori 1983; Yuma et al. 1998). The fish forage solitarily over a large area
Figure 1. Hunting behaviour of Neolamprologus fasciatus. (a) Hunting sequence: searching, approaching, aiming and darting. (b) Posture of aiming. The fish bends with the left side of its body abutting a rock and then darts at the prey (leftward hunting).
TAKEUCHI & HORI: BEHAVIOURAL LATERALITY IN CICHLID
mouth or lack thereof, respectively. After observation, the fish were caught using a portable short gill net and put in a container. Because of difficulty of preparing the large amount of an anaesthetic agent in the field, these fish removed from the container were anesthetized by placement into an ice-water bath (15 9 11 cm), which was kept in the freezer (20 C), for more than 20 min. Whether or not the ice-water bath works as effectively as an anaesthetic, we confirmed that the fish placed in the ice-water bath immediately quit their respiration (within <1 min). Subsequently, these fish were fixed in 10% formalin. Although the best way to guarantee brain death is to crush the head of the fish, we could not do so in this study because we needed to measure the cranial and external body morphology in detail. All of the morphological measurements were performed on the fixed samples. Thus, these fish were treated as well as possible. These operations were performed in accordance with the Regulation on Animal Experimentation at Kyoto University. Behavioural data on the proportion of rightward hunting were analysed for each individual, as follows: ½no: of rightward hunting=ðno: of rightward hunting þ no: of leftward huntingÞ: Significant departures from a random level (0.5) were examined by a one-sample Wilcoxon signed-ranks test to examine the tendency of behavioural laterality within populations. ShapiroeWilk tests and binomial tests were performed to examine deviance of the individuals as a group from a normal distribution and the tendency of each individual’s behaviours to differ from a frequency of 0.5, respectively.
Lateral Differences in Morphological Traits We examined the differences between the morphological traits on the left and those on the right sides of individuals, which were expected to relate to asymmetrical hunting behaviour. In scale-eaters, lateral dimorphism of the mouth is associated with hunting behaviour (Hori 1993). This dimorphism is defined by the direction of the skewed mouth opening (Hori 1991, 1993). In lefties, the left side of the head more or less faces the front, whereas, in righties, the right side of the head more or less faces frontward (Nakajima et al. 2004). The mouth of N. fasciatus opens widely in either direction, although the directional deviation of the mouth opening is difficult to quantify. In this study, following Hori et al. (2007), the mouth laterality of N. fasciatus was quantified as the difference in height between the right and the left mandible posterior ends (MPEs; the distance between the bottom of the suspensoriad articulation facet of the anguloarticular and the ventral end of the retroarticular process). MPE height was measured to 0.001-mm accuracy using a digital microscope (VHX-100, Keyence Ltd., Osaka, Japan) at 40 magnification. The mandibles were independently positioned on the microscope for each of three replicate measurements. Because measuring the distance between any
two points on a three-dimensional object is prone to yield some extreme values, the median value rather than the mean value was used for analysis. Measurements were conducted without reference to hunting behaviour data. The asymmetry index (AI) of the traits was quantified as RL 100; ðR þ LÞ=2 where R and L are the measured values of the right side and left side, respectively (Hori et al. 2007). Fish that showed large AI were those whose mouth opened more towards the direction with the smaller MPE. The mouth laterality of each individual was categorized by the index, such that an AI < 0 was considered lefty and an AI > 0 was righty. The laterality of individuals defined by this method accords with mouth laterality defined on the basis of the direction of the skewed mouth opening used in previous studies of other fish (Hori 1991, 1993; Mboko et al. 1998; Seki et al. 2000). Following Snoeks (2004), six bilateral meristic characters were also examined to determine basic morphological asymmetry: (1) outer teeth (OT) in the upper jaw, (2) gill rakers (GR), which are present on the first ceratobranchial arch, (3) pectoral fin rays (PFR), (4) upper lateral line scales (ULS), (5) lower lateral line scales (LLS), and (6) longitudinal line scales (LS). These were counted under a binocular microscope. The asymmetry index of these traits was calculated as the right side value minus the left side value (R L).
Statistics Palmer & Strobeck (1986) categorized three types of morphological asymmetries as follows: FA, in which traits have a normal distribution, with a L R difference whose mean is zero; directional asymmetry (DA), for which traits the mean is expected to be significantly different from zero; and antisymmetry (AS), in which traits are distinguished by a bimodal distribution in L R differences. To examine the distribution of AI of the morphological traits, we performed the following analyses. DA was tested using a one-sample Wilcoxon signed-ranks test for deviation from 0. Kurtosis (b2) was calculated, and an AnscombeeGlynn kurtosis test (Anscombe & Glynn 1983) was performed. Kurtosis is equal to 3 in a normal distribution, >3 in a leptokurtic distribution and <3 in a platykurtic distribution. We regarded kurtosis of <3 as an indicator of AS, following Palmer (1994). This method is commonly used, although it does not positively test whether traits show AS. Because the height of MPE was a metric variable, the deviance from the normal distribution was tested using a ShapiroeWilk test. Because the six meristic characters were discrete variables, a chi-square goodness-of-fit test was used. To assess the morphological factors affecting the degree of behavioural laterality of each individual, a generalized linear model (GLM) analysis was performed. The dependent variable was defined as the bias of hunting behaviour as
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ANIMAL BEHAVIOUR, 75, 4
arcsin
ðno:of rightwardhuntingno:of leftwardhuntingÞ ; ðno:of rightwardhuntingþno:of leftwardhuntingÞ
and the independent variables were the degrees of AI for the selected morphological traits. To assess the effect of the relationship between mouth laterality and asymmetrical hunting behaviour on the success ratio of hunting, a generalized linear mixed-model (GLMM) analysis was performed. Individuals with low foraging motivation (total number of darts at prey 4 in 1 h) were omitted from the analysis. We designed a GLMM with hunting success (hit or miss) as the dependent variable and the following as the independent variables: mouth laterality (lefty or righty), lateral type of hunting behaviour (leftward or rightward), combination of mouth laterality and lateral type of hunting behaviour as the fixed effect and individual as a random effect. Furthermore, we calculated the weighted mean of the success ratio of hunting of each combination, which was the value-weighted average of the ratios where the weight was proportionate to the number of darts at prey. The success ratio of hunting was compared between the combinations of the mouth laterality and the lateral type of hunting behaviour. The AnscombeeGlynn kurtosis test and chi-square goodness-of-fit test were performed using R statistical package. Other statistical analyses were performed using JMP version 5 (SAS Institute Inc., Cary, NC, U.S.A.).
RESULTS
Hunting Behaviour The average number of hunting behaviours observed per hour (X SD) was 31.3 9.5 (N ¼ 44). There was no significant bias in behavioural laterality across the whole population (i.e. no overall tendency to deviate towards either left or right for all individuals observed; X SD ¼ 0:505 0:193; one-sample Wilcoxon signedranks test: T ¼ 24.50, P ¼ 0.763). We analysed whether the distribution of the proportion of rightward hunting for each individual followed a normal distribution (Fig. 2). The distribution differed significantly from normal (ShapiroeWilk test: W ¼ 0.942, P ¼ 0.041) and was bimodal. Individual fish tended to show a bias for either leftward or rightward hunting; 14 of 44 fish had a significant bias (five fish showed left bias and nine fish right bias; binomial test: P < 0.05); when assuming P < 0.10, 22 of 44 fish showed bias (nine fish showed left bias and 13 fish right bias). A 1000-times simulation in which 44 fish each hunted randomly 31 times indicated that the average number of individuals that showed significant bias (binominal test: P < 0.05) was 1.3 fish (or 3.1 fish, if P < 0.10). Therefore, the observed values are higher than those expected from a population in which every fish behaves in an unbiased way relative to a normal distribution.
6
Number of individuals
1362
4
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Proportion of rightward hunting Figure 2. Frequency distribution of the proportion of rightward hunting in a field population of N. fasciatus. The dashed line indicates a normal curve fitted to the data. The solid line indicates the smoothing curve estimated by using Kernel smoothing (Kanel standard deviation ¼ 0.081).
Morphological Asymmetry The mean AI for each bilateral character was approximately 0 (Table 1). The frequency distribution of the AI of the height of MPE strongly deviated from normal (ShapiroeWilk test: W ¼ 0.879, P < 0.001), and its kurtosis was <3, indicating a clearly bimodal distribution (Fig. 3). Therefore, the heights of MPE were regarded as indicating AS. In PFR and LS, the variations in the difference between right and left sides were extremely small, and a chi-square goodness-of-fit test could not be performed. Thus, these two characters were excluded from subsequent analysis because they were inappropriate for examining the relationship of morphological asymmetry and behavioural laterality. The other four meristic traits (OT, GR, ULS and LLS) were regarded as showing FA: the mean values (R L) were approximately 0 and the distributions were normal, following the definition of Palmer (1994). No significant relationship existed among the degrees of asymmetry for MPE, OT, GR, ULS and LLS (Spearman rank correlation: MPE versus OT: P ¼ 0.966; MPE versus GR: P ¼ 0.913; MPE versus ULS: P ¼ 0.642; MPE versus LLS: P ¼ 0.990; OT versus GR: P ¼ 0.594; OT versus ULS: P ¼ 0.623; OT versus LLS: P ¼ 0.837; GR versus ULS: P ¼ 0.863; GR versus LLS: P ¼ 0.102; ULS versus LLS: P ¼ 0.663). The mouths of all sampled fish opened to either the left side or the right side; no fish had a mouth that opened straight forward. Because the AI of the height of MPE of 26 fish was negative and that of the other 18 fish was positive, the former and the latter were termed lefty and righty, respectively. In lefties (righties), the left (right) side of the head was more or less larger than the right (left) side, with the mouth opening rightward (leftward).
TAKEUCHI & HORI: BEHAVIOURAL LATERALITY IN CICHLID
Table 1. Statistics of the frequency distribution of the asymmetry index (AI) in selected morphological traits in N. fasciatus (N ¼ 44) Normal distribution test
Trait
AI: meanSD
Wilcoxon signed-ranks test (P)
MPE OT GR PFR ULS LLS LS
0.844.95 0.252.26 0.050.89 0.000.22 0.321.54 0.272.63 0.110.62
0.469 0.490 0.770 1.000 0.198 0.643 0.332
Kurtosis (b2)
AnscombeeGlynn kurtosis test (P)
ShapiroeWilk test (P)
Chi-square goodness-of-fit test (P)
1.56 2.79 2.64 22.00 4.52 2.97 2.61
<0.001 0.940 0.832 <0.001 0.042 0.703 0.787
<0.001 d d d d d d
d 0.988 0.815 NA 0.718 0.166 NA
A Wilcoxon signed-ranks test shows whether each mean significantly differs from the symmetric point (0). MPE, the height of mandible posterior end; the distance between bottom of the suspensoriad articulation facet of the anguloarticular and ventral end of the retroarticular process; the other traits are the number of OT, outer teeth in upper jaw; GR, gill rakers; PFR, pectoral fin rays; ULS, upper lateral line scales; LLS, lower lateral line scales; LS, longitudinal line scales. NA indicates that the test could not be performed for the trait.
Factors Affecting Behavioural Laterality We assessed whether the degree of behavioural laterality was affected by five morphological traits (MPE, OT, GR, ULS and LLS) using GLM analysis (Table 2). The GLM for the degree of behavioural laterality was significant (c25 ¼ 2:55, P < 0.001). Only the height of MPE had a significant effect. The fish with higher degrees of mouth laterality showed higher degrees of behavioural laterality. Thus the lateral difference in the height of MPE (i.e. mouth laterality) is a suitable predictor of behavioural laterality. The proportion (X SE) of rightward hunting by lefties was 0.57 0.04, whereas that of righties was 0.41 0.04. The proportion of rightward hunting by lefties was significantly higher than random (one-sample Wilcoxon signed-ranks test: P ¼ 0.041), and that of righties
was significantly lower than random (P ¼ 0.036). Thus, lefties bent mainly with the right side of their body abutting the rock, whereas righties tended to bend mainly the left side.
Factor Affecting Hunting Success The hunting success of righties was significantly higher than that of lefties on average, but the hunting success was different for each lateral type of hunting behaviour (Table 3). The success ratio of hunting of righties that adopted leftward hunting was significantly higher than both that of lefties that adopted leftward hunting and that of righties that adopted rightward hunting(GLMM: lefty leftward: coefficient ¼ 0.653, z ¼ 2.13, P ¼ 0.033; lefty rightward: coefficient ¼ 0.377, z ¼ 1.31, P ¼ 0.190; righty rightward: coefficient ¼ 0.704, z ¼ 2.07, P ¼ 0.038; Fig. 4).
Number of individuals
6
DISCUSSION Morphological traits in animals can show FA, DA or AS (van Valen 1962; Palmer & Strobeck 1986). The mouth laterality in N. fasciatus was considered to show AS, rather than FA or DA, because the frequency of AI in the mouths (MPE) of this species showed a bimodal distribution, with no symmetric individuals (Fig. 3). The deviation from symmetry of the mouth of N. fasciatus is at a level comparable to that of other fish of various feeding habits in
4
2 Table 2. Result of a GLM analysis on the degree of asymmetry index of hunting on the lateral difference of five morphological traits Variable
0
−10 −8
−6
−4
−2
0
2
4
6
8
10
Asymmetry index in height of MPE Figure 3. Frequency distribution of the AI of the height of the mandible posterior end (MPE) of mouths of N. fasciatus whose hunting was observed. The dashed line indicates the normal curve fitted to the data.
MPE OT GR ULS LLS
b
SE
t
P
0.04 0.03 0.05 0.04 0.04
0.01 0.02 0.06 0.03 0.02
3.86 1.17 0.75 1.12 1.83
<0.001 0.248 0.456 0.269 0.076
b indicates estimate of regression coefficient.
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ANIMAL BEHAVIOUR, 75, 4
Table 3. Result of GLMM analysis on the hunting success of N. fasciatus on the effects of the morphological and behavioural variables Variable
Coefficient
Mouth laterality (righty) Lateral type of hunting behaviour (rightward) Laterality (righty) Hunting (rightward)
0.710 0.303 1.001
SE
z
0.305 0.268
P
2.33 0.020 1.13 0.259
0.432 2.33 0.021
previous studies on asymmetry (Hori 1991; Mboko et al. 1998; Seki et al. 2000; Hori et al. 2007). The number of N. fasciatus individuals that performed significantly lateralized hunts in the population was larger than that expected in a population performing random hunting, and the proportion of rightward hunts of each individual in the population showed a bimodal distribution. The mouth-opening direction was consistent with the bias in direction of hunting behaviour. Behavioural laterality has often been interpreted as arising from FA (e.g. Collins et al. 1993). According to the FA hypothesis, an individual is unable to undergo symmetrical development and thus shows some behavioural laterality (Leary & Allendorf 1989; Bisazza et al. 1998). However, our results show that the behavioural laterality has no relation to the four traits showing FA (OT, GR, ULS and LLS) (Table 2). Behavioural laterality was obviously correlated to mouth laterality. Lefties and righties showed significantly more rightward and leftward hunting than random, respectively. The lateral dimorphism of mouths in fish seems to be determined by a Mendelian system of one locus-two alleles, with the lefty dominant over the righty and the dominant gene acting as either lethal or incompatible in homozygotes (Hori 1993; Seki et al. 2000; Hori et al. 2007). Thus, the bias in hunting behaviour may be consistently retained during the lifetime of each
0.6 Success ratio of hunting
1364
a 0.5
0.4
b
b
ab
0.3
0.2 leftward
rightward
Lefties
leftward
rightward
Righties
Figure 4. Comparisons of weighted means of the success ratio of hunting for lateral types of hunting behaviour and mouth laterality of N. fasciatus. The weighted mean ratio is the value-weighted average of the ratios where the weight is proportionate to the number of darts at prey. Error bars indicate 95% confidence intervals. Different letters above the bars represent significant differences of pairwise comparisons in the GLMM (P < 0.05).
individual. Behaviourally lateralized animals have enhanced skill performance and faster reaction times than nonlateralized animals, and thus their fitness should increase (Rogers 2000). Righties had more hunting success than lefties. The difference between phenotypes might be attributable to the brain lateralization. Pigeons tend to process a visual object using left hemisphere dominance (they use the right-eye field), and the discrimination performance of right-eye-dominant birds was superior to that of left¨ ntu ¨ rku ¨ n et al. 2000). Some reeye-dominant birds (Gu searchers have suggested that behavioural laterality arises from asymmetries of brain function, but cerebral asymmetry in fish remains an open question. Adding to this is that the explanation for the difference of hunting success from the brain lateralization has a logical defect from the point of view of population dynamics. If the higher hunting success of N. fasciatus in the study arose from brain lateralization and righties always had a hunting advantage over the lefties, righties should have been a majority in a population and the lefties might have perished sooner or later. But, in fact, the number of righties was less than that of lefties in 2005 (lefty:righty ¼ 26:18), and the proportion appears to have changed year by year during the past 10 years (Y. Takeuchi & M. Hori, personal observation). Alternatively, the consistent behavioural laterality may result from ‘dominance’ and the higher hunting success in righties may be a temporal phenomenon during a dynamic balancing between the two phenotypes. Seki et al. (2000) suggested that the mouth laterality correlated with some functional lateral differentiation, such as dominance in sensory abilities and/or in locomotion of one side of the body over the other. Consequently, lateralized behaviour corresponding to the dominance of each individual may increase that individual’s fitness. Furthermore, the presence and the ratio of two types of dominance in a population seem to affect their hunting efficiency (Hori 1993). In predatoreprey interactions, any attack from an unpredictable direction may lead to an increase in hunting success. If most individuals attack from a predetermined direction, their behaviour will become more predictable for prey animals, and as a result, their hunting success will be lowered. When both phenotypes co-occur, fish with the less frequent phenotype may enjoy a higher hunting success by confounding the vigilance of prey. In scale-eating cichlids, the minor morphs of mouth laterality could enjoy a higher hunting success than the dominant morphs and then become dominant in number after 1 or 2 years (Hori 1993). Our results show that, when righties of N. fasciatus abutted leftwards, their hunting success increased compared to rightward hunting, but when lefties abutted rightwards, their hunting success did not increase compared to leftward hunting (Fig. 4). Because the lefties were dominant in number in 2005, they might have had a disadvantage in hunting under frequency-dependent selection. This hypothesis needs to be tested with regard to whether lefties have a higher hunting success using rightward hunting when they are rare. Another factor that may be involved in behavioural antisymmetry is predation avoidance (Ghirlanda & Vallortigara 2004). Social fish (e.g. school-forming poeciliids
TAKEUCHI & HORI: BEHAVIOURAL LATERALITY IN CICHLID
such as Girardinus falcatus) tend to show behavioural laterality within populations, whereas solitary fish show two preferential directions of turning in a population (Rogers 1989; Bisazza et al. 2000; Vallortigara & Bisazza 2002). Because N. fasciatus rarely swims in groups, even in the young stage after independence, this fish is considered solitary, and young N. fasciatus are exposed to heavy predation pressure from many piscivorous cichlids (Hori 1983; Hori et al. 1993). When a predator attacks N. fasciatus, if individuals escape to either the left or the right, the predator may have difficulty in predicting the direction of escape compared to a situation in which individuals escape to only one side. Thus, predation avoidance may also be in play. About half of the observed N. fasciatus individuals showed a marginally significant lateral bias in hunting behaviour (P < 0.10), whereas the remaining half did not show any detectable bias. It is difficult to determine whether individuals with little bias were ‘ambidextrous’ under natural conditions. Behavioural tests should be designed to determine whether the individuals choose to hunt leftwards and rightwards equally. On the whole, the hunting behaviour of N. fasciatus under natural conditions showed laterality that was retained as AS (at the so-called individual level; however, this nomenclature may be troublesome when we discuss the meaning of laterality in a population, because the proportion of left- or right-biased individuals may change periodically (Hori 1993); we therefore refrain from using this term). The behavioural laterality was associated with mouth laterality and appeared to increase the hunting success of each individual. This phenomenon seems to be that of ‘behavioural dominance’. The behavioural dominance retained in a population may play a role in predatoreprey interactions in which an advantage for foraging or predator avoidance for each individual would depend on the ratio of the two morphs. Future studies should be focused on the dynamics of the laterality between animals of the two trophic levels: the focal species and their prey (shrimp) or enemies (fish predators).
Acknowledgments We thank the staff of the Lake Tanganyika Research Unit, Department of Research and Specialist Services, Republic of Zambia for providing facilities for field research and M. Kohda, H. Ochi, and other members of the Japanese Tanganyikan Research Team for their cooperation, encouragement and discussion during the field work. We are grateful to K. Watanabe, T. Takahashi, T. Sota, M. Yasugi, S. Tobo, Y. Takami, M. Sasabe and other members of the Laboratory of Animal Ecology, Graduate School of Science, Kyoto University for their valuable and critical comments on an early draft. Early versions of the manuscript were greatly improved by the comments of the editor and the three anonymous referees. This study was partly supported by the Grant for the Biodiversity Research of the 21st Century COE (A14), grants-in-aid of Scientific Research on Priority Areas (14087203), and the Global Center of Excellence Program ‘‘Formation of
a Strategic Base for Biodiversity and Evolutionary Research: from Genome to Ecosystem’’, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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