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the twisted smiles of left- and right - bending scale - eating cichlids ( figure 1 ) have achieved near - legendary status among evolutionary biologists since their explosion onto the scene in 1993 . as more data have accumulated , though , the story has become both more interesting and decidedly more puzzling .
two recent papers , by stewart and albertson in bmc biology   and van dooren et al . in evolution , add valuable new perspectives , including the exciting possibility that handed behavior may amplify mouth asymmetry during growth . but these and other studies also raise important questions about this textbook case of frequency - dependent selection .
conspicuously asymmetrical left - bending ( left ) and right - bending ( right ) individuals of the scale - eating cichlid fish perissodus microlepis from lake tanganyika .
the original story   is inescapably seductive because it ' makes sense ' and is easy to tell .
perissodus microlepis is a specialized scale - eater from lake tanganyika that nips scales off the posterior flanks of larger prey fish .
their mouths bend to one side of the head , which allows them to strike from a more posterior orientation that makes them less visible to intended victims .
mouths bend to the right in some individuals and to the left in others , and a single locus , two - allele polymorphism is thought to control the direction of mouth - bending , with right bending being dominant . finally , frequencies of right- and left - bending individuals appear to vary cyclically around 50:50 over time , as if negative frequency - dependent selection were maintaining this polymorphism .
so , the logic goes , when right - bending individuals become more common , potential victims are thought to become more watchful of their left sides , thereby increasing the feeding success and therefore the fitness of the rarer , left - bending individuals .
but puzzling results regarding the extent of mouth bending in adults and juveniles , and its genetic basis , have emerged from several subsequent studies .
differences between right- and left - bending morphs of adult p. microlepis can sometimes be dramatic ( figure 1 ) .
however , careful geometric morphometric analyses of head and jaw asymmetry by stewart and albertson   and van dooren et al
these new results help to explain why a revision of the genus perissodus reported conspicuous asymmetry in only one species , p. eccentricus ( thought at the time to be a sister species to p. microlepis ) , and are also consistent with earlier quantitative evidence for subtle ( 2 - 3% ) and more continuous variation in mouth asymmetry . among six species of scale - eating cichlids , including p. microlepis , geometric morphometric measures of overall head - shape asymmetry ( when viewed laterally ) were only significant in adults of one of the two most derived species , p. straeleni .
measurements of lower jaw mechanical advantage differed between right- and left - bending forms in the two most derived scale - eaters , but these differences were only marginally significant for three of the four tests .
so , although the four most derived species eat scales almost exclusively , the statistical signal for asymmetry in the heads and jaws of adults appears surprisingly weak .
similarly , when viewed from above , the mouth - bending angle also appears to be less discrete than generally believed .
careful head - orientation measures of live , adult p. microlepis confirmed that some bent towards the right and others to the left .
but the frequency distribution of asymmetry values was only weakly bimodal , with modes corresponding to bending angles of 2 - 3 degrees . here again
, right and left bending did not appear to be discrete states in adults , and more then 10% of individuals were nearly symmetrical ( mouth bend less than 1 degree ) .
finally , temporal variation in the proportions of left- and right - bending p. microlepis is much less pronounced than generally believed . unlike for total population size , where a confidence limit is difficult to estimate , confidence limits to the proportions of right- and left - bending individuals within a sample are easily obtained from a binomial distribution .
of 13 samples taken over an 11-year period ( see figure 2a of ) , only two approached statistical significance .
furthermore , in all three populations where two samples were taken concurrently , one yielded an excess of left - bending individuals and the other a deficit .
in other words , variation among samples , and therefore among years , was not significantly greater than expected due to simple sampling error .
parent phenotypes , offspring phenotypes , and expected frequencies of right - bending ( r ) and left - bending ( l ) forms of the scale - eating cichlid fish perissodus microlepis .
each box represents offspring from a single cross ( except rows ( a ) and ( b ) , where results are pooled from all crosses in that study ) .
parent and offspring phenotypes were scored visually , except in row ( c ) where offspring phenotypes were measured and parent phenotypes inferred from putative microsatellite markers for right- and left - bending .
colors indicate statistically significant departures from expectations for model i ( orange box ) , model ii ( yellow box ) , or both models i and ii ( blue box ) .
parent phenotypes follow the original convention of hori , who referred to right- and left - bending as dextral and sinistral .
this convention differs from , who follow the later convention of referring to right - bending fish as ' lefties ' because they attack prey from the left side . in regard to model
ii , if the rr genotype is lethal as proposed in , then all right - bending parents ( r ) must be rl heterozygotes .
the quantitative analyses of head - bending variation among brooded offspring from a single female   yielded two puzzling observations . first
second , although some individuals departed from symmetry by several degrees , the distribution of jaw bending angles was clearly unimodal and centered on zero ( see figure 4c of ) .
if two alleles at a single locus induce discrete right- and left - bending morphs , then two questions arise .
why do stewart and albertson   find that so many juveniles ( close to one - third ) exhibit no significant mouth bending ? and why does asymmetry in juveniles not materialize as a bimodal distribution of right - left differences ?
to complicate matters further , the genetic basis of mouth - bending has become more rather than less puzzling as more data accumulate .
two alleles , r and l , at a single locus are thought to direct bending to the right or the left , respectively .
the first is a simple model where r is dominant   ( figure 2 , model i ) , whereas the second is a more complex model , where r is both dominant and homozygous lethal   ( figure 2 , model ii ) .
published offspring frequencies from four crosses are inconsistent with model i ( figure 2 , orange and blue boxes , ) and those from six crosses are inconsistent with model ii ( figure 2 , yellow and blue boxes ) .
furthermore , none of the pooled results from the original study   ( figure 2a ) is consistent with model ii , a model where rr homozygotes are lethal so all right - bending parents must be heterozygotes .
stewart and albertson   measured mouth asymmetry quantitatively in juveniles and genotyped them for two microsatellite markers , a and b , putatively linked to right - bending ( r ) and left - bending ( l ) alleles , respectively .
first , nearly half of left - bending juveniles ( 20 of 45 ) carried marker a , a result inconsistent with the dominance of allele r. second , nearly 10% of individuals ( 9 of 93 ) , regardless of mouth bend , were homozygous for marker a , a result inconsistent with rr lethality ( figure 2 , model ii ) .
third , over 12% of right - bending juveniles ( 6 of 48 ) were homozygous for marker b , a result inconsistent with being homozygous recessive for allele l. finally , although both parents of this brood were inferred to be heterozygotes , and other microsatellite evidence pointed to a single male parent , equal frequencies of right- and left - bending offspring are not consistent with either model i or ii ( figure 2c ) . in the second genetic marker study , which used both nuclear and mitochondrial dna markers , lee et al .  
report widespread interbreeding between right- and left - bending morphs , consistent with random mating . however , these results are not consistent with the predictions of model ii , or with published reports , that right - bending males should or do prefer to mate with left - bending females and vice versa ( disassortative mating ) because rr homozygotes are lethal .
cichlid mouth asymmetry might be inherited like visceral asymmetry in mice   ( that is , like left - right asymmetry of internal organs such as the heart ) , where a dominant allele causes mouth bending towards one side but a recessive allele yields random phenotypes ( equal numbers of right and left ) ( figure 2 , model iii ) .
alternatively , the direction of asymmetry might be purely random and not inherited at all , as observed in 27 of 28 cases of random asymmetries in other organisms ( model iv ) . left - bending offspring from two right - bending parents are either too common for model iii or too few for model iv ( 9 of 10 crosses in both cases ; figure 2 ) .
similarly , right - bending offspring from two left - bending parents are too few for either model iii or iv ( figure 2 ) .
unfortunately , direction of mouth - bending of both parents and offspring was scored by eye in all but one cross ( that in figure 2c ) , even though mouth - bending angles can be difficult to detect in adults   and are so small as to be ' ambiguous ' in more than one - third of fry measured .
clearly , more studies like that of stewart and albertson   are needed to resolve these unsettling inconsistencies .
despite the questions raised above , these recent studies   suggest an exciting new dimension to the scale - eating cichlid story . handed or lateralized
first , mouth asymmetry increases with increasing body size , as would be expected if repeated lateralized behavior induced greater mouth asymmetry over time .
second , the frequency distribution of mouth asymmetry is not bimodal in brooded juveniles that have not yet begun to feed on scales , suggesting that scale - eating itself may amplify morphological asymmetry later in life .
third , mouth asymmetry of fish forced experimentally to attack prey from their non - preferred side tended to become less pronounced over time , whereas it increased in those allowed to attack from their preferred side .
fourth , the stronger the lateralized behavior of individual fish , the more pronounced the mouth asymmetry
. finally , cichlids , and many other fish , exhibit plastic jaw form that responds to the diet they experience : side of attack may therefore matter just as much as kind of prey eaten .
several puzzling aspects of the scale - eating cichlid story could therefore be more easily explained if mouth asymmetry is actually induced or amplified by lateralized behavior .
does consistency of lateralized behavior increase with increasing body size , as expected if individuals learn to become more proficient at attacking one side with increasing age ?
does lateralized behavior or mouth asymmetry of fish reared in the laboratory on a diet of free - floating scales become less pronounced compared to fish consistently forced to obtain scales by striking the sides of their prey , as expected if lateralized behaviors are learned and morphological asymmetry is amplified by lateralized behavior ?
do offspring from left - bending parents exhibit a greater tendency to attack the right sides of their prey even before mouth asymmetry becomes pronounced , as expected if greater genetic variation exists for the degree of lateralized behavior   than for the degree of morphological asymmetry ?
do those individuals that attack prey consistently from one side obtain more scales than those that sometimes attack from the right or left , as expected if learned behavior improves feeding performance ? as these examples show , studies of right - left asymmetries offer valuable clues about how development evolves because the questions are clear and tests of developmental mechanisms are possible in many different organisms .
the freedom to study many aspects of biological asymmetries has been generously supported by nserc canada ( discovery grant a7245 ) and a sabbatical leave from the university of alberta .
two recent studies in bmc biology and evolution raise important questions about a textbook case of frequency - dependent selection in scale - eating cichlid fishes . 
 they also suggest a fascinating new line of research testing the effects of handed behavior on morphological asymmetry.see research article http://www.biomedcentral.com/1741-7007/8/8 .