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Synchronous claw-waving of fiddler crabs
SYNCHRONOUS CLAW-WAVING OF FIDDLER CRABS BY HELEN R . S . GORDON
Department of Zoology . Unlver,cit,r ofthe Witwatersrand . Johannesburg Introduction A recent study was made of the ecology and behaviour of the five species of flea crab, which occupy the mangrove swamps of Inhaca Island, off the coast of Mozambique, The species studied were U, annullpes, U. inversa. U. ehl®rophthalmus, U. marlonis and U. urvlllei, The crabs were observed during several seasons, extending over a p eriod of about a year, that is, from April, 1954, to May, 1955, One of the aspects studied was the characteristic wavin movement of the major cheliped of mate" crabs, a phenomenon generally referred to as "claw-waving" or "claw-beckoning ." Various workers on flea have already expressed their views on the function served by the claw-waving of the male, Verwey (1950), for example, caims that claw-waving is used exclusively as a means of threat, to warn intruders off the territory of the beckoning individual, Crane (1941), however, while admitting . that claw-waving is used as a means of territorial defense and, thus, as a means of delimiting the territory of each individual, asserts nevertheless that claw-waving plays a major role in courtship, it being the chief method used to attract Individuals of the opposite sex . Crane further points out that this latter function is especially noticeable in those species in which waving is limited to the reproductive season, Peters (1955), while studying the variations in the methods of claw-waving shown by certain species of Lieu and Minuea, found that the method of waving varies from one species to another, particular) in the duration of the movements involve, Within any one of his species, however, the method of beckoning a ppears remarkably stereotyped, Peters asserts that claw-waving in Lieu crabs is chiefly a means of territorial efence, because the individuals occur generally in very dense populations and, not only do the spend a long period of every day out of the r burrows, feeding, but they have an unobstructed view of one another on the feeding-grounds . Altevogt (1957), on the other hand . asserts that, in the three Ue i species studied by him, claw-waving has no function as a means of delimiting
territories, but serves only in attracting females during the breeding-season . He found that there was, in fact, very little claw-waving taking place during the normal feeding and burrow-digging periods of these species . The male crabs of the five Inhaca species showed claw-waving both as a means of t hreat . to a varying degree throughout the year, andwith the exception of U. inversa, in which no real courtship-display was ever observed-as a means of attracting females, only during the breeding-season, It should be noted, in passing, that the breeding-seasons of the five species do not quite coincide with each other, some of the species starting to show courtship behaviour earlier than others, As a general rule, aggressive behaviour towards intruders was greater during the courting and breeding-season of the species than during the rest of the year, and actual fighting-involving the gripping of one another's claws-was at all times relatively rare as compared with threat, Crane (1941) found that, in the Uea species of Panama, threat-display only very rarely preceded a duel, but, in the Inhaca species, actual fighting took place only on the relatively rare occasions when threatdisplay had failed, Tinbergen (1951) has noted that fighting in animals usually consists of threatening or bluff, and this is certainly the case in the (Jea species of Inhaca, Claw-waving is used both as a means of threat and as a means of courtship-disp lay, but the method used was found to vary from species to species, while remaining very constant within any one species . In this way, the Inhaca species resemble the P,l Salvador species of Peters (1955), Generally speaking, the male UFea crabs of Inhaca use almost the same basic movement of the major eheliped, whether such a male is threatening off some other individual from his territory, or courting a female crab . In most species, however, when a male is trying to attract the attention of a passing female, the display-movement used is generally faster, more definite, and of a somewhat higher intensity than that observed during threat-display . It was found, too, that in V, annulipes, for example, a crab engaged in threat-display may ,jerk about, 219
GORDON : SYNCHRONOUS CLAW-WAVING OF FIDDLER CRABS but to a lesser degree than in courtship . Moreover, during threat, he tends to remain more or less in one spot while waving his cheliped, and nearly always stays in the near vicinity of his burrow . Thus, the running about and the jerking movements of the body from side to side are generally associated with the attracting of a female's attention during courtship, and not so much with threat . Unlike Altevogt's species, which nevertheless also include U. annulipes, the Inhaca species certainly show threat and a certain amount of actual fighting, chiefly centring around the ownership of burrows, during normal feeding and burrow-digging periods . In addition to the usual claw-waving associated with threat and courtship, however, various modifications of the phenomenon of clawwaving were also shown by individuals of four of the five Inhaca species . In each case, this special type of claw-waving appeared to serve no particular purpose . The most striking, and certainly the commonest of these variations was what can best be described as a widespread "synchronous claw-waving", shown only by males of U. annulipes . Synchronous Claw-waving in U. annulipes Synchronous claw-waving-a peculiar type of rhythmic claw-waving, involving small groups of male individuals-was first noticed during February, 1955, at two of the Inhaca mangrove swamps, where U. annulipes individuals abound . This type of concerted waving appeared to be more or less at its peak during February, when it was mid-summer, for every U. annulipes population in both the swamps seemed to have a few groups of synchronous claw-wavers . During another visit, towards the end of April and beginning of May, when the weather was cooler, synchronous claw-waving was again observed at both these swamps, but to a lesser extent . Not only was the number of synchronising groups noticed in any one population considerably fewer, but the synchronous waving itself was, generally speaking, somewhat less intense . As a result of this, most of the observations on synchronous claw-waving were recorded during February . Apart from being at its maximum during the warmer months of the year, synchronous clawwaving showed also a daily peak, which was reached during the hottest part of the day, the waving being at its most intense during these hours . Early in the morning and in the late afternoon, there were fewer groups of
239
synchronous wavers to be seen, and most of the claw-waving taking place at these times was performed by scattered males independently of one another . There was always a greater number of waving groups in sunlit areas than in the shade where, as a general rule, the individuals were definitely rather sluggish, being far less active than those exposed to the sun . Synchronous claw-waving generally seemed to start when one or rarely two individuals would begin to wave with their major chelipeds, occasionally as a reaction to a passing female but nearly always for no apparent reason . Almost immediately, one male after another in the vicinity of the beckoning individual or individuals would generally begin to wave his major cheliped until there was a small group of individuals, moving automatically in unison, the movements of their bodies and chelipeds being synchronised to a remarkable degree . The individuals in each group appeared to be in some way lulled by the rhythmic movements of their fellows, for they generally continued waving thus for an average of about ten to fifteen minutes . There were usually about five to seven crabs in a group, all facing the same way or in different directions, some even standing back to back with one another . Very occasionally, when a particular female happened to be the "trigger" setting off the claw-waving mechanism in the first male individual, she would gradually become more or less surrounded by a ring of males, waving synchronously . Such claw-waving however, would invariably continue long after the female had moved away, and new individuals that had never even seen the female responsible might join the group . Frequently, there were as many as ten small groups of synchronous wavers in a small area, and these groups would usually overlap one another to a certain extent . For example, a particular male involved in the synchronised waving of one group might suddenly, and for no apparent reason, become involved in the synchronised waving of a neighbouring group . Moreover, adjacent groups of synchronous wavers might gradually merge together to form a larger group, in which all the individuals would eventually begin to wave their chelipeds in unison . The individuals did not appear to feed whilst they were engaged in synchronous waving, but an individual in a group might gradually stop waving, and possibly, move a short distance away, in order to feed, often quite busily . Should such an individual by any chance move into the
240
ANIMAL BEHAVIOUR, Vl, 3-4
midst of a neighbouring group of synchronous wavers, however, he would generally . stop feeding almost at once and automatically join in the waving of this group . The fact that the individuals comprising a group moved about and changed their group from time to time was another way in which overlapping of the groups would take place . Each individual, then, would tend to alternate spells of synchronous claw-waving with bouts of feeding, or such a crab might begin to show independent claw-waving, usually to threaten other males but occasionally, in February anyway, to attract the attention of passing females . As gradually as it began, synchronous clawwaving would end . Just as one individual after another gradually took up the movement until a group was formed, so one individual after another would gradually stop waving, again for no apparent reason . Although the majority of synchronous claw-wavers were adult, some rather small young forms were noticed in many of the groups .
cheliped as high as possible, so that the claw was pointing upward, then swing it first down and in towards his body and finally out to the side again, in a smooth, continuous movement .
0 6?
The Movements Involved in Synchronous Waving The claw-waving movements of each individual in a synchronising group were found to be essentially similar to the normal beckoning movements used by a male when threatening another male or when attracting the attention of a passing female . The movements involved, however, seemed to be even more exaggerated than those used in courtship-display which, in its turn, as has already been pointed out, involves movements which are not only faster and more jerky than those of threat-display . but which are also more clearly defined and of a higher intensity. The movement of each individual in the group was seen to start originally with the position of rest : that is, the ambulatories were slightly bent and the major cheliped was held bent and in the normal horizontal plane, with the claw itself pointing slightly forward . The cheliped would then be extended and stretched out sideways to the very fullest extent and, with the cheliped held in this position, the crab would give a pronounced jerk or jump, reminiscent of courtship-display, generally moving a short distance to one side but, very occasionally, landing on the same spot . Next, while raising himself on to the tips of his now straightened ambulatories and lifting his minor cheliped, the individual would raise his major
Fig . 1 . The main positions of the chelipeds and ambulatories during synchronous claw-waving of U . annulipes.
GORDON : SYNCHRONOUS CLAW-WAVING OF FIDDLER CRABS
As the major cheliped was raised to its highest point, a few of the ambulatories on the side of the major cheliped might even be lifted momentarily from the ground. As the major cheliped was lowered and restored to the position out at the side of the individual, the minor cheliped would also be somewhat lowered, and the ambulatories bent slightly once again . The main positions of the major cheliped in relation to the body are illustrated in Fig . 1 . Following each beckoning-movement, there was a definite pause of about a second or two, after which the crab would generally jerk once again, this time usually to the side opposite but occasionally to the same side as previously, and then the whole claw-beckoning movement would be repeated . The individuals in a synchronising group would generally keep nearly perfect time with one another, but occasionally the beckoning movements of individuals would tend to overlap one another very slightly, one individual starting to move his major cheliped before another which, in his turn, would start the waving-movement before yet another individual, and so on . On the whole, too, the crabs would all jerk at approximately the same instant, and they would generally all jerk to the same side, whether or not they were all facing in the same direction . Discussion The synchronous claw-waving shown by males of U. annulipes is obviously a highly specialised form of beckoning, the function of which is not clear . It certainly does not form part of either the threat- or courtship-pattern, although the first male of a group sometimes begins his initial waving as a reaction to a passing female. Since it is a phenomenon which appears to be correlated with temperature, synchronous clawwaving can probably best be described as a physical expression of abundant energy in a sun-loving genus . The fact that this energy finds expression in this particular way in only
241
one of the five Inhaca species can perhaps best be explained by the fact that U. annulipes individuals spend the longest daily period of all five Uca species feeding, out of their burrows and exposed to the heat of the sun . Summary 1 . An ecological and behavioural study was made of the five Uca species of Inhaca Island . 2 . In addition to the claw-waving shown by males as a threat-display or as part of courtship behaviour, various modifications of the phenomenon of claw-waving were also noticed . The most widespread of these modifications was a type of synchronous claw-waving, shown only by males of U. annulipes . 3 . Synchronous claw-waving is a form of concerted waving by small groups of individuals . 4 . The movements involved in synchronous claw-waving are described in detail. 5 . Synchronous claw-waving showed both a seasonal and a diurnal peak, and thus has a definite connection with temperature . 6. This highly specialised form of clawwaving is probably due to the abundant energy of the U. annulipes individuals, which spend the most time of all the species above-ground, feeding. REFERENCES Altevogt, R . (1957) . Untersuchungen zur Biologie, Okologie and Physiologie lndischer Winkerkrabben . Z. Morph . u . Okol. Tiere, 46, 1-110 . Crane, J. (1941). Crabs of the genus Uca from the West Coast of Central America . Zoologica, XXVI, 145208 . Peters, H. M . (1955) . Die Winkgebarde von Uca and Minuca (Brachyura) in vergleichend-ethologischer, -okologischer and -morphologisch-anatomischer Betrachtung . Z. Morph . u. Okol. Tiere ., 43,425-500 . Tinbergen, N. (1951). The study of instinct . Oxford : Clarendon Press. Verwey, J . (1930) . Einiges uber die Biologie ostindischer Mangrove-krabben . Treubia, 12, 169-261 . Accepted for publication 14th January, 1958 .
Department of Zoology . Unlver,cit,r ofthe Witwatersrand . Johannesburg Introduction A recent study was made of the ecology and behaviour of the five species of flea crab, which occupy the mangrove swamps of Inhaca Island, off the coast of Mozambique, The species studied were U, annullpes, U. inversa. U. ehl®rophthalmus, U. marlonis and U. urvlllei, The crabs were observed during several seasons, extending over a p eriod of about a year, that is, from April, 1954, to May, 1955, One of the aspects studied was the characteristic wavin movement of the major cheliped of mate" crabs, a phenomenon generally referred to as "claw-waving" or "claw-beckoning ." Various workers on flea have already expressed their views on the function served by the claw-waving of the male, Verwey (1950), for example, caims that claw-waving is used exclusively as a means of threat, to warn intruders off the territory of the beckoning individual, Crane (1941), however, while admitting . that claw-waving is used as a means of territorial defense and, thus, as a means of delimiting the territory of each individual, asserts nevertheless that claw-waving plays a major role in courtship, it being the chief method used to attract Individuals of the opposite sex . Crane further points out that this latter function is especially noticeable in those species in which waving is limited to the reproductive season, Peters (1955), while studying the variations in the methods of claw-waving shown by certain species of Lieu and Minuea, found that the method of waving varies from one species to another, particular) in the duration of the movements involve, Within any one of his species, however, the method of beckoning a ppears remarkably stereotyped, Peters asserts that claw-waving in Lieu crabs is chiefly a means of territorial efence, because the individuals occur generally in very dense populations and, not only do the spend a long period of every day out of the r burrows, feeding, but they have an unobstructed view of one another on the feeding-grounds . Altevogt (1957), on the other hand . asserts that, in the three Ue i species studied by him, claw-waving has no function as a means of delimiting
territories, but serves only in attracting females during the breeding-season . He found that there was, in fact, very little claw-waving taking place during the normal feeding and burrow-digging periods of these species . The male crabs of the five Inhaca species showed claw-waving both as a means of t hreat . to a varying degree throughout the year, andwith the exception of U. inversa, in which no real courtship-display was ever observed-as a means of attracting females, only during the breeding-season, It should be noted, in passing, that the breeding-seasons of the five species do not quite coincide with each other, some of the species starting to show courtship behaviour earlier than others, As a general rule, aggressive behaviour towards intruders was greater during the courting and breeding-season of the species than during the rest of the year, and actual fighting-involving the gripping of one another's claws-was at all times relatively rare as compared with threat, Crane (1941) found that, in the Uea species of Panama, threat-display only very rarely preceded a duel, but, in the Inhaca species, actual fighting took place only on the relatively rare occasions when threatdisplay had failed, Tinbergen (1951) has noted that fighting in animals usually consists of threatening or bluff, and this is certainly the case in the (Jea species of Inhaca, Claw-waving is used both as a means of threat and as a means of courtship-disp lay, but the method used was found to vary from species to species, while remaining very constant within any one species . In this way, the Inhaca species resemble the P,l Salvador species of Peters (1955), Generally speaking, the male UFea crabs of Inhaca use almost the same basic movement of the major eheliped, whether such a male is threatening off some other individual from his territory, or courting a female crab . In most species, however, when a male is trying to attract the attention of a passing female, the display-movement used is generally faster, more definite, and of a somewhat higher intensity than that observed during threat-display . It was found, too, that in V, annulipes, for example, a crab engaged in threat-display may ,jerk about, 219
GORDON : SYNCHRONOUS CLAW-WAVING OF FIDDLER CRABS but to a lesser degree than in courtship . Moreover, during threat, he tends to remain more or less in one spot while waving his cheliped, and nearly always stays in the near vicinity of his burrow . Thus, the running about and the jerking movements of the body from side to side are generally associated with the attracting of a female's attention during courtship, and not so much with threat . Unlike Altevogt's species, which nevertheless also include U. annulipes, the Inhaca species certainly show threat and a certain amount of actual fighting, chiefly centring around the ownership of burrows, during normal feeding and burrow-digging periods . In addition to the usual claw-waving associated with threat and courtship, however, various modifications of the phenomenon of clawwaving were also shown by individuals of four of the five Inhaca species . In each case, this special type of claw-waving appeared to serve no particular purpose . The most striking, and certainly the commonest of these variations was what can best be described as a widespread "synchronous claw-waving", shown only by males of U. annulipes . Synchronous Claw-waving in U. annulipes Synchronous claw-waving-a peculiar type of rhythmic claw-waving, involving small groups of male individuals-was first noticed during February, 1955, at two of the Inhaca mangrove swamps, where U. annulipes individuals abound . This type of concerted waving appeared to be more or less at its peak during February, when it was mid-summer, for every U. annulipes population in both the swamps seemed to have a few groups of synchronous claw-wavers . During another visit, towards the end of April and beginning of May, when the weather was cooler, synchronous claw-waving was again observed at both these swamps, but to a lesser extent . Not only was the number of synchronising groups noticed in any one population considerably fewer, but the synchronous waving itself was, generally speaking, somewhat less intense . As a result of this, most of the observations on synchronous claw-waving were recorded during February . Apart from being at its maximum during the warmer months of the year, synchronous clawwaving showed also a daily peak, which was reached during the hottest part of the day, the waving being at its most intense during these hours . Early in the morning and in the late afternoon, there were fewer groups of
239
synchronous wavers to be seen, and most of the claw-waving taking place at these times was performed by scattered males independently of one another . There was always a greater number of waving groups in sunlit areas than in the shade where, as a general rule, the individuals were definitely rather sluggish, being far less active than those exposed to the sun . Synchronous claw-waving generally seemed to start when one or rarely two individuals would begin to wave with their major chelipeds, occasionally as a reaction to a passing female but nearly always for no apparent reason . Almost immediately, one male after another in the vicinity of the beckoning individual or individuals would generally begin to wave his major cheliped until there was a small group of individuals, moving automatically in unison, the movements of their bodies and chelipeds being synchronised to a remarkable degree . The individuals in each group appeared to be in some way lulled by the rhythmic movements of their fellows, for they generally continued waving thus for an average of about ten to fifteen minutes . There were usually about five to seven crabs in a group, all facing the same way or in different directions, some even standing back to back with one another . Very occasionally, when a particular female happened to be the "trigger" setting off the claw-waving mechanism in the first male individual, she would gradually become more or less surrounded by a ring of males, waving synchronously . Such claw-waving however, would invariably continue long after the female had moved away, and new individuals that had never even seen the female responsible might join the group . Frequently, there were as many as ten small groups of synchronous wavers in a small area, and these groups would usually overlap one another to a certain extent . For example, a particular male involved in the synchronised waving of one group might suddenly, and for no apparent reason, become involved in the synchronised waving of a neighbouring group . Moreover, adjacent groups of synchronous wavers might gradually merge together to form a larger group, in which all the individuals would eventually begin to wave their chelipeds in unison . The individuals did not appear to feed whilst they were engaged in synchronous waving, but an individual in a group might gradually stop waving, and possibly, move a short distance away, in order to feed, often quite busily . Should such an individual by any chance move into the
240
ANIMAL BEHAVIOUR, Vl, 3-4
midst of a neighbouring group of synchronous wavers, however, he would generally . stop feeding almost at once and automatically join in the waving of this group . The fact that the individuals comprising a group moved about and changed their group from time to time was another way in which overlapping of the groups would take place . Each individual, then, would tend to alternate spells of synchronous claw-waving with bouts of feeding, or such a crab might begin to show independent claw-waving, usually to threaten other males but occasionally, in February anyway, to attract the attention of passing females . As gradually as it began, synchronous clawwaving would end . Just as one individual after another gradually took up the movement until a group was formed, so one individual after another would gradually stop waving, again for no apparent reason . Although the majority of synchronous claw-wavers were adult, some rather small young forms were noticed in many of the groups .
cheliped as high as possible, so that the claw was pointing upward, then swing it first down and in towards his body and finally out to the side again, in a smooth, continuous movement .
0 6?
The Movements Involved in Synchronous Waving The claw-waving movements of each individual in a synchronising group were found to be essentially similar to the normal beckoning movements used by a male when threatening another male or when attracting the attention of a passing female . The movements involved, however, seemed to be even more exaggerated than those used in courtship-display which, in its turn, as has already been pointed out, involves movements which are not only faster and more jerky than those of threat-display . but which are also more clearly defined and of a higher intensity. The movement of each individual in the group was seen to start originally with the position of rest : that is, the ambulatories were slightly bent and the major cheliped was held bent and in the normal horizontal plane, with the claw itself pointing slightly forward . The cheliped would then be extended and stretched out sideways to the very fullest extent and, with the cheliped held in this position, the crab would give a pronounced jerk or jump, reminiscent of courtship-display, generally moving a short distance to one side but, very occasionally, landing on the same spot . Next, while raising himself on to the tips of his now straightened ambulatories and lifting his minor cheliped, the individual would raise his major
Fig . 1 . The main positions of the chelipeds and ambulatories during synchronous claw-waving of U . annulipes.
GORDON : SYNCHRONOUS CLAW-WAVING OF FIDDLER CRABS
As the major cheliped was raised to its highest point, a few of the ambulatories on the side of the major cheliped might even be lifted momentarily from the ground. As the major cheliped was lowered and restored to the position out at the side of the individual, the minor cheliped would also be somewhat lowered, and the ambulatories bent slightly once again . The main positions of the major cheliped in relation to the body are illustrated in Fig . 1 . Following each beckoning-movement, there was a definite pause of about a second or two, after which the crab would generally jerk once again, this time usually to the side opposite but occasionally to the same side as previously, and then the whole claw-beckoning movement would be repeated . The individuals in a synchronising group would generally keep nearly perfect time with one another, but occasionally the beckoning movements of individuals would tend to overlap one another very slightly, one individual starting to move his major cheliped before another which, in his turn, would start the waving-movement before yet another individual, and so on . On the whole, too, the crabs would all jerk at approximately the same instant, and they would generally all jerk to the same side, whether or not they were all facing in the same direction . Discussion The synchronous claw-waving shown by males of U. annulipes is obviously a highly specialised form of beckoning, the function of which is not clear . It certainly does not form part of either the threat- or courtship-pattern, although the first male of a group sometimes begins his initial waving as a reaction to a passing female. Since it is a phenomenon which appears to be correlated with temperature, synchronous clawwaving can probably best be described as a physical expression of abundant energy in a sun-loving genus . The fact that this energy finds expression in this particular way in only
241
one of the five Inhaca species can perhaps best be explained by the fact that U. annulipes individuals spend the longest daily period of all five Uca species feeding, out of their burrows and exposed to the heat of the sun . Summary 1 . An ecological and behavioural study was made of the five Uca species of Inhaca Island . 2 . In addition to the claw-waving shown by males as a threat-display or as part of courtship behaviour, various modifications of the phenomenon of claw-waving were also noticed . The most widespread of these modifications was a type of synchronous claw-waving, shown only by males of U. annulipes . 3 . Synchronous claw-waving is a form of concerted waving by small groups of individuals . 4 . The movements involved in synchronous claw-waving are described in detail. 5 . Synchronous claw-waving showed both a seasonal and a diurnal peak, and thus has a definite connection with temperature . 6. This highly specialised form of clawwaving is probably due to the abundant energy of the U. annulipes individuals, which spend the most time of all the species above-ground, feeding. REFERENCES Altevogt, R . (1957) . Untersuchungen zur Biologie, Okologie and Physiologie lndischer Winkerkrabben . Z. Morph . u . Okol. Tiere, 46, 1-110 . Crane, J. (1941). Crabs of the genus Uca from the West Coast of Central America . Zoologica, XXVI, 145208 . Peters, H. M . (1955) . Die Winkgebarde von Uca and Minuca (Brachyura) in vergleichend-ethologischer, -okologischer and -morphologisch-anatomischer Betrachtung . Z. Morph . u. Okol. Tiere ., 43,425-500 . Tinbergen, N. (1951). The study of instinct . Oxford : Clarendon Press. Verwey, J . (1930) . Einiges uber die Biologie ostindischer Mangrove-krabben . Treubia, 12, 169-261 . Accepted for publication 14th January, 1958 .