Aggregation and foraging behavior of whirligig beetles (Gyrinidae)

Behav. Ecol. Sociobiol. 7, 179-186 (1980) Behavioral Ecology and Sociobiology �9 by Springer-Verlag 1980

Aggregation and Foraging Behavior of Whirligig Beetles (Gyrinidae)

Bernd Heinrich 1 and F. Daniel Vogt 2 1 Division of Entomology and Parasitology, University of California, Berkeley, California 94720, USA z Department of Biology, Wesleyan University, Middletown, Connecticut 06457, USA

Received October 29, 1979 / Accepted March 22, 1980

Summary. Whirligig beetles aggregate in the daytime into dense single- and multispecies groups ( 'rafts ') of hundreds or thousands of individuals. On the 22- km shoreline of Lake Itasca in northern Minnesota, these aggregations were on the average 0.8 km apart, and they were usually found day after day in the same locations.

Most beetles apparently do no t ' home ' to the aggregation of their origin after dispersing at night because (a) the species composition of some aggregations changed greatly, and (b) paint-marked beetles (Dineu- tus homo moved overnight from one aggregation as far as 4 km, joining 11 of the 14 large (> 300 beetles) D. horni groups on the lake.

Throughout the night, the largest concentrations of beetles remained within 100 m of the diurnal aggregation sites. Beetles reconvened into the compact rafts before daybreak, in part by following each other in sometimes long single files or ' t rains. ' Their forward motion stopped after they joined large numbers of other beetles. We infer that following behavior en- ables those individuals that have dispersed from their original aggregations (during their nocturnal foraging) to find and join other aggregations before daylight.

Naive fish ate the beetles despite their noxious secretions. However, fish living near rafting sites and feeding on insects on the water surface in daylight should soon learn to avoid the beetles. The rafting sites would then become 'safe ' places. We observed fish attacking only those beetles that had been either dispersed from their rafts or released into open water away from raft sites in the daytime. We speculate that the evolutionary significance of the aggregation behavior is related to predator (fish) avoidance.

Introduction

In insects, as in other animals, aggregation of individuals (depending on the species) facilitates success in

mating, defense from predators and parasites (Alex- ander 1974), acquisition of resources by group foraging (Wilson 1971; Alexander 1974), protection from the physical environment (Wagner 1954), or combina- tions of some or all of these (Wilson 1971).

Whirligig beetles dwell on the water surface of quiet streams and lakes where they are predators and scavengers of floating insects. They are well known to congregate in large groups or ' raf ts ' and to be equipped with defensive secretions (Benfield 1972; Meinwald et al. 1972; Newhart and Mumma 1978), but neither the immediate behavioral mechanisms nor the ultimate evolutionary significance of the rafting behavior are known. Benfield (1972) suggests that by aggregating in groups, they may 'advertise ' their defensive secretions en masse. However, the beetles do not discharge their noxious chemicals until they are attacked. Naive fish eat beetles (Benfield 1972), but the chemicals are of sufficient quantity in even one beetle (Newhart and Mumma 1978) to repel predators after a single encounter. The groups do not continuously emit offensive chemicals that keep predators at bay.

We here examine the immediate behavioral aspects of rafting, and make inferences about its ultimate, evolutionary signifcance.

Materials and Methods

The investigations were made during August 1979, on Lake Itasca in northern Minnesota, USA. Lake Itasca has a shoreline of approximately 22 km, and except for the northeast shore of the south- ern arm (see Fig. 5), most of the lake is bordered by a band of aquatic vegetation, principally wild rice, Zizania aquatica, and bulrushes, Scirpus acutis.

We surveyed for beetles in and along the vegetation on the shoreline, and over the lake surface, either from a paddle-driven canoe or from a boat with a 4-hp outboard motor. On wind-still days when the water surface was smooth, single beetles could be seen from at least 20 m.

Nighttime surveys were made under wind-still conditions when the lake surface was glassy smooth. While anchored approximately

0340-5443/80/0007/0179/$01.60

180 B. Heinrich and F.D. Vogt: Aggregation in Whirligig Beetles

20 m offshore, we observed beetles (by the wake they left on the water surface) passing right or left over a ribbon of light reflected onto the water from a light source on shore. We also surveyed beetle activity along shoreline transects of known distance by traveling at constant speed (40 m/rain). An observer in the bow counted beetles seen by shining a flashlight along the water surface. The area surveyed by this method encompassed approximately 2 m on each side of the path of the boat. The beetles did not appear to be disturbed by the light. Unlike in the daytime, they did not evade the boat, and they did not swim out of the beam of the light.

The numbers of beetles in rafts were estimated by visual inspec- tion. We were probably accurate to within + 10% in absolute counts, although we presume greater accuracy for comparative counts between different rafts. In one instance we photographed a group of beeries (D. horn o before they started moving, after estimating it to contain 250 and 300 beetles, respectively; 295 beetles were subsequently counted on a projection of the Kodachrome slide.

In the field we could with confidence only distinguish between 'large' and 'other ' (medium and small) beetles. Later identification of sample specimens showed that the large beeries were Dineutus horni Roberts and the medium and small ones were a mixture of Gyrinus impressicollis Kirby, G. dichrous LeC., and Gyrinus spp.

Results

The Diurnal Rafts

Leech and Chandler (1956) reported that 'most species are diurnal, but in at least certain unfavorable habitats some become nocturnal.' We do not know what constitutes favorable or 'unfavorable' habitats, but beetles are known to flourish in diverse environ- ments. We observed similar rafts consisting of thousands of quiescent individuals in a small quiet boggy stream in Maine, as well as along large open waters (Lake Itasca) in northern Minnesota. In both habitats the beetles appeared to be quiescent during the daytime, although they became highly active in attempt- ing to escape when approached within several meters.

The beetles on Lake Itasca were almost exclusively aggregated into rafts during the daytime (see Fig. 1). For example, during a survey on August 17 of the whole 22-kin lakeshore, we observed only 11 single

Fig. 1. Part of a mnltispecies aggregation of whirligig beetles along the edge of bulrushes, Scirpus acutis, on Lake Itasca. The raft in the foreground contains approximately 980 beetles

B. Heinrich and F.D. Vogt: Aggregation in Whirligig Beetles 181

Table 1. Approximate numbers of D. horni and other whirligig beetles at 15 of the 36 diurnal aggregations numbered consecutively around Lake Itasca on August 9 and August 17

Aggrega- Distance a D. horni tion no.

Aug. 9

Other

Aug. 17 Aug. 9 Aug. 17

1 0.71 10,500 0 9,000 1.500 2 0.13 1,500 1,000 5,800 9,000 3 0.16 800 400 8,200 10,700 4 400 100 0 0

0.65 5 0.21 4,300 4,300 0 19,200 6 300 0 0 0

1.0 7 0.51 0 0 0 1,100 8 1.10 350 0 200,000 110,000 9 1.11 300 4.100 15,000 13,900

10 0.32 0 0 0 1,400 11 0.71 9,000 0 34,000 2,600 12 0.40 0 0 0 7,600 13 0.21 0 0 11,000 21,000 14 1.63 2,300 0 0 0 15 200 4 0 , 0 0 0 6 0 , 0 0 0 50,000

" Distance: linear dimensions in km along the shoreline between consecutive aggregations along the SW arm of the lake

beetles out of a total estimated 401,300 in 36 aggregations (see Table 1). Total count for D. horni was 45,500 and 57,300 for August 9 and August 17, respectively. The total corresponding counts for the other beetles were 396,000 and 344,000.

Single beetles were invariably swimming actively with net forward motion on the water surface, while the beetles in rafts were quiescent until we came within approximately 4-5 m of them. Individual aggregations contained from approximately 50 to 200,000 beetles. The small aggregations (<500 beetles) tended to be compacted into single rafts, but the large aggregations were generally composed of numerous (up to a dozen or more) separate rafts dispersed along 40-50 m of shoreline (Fig. 1).

Most of the aggregations were found day after day in the same location. We surveyed portions of the lake almost every day for two weeks, and the entire lakeshore on August 9 and August 17. Between these two dates, 67% of all of the aggregations coin- cided in location, but generally not in size (Table 1). The larger aggregations tended to be more permanent. The coincidence of aggregations of more than 500 beetles was 78% between August 9 and 17. However, we did observe one large concentration of approximately 15,000 beetles appear overnight at one site and then disappear the following morning. The five large aggregations in the vicinity of the Biological

Y- tJ

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0 I - I0 I I - 51- I01- 201- 301- 500- 1,001- 2,001- I0,001- 20,001-

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Fig. 2. Frequency of different raft sizes in P. horni (solid bars) and primarily other beetles (open bars). The data were collected during the daylight hours of August 15

Station remained in the same places every day for at least two weeks.

Several species of beetles sere involved in the rafting behavior. We observed rafts of exclusively large beetles (D. homO, rafts of primarily medium-sized beetles (G. impressicoIlis) or small beetles (G. dichrous), and mixed rafts of the three size classes in various proportions (Table 1). In any one area of aggregation, the beetles of various species could be either in separate rafts, or completely intermingled (see Fig. 1). In general the small and medium-sized beetles (these species were not identified in the field) tended to be in separate rafts from the large beetles, D. horni. Rafts of D. horni tended to be small, while the smaller beetles had much larger rafts (Fig. 2).

The species segregation was probably in part due to habitat differences. The small and medium-sized beetles tended to raft outside the vegetation, sometimes 2-3 m out onto the lake surface (where they were exposed to wave action). The D. horni, in contrast, tended to raft in quiet, protected water, as among the vertical stems of bulrushes, or in small sheltered inlets scattered within patches of wild rice. tn some large protected coves, however, D. horni were found many meters from shore, aggregated along with several other species into huge rafts of many thousands. On the other hand, the small and medium-sized beetles were rarely found in the small rafts of D. horni that were relatively 'hidden' in bays of the spreading wild rice.

Although the spatial distribution of the beetle aggregations was relatively little changes between Au- gust 9 and 17, their species composition differed wide- ly (Table 1). In some instances all of the D. horni disappeared from an aggregation, while in other instances large numbers of them appeared in areas that previously contained only members of the other species. Distances between all of the 36 'permanent' and two 'temporary' aggregations around the periphery

182

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1 0 -

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I 2 3 4 5 6 - 1 0 11-20 2 1 - 5 0

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Fig. 3. Frequency of different group sizes of D. horni observed at 23.00-23.30 hours

of the lake averaged 0.59 km (SE=0.02). Excluding the six small (< 500 beetle) aggregations, the mean interaggregation distance was 0.78 km (range=0.21- 2.9 kin). The locations of the aggregations had no apparent relationships to the distribution of the vege- tations; except for the southeast shore of the south- eastern arm of the lake, nearly the entire lakeshore was bordered by a strip of aquatic vegetation.

Nocturnal Activity

All of the beetle species began their foraging activity shortly after sunset, returning to form their rafts just before dawn. Some beetles left the aggregations at night, swimming on the water surface in relatively straight lines along the shoreline at approximately 30 m/min. However, most of the beetles, which dispersed into small groups or moved singly (see Fig. 3), remained within 100 m of their rafts throughout the night. Foraging beetles tended to zig-zag nearly continuously or move in small circles, and although their movements were rapid, they did not travel far. At 21.20 hours, individual beetles (n = 21) at one raft site were moving an average net distance of 1.19 m/min (range = 0.0-4.5). The beetles were apparently not disturbed by our boat, moving within several cm of it and 'instantly' grabbing disabled mosquitoes dropped down to them. In contrast, in the daytime when the undisturbed beetles were stationary, rafts could not be approached within 4-5 m without caus- ing immediate activity.

Although thousands of beetles remained at or near the large diurnal aggregation sites throughout the

B. Heinrich and F.D. Vogt: Aggregation in Whirligig Beetles

6 0

4O

20

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NORTH ~' r E

4 0

6 0 ~ r i t i i ~ 1 r p p 16 18 20 2 2 24 2 4.

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Fig. 4. Numbers of beetles observed passing south away from a diurnal aggregation of approximately 15,000 beetles (100 D. horni, 15,000 other) and north (toward the aggregation) along a N-S shoreline. The observations were made at a point 300 m south of the rafts (except for the cross-hatched bar at 21.35 hours, which represents observations made at 700 m south of the rats). Late evening observations (10 min each) were made against reflections of fading sunlight, and the others (15 min each) were made against a beam of reflected light from shore; 15-min observations at sunset and sunrise revealed not a single beetle

night, others moved relatively long distances from the raft sites along the shoreline after nightfall. For example, 1.5 h after sunset during the night of August 17, we observed (over a 10-rain observation period) 42 beetles moving south, away from the rafts, at a point 300 m south of a large aggregation (Fig. 4). Approximately 10 rain later, 700 m south of the aggregation, only seven beetles were seen going south during a similar duration. There were no beetles moving north toward the aggregation at either the near or the far observation point (Fig. 4). Between midnight and 04.30 hours, the number of beetles moving toward and away from the aggregation was approximately equal. However, at approximately 05.00 hours, 70 rain before sunrise, most of the beetle traffic was toward the diurnal rafting site. The beetles were moving rapidly, maintaining relatively constant direction.

Mark-Recapture Studies

We painted the elytra of D. horni from one aggregation with quick-drying paint (red butyrate dope) and released the marked beetles in mid-lake after dark on the same day (at 21.00 hours) to determine whether they would return to their 'home' rafts at night. A1-

B. Heinrich and F.D. Vogt: Aggregation in Whirligig Beetles 183

22

E

Site of capture (Expt. 1 + 2 ) J 6~ 35 H and re lease(Expt .2} ~ . _ J ~ ~ Site of release

/ _ ~ 7 - - (Expt. I)

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Fig. 5. All of the known aggregations of D. horni (asterisks) along Lake Itasca (August 9-17), and the dispersion of marked D. horni to these aggregations. Experiment 1 : 680 beetles were marked with red paint and released at night (August 10) in mid-lake. Numerals on the outside of the lake boundary indicate the approximate number, and location, of marked beetles seen during the second morning after the release. Experiment 2:690 beetles were captured (August 11) at the same site, marked with white paint, and released that same afternoon back into the aggregation from which they originated. Numerals inside the lake boundary indicate the approximate number, and location, of these marked beetles seen the following morning. The aggregation from which the beetles were captured disappeared after the second night (and the aggregation that is circled appeared the same night) after the beetles for experiment 2 were captured there

though many of these beetles did return to the aggregation of their origin, the majority that we resighted were found with other groups of their own species the following morning and the next day. The paint- marked beetles from the one aggregation were recov- ered at D. horni aggregations occurring over two of the three arms of Lake Itasca after two nights (Fig. 5). No marked beetles joined rafts containing only the other species, and none were observed singly. The oily secretions of the beetles tended to make the paint marks slough off after one or two days, but approximately 117, 64, and 6 marked beetles were seen the first, second, and sixth days after release. (The whole lakeshore was searched only on the second and sixth days.) On the second day, one marked beetle had dispersed to join another aggregation approximately 4 km from the release site (Fig. 5). It is doubtful that the beetles flew: their elytra were usually sealed shut by the paint. Also, of the hundreds of beetles

that we placed into trays during paint-marking, none escaped by flying. As many or more beetles joined D. horni aggregations on the eastern side of the lake as on the west, where they had originated. We conclude that the beetles were not homing to the aggregation of their origin.

It was possible that the wide dispersion of the beetles released at night in mid-lake could have been due to disorientation in new surroundings. We therefore marked another group of 690 D. horni beetles (with white butyrate dope) from the prior capture site and released them back into the same rafts 3 h before sunset. Over the next two days, these beetles dispersed at least as far, and possibly farther, than the first group. They were found in D. horni aggregations at the tips of all three branches of the huge lake (Fig. 5). Interestingly, no D. horni returned to the original aggregation site where we had captured beetles for two days, but new beetle aggregations appeared approximately 1 km south.

Raft Breakup and Aggregation Behavior

We observed the natural breakup and reaggregation of beetles at their rafts at dusk and in the morning, as well as after we had disturbed them in the daytime, in order to gain further insights into the mechanics of group formation and cohesion.

The natural dispersal of rafts after dark was a gradual process. At approaching dusk the beetles began to ' mill' around. The speed of this activity within each raft, as well as raft circumference, increased with increasing darkness. The following account from the evening of August 14 describes the breakup of an aggregation of four adjacent rafts totaling about 15,000 beetles.

The milling, circular movement of the beetles in the rafts began at 20.45 hours, 20 rain after official sunset. At 20.50 hours, milling movement of all of the beetles seemed to be nearly continuous, but the rafts were still intact. At 20.55 hours, the neighboring rafts (see Fig. 1) were intermingled, raft boundaries had become' fuzzy, ' and the first beetles were leaving. At 21.00 hours, the beetles were dispersed over a wide area of approximately 50 m. They approached the sides of the boat from which we were watching. Dead or disabled mosquitoes dropped onto the water were grabbed nearly ' instantly. ' At 21.20 hours, when it was dark, beetles could be seen against reflected light up to 20 m out into the lake, but large numbers of beetles remained near the original raft sites. Some beetles were traveling in relatively straight lines away from the aggregation along the shoreline, moving at about 30 m/rain.

184 B. Heinrich and F.D. Vogt : Aggregation in Whirligig Beetles

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I n=lo

~ n=lo _I n=8

I ~ rl=lO

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Number of bee~les/~jroup

Fig. 6. Net swimming velocities of beetles (primarily C. impressicollis) as a function of group size after disturbance by canoe. Vertical bars represent one standard error on each side of the mean

The same beetle aggregation was observed the following morning before dawn. At 05.05 hours, there were more beetles traveling (at approximately 30 m/ rain) in straight lines toward the rafting sites than leaving (Fig. 4). At 05.30 hours, when dawn was approaching, beetles were still moving erratically in all directions over the water surface near the rafting sites, but the rafts had not yet reformed. At 05.40 hours, as it was getting light, many ' t rains ' of beetles were joining the rafts. The individual beetles were still moving continuously, but rather than moving in erratic circles, their net movement was toward rafting sites. Single beetles that approached the periphery of the aggregation area joined ' trains, ' and trains joined each other. Small groups moving rapidly attached themselves to larger groups that moved more slowly. Beetles became quiescent or milled about in small circles when forming large groups (see Fig. 6). The trains of beetles moved in relatively straight lines, but the individuals within a train tended to zig-zag back and forth while maintaining movement in the forward direction.

Individual beetles (n =20) showed an average net movement of 0.36 m/rain (range =2.8-16 m/rain). At 06.00 hours, when the rafts were beginning to con- dense, the beetles were still milling, as at dusk. The mean number of individuals per train was 10.6 beetles (n = 55, SE = 27, range = 2-50). A few trains of beetles were still returning to the rafts at dawn. Some of these trains were several meters long. At 06.07 hours, 7 min before official sunrise, the beetles were aggregated into one huge raft about 30 m long. At sunrise, the 30-m-long beetle aggregations has contracted in various places to produce a patchwork of several tight rafts, and the beetles had nearly stopped moving.

We disrupted aggregations in the daytime in order to further observe the beetles' behavior. The beetles of a large raft (15,000) left in trains of hundreds after one pass through them with a motorboat, but they

regrouped within 5-10 s. However, after the sixth consecutive pass through them, the trains dispersed up and down the shore, and within 15 rain, long lines of beetles were proceeding up and down the shoreline as far as 1/2 km from their origin. Several small groups had stopped to coalesce along the shore. How- ever, in nine similar trials at different locations, the beetles reconverged within one to several minutes at the same location or several meters from it.

After raft disturbance, individual beetles first appeared to move randomly. However, the dispersed beetles soon began to follow one another and within a few seconds we observed ' t ra ins ' of from two to three dozens of beetles, which coalesced into ribbons of hundreds, that congealed into tight groups of thousands. The velocity of the beetles and the distance traveled was a function of group size (Fig. 6).

Beetle Predation

Lake Itasca contains several fish species in abun- dance, all of which, for at least part of their lives, feed on insects. Gyrinid beetles swimming on the water surface are highly visible. However, the beetles are well known to emit defensive secretions (Benfield 1972; Meinwald et al. 1972). We routinely smelled these secretions near those rafts that we disturbed, as well as in individual beetles that we captured.

We do not know whether the beetles have at one time in their recent evolutionary history suffered heavy predation by fish, but the potential for fish predation seems to be a distinct possibility, at least for beetles that stray from their rafts and contact naive fish. A captive rock bass, Ambloplites rupestris, ' instantly ' grabbed a live beetle dropped into its aquarium. The second beetle presented to it was also swallowed within a minute, but it was then regurgitat- ed. The fish only examined the third beetle presented to it within 5 rain. On the other hand, several pumkin- seed, Lepomis gibbosus, in another aquarium tank, ate five beetles, one after another, as they were released one after another above them. A largemouth bass, Micropterus salmoides, in the aquarium also ate three beetles.

We occasionally observed apparent predation in the field. On ten different occasions, we captured several hundred beetles and released them at sites where there were no rafted beetles. On two occasions, fish ' jumped ' into these released beetles. We also saw a fish ' jump ' directly into schools of beetles that we were dispersing from their original raft site.

It is probable, however, that most fish in the field learn to avoid eating the beetles. We sometimes saw fish directly underneath the large diurnal beetle concentrations, and we did not observe these fish disturb- ing the beetles. No water beetles were found in the

B. Heinrich and F.I2). Vogt: Aggregation in Whirligig Beetles 185

stomachs of the following fish caught in mid-July to early August in Lake Itasca: 49 yellow perch, 20 rock bass, 30 largemouth bass, and 30 pumpkinseed (Michael R. Ross, personal communication).

Discussion

Whirligig beetles are well known to congregate into large floating aggregations, and it was thought that they were diurnal except in ' certain unfavorable habitats' (Leech and Chandler 1956). Lake Itasca appeared to be an ideal habitat, inasmuch as it supported large beetle populations.

We found that the beetles were almost exclusively nocturnal, sometimes foraging long distances from their diurnal rafts where all beetles were quiescent in the daytime. The 'central place' system of the beetles presumably reduces the local food resources, thus increasing the mean locomotion energy required to reach new resources. As discussed by Hamilton and Watt (1970), the aggregation of large numbers of individuals is only advantageous when this provides some benefits unavailable to one nomad or to a group of nomadic individuals. However, the mechanisms of the aggregation behavior indicate, at least in part, some aspects of nomadism, and it becomes necessary to examine the proximal mechanisms before the ultimate.

Mechanisms

The beetles aggregated into tight groups of many thousands in the daytime, while dispersing at night to forage. Since the aggregations were found, for the most part, in the same locations each day, this raises the question of how the beetles reassemble. Beetles leave the aggregations at night as individuals moving independently, but at and before dawn they follow one another, and return to the aggregations in ' trains.' This suggests that the aggregations are creat- ed and maintained primarily by beetles orienting to one another presumably by use of their Johnston's organs (Eggers 1927).

According to his hypothesis, a beetle that has en- gaged in solitary foraging most of the night need only cruise along the shoreline in the morning until it meets up with others (which are likely to be close to a main concentration) and follow them in order to end up in a raft by daylight. We estimated that the beetles cruising along the shoreline had a net forward velocity of 30 m/min. Tucker (1969) calculat- ed swimming speeds of D. eorolinus at 32 m/min, and G. natator has been estimated to swim at 60 m/min in short bursts (Nachtigall 1965). Since the large beetle concentrations were, on the average, 0.78 km apart, it should take a beetle located halfway between

two concentrations cruising at 30 m/min only 13 min to reach a raft.

For this hypothesis to explain why the rafts are found in the same place each day, it is necessary that some beetles remain both night and day at the raft sites. This condition is met; large concentrations of beetles remained at night in the vicinity of the diurnal aggregations.

If aggregations dispersed totally each night, we would predict that the probability of their reaggregat- ing at the same location would be miniscule since the habitat was nearly uniformly suitable for rafting sites. Indeed, we observed that when we succeeded in dispersing all or most of the beetles from any one site, then that site remained free of beetles the next day. Secondly, marked beetles taken from rafts and returned to them dispersed nearly randomly overnight to most or all available other rafts within several km. Thirdly, if all of the beetles of a group 'know' where to return to, then one would predict that group size and species composition at any one aggregation would remain relatively constant. This condition was not fulfilled; at one lecation the beetle numbers could decrease greatly, while at another location the numbers could increase, even though the beetles left their concentrations at night solitarily. From this diverse evidence we conclude therefore that beetles (the well-fed individuals?) remaining at or near the rafts are used as orientation guides to those returning. In the predawn hour, beetles that have been solitary apparently begin to follow other beetles that they meet while cruising along the lakeshore.

We found a net movement of beetles in the direction away from the aggregations in late evening, and a net movement of beetles toward them before dawn (Fig. 4). While these data could indicate homing behavior, they can also be interpreted in terms of passive dispersal and following behavior: At dawn there could be a few beetles moving away from a particular raft under observation (south, in Fig. 4) simply because those beetles traveling in that direction stay at the first beetle aggregations encountered, while none leave it.

Functional Significance

The social and other aggregations of insects serve a wide variety of functions (Wilson 1971 ; Alexander 1974). Aggregations of thousands of beetles may be incidental to localized food availability (e.g., dung beetles, Heinrich und Bartholomew, 1979) and possibly to bring the sexes together for mating (e.g., coc- cinellids, Hagen 1962). Benfield (1972) suggests that by aggregating in groups, gyrinid beetles may 'advertise' their defensive secretions (Meinwald et al. 1972; Newhart and Mumma 1978) en masse.

186 B. Heinrich and F.D. Vogt: Aggregation in Whirligig Beetles

We infer that it is unlikely that the primary significance of the aggregations relates to mating. We observed the beetles in late summer, but the beetles overwinter as adults and mate in early spring (Ben- field 1972). In addition, we at no time observed mating behavior, and formation of the multispecies rafts also argues against the mating hypothesis in these insects.

We speculate that the primary functional significance of the beetle aggregations is defense, probably from fish predation, though not necessarily to advertise the defensive secretions. As has been observed previously (Benfield 1972), we found that naive fish initially eat gyrinid beetles, but learn to reject them, probably because of their defensive secretions (Eisner and Meinwald 1966). Stomach analyses of fish from the field in this study, and in previous ones (Forbes 1888), have generally failed to disclose gyrinids. How- ever, predation pressures on gyrinids could vary, depending on hunger and availability of alternate prey. In addition, fish may forget, and new naive individuals are constantly recruited into the population. Largemouth bass, Micropterus salmoides, for example, will not ordinarily eat gyrinids when brought into the laboratory from a gyrinid-infested pond, but they will eat a few beetles after a long delay when they have presumably forgotten what they had learned (T. Eisner, personal communication). In summary, the beetles' strategy may generally be highly effective, but it is probably not fail-safe. Some individual beetles are sacrificed to predators. The pertinent question then is: Although the probability of being eaten by fish is low, how do the beetles minimize this probability?

Several hypotheses can be suggested for protective functions of aggregating in the daytime. First, a predator being confronted by thousands of randomly moving beetles of similar appearance decreases capture success of any one of these (Ohguchi 1978). How- ever, the beetles are so densely aggregated that a fish could easily capture many with one bite without at- tempting to pursue any one, i.e., it should be easier for fish to capture rafted than single beetles. Second, although large concentrations of beetles could by their combined effort emit large amounts of defensive chemical, this is probably also not the whole, explana- tion for rafting, because fish are repelled even after contacting single beetles. A third, more plausible ex- planation is that beetles that join aggregations are moving into areas where there either are no hungry predators, or where the available predators have al- ready had ample opportunity to catch beetles and to have learned to avoid catching more. The predator 's satiation could also play a role. The larger the aggregation, the less likely the newcomer will be cap-

tured by a naive or a hungry fish. Freshwater teleost, known to restrict themselves to specific leks (Loiselle and Barlow 1978), have the opportunity to be fre- quently ' reminded ' of the beetles' noxiousness, should they forget. By straying from their aggregation in the daytime, beetles risk being eaten or damaged by the visually, oriented surface-feeding predators that either have not previously encountered the beetles, or have forgotten their previous exposure.

Acknowledgements. We are indebted to the Itasca Biological Station for providing the facilities and equipment that made the study possible. Dave Bosensko provided captive fish from Lake Itasca for some of our experiments. We also thank John Doyen for identification of the species. This work was supported by NSF grant DEB77-08430.

References

Alexander RD (1974) The evolution of social behavior. Annu Rev Ecol Syst 5 : 325-383

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Documents

Aggregation and foraging behavior of whirligig beetles (Gyrinidae)