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Modulation of pheromone trail strength with food quality in Pharaoh's ant, Monomorium pharaonis
ANIMAL BEHAVIOUR, 2007, 74, 463e470 doi:10.1016/j.anbehav.2006.11.027
Modulation of pheromone trail strength with food quality in Pharaoh’s ant, Monomorium pharaonis ˆ LINE †‡ D UN CA N E . J AC KS O N* & N ICO LAS C H A
*Department of Computer Science, University of Sheffield yDepartment of Animal and Plant Sciences, University of Sheffield zLEEC, UMR CNRS 7153, Universite´ Paris 13, 93430 Villetaneuse, France (Received 22 June 2006; initial acceptance 19 September 2006; final acceptance 16 November 2006; published online 22 August 2007; MS. number: 9009R)
Pheromone trails are self-organized processes, where colony-level behaviour emerges from the activity of many individuals responding to local information. The Pharaoh’s ant is an important model species for investigating pheromone trails. Here we show that Pharaoh’s ant foragers mark with trail pheromones, using their stinger, on both the outward and return leg of foraging trips. Examination of trail markings showed that 10.5% of returning fed ants simply made marks by dragging their engorged gaster, because stinger marks were absent. After discounting gaster-dragging hair marks we found that fed ants (42.5%) did not mark significantly more frequently than unfed ants (36.0%). However, we found that trail-marking fed ants marked pheromone trails with a significantly greater intensity, as compared to trail-marking unfed ants, if the food source was high quality (1.0 M sucrose). When the food quality was low (0.01 M sucrose) we detected no significant difference in marking intensity between fed and unfed trail-marking ants. Our results show that in Pharaoh’s ants individual trail marking occurs at a frequency of w40% among fed and unfed foragers, but the frequency of individuals marking with high intensity (continuous marking) is significantly greater when a food source is high quality. This contrasts with another model species, Lasius niger, where trail strength is modulated by an all-or-nothing individual response to food quality. The reason for this fundamental difference in mechanism is that Pharaoh’s ant is highly reliant on pheromone trails for environmental orientation, so must produce trails, whereas L. niger is proficient at visual-based orientation. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Keywords: communication; Monomorium pharaonis; Pharaoh’s ant; pheromone trail; self-organization
The foraging trails of ants are often presented as a paradigm of self-organization (e.g. Camazine et al. 2001). The behavioural feedback system found in foraging trails perfectly illustrates the principle of emergence, where the activities of many agents responding only to local information leads to a global adaptive process. Simple mathematical and computational models have been used to show how adaptive global solutions can arise in such a system, where a trail pheromone provides positive feedback. However, from a biological perspective, models may have led to an oversimplified view of this process. Increasingly research is uncovering great sophistication in the use
Correspondence: D. E. Jackson, Department of Computer Science, University of Sheffield, Regent Court, 211 Portobello Street, Sheffield S1 4DP, U.K. (email: [email protected]field.ac.uk). 0003e 3472/07/$30.00/0
of pheromone communication throughout ant trail networks. For example, most trail-following ants actually use multiple trail pheromones secreted from one or more glands (Witte & Maschwitz 2002; Jackson et al. 2006). Furthermore, behavioural specialization has been found in the response to particular pheromones and in their selective deposition (Jackson et al. 2006; Hart & Jackson 2006). Understanding how pheromone trails work clearly demands a deeper investigation of the behaviour of individual ants throughout the foraging process. The Pharaoh’s ant is an important model species for conducting empirical research into pheromone trails. Foraging trails produced by Pharaoh’s ant form complex branching networks that can extend up to 10 m from the nest (Sudd 1960). The characteristic geometrical structure of trail networks is exploited as an aid in orientation, whilst the long-lived nature of the pheromone used in
463 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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ANIMAL BEHAVIOUR, 74, 3
networks facilitates rapid exploration of the environment from day-to-day (Jackson et al. 2004, 2006). Pheromone trails of Pharaoh’s ant also contain a short-lived component that lasts for only 25 min (Jeanson et al. 2003), which contrasts with the long-lived component that lasts for over 2 days (Jackson et al. 2006). Seven pheromones known to elicit trail following in Pharaoh’s ant have been chemically isolated and synthesized. Monomorines, isolated from the poison glands, have trail following activity at concentrations as low as 108 g/cm, with blends of monomorines I and III being the most active (Ritter et al. 1977a). Faranal, found in trace amounts in the Dufour’s glands (Ritter et al. 1977b), is active at concentration as low as 1012 g/cm, although maximal activity occurs at 109 g/cm (Ritter et al. 1977b; Ho¨lldobler & Wilson 1990). Trail pheromones are deposited on surfaces using the extruded stinger. Both the poison gland and the Dufour’s gland empty through the stinger, but secretions from either gland can be deposited separately (or simultaneously) because both glands possess reservoirs with sphincters to control release (Billen 1987). It is frequently assumed that ants only lay pheromone after discovering food, whilst they are returning to the nest. This is only true of a returning ant that can navigate back to the nest using her individual orientation skills, such as memory of visual landmarks, path integration or a sun compass (Ho¨lldobler & Wilson 1990). Once the successful forager returns to the nest its pheromone trail can then guide recruited ants to the food source. However, many ant species are reliant on pheromone trails for navigation during foraging and are obliged to lay trail pheromone to guide them back to the nest. Pharaoh’s ant is reliant on pheromone trails for orientation in the foraging environment. Foragers on active Pharaoh’s ant trails are known to deposit trail pheromone both when fed and unfed, and when walking to or from the nest (Hart & Jackson 2006). It has been observed that Pharaoh’s ant produces ‘exploratory’ trails in the absence of food, but only after the discovery of food are such trails termed foraging trails, or ‘exploitation’ trails (Fourcassie´ & Deneubourg 1994). This terminology is confusing and suggests that there are qualitative differences between exploratory trails and foraging trails, where none has been shown. A major problem in understanding pheromone trail dynamics is determining the contribution individual ants make to a pheromone trail and how that contribution leads to the colony-level selection of food sources. Previous studies have suggested that fed ants contribute most to trails but these studies have relied on indirect measures of trail deposition, particularly behavioural observation of body postures such as gaster curling that might indicate trail marking (Beckers et al. 1993; Mailleux et al. 2000). We determined the contribution that fed and unfed Pharaoh’s ant foragers made to pheromone trails, but used a direct measure of pheromone deposition. We studied the physical marks made by ants walking on a smoked glass surface (Hangartner 1969; Jackson et al. 2004). Using this direct measure we analysed trail-marking frequency on active foraging trails and the intensity of marking with variable food quality.
METHODS
Study Species Four study colonies of Pharaoh’s ant (Formicidae: Myrmicinae) each contained 1200e2500 workers, brood of all stages and multiple queens (12e50). Pharaoh’s ants are small (ca. 2 mm in length), monomorphic ants and easily maintained in the laboratory. Colonies were housed in wooden nestboxes (11 8 cm and 2 cm high) held within a large plastic foraging box (45 30 cm and 15 cm high) in a climate controlled room (24 2 C, RH ¼ 30%, 12:12 h light:dark cycle). Colonies were given fresh water ad libitum in glass tubes sealed with cotton wool and fed with Tenebrio larvae, sugar syrup, dried liver and dried egg yolk.
Trail-marking Frequency To determine the frequency of pheromone trail marking when walking to and from a feeder, we constrained ants to produce pheromone trails in a narrow corridor placed upon smoked glass (Fig. 1). A sheet of toughened glass (74 28 cm) was held over a wax candle flame so as to coat it with a fine layer of soot. A glass sheet was then placed on top of a clear glass tank. A colony accessed one edge of the smoked glass via a bridge, which led to a corridor (1 cm internal width) made of two polycarbonate strips (60 4 cm and 0.5 cm). The inner surfaces of the corridor were coated with Fluon to prevent ants from climbing them. A syrup feeder was placed at the far end of the corridor. Observations began when colonies had successfully established foraging trails, which we operationally defined as when 20 ants had returned from the feeder. This was to ensure pheromone trails were not heavily marked before we began observations, but also to ensure a continuous trail had been established. In the early stages of the trail establishment process the trails of ants are still very weak, or possibly discontinuous, and trail following behaviour on such unestablished trails may differ from that on strong, established trails. We chose the point after which 20 ants had returned from the feeder because trail traffic was regular at this point, indicating trail establishment. Strong light sources were placed above and below the corridor to facilitate observation of marking by individual ants. Individuals were observed as they walked the full length of the corridor from the feeder to the bridge or vice versa. The corridor usually contained multiple ants, because this was an active trail. Focal ants were randomly selected as they either left the feeder, or stepped off the bridge into the corridor, and they were followed until they either reached the bridge (fed) or feeder (unfed). We observed each ant continually using a 20 magnifying hand lens, which was moved along the top of the corridor as the focal ant walked along. We paid close attention to the smoked glass for any markings made by the focal ant. Any trail marking was recorded (excluding footprint marks). We observed 50 ants walking towards the food source (unfed) and 50 ants returning to the nest from the
ˆ LINE: ANT PHEROMONE TRAIL MARKING JACKSON & CHA
Light source Smoked glass sheet Syrup feeder Foraging box
Bridge
Corridor
Glass tank Light source Nestbox Figure 1. Experimental apparatus used for investigation of pheromone trails on smoked glass. Ants accessed the corridor leading to the sugar syrup feeder, via a bridge from the foraging box. The corridor constrained foraging ants to form a straight trail to the feeder on the smoked glass surface, whilst strong illumination from both above and below facilitated observation of trail marking. For closer examination of trail markings a smoked glass slide (7.5 2.5 cm) was placed lengthways in the centre of the corridor and removed with forceps when an ant stepped onto it. An ant was allowed to walk on the slide until it reached an edge. Fifty fed and 50 unfed ants from each of four colonies were tested.
feeder (fed). This procedure was repeated for four colonies using a fresh sheet of smoked glass for each colony.
Trail Marking We examined the isolated trail markings of individual ants to discriminate between different types of marking and also to measure the intensity of trail marking. We modified the apparatus shown in Fig. 1 and made the corridor 2.5 cm wide. Once colonies had established a foraging trail (>20 ants returned from the 1.0 M sugar syrup feeder) we placed a smoked glass microscopy slide (7.5 2.5 cm) lengthways in the middle of the widened foraging corridor. When an ant stepped on the slide, the slide was carefully removed from the corridor using forceps and elevated to a height of approximately 20 cm. The slide was kept in a horizontal position to minimize disturbance to the ant. We removed the slide to ensure only one ant walked on the slide. We allowed the ant to walk across the slide until it reached an edge at which point the ant was shaken from the slide. This procedure was repeated using four colonies in total, and for each colony we tested 50 unfed ants and 50 fed ants. A fresh slide was used for each ant tested. The same experimental operator always performed the slide manipulation so as to eliminate operator bias. Each slide was suspended above a bright light source and examined with a 20 magnifying lens. Using Hangartner’s (1969) scheme we classified the markings made by individual Pharaoh’s ants. Hangartner (1969) showed that the fire ant, Solenopsis geminata, produces easily classified trail markings on smoked glass, depending on whether the stinger is extended and the degree of contact with the substrate. We also measured the distance marked per unit distance walked upon the slide. Markings were classified as either trail markings (continuous streaks and discontinuous dots); hair markings produced by gaster dragging; or footprints. Combinations of these markings were also observed. Intensity of trail marking was measured as the proportion of the total distance walked on the slide when trail markings were made. To do this we placed transparent plastic over the
back-lit slide and traced the path of the ant onto the plastic with a pen. Some ants made sinuous paths across the slide, so intensity was normalized to give total trail marked as a proportion of total distance walked (trail marking per 1 cm walked). A further measure of intensity, also first noted by Hangartner (1969), is whether ants mark with continuous lines or discontinuous dots. These different markings indicate variability in pressure applied by the stinger to a surface and this determines the amount of pheromone likely to be deposited. We quantified the number of fed and unfed ants that marked continuously or discontinuously on slides.
Trail Marking with Low Quality Food Source We repeated this trail marking investigation procedure with a low quality (0.01 M) sugar syrup feeder using two colonies, each time for 50 fed and 50 unfed ants.
Classification of Trail Markings We produced images of the different markings made by Pharaoh’s ant by allowing a single colony to access a sheet of smoked glass using a bridge. The set-up used was as shown in Fig. 1 but the corridor was omitted, which allowed ants to forage without constraint. The smoked glass sheet was removed after 2 h and cleared of ants by shaking. We then produced an image of trail markings on the glass by scanning it using a Canon Canoscan Lide Scanner, with backlighting from a Canon slide copier illuminator. RESULTS
Trail-marking Frequency Table 1 shows the trail-marking frequency of fed and unfed ants from four test colonies. We found no significant colony effect (two-way ANOVA: F3,392 ¼ 0.32, P ¼ 0.814) on marking frequency, but found a highly significant effect of fed/unfed status (F1,392 ¼ 15.81,
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Table 1. Frequency of pheromone trail marking (þ) by individual ants foraging within a 60 cm corridor placed on a sheet of smoked glass Unfed
Fed
Trail marks Trail marks Trail marks Trail marks þ þ Colony Colony Colony Colony
1 2 3 4
15 17 20 18
35 33 30 32
All colonies 70 (35%) 130 (65%)
27 29 23 30
23 21 27 20
109 (54.5%)
91 (45.5%)
We recorded no trail marking () when only footprints were added to markings already present in the soot. We observed 50 fed ants walking to the nest and 50 unfed ants walking to the food source from each of four colonies.
P < 0.001). There was no significant interaction effect between colony and fed/unfed status (colony*fed/unfed: F3,392 ¼ 0.84, P ¼ 0.471). Overall (colonies pooled) 54.5% of fed ants made trail markings compared to only 35.0% unfed ants marked.
Trail Markings Categories of pheromone trail markings made by Pharaoh’s ant are shown in Fig. 2. Markings made by foragers took the form of discontinuous trails (spots: Fig. 2d) continuous trails (streaks: Fig. 2b, f), footprints (Fig. 2c), hair marks (Fig. 2e) or combinations of these markings. When we examined markings made on unconstrained smoked glass (no corridor) we also found that the regions between trails (Fig. 2a) were marked with discontinuous sections of trail and infrequent spots indicating sparing deposition of trail pheromone. Markings made in trials by four colonies are presented in Table 2. The categories we used for classification of markings were trail markings (continuous and discontinuous stinger markings or combined stinger plus hair markings); hair marks alone; and footprints alone (no trail marking). When hair marks were included as trail markings results were very similar to those shown in Table 1. We found no significant colony effect (two-way ANOVA: F3,392 ¼ 0.19, P ¼ 0.903) on marking frequency, but found a highly significant effect of fed/unfed status (F1,392 ¼ 10.45, P ¼ 0.001). There was no significant interaction effect between colony and fed/unfed status (colony*fed/unfed: F3,392 ¼ 0.35, P ¼ 0.786). However, when we excluded hair marking as trail markings we found there was no significant difference in the trail marking (stinger marks) frequency of fed and unfed ants (F1,392 ¼ 1.75, P ¼ 0.186). We found no significant colony effect (F3,392 ¼ 0.34, P ¼ 0.795) and there was no significant interaction effect between colony and fed/unfed status (colony*fed/unfed: F3,392 ¼ 0.29, P ¼ 0.835). Markings with abdomen hairs (stinger markings absent) were made by 10.5% of all fed ants (colonies pooled) compared to only 1% of all unfed ants (colonies pooled), and were a consequence of fed ants dragging their engorged gaster.
We found a significant difference in intensity of marking between trail-marking fed and unfed ants, using two separate measures. We found that fed ants laid trail (stinger marking) with a slight but significantly greater intensity (as proportion of distance marked) compared to unfed trail-laying ants (Student’s t test: t ¼ 2.06, P ¼ 0.044). Unfed ants made trail markings for 0.64 cm (SD ¼ 0.36) of every 1.0 cm walked, whilst fed ants marked 0.75 cm per 1.0 cm (SD ¼ 0.31). In our second measure of marking intensity we found that trail-marking fed ants made continuous markings on glass slides significantly more frequently (72.9%) than trail-marking unfed (50.0%) ants (chi-square test: c21 ¼ 8:75, N ¼ 157, P ¼ 0.003).
Trail Marking with Low Quality Food Source Table 3 shows results obtained when ants foraged to a 0.01 M feeder. We found no significant effect of fed/ unfed status (two-way ANOVA: F1,196 ¼ 0.33, P ¼ 0.564) on marking frequency. There was no colony effect (F1,196 ¼ 0.75, P ¼ 0.387) and no significant interaction effect between colony and fed/unfed status (colony*fed/ unfed: F1,196 ¼ 0.08, P ¼ 0.773). The intensity of marking as measured in marking per unit distance did not differ significantly between fed (0.68 cm marked per 1 cm, SD ¼ 0.41, N ¼ 40) and unfed (0.60 cm marked per 1 cm, SD ¼ 0.38, N ¼ 36) ants, with the low quality (0.01 M) feeder (Student’s t test: t ¼ 0.879, P ¼ 0.382). In our second analysis of intensity we found that only 55% of trailmarking fed ants (22/40) marked continuously, which was not significantly different to the frequency of continuously marking unfed trail-marking ants (16/36 ¼ 44.4%; chi-square test: c21 ¼ 0:844, N ¼ 76, P ¼ 0.358). The frequency of trail-marking fed ants that continuously marked when a 1.0 M feeder was present (62/ 85 ¼ 72.9%) was significantly greater than when a 0.01 M feeder (22/40 ¼ 55%) was used (chi-square test: c21 ¼ 3:962, N ¼ 125, P ¼ 0.046). We found no difference between the proportions of unfed trail-marking ants that continuously marked with the two different feeder qualities (c21 ¼ 0:297, N ¼ 108, P ¼ 0.586). Our results suggested that w50% of unfed trail-marking ants always marked trails with a consistent intensity, but that trail-marking fed ants increased the intensity of their marking as a function of feeder quality. This increased intensity was evident in an increased proportionate distance for which trail was laid and also in an increased proportion of ants marking continuously, when a 1.0 M feeder was used. DISCUSSION Our data confirm that Pharaoh’s ant foragers mark pheromone trails whilst walking to food and the nest. Initially, we found that the frequency of trail marking was higher in fed ants (54.5%) than unfed ants (35.0%). However, 10.5% of fed ants were making parallel hair markings (Fig. 2e) by dragging their engorged gaster. These hair markings were not accompanied by marking with the stinger, which is the source of all trail pheromones in Pharaoh’s ant
ˆ LINE: ANT PHEROMONE TRAIL MARKING JACKSON & CHA
Figure 2. Markings formed by a Monomorium pharaonis colony foraging on a large (39.2 27.6 cm) sheet of smoked glass, but without the corridor shown in Fig. 1. (a) Discontinuous, spotted markings typical of off-trail regions (bar ¼ 0.25 mm); (b) continuous streaked markings that form a main trail (bar ¼ 1.0 mm); (c) area of intense footprint marking only (bar ¼ 0.20 mm); (d) long section of spotted trail (bar ¼ 0.15 mm); (e) parallel markings made by hairs whilst dragging the abdomen (bar ¼ 0.02 mm); and (f) continuous, unbroken streak typical of main trail (bar ¼ 0.25 mm).
(monomorine trail pheromones were chemically undetectable in hair markings, D.E. Jackson, unpublished data). When hair markings were excluded from the analysis we found fed and unfed ants marked trails with equal frequency. We found a small, but significant, difference in the intensity of trail marking, where trail-marking fed ants laid trail for a greater proportion of the distance they walked on a slide. However, a more significant measure of intensity was that 50% of unfed marking ants laid discontinuous trail markings, but only 17.1% of fed marking ants made such intermittent markings. Fed and unfed Pharaoh’s ant workers laid trail with equal probability on an established trail to a high quality (1.0 M sucrose) feeder, but trail-marking fed ants laid trail with a much
greater intensity. When a low quality food source (0.01 M sucrose) was present fed and unfed ants marked with the same frequency and the same intensity. With a 1.0 M sucrose feeder we found that overall 39.3% of foragers (fed and unfed pooled) made trail markings indicative of pheromone deposition, which closely matches the proportion of ants (43.0%) found to make frequent U-turns on trails in the study of Hart & Jackson (2006). U-turners were shown to be specialized ants with a high probability (88%) of trail marking. In Lasius niger, a species where trail-laying behaviour (not physical marking) has been extensively investigated, trail-laying frequency varies from 34% to 77% on inbound and outbound legs of foraging trips, but the frequency declines as foraging bouts
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Table 2. Intensity and frequency of trail marking by individual ants on smoked glass slides Unfed
Colony Colony Colony Colony
1 2 3 4
Total Mean (%)SD
Fed
Footprints only
Trail marks
Trail marked per cm
Hair marks only
Footprints only
Trail marks
Trail marked per cm
Hair marks only
30 32 34 30
19 17 16 20
0.620.33 0.660.37 0.610.39 0.660.36
1 1 0 0
22 25 22 25
20 19 23 23
0.730.32 0.820.28 0.740.31 0.710.32
8 6 5 2
126 633.82
72 363.65
d 0.640.36
2 11.15
94 473.46
85 42.54.12
d 0.750.31
21 10.55.00
Food source was a 1.0 M sucrose syrup feeder. Fifty fed (returning) and 50 unfed (outbound) foraging ants, from each of four colonies were observed (fed ants: total N ¼ 200; unfed ants: total N ¼ 200). Trail markings were those made either by the stinger alone (continuous streaks, short dashes or dots) or a combination of hair and stinger markings. No trail marking was characterised by footprints alone. Hair markings were those made by abdomen hairs alone and were a consequence of engorged ants dragging their gaster. For trail-laying ants (stinger marking) we measured the proportion of the total distance walked in crossing the slide where trail was laid, as a measure of intensity (distance trail laid per cm walked).
progress (Detrain et al. 2001; Mailleux et al. 2005). It seems unlikely that a similar decline in marking frequency would be found during Pharaoh’s ant foraging bouts. This is because pheromone trails are essential for the orientation of Pharaoh’s ant, when searching for food and transporting food back to the nest. Because of this reliance on trails Pharaoh’s ant forms an ‘exploratory trail’ network whenever new territory is encountered (Fourcassie´ & Deneubourg 1994). Sections of these exploratory trails become active ‘exploitation’ trails once food is discovered. However, this frequently used distinction between ‘exploratory’ and ‘exploitation’ trails is an artificial one and a source of confusion. In every respect all pheromone trails formed by foragers are simply foraging trails, whether they currently lead to food sources or not. Pharaoh’s ant is obliged to maintain pheromone trails throughout foraging persists but L. niger can use visual cues, such as landmarks and polarised light, to navigate between the nest and food (Aron et al. 1993). We suggest that the proportion of individuals engaged in trail marking does not vary in Pharaoh’s ant because pheromone trails are so essential for orientation.
We found that trail marking intensity varied significantly between fed and unfed Pharaoh’s ant foragers with a 1.0 M feeder, but there was no difference in intensity of marking, between fed and unfed ants, when a 0.01 M feeder was present. In L. niger the probability of trail marking in response to a food source varies idiosyncratically between individuals as a function of the food quality (Mailleux et al. 2005). Individuals will only lay trail pheromone if the quality of a food source satisfies, or exceeds, their personal selection criteria (Mailleux et al. 2000). Thus if a food source is of poor quality, fewer individuals will choose to mark and the pheromone trail will be weak. In L. niger this all-or-nothing response is the key component of trail strength modulation (Mailleux et al. 2003). Our results with Pharaoh’s ant are similar to the situation found in the fire ant, Solenopsis geminata, where a constant proportion of individuals mark with trail pheromone, but they can modulate trail strength through variable intensity of deposition. Trail deposition intensity is modulated by varying the pressure of stinger contact with the surface (Hangartner 1969). The result is that either strong, continuous marks or weak, discontinuous
Table 3. Intensity and frequency of trail marking by individual ants on smoked glass slides, where the food source was a 0.01 M sucrose syrup feeder Unfed
Fed
Footprints only
Trail marks
Trail marked per cm
Hair marks only
Footprints only
Trail marks
Trail marked per cm
Hair marks only
Colony 1 Colony 2
28 34
20 16
0.600.38 0.590.38
2 0
27 29
21 19
0.690.38 0.670.43
2 2
Total Mean (%)SD
62 628.5
36 365.7
d 0.600.38
2 22.8
56 562.8
40 402.8
d 0.680.41
4 40
Fifty fed (returning) and 50 unfed (outbound) foraging ants, from each of two colonies were observed (fed ants: total N ¼ 100; unfed ants: total N ¼ 100).
ˆ LINE: ANT PHEROMONE TRAIL MARKING JACKSON & CHA
(spotted) marks are made. The surface area of the stinger making contact with the substrate determines the volume of pheromone deposited. In S. geminata the intensity of individual marking was shown to be modulated with food quality, food distance and volume consumed. As with Pharaoh’s ant, foragers of S. geminata are reliant on trails for orientation. Our results show that trail pheromone must be continually deposited throughout foraging bouts in species that rely on trails for orientation. A variation in trail marking intensity with food quality is found in Pharaoh’s ant, where fed ants marked trail with greatest intensity but this intensity was not different to unfed when a lower quality food source was present. Our results suggest that individual responses to food must be the key to modulating trail strength in Pharaoh’s ant. This contrasts with L. niger, where individual choice as to whether they mark, or not, is most important. The two different methods of modulating trail pheromone concentration have important consequences for the performance of ant colonies when faced with different foraging challenges. But modulation of trail concentration through variation of individual intensity of marking and all-or-nothing individual choice both achieve the same end for the colony; a graded response to food quality. However, for an obligate (or highly reliant) trail-following ant species a modulation of trail marking intensity is the only viable option. But how does this affect foraging performance? Sumpter & Beekman (2003) showed that Pharaoh’s ant selected the better of two food sources in 14 out of 18 trials and suggested that ‘the amount of pheromone that the ants deposit.depends on the strength of the food sources’. We have confirmed this suggestion and shown that the mechanism is quite different from that found in the model species L. niger. Likewise Beckers et al. (1993) found that L. niger selected the best feeder in 12 out of 14 trials. So both strategies perform almost equally when presented with a simple binary choice between two unequal food sources. However, a challenge of greater importance from an ecological viewpoint is the ability to select and concentrate efforts on the best available resource in a changing, competitive environment. In L. niger switching to a better resource is either very slow, or never occurs, when the introduction is after an initial resource has been selected (Beckers et al. 1990). Lasius niger usually becomes ‘stuck’ on an initial poor resource even though a better one is available. In L. niger visual memory of routes play a major part in foraging so switching from one source to another may require more than the decay of a pheromone trail. Sudd (1960) showed that Pharaoh’s ant is slow to redirect workers when better food resources are made available, but colonies can usually do so in about an hour. However, Sudd (1960) was working in a natural environment with trails of up to 10 m in length, compared to trails typically of less than 1 m used in experimental studies of L. niger (Beckers et al. 1990). Sudd (1960) also observed that in Pharaoh’s ant it is commonplace for a single colony to forage on multiple resources simultaneously, using several pheromone trails. A recognized benefit of trail marking in both directions is that the shortest path to food can be selected
without recourse to visual information (Goss et al. 1989). In a binary choice trail set-up, where two different length paths to the same food source, the shortest route can be selected when ants mark in both directions, because for an initial period the shortest branch is marked by ants moving in both directions whilst the longest branch is only marked by ants walking in one direction during that period. Thus the shortest branch accumulates pheromone faster and is selected. The greater the difference in the amount of marking made by ants walking to the food and those returning is, then the more random the overall choice between two different length branches. This mechanism predicts that L. niger will be poor at selecting the shortest of two paths, because it only marks trails after food is found, whereas in fact L. niger performs well. The solution to this apparent paradox is that L. niger can use additional information, such as visual cues, and can identify the most direct route back to the nest (Beckers et al. 1993; Camazine et al. 2001). Lasius niger foragers remember the general direction to the nest and those returning on the longer path can turn around, and then follow the shorter route, when they see that the route they are taking does not lead directly to the nest. However, some individuals also prefer to return to the nest using the same route they followed to the food (Beckers et al. 1992). It is clear that a foraging mechanism which is reliant on pheromones performs just as well as one with recourse to visual memory. Pheromone trails enable rapid mass recruitment to food ¨ lldobler & Wilson 1990) but they also discoveries (Ho impose constraints on the overall foraging strategy of a species. The characteristics of trail pheromones used, particularly their decay rate, impose major constraints on foraging flexibility. For example, if pheromone decay is slow the ability to switch to better food sources is similarly slow. Ant species vary in their reliance on pheromone trails and visual cues for orientation (Aron et al. 1993). A high reliance on pheromone trails requires that trails are marked constitutively as an essential aid for forager navigation between the food and nest. When trails are laid constitutively there is a requirement for a more sophisticated mechanism of redirecting foragers, one that does not rely on trails decaying until they are undetectable. When trails are important for orientation the modulation of trail pheromone concentration enables foragers to select the most rewarding of two or more food sources. As we have shown, this modulation can be achieved by two different means, depending on whether the trail is essential for orientation. In Pharaoh’s ant modulation of trail strength is by a change in the intensity of trail marking by fed workers returning to the nest. Orientation trails will be maintained but where they lead to food the trail pheromone concentration will be increased because of more intense marking by fed ants. Understanding the parameters constraining pheromone trail function provides a better appreciation of the ecological limits of this foraging strategy. In determining key parameters we can also better understand the many subtle variations on the general mechanism of self-organized information sharing using pheromone trails.
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Acknowledgments D.J. wishes to thank BT Exact and the EPSRC for funding this research through the project ‘Simulation of Social Insect Communities’. We also thank Francis Ratnieks for providing facilities and Mike Holcombe for contributing useful ideas. References Aron, S., Beckers, R., Deneubourg, J. L. & Pasteels, J. M. 1993. Memory and mass communication in the orientation of two mass-recruiting ant species. Insectes Sociaux, 40, 369e380. Beckers, R., Deneubourg, J. L., Goss, S. & Pasteels, J. M. 1990. Collective decision-making through food recruitment. Insects Sociaux, 27, 258e267. Beckers, R., Deneubourg, J. L. & Goss, S. 1992. Trails and U-turns in the selection of a path by the ant Lasius niger. Journal of Theoretical Biology, 159, 397e415. Beckers, R., Deneubourg, J. L. & Goss, S. 1993. Modulation of traillaying in the ant Lasius niger and its role in the collective selection of a food source. Journal of Insect Behaviour, 6, 751e759. Billen, J. P. 1987. New structural aspects of the Dufour’s and venom glands in social insects. Naturwissenschaften, 74, 340e341. Camazine, S., Deneubourg, J., Franks, N. R., Sneyd, J., Theraulaz, G. & Bonabeau, E. 2001. Self-organisation in Biological Systems. Princeton, New Jersey: Princeton University Press. Detrain, C., Natan, C. & Deneubourg, J. L. 2001. The influence of the physical environment on the self-organised foraging patterns of ants. Naturwissenschaften, 88, 171e174. Fourcassie´, V. & Deneubourg, J. L. 1994. The dynamics of collective exploration and trail-formation in Monomorium pharaonis: experiments and model. Physiological Entomology, 19, 291e300. Goss, S., Aron, J. L., Deneubourg, J. L. & Pasteels, J. M. 1989. Self-organised shortcuts in the Argentine ant. Naturwissenschaften, 76, 579e581. Hangartner, W. 1969. Structure and variability of the individual odor trail in Solenopsis geminata. Zeitscrift fuer Vergleichende Physiologie, 62, 111e120. Hart, A. & Jackson, D. E. 2006. U-turns on ant pheromone trails. Current Biology, 16, 42e43.
¨ lldobler, B. & Wilson, E. O. 1990. The Ants. Cambridge, MassaHo chusetts: Belknap Press of Harvard University. Jackson, D. E., Holcombe, M. & Ratnieks, F. L. W. 2004. Trail geometry gives polarity to ant foraging networks. Nature, 432, 907e909. Jackson, D. E., Martin, S. J., Holcombe, M. & Ratnieks, F. L. W. 2006. Longevity and detection of persistent foraging trails in Pharaoh’s ants, Monomorium pharaonis (L.). Animal Behaviour, 71, 351e359. Jeanson, R., Ratnieks, F. L. W. & Deneubourg, J. L. 2003. Pheromone trail decay rates on different substrates in the Pharaoh’s ant, Monomorium pharaonis. Physiological Entomology, 28, 192e198. Mailleux, A. C., Deneubourg, J. M. & Detrain, C. 2000. How do ants assess food volume? Animal Behaviour, 59, 1061e1069. Mailleux, A. C., Deneubourg, J. M. & Detrain, C. 2003. Regulation of ants’ foraging to resource productivity. Proceedings of the Royal Society of London, Series B, 270, 1609e1616. Mailleux, A. C., Detrain, C. & Deneubourg, J. M. 2005. Triggering and persistence of trail-laying in foragers of the ant Lasius niger. Journal of Insect Physiology, 51, 297e304. Ritter, F. J., Bruggemann, I. E. M., Persoons, C. J., Talman, E., van Oosten, A. M. & Verwiel, P. E. J. 1977a. Evaluation of social insect pheromones in pest control, with special reference to subterranean termites and Pharaoh’s ants. In: Crop Protection Agents e Their Biological Evaluation (Ed. by N. R. McFarlane), pp. 201e222. London: Academic Press. Ritter, F. J., Bruggemann, I. E. M., Verwiel, P. E. J., Persoons, C. J. & Talman, E. 1977b. Trail pheromone of the Pharaoh’s ant, Monomorium pharaonis: isolation and identification of faranal, a terpenoid related to juvenile hormone II. Tetrahedron Letters, 30, 2617e2618. Sudd, J. H. 1960. The foraging method of the Pharaoh’s ant. Animal Behaviour, 8, 67e75. Sumpter, D. J. T. & Beekman, M. 2003. From nonlinearity to optimality: pheromone trail foraging by ants. Animal Behaviour, 66, 273e280. Witte, V. & Maschwitz, U. 2002. Coordination of raiding and emigration in the ponerine army ant Leptogenys distinguenda: a signal analysis. Journal of Insect Behaviour, 15, 195e217.
Modulation of pheromone trail strength with food quality in Pharaoh’s ant, Monomorium pharaonis ˆ LINE †‡ D UN CA N E . J AC KS O N* & N ICO LAS C H A
*Department of Computer Science, University of Sheffield yDepartment of Animal and Plant Sciences, University of Sheffield zLEEC, UMR CNRS 7153, Universite´ Paris 13, 93430 Villetaneuse, France (Received 22 June 2006; initial acceptance 19 September 2006; final acceptance 16 November 2006; published online 22 August 2007; MS. number: 9009R)
Pheromone trails are self-organized processes, where colony-level behaviour emerges from the activity of many individuals responding to local information. The Pharaoh’s ant is an important model species for investigating pheromone trails. Here we show that Pharaoh’s ant foragers mark with trail pheromones, using their stinger, on both the outward and return leg of foraging trips. Examination of trail markings showed that 10.5% of returning fed ants simply made marks by dragging their engorged gaster, because stinger marks were absent. After discounting gaster-dragging hair marks we found that fed ants (42.5%) did not mark significantly more frequently than unfed ants (36.0%). However, we found that trail-marking fed ants marked pheromone trails with a significantly greater intensity, as compared to trail-marking unfed ants, if the food source was high quality (1.0 M sucrose). When the food quality was low (0.01 M sucrose) we detected no significant difference in marking intensity between fed and unfed trail-marking ants. Our results show that in Pharaoh’s ants individual trail marking occurs at a frequency of w40% among fed and unfed foragers, but the frequency of individuals marking with high intensity (continuous marking) is significantly greater when a food source is high quality. This contrasts with another model species, Lasius niger, where trail strength is modulated by an all-or-nothing individual response to food quality. The reason for this fundamental difference in mechanism is that Pharaoh’s ant is highly reliant on pheromone trails for environmental orientation, so must produce trails, whereas L. niger is proficient at visual-based orientation. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Keywords: communication; Monomorium pharaonis; Pharaoh’s ant; pheromone trail; self-organization
The foraging trails of ants are often presented as a paradigm of self-organization (e.g. Camazine et al. 2001). The behavioural feedback system found in foraging trails perfectly illustrates the principle of emergence, where the activities of many agents responding only to local information leads to a global adaptive process. Simple mathematical and computational models have been used to show how adaptive global solutions can arise in such a system, where a trail pheromone provides positive feedback. However, from a biological perspective, models may have led to an oversimplified view of this process. Increasingly research is uncovering great sophistication in the use
Correspondence: D. E. Jackson, Department of Computer Science, University of Sheffield, Regent Court, 211 Portobello Street, Sheffield S1 4DP, U.K. (email: [email protected]field.ac.uk). 0003e 3472/07/$30.00/0
of pheromone communication throughout ant trail networks. For example, most trail-following ants actually use multiple trail pheromones secreted from one or more glands (Witte & Maschwitz 2002; Jackson et al. 2006). Furthermore, behavioural specialization has been found in the response to particular pheromones and in their selective deposition (Jackson et al. 2006; Hart & Jackson 2006). Understanding how pheromone trails work clearly demands a deeper investigation of the behaviour of individual ants throughout the foraging process. The Pharaoh’s ant is an important model species for conducting empirical research into pheromone trails. Foraging trails produced by Pharaoh’s ant form complex branching networks that can extend up to 10 m from the nest (Sudd 1960). The characteristic geometrical structure of trail networks is exploited as an aid in orientation, whilst the long-lived nature of the pheromone used in
463 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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networks facilitates rapid exploration of the environment from day-to-day (Jackson et al. 2004, 2006). Pheromone trails of Pharaoh’s ant also contain a short-lived component that lasts for only 25 min (Jeanson et al. 2003), which contrasts with the long-lived component that lasts for over 2 days (Jackson et al. 2006). Seven pheromones known to elicit trail following in Pharaoh’s ant have been chemically isolated and synthesized. Monomorines, isolated from the poison glands, have trail following activity at concentrations as low as 108 g/cm, with blends of monomorines I and III being the most active (Ritter et al. 1977a). Faranal, found in trace amounts in the Dufour’s glands (Ritter et al. 1977b), is active at concentration as low as 1012 g/cm, although maximal activity occurs at 109 g/cm (Ritter et al. 1977b; Ho¨lldobler & Wilson 1990). Trail pheromones are deposited on surfaces using the extruded stinger. Both the poison gland and the Dufour’s gland empty through the stinger, but secretions from either gland can be deposited separately (or simultaneously) because both glands possess reservoirs with sphincters to control release (Billen 1987). It is frequently assumed that ants only lay pheromone after discovering food, whilst they are returning to the nest. This is only true of a returning ant that can navigate back to the nest using her individual orientation skills, such as memory of visual landmarks, path integration or a sun compass (Ho¨lldobler & Wilson 1990). Once the successful forager returns to the nest its pheromone trail can then guide recruited ants to the food source. However, many ant species are reliant on pheromone trails for navigation during foraging and are obliged to lay trail pheromone to guide them back to the nest. Pharaoh’s ant is reliant on pheromone trails for orientation in the foraging environment. Foragers on active Pharaoh’s ant trails are known to deposit trail pheromone both when fed and unfed, and when walking to or from the nest (Hart & Jackson 2006). It has been observed that Pharaoh’s ant produces ‘exploratory’ trails in the absence of food, but only after the discovery of food are such trails termed foraging trails, or ‘exploitation’ trails (Fourcassie´ & Deneubourg 1994). This terminology is confusing and suggests that there are qualitative differences between exploratory trails and foraging trails, where none has been shown. A major problem in understanding pheromone trail dynamics is determining the contribution individual ants make to a pheromone trail and how that contribution leads to the colony-level selection of food sources. Previous studies have suggested that fed ants contribute most to trails but these studies have relied on indirect measures of trail deposition, particularly behavioural observation of body postures such as gaster curling that might indicate trail marking (Beckers et al. 1993; Mailleux et al. 2000). We determined the contribution that fed and unfed Pharaoh’s ant foragers made to pheromone trails, but used a direct measure of pheromone deposition. We studied the physical marks made by ants walking on a smoked glass surface (Hangartner 1969; Jackson et al. 2004). Using this direct measure we analysed trail-marking frequency on active foraging trails and the intensity of marking with variable food quality.
METHODS
Study Species Four study colonies of Pharaoh’s ant (Formicidae: Myrmicinae) each contained 1200e2500 workers, brood of all stages and multiple queens (12e50). Pharaoh’s ants are small (ca. 2 mm in length), monomorphic ants and easily maintained in the laboratory. Colonies were housed in wooden nestboxes (11 8 cm and 2 cm high) held within a large plastic foraging box (45 30 cm and 15 cm high) in a climate controlled room (24 2 C, RH ¼ 30%, 12:12 h light:dark cycle). Colonies were given fresh water ad libitum in glass tubes sealed with cotton wool and fed with Tenebrio larvae, sugar syrup, dried liver and dried egg yolk.
Trail-marking Frequency To determine the frequency of pheromone trail marking when walking to and from a feeder, we constrained ants to produce pheromone trails in a narrow corridor placed upon smoked glass (Fig. 1). A sheet of toughened glass (74 28 cm) was held over a wax candle flame so as to coat it with a fine layer of soot. A glass sheet was then placed on top of a clear glass tank. A colony accessed one edge of the smoked glass via a bridge, which led to a corridor (1 cm internal width) made of two polycarbonate strips (60 4 cm and 0.5 cm). The inner surfaces of the corridor were coated with Fluon to prevent ants from climbing them. A syrup feeder was placed at the far end of the corridor. Observations began when colonies had successfully established foraging trails, which we operationally defined as when 20 ants had returned from the feeder. This was to ensure pheromone trails were not heavily marked before we began observations, but also to ensure a continuous trail had been established. In the early stages of the trail establishment process the trails of ants are still very weak, or possibly discontinuous, and trail following behaviour on such unestablished trails may differ from that on strong, established trails. We chose the point after which 20 ants had returned from the feeder because trail traffic was regular at this point, indicating trail establishment. Strong light sources were placed above and below the corridor to facilitate observation of marking by individual ants. Individuals were observed as they walked the full length of the corridor from the feeder to the bridge or vice versa. The corridor usually contained multiple ants, because this was an active trail. Focal ants were randomly selected as they either left the feeder, or stepped off the bridge into the corridor, and they were followed until they either reached the bridge (fed) or feeder (unfed). We observed each ant continually using a 20 magnifying hand lens, which was moved along the top of the corridor as the focal ant walked along. We paid close attention to the smoked glass for any markings made by the focal ant. Any trail marking was recorded (excluding footprint marks). We observed 50 ants walking towards the food source (unfed) and 50 ants returning to the nest from the
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Light source Smoked glass sheet Syrup feeder Foraging box
Bridge
Corridor
Glass tank Light source Nestbox Figure 1. Experimental apparatus used for investigation of pheromone trails on smoked glass. Ants accessed the corridor leading to the sugar syrup feeder, via a bridge from the foraging box. The corridor constrained foraging ants to form a straight trail to the feeder on the smoked glass surface, whilst strong illumination from both above and below facilitated observation of trail marking. For closer examination of trail markings a smoked glass slide (7.5 2.5 cm) was placed lengthways in the centre of the corridor and removed with forceps when an ant stepped onto it. An ant was allowed to walk on the slide until it reached an edge. Fifty fed and 50 unfed ants from each of four colonies were tested.
feeder (fed). This procedure was repeated for four colonies using a fresh sheet of smoked glass for each colony.
Trail Marking We examined the isolated trail markings of individual ants to discriminate between different types of marking and also to measure the intensity of trail marking. We modified the apparatus shown in Fig. 1 and made the corridor 2.5 cm wide. Once colonies had established a foraging trail (>20 ants returned from the 1.0 M sugar syrup feeder) we placed a smoked glass microscopy slide (7.5 2.5 cm) lengthways in the middle of the widened foraging corridor. When an ant stepped on the slide, the slide was carefully removed from the corridor using forceps and elevated to a height of approximately 20 cm. The slide was kept in a horizontal position to minimize disturbance to the ant. We removed the slide to ensure only one ant walked on the slide. We allowed the ant to walk across the slide until it reached an edge at which point the ant was shaken from the slide. This procedure was repeated using four colonies in total, and for each colony we tested 50 unfed ants and 50 fed ants. A fresh slide was used for each ant tested. The same experimental operator always performed the slide manipulation so as to eliminate operator bias. Each slide was suspended above a bright light source and examined with a 20 magnifying lens. Using Hangartner’s (1969) scheme we classified the markings made by individual Pharaoh’s ants. Hangartner (1969) showed that the fire ant, Solenopsis geminata, produces easily classified trail markings on smoked glass, depending on whether the stinger is extended and the degree of contact with the substrate. We also measured the distance marked per unit distance walked upon the slide. Markings were classified as either trail markings (continuous streaks and discontinuous dots); hair markings produced by gaster dragging; or footprints. Combinations of these markings were also observed. Intensity of trail marking was measured as the proportion of the total distance walked on the slide when trail markings were made. To do this we placed transparent plastic over the
back-lit slide and traced the path of the ant onto the plastic with a pen. Some ants made sinuous paths across the slide, so intensity was normalized to give total trail marked as a proportion of total distance walked (trail marking per 1 cm walked). A further measure of intensity, also first noted by Hangartner (1969), is whether ants mark with continuous lines or discontinuous dots. These different markings indicate variability in pressure applied by the stinger to a surface and this determines the amount of pheromone likely to be deposited. We quantified the number of fed and unfed ants that marked continuously or discontinuously on slides.
Trail Marking with Low Quality Food Source We repeated this trail marking investigation procedure with a low quality (0.01 M) sugar syrup feeder using two colonies, each time for 50 fed and 50 unfed ants.
Classification of Trail Markings We produced images of the different markings made by Pharaoh’s ant by allowing a single colony to access a sheet of smoked glass using a bridge. The set-up used was as shown in Fig. 1 but the corridor was omitted, which allowed ants to forage without constraint. The smoked glass sheet was removed after 2 h and cleared of ants by shaking. We then produced an image of trail markings on the glass by scanning it using a Canon Canoscan Lide Scanner, with backlighting from a Canon slide copier illuminator. RESULTS
Trail-marking Frequency Table 1 shows the trail-marking frequency of fed and unfed ants from four test colonies. We found no significant colony effect (two-way ANOVA: F3,392 ¼ 0.32, P ¼ 0.814) on marking frequency, but found a highly significant effect of fed/unfed status (F1,392 ¼ 15.81,
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Table 1. Frequency of pheromone trail marking (þ) by individual ants foraging within a 60 cm corridor placed on a sheet of smoked glass Unfed
Fed
Trail marks Trail marks Trail marks Trail marks þ þ Colony Colony Colony Colony
1 2 3 4
15 17 20 18
35 33 30 32
All colonies 70 (35%) 130 (65%)
27 29 23 30
23 21 27 20
109 (54.5%)
91 (45.5%)
We recorded no trail marking () when only footprints were added to markings already present in the soot. We observed 50 fed ants walking to the nest and 50 unfed ants walking to the food source from each of four colonies.
P < 0.001). There was no significant interaction effect between colony and fed/unfed status (colony*fed/unfed: F3,392 ¼ 0.84, P ¼ 0.471). Overall (colonies pooled) 54.5% of fed ants made trail markings compared to only 35.0% unfed ants marked.
Trail Markings Categories of pheromone trail markings made by Pharaoh’s ant are shown in Fig. 2. Markings made by foragers took the form of discontinuous trails (spots: Fig. 2d) continuous trails (streaks: Fig. 2b, f), footprints (Fig. 2c), hair marks (Fig. 2e) or combinations of these markings. When we examined markings made on unconstrained smoked glass (no corridor) we also found that the regions between trails (Fig. 2a) were marked with discontinuous sections of trail and infrequent spots indicating sparing deposition of trail pheromone. Markings made in trials by four colonies are presented in Table 2. The categories we used for classification of markings were trail markings (continuous and discontinuous stinger markings or combined stinger plus hair markings); hair marks alone; and footprints alone (no trail marking). When hair marks were included as trail markings results were very similar to those shown in Table 1. We found no significant colony effect (two-way ANOVA: F3,392 ¼ 0.19, P ¼ 0.903) on marking frequency, but found a highly significant effect of fed/unfed status (F1,392 ¼ 10.45, P ¼ 0.001). There was no significant interaction effect between colony and fed/unfed status (colony*fed/unfed: F3,392 ¼ 0.35, P ¼ 0.786). However, when we excluded hair marking as trail markings we found there was no significant difference in the trail marking (stinger marks) frequency of fed and unfed ants (F1,392 ¼ 1.75, P ¼ 0.186). We found no significant colony effect (F3,392 ¼ 0.34, P ¼ 0.795) and there was no significant interaction effect between colony and fed/unfed status (colony*fed/unfed: F3,392 ¼ 0.29, P ¼ 0.835). Markings with abdomen hairs (stinger markings absent) were made by 10.5% of all fed ants (colonies pooled) compared to only 1% of all unfed ants (colonies pooled), and were a consequence of fed ants dragging their engorged gaster.
We found a significant difference in intensity of marking between trail-marking fed and unfed ants, using two separate measures. We found that fed ants laid trail (stinger marking) with a slight but significantly greater intensity (as proportion of distance marked) compared to unfed trail-laying ants (Student’s t test: t ¼ 2.06, P ¼ 0.044). Unfed ants made trail markings for 0.64 cm (SD ¼ 0.36) of every 1.0 cm walked, whilst fed ants marked 0.75 cm per 1.0 cm (SD ¼ 0.31). In our second measure of marking intensity we found that trail-marking fed ants made continuous markings on glass slides significantly more frequently (72.9%) than trail-marking unfed (50.0%) ants (chi-square test: c21 ¼ 8:75, N ¼ 157, P ¼ 0.003).
Trail Marking with Low Quality Food Source Table 3 shows results obtained when ants foraged to a 0.01 M feeder. We found no significant effect of fed/ unfed status (two-way ANOVA: F1,196 ¼ 0.33, P ¼ 0.564) on marking frequency. There was no colony effect (F1,196 ¼ 0.75, P ¼ 0.387) and no significant interaction effect between colony and fed/unfed status (colony*fed/ unfed: F1,196 ¼ 0.08, P ¼ 0.773). The intensity of marking as measured in marking per unit distance did not differ significantly between fed (0.68 cm marked per 1 cm, SD ¼ 0.41, N ¼ 40) and unfed (0.60 cm marked per 1 cm, SD ¼ 0.38, N ¼ 36) ants, with the low quality (0.01 M) feeder (Student’s t test: t ¼ 0.879, P ¼ 0.382). In our second analysis of intensity we found that only 55% of trailmarking fed ants (22/40) marked continuously, which was not significantly different to the frequency of continuously marking unfed trail-marking ants (16/36 ¼ 44.4%; chi-square test: c21 ¼ 0:844, N ¼ 76, P ¼ 0.358). The frequency of trail-marking fed ants that continuously marked when a 1.0 M feeder was present (62/ 85 ¼ 72.9%) was significantly greater than when a 0.01 M feeder (22/40 ¼ 55%) was used (chi-square test: c21 ¼ 3:962, N ¼ 125, P ¼ 0.046). We found no difference between the proportions of unfed trail-marking ants that continuously marked with the two different feeder qualities (c21 ¼ 0:297, N ¼ 108, P ¼ 0.586). Our results suggested that w50% of unfed trail-marking ants always marked trails with a consistent intensity, but that trail-marking fed ants increased the intensity of their marking as a function of feeder quality. This increased intensity was evident in an increased proportionate distance for which trail was laid and also in an increased proportion of ants marking continuously, when a 1.0 M feeder was used. DISCUSSION Our data confirm that Pharaoh’s ant foragers mark pheromone trails whilst walking to food and the nest. Initially, we found that the frequency of trail marking was higher in fed ants (54.5%) than unfed ants (35.0%). However, 10.5% of fed ants were making parallel hair markings (Fig. 2e) by dragging their engorged gaster. These hair markings were not accompanied by marking with the stinger, which is the source of all trail pheromones in Pharaoh’s ant
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Figure 2. Markings formed by a Monomorium pharaonis colony foraging on a large (39.2 27.6 cm) sheet of smoked glass, but without the corridor shown in Fig. 1. (a) Discontinuous, spotted markings typical of off-trail regions (bar ¼ 0.25 mm); (b) continuous streaked markings that form a main trail (bar ¼ 1.0 mm); (c) area of intense footprint marking only (bar ¼ 0.20 mm); (d) long section of spotted trail (bar ¼ 0.15 mm); (e) parallel markings made by hairs whilst dragging the abdomen (bar ¼ 0.02 mm); and (f) continuous, unbroken streak typical of main trail (bar ¼ 0.25 mm).
(monomorine trail pheromones were chemically undetectable in hair markings, D.E. Jackson, unpublished data). When hair markings were excluded from the analysis we found fed and unfed ants marked trails with equal frequency. We found a small, but significant, difference in the intensity of trail marking, where trail-marking fed ants laid trail for a greater proportion of the distance they walked on a slide. However, a more significant measure of intensity was that 50% of unfed marking ants laid discontinuous trail markings, but only 17.1% of fed marking ants made such intermittent markings. Fed and unfed Pharaoh’s ant workers laid trail with equal probability on an established trail to a high quality (1.0 M sucrose) feeder, but trail-marking fed ants laid trail with a much
greater intensity. When a low quality food source (0.01 M sucrose) was present fed and unfed ants marked with the same frequency and the same intensity. With a 1.0 M sucrose feeder we found that overall 39.3% of foragers (fed and unfed pooled) made trail markings indicative of pheromone deposition, which closely matches the proportion of ants (43.0%) found to make frequent U-turns on trails in the study of Hart & Jackson (2006). U-turners were shown to be specialized ants with a high probability (88%) of trail marking. In Lasius niger, a species where trail-laying behaviour (not physical marking) has been extensively investigated, trail-laying frequency varies from 34% to 77% on inbound and outbound legs of foraging trips, but the frequency declines as foraging bouts
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Table 2. Intensity and frequency of trail marking by individual ants on smoked glass slides Unfed
Colony Colony Colony Colony
1 2 3 4
Total Mean (%)SD
Fed
Footprints only
Trail marks
Trail marked per cm
Hair marks only
Footprints only
Trail marks
Trail marked per cm
Hair marks only
30 32 34 30
19 17 16 20
0.620.33 0.660.37 0.610.39 0.660.36
1 1 0 0
22 25 22 25
20 19 23 23
0.730.32 0.820.28 0.740.31 0.710.32
8 6 5 2
126 633.82
72 363.65
d 0.640.36
2 11.15
94 473.46
85 42.54.12
d 0.750.31
21 10.55.00
Food source was a 1.0 M sucrose syrup feeder. Fifty fed (returning) and 50 unfed (outbound) foraging ants, from each of four colonies were observed (fed ants: total N ¼ 200; unfed ants: total N ¼ 200). Trail markings were those made either by the stinger alone (continuous streaks, short dashes or dots) or a combination of hair and stinger markings. No trail marking was characterised by footprints alone. Hair markings were those made by abdomen hairs alone and were a consequence of engorged ants dragging their gaster. For trail-laying ants (stinger marking) we measured the proportion of the total distance walked in crossing the slide where trail was laid, as a measure of intensity (distance trail laid per cm walked).
progress (Detrain et al. 2001; Mailleux et al. 2005). It seems unlikely that a similar decline in marking frequency would be found during Pharaoh’s ant foraging bouts. This is because pheromone trails are essential for the orientation of Pharaoh’s ant, when searching for food and transporting food back to the nest. Because of this reliance on trails Pharaoh’s ant forms an ‘exploratory trail’ network whenever new territory is encountered (Fourcassie´ & Deneubourg 1994). Sections of these exploratory trails become active ‘exploitation’ trails once food is discovered. However, this frequently used distinction between ‘exploratory’ and ‘exploitation’ trails is an artificial one and a source of confusion. In every respect all pheromone trails formed by foragers are simply foraging trails, whether they currently lead to food sources or not. Pharaoh’s ant is obliged to maintain pheromone trails throughout foraging persists but L. niger can use visual cues, such as landmarks and polarised light, to navigate between the nest and food (Aron et al. 1993). We suggest that the proportion of individuals engaged in trail marking does not vary in Pharaoh’s ant because pheromone trails are so essential for orientation.
We found that trail marking intensity varied significantly between fed and unfed Pharaoh’s ant foragers with a 1.0 M feeder, but there was no difference in intensity of marking, between fed and unfed ants, when a 0.01 M feeder was present. In L. niger the probability of trail marking in response to a food source varies idiosyncratically between individuals as a function of the food quality (Mailleux et al. 2005). Individuals will only lay trail pheromone if the quality of a food source satisfies, or exceeds, their personal selection criteria (Mailleux et al. 2000). Thus if a food source is of poor quality, fewer individuals will choose to mark and the pheromone trail will be weak. In L. niger this all-or-nothing response is the key component of trail strength modulation (Mailleux et al. 2003). Our results with Pharaoh’s ant are similar to the situation found in the fire ant, Solenopsis geminata, where a constant proportion of individuals mark with trail pheromone, but they can modulate trail strength through variable intensity of deposition. Trail deposition intensity is modulated by varying the pressure of stinger contact with the surface (Hangartner 1969). The result is that either strong, continuous marks or weak, discontinuous
Table 3. Intensity and frequency of trail marking by individual ants on smoked glass slides, where the food source was a 0.01 M sucrose syrup feeder Unfed
Fed
Footprints only
Trail marks
Trail marked per cm
Hair marks only
Footprints only
Trail marks
Trail marked per cm
Hair marks only
Colony 1 Colony 2
28 34
20 16
0.600.38 0.590.38
2 0
27 29
21 19
0.690.38 0.670.43
2 2
Total Mean (%)SD
62 628.5
36 365.7
d 0.600.38
2 22.8
56 562.8
40 402.8
d 0.680.41
4 40
Fifty fed (returning) and 50 unfed (outbound) foraging ants, from each of two colonies were observed (fed ants: total N ¼ 100; unfed ants: total N ¼ 100).
ˆ LINE: ANT PHEROMONE TRAIL MARKING JACKSON & CHA
(spotted) marks are made. The surface area of the stinger making contact with the substrate determines the volume of pheromone deposited. In S. geminata the intensity of individual marking was shown to be modulated with food quality, food distance and volume consumed. As with Pharaoh’s ant, foragers of S. geminata are reliant on trails for orientation. Our results show that trail pheromone must be continually deposited throughout foraging bouts in species that rely on trails for orientation. A variation in trail marking intensity with food quality is found in Pharaoh’s ant, where fed ants marked trail with greatest intensity but this intensity was not different to unfed when a lower quality food source was present. Our results suggest that individual responses to food must be the key to modulating trail strength in Pharaoh’s ant. This contrasts with L. niger, where individual choice as to whether they mark, or not, is most important. The two different methods of modulating trail pheromone concentration have important consequences for the performance of ant colonies when faced with different foraging challenges. But modulation of trail concentration through variation of individual intensity of marking and all-or-nothing individual choice both achieve the same end for the colony; a graded response to food quality. However, for an obligate (or highly reliant) trail-following ant species a modulation of trail marking intensity is the only viable option. But how does this affect foraging performance? Sumpter & Beekman (2003) showed that Pharaoh’s ant selected the better of two food sources in 14 out of 18 trials and suggested that ‘the amount of pheromone that the ants deposit.depends on the strength of the food sources’. We have confirmed this suggestion and shown that the mechanism is quite different from that found in the model species L. niger. Likewise Beckers et al. (1993) found that L. niger selected the best feeder in 12 out of 14 trials. So both strategies perform almost equally when presented with a simple binary choice between two unequal food sources. However, a challenge of greater importance from an ecological viewpoint is the ability to select and concentrate efforts on the best available resource in a changing, competitive environment. In L. niger switching to a better resource is either very slow, or never occurs, when the introduction is after an initial resource has been selected (Beckers et al. 1990). Lasius niger usually becomes ‘stuck’ on an initial poor resource even though a better one is available. In L. niger visual memory of routes play a major part in foraging so switching from one source to another may require more than the decay of a pheromone trail. Sudd (1960) showed that Pharaoh’s ant is slow to redirect workers when better food resources are made available, but colonies can usually do so in about an hour. However, Sudd (1960) was working in a natural environment with trails of up to 10 m in length, compared to trails typically of less than 1 m used in experimental studies of L. niger (Beckers et al. 1990). Sudd (1960) also observed that in Pharaoh’s ant it is commonplace for a single colony to forage on multiple resources simultaneously, using several pheromone trails. A recognized benefit of trail marking in both directions is that the shortest path to food can be selected
without recourse to visual information (Goss et al. 1989). In a binary choice trail set-up, where two different length paths to the same food source, the shortest route can be selected when ants mark in both directions, because for an initial period the shortest branch is marked by ants moving in both directions whilst the longest branch is only marked by ants walking in one direction during that period. Thus the shortest branch accumulates pheromone faster and is selected. The greater the difference in the amount of marking made by ants walking to the food and those returning is, then the more random the overall choice between two different length branches. This mechanism predicts that L. niger will be poor at selecting the shortest of two paths, because it only marks trails after food is found, whereas in fact L. niger performs well. The solution to this apparent paradox is that L. niger can use additional information, such as visual cues, and can identify the most direct route back to the nest (Beckers et al. 1993; Camazine et al. 2001). Lasius niger foragers remember the general direction to the nest and those returning on the longer path can turn around, and then follow the shorter route, when they see that the route they are taking does not lead directly to the nest. However, some individuals also prefer to return to the nest using the same route they followed to the food (Beckers et al. 1992). It is clear that a foraging mechanism which is reliant on pheromones performs just as well as one with recourse to visual memory. Pheromone trails enable rapid mass recruitment to food ¨ lldobler & Wilson 1990) but they also discoveries (Ho impose constraints on the overall foraging strategy of a species. The characteristics of trail pheromones used, particularly their decay rate, impose major constraints on foraging flexibility. For example, if pheromone decay is slow the ability to switch to better food sources is similarly slow. Ant species vary in their reliance on pheromone trails and visual cues for orientation (Aron et al. 1993). A high reliance on pheromone trails requires that trails are marked constitutively as an essential aid for forager navigation between the food and nest. When trails are laid constitutively there is a requirement for a more sophisticated mechanism of redirecting foragers, one that does not rely on trails decaying until they are undetectable. When trails are important for orientation the modulation of trail pheromone concentration enables foragers to select the most rewarding of two or more food sources. As we have shown, this modulation can be achieved by two different means, depending on whether the trail is essential for orientation. In Pharaoh’s ant modulation of trail strength is by a change in the intensity of trail marking by fed workers returning to the nest. Orientation trails will be maintained but where they lead to food the trail pheromone concentration will be increased because of more intense marking by fed ants. Understanding the parameters constraining pheromone trail function provides a better appreciation of the ecological limits of this foraging strategy. In determining key parameters we can also better understand the many subtle variations on the general mechanism of self-organized information sharing using pheromone trails.
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Acknowledgments D.J. wishes to thank BT Exact and the EPSRC for funding this research through the project ‘Simulation of Social Insect Communities’. We also thank Francis Ratnieks for providing facilities and Mike Holcombe for contributing useful ideas. References Aron, S., Beckers, R., Deneubourg, J. L. & Pasteels, J. M. 1993. Memory and mass communication in the orientation of two mass-recruiting ant species. Insectes Sociaux, 40, 369e380. Beckers, R., Deneubourg, J. L., Goss, S. & Pasteels, J. M. 1990. Collective decision-making through food recruitment. Insects Sociaux, 27, 258e267. Beckers, R., Deneubourg, J. L. & Goss, S. 1992. Trails and U-turns in the selection of a path by the ant Lasius niger. Journal of Theoretical Biology, 159, 397e415. Beckers, R., Deneubourg, J. L. & Goss, S. 1993. Modulation of traillaying in the ant Lasius niger and its role in the collective selection of a food source. Journal of Insect Behaviour, 6, 751e759. Billen, J. P. 1987. New structural aspects of the Dufour’s and venom glands in social insects. Naturwissenschaften, 74, 340e341. Camazine, S., Deneubourg, J., Franks, N. R., Sneyd, J., Theraulaz, G. & Bonabeau, E. 2001. Self-organisation in Biological Systems. Princeton, New Jersey: Princeton University Press. Detrain, C., Natan, C. & Deneubourg, J. L. 2001. The influence of the physical environment on the self-organised foraging patterns of ants. Naturwissenschaften, 88, 171e174. Fourcassie´, V. & Deneubourg, J. L. 1994. The dynamics of collective exploration and trail-formation in Monomorium pharaonis: experiments and model. Physiological Entomology, 19, 291e300. Goss, S., Aron, J. L., Deneubourg, J. L. & Pasteels, J. M. 1989. Self-organised shortcuts in the Argentine ant. Naturwissenschaften, 76, 579e581. Hangartner, W. 1969. Structure and variability of the individual odor trail in Solenopsis geminata. Zeitscrift fuer Vergleichende Physiologie, 62, 111e120. Hart, A. & Jackson, D. E. 2006. U-turns on ant pheromone trails. Current Biology, 16, 42e43.
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