Army Ant (Dorylus sp.) Raid Attendance and Behavior by Myrmecophiles at Different Elevations at Moka, Bioko Island, Equatorial Guinea

18 November 2012

Michelle Carroll

207 Heritage Road, Cherry Hill, NJ 08034

mac396@drexel.edu

Abstract

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For more than a century we have known that many species of insects and other

arthropods live with ants and have developed thriving symbiotic relationships with them.

Occasionally, some species can be found as casual predators or temporary nest

commensals. These ant guests, commonly known as myrmecophiles, include a great

variety of beetles, mites, collembolans, flies, and wasps as well as other less abundant

relatives of most other insect groups (Hölldobler and Wilson 1990). The purpose of this

study was to provide the first database of myrmecophile morphotypes seen within

Dorylus sp. migration and foraging columns on Bioko Island, Equatorial Guinea. An

elevation gradient was set up ranging from 526 to 1942 meters to observe potential

differences in morphotype densities. Over a 10-day period, sixteen samples were

collected. Two samples taken from bivouacs, four from migrating columns, eight from

foraging columns, and two from foraging raids. Fifteen different myrmecophile

morphotypes were found. Morphotypes 12 (Acari sp.), 4 (Melopina sp.) and 5

(Staphilindae sp.) had the highest individual totals respectively. No significant difference

was observed in myrmecophile morphotype presence/absence or density at different

elevations. However, migration columns were only observed in primary and secondary

forests, whereas foraging columns and raids were found in all habitat types, but were

often present in areas with high rates of human disturbance (e.g. residential and

agricultural areas). In general, more myrmecophiles were found in migrating columns,

but further studies need to be conducted to confirm this hypothesis. Future studies should

focus on standardizing the methods used to collect and analyze data.

Introduction

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Ants run much of the terrestrial world as the premier soil turners, channelers of energy,

and dominatrices of the insect fauna – yet many aspects of their biology remain

insufficiently understood (Hölldobler and Wilson 1990). The African driver ants of the

genus Dorylus live in colonies of millions of workers and are most famously known for

their fear-striking raids. Daily, a base column which extends from the nest to the raid,

branches into smaller narrow columns which terminate in groups of advancing workers

on the lookout for social insect colonies. During swarm raids the Dorylus ants sweep

almost all forms of animal life before them, killing insects and larger creatures too

sluggish to get out of the way (Hölldobler and Wilson 1990). The colonies are

surprisingly numerous in many parts of Africa with colonies occurring at a density of

about one colony per 10 hectares in the Guinea savanna of the Ivory Coast and three

colonies per 10 hectares in the nearby forest (Hölldobler and Wilson 1990).

Ants possess a highly complex system of communication, which not only enables

them to gather food and care for young, but also allows them to detect nest and non-

nestmates. However, there are a great deal of arthropods, often referred to as

myrmecophiles that simulate ant mechanical and chemical cues in order to maximize all

of the social benefits from their hosts. Benefits include increased prey availability, self

and brood protection, and a controlled climate within the bivouac. In many cases,

myrmecophiles regularly accompany raiding columns, where they prey on the captured

food of their hosts or the host ants themselves (Kronauer and Pierce 2011).

While there have been several studies about ants in Africa, little if anything at all

is known about army ant raids or their interactions with myrmecophiles on Bioko Island,

Equatorial Guinea. The purpose of this study was to characterize the diversity and

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abundance of myrmecophiles associated with Dorylus ants across an elevation gradient as

well as to describe the interactions between some of the myrmecophiles and their ant

hosts. This project will provide pilot data useful for researchers studying Dorylus ants

and their myrmecophiles on Bioko more comprehensively in the future.

Objective

This project sought to provide preliminary data on myrmecophile diversity, abundance

and interactions with army ants at different elevations on Bioko Island, Equatorial

Guinea.

Hypothesis

The abundance of myrmecophile morphotypes present at raids of Dorylus sp. will differ

with elevation due to changes in the environment.

Methods

Observations of Dorylus were made in the montane forests of Moka, Bioko Island,

Equatorial Guinea at an altitude of approximately 1,300 meters. As the site is located on

a mountain slope it consists of moderately, sometimes steeply rising and falling slopes

with streams. The area directly surrounding the Bioko Biodiversity Protection Program

(BBPP) Moka Wildlife Center (MWC) is an agricultural matrix. About 1000 meters

down the road, high levels of human disturbance are present including two villages

(Moka-Malabo and Moka-Bioko) and a church under construction. Three pre-existing

transects were walked in the areas immediately adjacent to Moka in order to locate

Dorylus activity. Moka-Malabo connects directly to the Lago Biao trail and extends

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upward for 3,100 meters through predominantly secondary forest. The Lago Biao trail

begins in the village (~1,300m) and extends upwards to the rim of the Pico Biao crater

lake (~1,900m) Balacha Sur begins at 1400 meters and extends downward for 3,995

meters through secondary and primary forests. One large road intersects Moka,

connecting the village to various other points on Bioko Island, Equatorial Guinea.

Samples at higher elevations, 1,821 m to 1,942 m, were taken over the course of two days

on the trails near crater lake, Lago Biao. Lago Biao 1 refers to the trail from the Moka-

Riaba Road to the mirador on the rim of the Lago Biao crater. Lago Biao 2 refers to the

trail from the mirador down to the lake edge/outflow point. Primary and secondary

forests as well as residential and agricultural areas were sampled to provide a range of

habitats

Locating and preserving ants

Between November 02, 2012 and November 14, 2012 sixteen samples were collected at

elevations ranging from 526 to 1942 meters from various areas around Moka and Calabo-

Riaba using both marked and unmarked trails. When an ant column or raid was

discovered, the following data were recorded: time, location, weather, elevation,

coordinates and type of forest. Distinction between the methods of travel (foraging or

migrating) was differentiated by the presence of larvae. Unilateral columns traveling with

large amounts of larvae were noted as migrating, while bidirectional columns carrying

food were noted as foraging (Table 1). Using an aspirator, sections of the column were

randomly collected and placed in 100% ethanol for preservation. 50 mL tubes were used

to collect large samples of at least 100 ants, and smaller 1.5 mL tubes were used when

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Table 1: Character Comparison between Migrating and Foraging in Dorylus sp. on Bioko. Island.

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separating myrmecophile morphotypes. Using a numbered morphotype system, the

diversity and density of each morphotype were recorded. Morphotypes were

characterized by size, shape, color, and presence/absence of wings. Myrmecophiles were

found freely traveling with the raid or riding on members of the colony.

Results

Over the 10-day collection period, 16 different columns were sampled across an elevation

gradient ranging from 526 to 1,942 meters. Three samples fewer than 1,000 meters were

taken on the same day along Moka-Riaba road with an elevational difference of about

100 meters between each sample. Due to unpredictable weather and limited collection

time, no samples were collected at elevations between 750 and 1,360 meters. Samples

from 1,360 m and 1,384 m in elevation were taken along the Balacha Sur trail. The

samples taken around the MWC and the base of Lago Biao were collected roughly every

20 meters of elevation gain and range from 1,400 m to 1,591 m. Three different

morphotypes of army ants were discovered: A: Large, Red head/Black Abdomen, B:

Small, Read head/black abdomen, and C: Large, Black head and abdomen.

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Table 2: Myrmecophile morphotypes present in Dorylus sp.

Figure 1: Average encounter elevation per myrmecophile morphotype in Dorylus sp. columns across an elevation gradient in Moka, Bioko Island, Equatorial Guinea. Capped bars show the highest and lowest elevations of each given morphotype. Points without bars were only found at one elevation.

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16400

700

1000

1300

1600

1900

2200

Myrmecophile Morphotype

Ele

vati

on

(m

)

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Migration Column Foraging Column Foraging Raid

Column Structure

Concentrated column 1-2 in. thick.

Canal system in place

Slightly dispersed column 2-4 in. thick.

No canal system in place

Widely dispersed 3-6 m wide. No canal system in

place.Direction of Flow

Unidirectional, only on ground

Bidirectional, only on ground

Bidirectional, can be found on ground

and plantsProtection

LevelHeavily protected Lightly protected Little to no

protection

Presence of Food

No Yes Yes

Presence of Larvae

Yes No No

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Discussion

Agricultural Secondary/Primary Forest0

1

2

3

4

5

6

Migrating Column

Foraging Column/RaidBivouac

Nu

mb

er o

f Sam

ple

s

Figure 3: Location and Quantity of bivouacs, migration columns, and foraging columns/raids.

A-C. Bolded X’s indicate morphotypes found in only one Dorylus sp.

8Dorylus sp. A

(Large: Red Head/Black Abdomen)

Dorylus sp. B(Small: Red Head/Black

Abdomen)

Dorylus sp. C(Large: Black Head/Black

Abdomen)Morphotype1 X X XMorphotype 2 − X −Morphotype 3 X X XMorphotype 4 X X XMorphotype 5 X X XMorphotype 6 X − −Morphotype 7 X X XMorphotype 8 − X −Morphotype 9 X X XMorphotype 10 − − XMorphotype 11 − − XMorphotype 12 X X XMorphotype 13 X −  −Morphotype 14 − − XMorphotype 15 − X  −

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This study was conducted to assess the attendance rate and behavior of myrmecophiles

associated with Dorylus sp. Information collected served as a pilot study of the army ants

and their cohabitants on Bioko Island, Equatorial Guinea. Sixteen samples were collected

across an elevation gradient ranging from 526 to 1942. Marked trails (e.g. Moka Nature

Trail, Balacha Sur and Lago Biao) were primarily used, although several unmarked trails

were used to assess myrmecophile morphotype and density at lower elevations.

Fifteen different myrmecophile morphotypes were found in cohabitation with

three different species of Dorylus. Different species of Beetle (Coleoptera sp.), Fly

(Diptera sp.), arachnid (Arachnida sp.), and mite (Acari sp.) composed the different

morphotypes. Morphotype 1, a Staphilinidae likely of the genus Pella or Dinarda, was

the only myrmecophile large enough to be seen traversing the column with the naked eye.

Morphotype 1 was often seen traveling with the column but frequently lifting its

abdomen to appease its host. The behavior of these two staphylinid species is briefly

described in the following passage; “ Each time [they] encounter ants they flex their

abdomen forward and point the abdominal tip toward the head of their adversaries.

Usually the ants respond by antennating the tip and licking it briefly. This ordinarily

damps the ants’ aggression, allowing the beetle to escape” (Hölldobler and Wilson 1990).

All remaining morphotypes were too small to view their behavior within the colony.

Further analysis revealed that morphotypes 12, 4 and 5 (Acari sp., Diptera sp. and

Staphilinidae sp.) were the most present.

There was no significant difference between myrmecophile morphotypes across

an elevation gradient; however, extreme elevations appeared to have myrmecophile

specialists (Figure 1). At the lowest elevation, 526 meters, only morphotype 4, a phorid

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fly, was present. Morphotype 12, a mite, was the only morphotype encountered at 631

meters, the second lowest elevation and again at the highest elevation sampled, 1942

meters. Further testing is suggested to support or reject these data.

Although no significant difference was found across elevations, changes in

column types did directly correlate to changes in habitat. Emigrations of Dorylus sp. take

several days to complete and are separated by stationary phases that vary in duration from

six days to two or three months (Hölldobler and Wilson 1990). Due to the relatively

stable nature of the colony, Dorylus sp. usually create bivouacs of depths between 1-4

meters in areas far from human disturbance. Both bivouacs were found in the primary

forests near Lago Biao, emphasizing the ants’ high level of colony protection. All

migration columns were found in secondary forests, further illustrating their innately

protective behavior. Foraging columns and raids were found across all terrain types, thus

confirming that high levels of protection are not needed when in search of food. Often,

foraging columns and raids were found in areas of high human disturbance, such as

residential and agricultural matrices, which provide large quantities of easily accessible

food.

In general, myrmecophile abundance was higher in migrating samples, but

diversity was higher in foraging columns/raids. Specialized myrmecophiles are assumed

to be in larger quantities within migrating columns due to their dependency on the ants

for food, shelter and protection. On the other hand, a wider variety of myrmecophiles are

found in foraging columns and raids as opportunists searching for a quick meal. Finally,

myrmecophile densities do not differ significantly across elevations sampled here,

however, there does appear to be specialized morphotypes fully dependent on Dorylus sp.

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at extreme elevations. In addition, changes in habitat directly affect the rate at which

bivouacs and migration or foraging columns are encountered, with bivouacs only seen in

primary forests, migration columns in secondary forests and foraging columns/raids

across all habitats.

Recommendations

Due to the lack of prior research, there was a period of trail-and-error to find the

optimal way to record data. Future studies should focus on standardizing the methods

used to collect and analyze data. For example, time spent at each location should be

equal, to eliminate sample biases. Samples should only be collected randomly, as there

are not many myrmecophiles visible to the naked eye. A sturdy frame coated in baby

powder will help to standardize the sample size and allow for the capture of all castes

present in a certain section of each column or raid. Analysis in the lab should be done

with the same microscope each time (A compound microscope was used in the pilot

study, but a more powerful microscope is suggested for future research). Students should

make sure to bring the following materials from the United States as they are hard to

obtain in Equatorial Guinea: Aspirator, micropipette, and needle-nose tweezers. If

possible, a microscope or camera with a macro setting should be used to photograph

myrmecophile morphotypes. Lastly, a minimum of four samples should be collected each

day to stabilize the data. If possible, surveys should also be conducted at night, as this

seems to be the optimal time for raids.

Acknowledgments

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I would like to thank my professor Drew Cronin for his guidance throughout this

process. I would also like to thank David Montgomery for joining Pastor and I on our

adventures, helping us collect ants and providing photographs for our report. Lastly I

would like to thank Piotr Lukasik as he provided a lot of materials and guidance prior to

coming to Equatorial Guinea.

Appendix 1: Refer to Folder FRTE – Carroll – Assignment 11 and 12:

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Document FRTE – Carroll – Army Ant Data Sheet

Appendix 2: Refer to Folder FRTE – Carroll – Assignment 11 and 12:

Document FRTE – Carroll – Daily Research Log

References Cited

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Hölldobler, B. and Wilson, E.O. 1990. The Ants. 1st edition. Belknap Press of Harvard

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Kistner, D.H., Berghoff, S.M., Maschwitz, U. 2003. Mymecophilous Staphylinidae

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Formicidae) in Malaysia with studies of their behavior. Sociobiology. 41: 208-

265.

Kronauer, D.J & Pierce, N.E. 2011. Myrmecophiles. Current Biology 21: R208

O’Donnell, S. & Kumar, A. 2006. Microclimatic factors associated with elevational

changes in army ant density in tropical montane forest. Ecological Entomology.

31:491-498.

Schoning, C., Njagi, W.M., Franks, N.R. 2005. Temporal and spatial patterns in the

emigrations of the army ant Dorylus (Anomma) molestus in the montane forest of

Mt Kenya. Ecological Entomology. 30:532-540.

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