Robert Droual and Howard Topoff.
The Emigration Behavior of Two Species of the Genus Pheidole (Hymenoptera: Formicidae).
Psyche 88:135-150, 1981.

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THE EMIGRATION BEHAVIOR OF TWO SPECIES OF THE GENUS PHEIDOLE (FORMICIDAE: MYRMICINAE). Colony emigrations are common among ants (Wilson 1971) and occur for a diversity of reasons. However, except for the legionary ants,
in which colony emigrations are an inherent part of the foraging ecology (Wilson 1971), and species which inhabit delicate and easily disturbed nests (Holldobler and Wilson 1977, Moglich 1979), emigrations are thought to occur infrequently. Here we present evidence that two species of the genus Pheidole, P. deser- torum Wheeler and P. hyatti Emery, emigrate frequently under environmentally stable conditions. We further advance the hy- pothesis that the surplus nests resulting from these emigrations, reduce the secondary losses which occur as a consequence of the panic-alarm defense these species employ against army ants of the genus Neivamyrmex, by serving as temporary shelters and centers for colony reorganization.
This investigation was conducted during the months of June, July and August, 1980, at two different study sites. One site was an oak- juniper woodland located on the grounds of the Southwestern Research Station of the American Museum of Natural History near Portal, Arizona (elev. 1646 m). The other site was a desert-grassland located 8 km N.W. of Rodeo, Hidalgo Co., New Mexico (elev. 1250 m). In both habitats a winter (Dec., Jan., Feb. and March) and a summer (July, June and August) rainy season occur. On the oak- juniper woodland site colonies of both P. desertorum and P. hyatti were located and marked; on the desert-grassland site only colonies of P. desertorum were located and marked. å´Biolog Program, City College of C.U.N.Y., New York, N.Y. 10031 2Psychology Department, Hunter College of C.U.N.Y., New York, N.Y. 10021 and Department of Entomology, The American Museum of Natural History, New York, N.Y. 10024
Manuscript received by the editor June 8, 1981 Pu&e 88:135-151 t 1981). http:llpsyche cnlchb ore/SM%l3S html



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Psyche
[Vol. 88
Table 1.
Emigration characteristics of P. hyatti. Colony
Days
Observed
Distance Between First
Number of Returns to a
and Last Observed
Emigrations
Former Nest Nests (m)
Total
Colony designations were based on the species (D - desertorurn, H - hyatti), the date when the colony was found (Jn - June, Jl - July, A - August) and the order in which it was found on that date. For example, H-Jnl8-2 is the second P. hyatti colony found on June 18. Activity for both species began at approximately 2000 hr (MST) and ceased 0500 hr. To determine emigration frequency all colonies were inspected at least once daily between 2000 and 2400 hr. In order to avoid disturbing the colony any prolonged observations were made using red light. About two-thirds of the emigrations for each species were documented indirectly when a colony occupying a nest one night was found at another nest the following night. A colony was assumed to be occupying a nest if 10 foragers were



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19811 Droual & Topoff- Genus Pheidole 137 Table 2. Emigration characteristics of P. desertorurn. Days
Colony Observed
D-Jn 10- la* 70
D-JnI I-la 63
D-Jnl l-2a 68
D-Jnl2-la 60
D-Jn 12-2a 6 8
Distance Between First
Number of Returns to a
and Last Observed
Emigrations Former Nest Nests (m)
D-Jn 15-2a
D-Jnl6-lb
D-Jnl7-! b
D-Jnl7-2b
D-Jnl8-la
D-Jn20- I b
D-Jn25-la
D-Jn28-l a
D-J1 1-la
D-J1 13-lb
D-J1 13-2b
D-J1 15-1 b
D-J130-l b
D-J1 30-2a
D-A 1-la
Total
*a: desert-grassland; b: oak-juniper woodland. observed leaving the nest during a 1 min period. If this criterion was not met, or if there was some other reason to doubt the location of the colony, a small peanut butter bait was used to locate the colony. To avoid confusion when using this indirect method, an attempt was made to locate and mark any neighboring conspecific colonies. The remainder of the emigrations were observed directly when an emigration was discovered in progress. The nests were marked and the distance between the old and the new nests measured. With the statistical tests employed in this paper probabilities of .05 or less were accepted as significant.



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138 Psyche [Vol. 88
RESULTS
Colonies of both P. desertorurn and P. hyatti showed consider- able variation in their frequencies of emigration (see Tables 1 and 2). One colony of each species (D-Jnl l-2 and H-Jnl7-1) did not emigrate at all, while one P. desertorurn colony (D-Jn20-1) erni- grated 8 times, and one P. hyatti colony (H-Jnl4-2) emigrated 16 times. Despite this variability, both species showed a clear tendency to emigrate frequently: over one-half of the P. desertorurn colonies emigrated at least 4 times, and over one half of the P. hyatti colonies emigrated at least 6 times. To statistically compare the emigration frequency of the two species, the percentage days for which an emigration was documented was calculated for each colony, and the percentages for each species were compared using the Wilcoxon two-sample test (Sokal and Rohlf 1969). No significant difference was found in the emigration frequency between the two species (.l > P > .05).
This similarity between species in emigration frequency can also be seen if the frequency of time interval between emigrations is compared. Figures 1 and 2 show the frequency of emigration interval for P. hyatti and P. desertorurn, respectively. Both distri bu- tions are strongly skewed to the right with a surprisingly high number of emigrations occurring 1 to 2 days after the previous emigration. No significant difference was found in the frequency distribution between the two species (Wilcoxon two-sample test: 4 > P > .2)
The daily occurrence of emigrations among all colonies is shown in the form of frequency histograms in Figs. 3, 4 and 5. The upper line in the graphs outlines the number of colonies which were included in the sample size each night. Excluded from the sample were colonies which were raided by army ants, or were still suffering from the effects of an army ant raid (see Discussion). Superimposed over the graphs is a bar diagram showing the daily rainfall. A positive correlation was found to exist between emigration frequency and daily rainfall in all three cases (Spearman rank correlation coefficient: P. hyatti : rs = .28, N = 66; P. desertorurn in oak-juniper woodland : rs = .25, N = 64; 7'. desertorurn in desert- grassland : rs = .32, N = 70). The effect of rainfall on emigration frequency is most clearly seen in P. desertorurn in the desert- grassland habitat (Fig. 5). During the 29 days before the first heavy



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19811 Droual & Topoff- Genus Pheidole 139 rainfall on July 9 only three emigrations occurred, but within 9 days after this rainfall 29 emigrations occurred. During this 9 day period 13 of the 15 colonies being observed on this site emigrated at lea st once.
The emigration distance for both species was variable. Mean emigration distance for P. hyatti was 1.8 k 1.0 m (N=137; range 0 - 3 - 4.9) and mean emigration distance for P. desertorurn was 2.5 =!z 1.4 m (N=102; range 0.4 - 6.9). The larger emigration distance of P. desertorurn over that of P. hyatti correlates with the larger size of this species (mean length of P. hyatti minor = 2.64 k 0.04 mrn, N=50; mean length of P. desertorurn minor = 3.14 k 0.03 mrn, N=57).
Despite the high emigration frequency of both species, colonies of neither species tended to move far from the nests at which they were first discovered. Tables 1 and 2 show the number of times each colony returned to a former nest, and the distance between the first and last nests. As can be seen, 49% of P. desertorurn's emigrations, and 42% of P. hyatti's emigrations were to former nest sites, and 1 1
P. desertorurn colonies and 5 P. hyatti colonies at the end of the study were at the nest at which they were first discovered. This crisscrossing pattern of emigrations is illustrated for three colonies of each species in Figs. 6 and 7. The relative location of the nests reveal a clumped rather than a linear arrangement which would "be expected if the colony were emigrating out of an area. The dates of nest movements for each colony show that the variability of emigration interval within each colony was considerable. This can be readily seen by examining the ranges of emigration intervals for the colonies shown in Figs. 6 and 7: for the P. hyatti colonies the ranges are, H-Jnl4-2: 1-17 days; H-Jnl4-3: 3-18 days; H-Jnl8- 1 : 1-8 days; for the P. desertorurn colonies the ranges are D-Jn25- 1: 1-19 days; D-Jnl2-4: 1-21 days; D-J130-1: 1-4 days. Because P. desertorurn and P. hyatti emigrated so frequently about 33% of the emigrations of both species were discovered in progress. These emigrations were readily noticed as hundreds to thousands of workers, most carrying brood, formed a column connecting the old nest to the new nest. The width of this column for P. hyatti was about 3 cm, while for P. desertorurn the column tended to be wider (on one occasion reaching a width of 15 cm). Laboratory experiments have revealed that P. hyatti's emigrations



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Psyche
P. HYATTI
[Vol. 88
10 15 20 25
EMIGRATION INTERVAL ( DAYS
Figure 1.
Frequency of the time interval between emigrations for Pheidole hyatri. are totally organized by a substance secreted by the poison gland (Droual et al., in prep.). The queen of both species moved inde- pendently in the emigrations although she was usually surrounded by a retinue consisting mostly of minor workers (workers of the genus Pheidole are dimorphic) who tugged her by the mandibles or antennae if she hesitated en route to the new nest. During June and the early part of July alates were frequently seen in the column also moving independently.
However, on one occasion, during a P.
desertorum emigration, workers were observed carrying some of the males.
A number of phenomena related to these species' high emigration frequencies were observed. One colony of each species (D-Jn20-1 and H-Jnl9-2) performed what we call an aborted emigration. In these cases the colony was observed emigrating to a new nest but on the following night was found to be back at its old nest. One P. desertorum colony (D-Jn-25-1) appeared to perform two emigra- tions in one night. On August 17 the colony was observed emigrat- ing from nest 2 to nest 1 (see Fig. 6). However on the following night the colony was found at nest 3. On a number of occasions an



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Droual & TopoffÌÔGenu Pheidole
P. DESERTORUM
5 10 15 20 25
EMIGRATION INTERVAL ( DAYS 1
Figure 2.
Frequency of the time interval between emigrations for desertorum.
emigration could be predicted in advance by the colony's excava activity at another site. For example, before colony D-Jn 1 =
emigrated to its second nest site on 81 17, workers from the col - were observed excavating at the site on 8/4,8/ 5, 81 7 and 8/ 10-8 ,/---/ However two colonies of both P. desertorum and P. hyatti - observed excavating at sites to which they did not emigrate -
though they emigrated later to other nests. In this paper we have shown that P. desertorum and P. hv - - emigrate frequently and that the emigration frequencies of the -
species are similar. This similarity in emigration frequency beco - even more marked
when it is taken into account that most o r desertorum's emigrations in the desert-grassland occurred after- first rainfall. The sharp increase in emigration activity after the - can possibly be explained by the affect of the rainfall upon the - Before the rains began the soil was very hard and compact, but a 3



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Droual & TopoffÌ Genus Pheidole
P. DESERTORUM
5 10 15 20 25
EMIGRATION INTERVAL ( DAYS 1
Figure 2. Frequency of the time interval between emigrations for Pheidole
desertorurn.
emigration could be predicted in advance by the colony's excavation activity at another site. For example, before colony D-Jnl2-2 emigrated to its second nest site on 8/ 17, workers from the colony were observed excavating at the site on 8/4,8/ 5,8/ 7 and 8 / 10-81 17. However two colonies of both P. desertorum and P. hvatti were observed excavating at sites to which they did not emigrate even though they emigrated later to other nests. In this paper we have shown that P. desertorum and P. h+~,a//i emigrate frequently and that the emigration frequencies of the two species are similar. This similarity in emigration frequency becomes even more marked when it is taken into account that most of P.
desertorurn's emigrations in the desert-grassland occurred after the first rainfall. The sharp increase in emigration activity after the rain can possibly be explained by the affect of the rainfall upon the soil. Before the rains began the soil was very hard and compact, but after



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Psyche
P HYATTI
[Vol. 88
DATE
Figure 3. Daily occurrence of emigrations for Pheidole h,~,atti. Black bars indicatethe number of colonies which emigrated each night. Upper line outlines the number colonies included in the sample each night. Right ordinate indicates rainfall for the superimposed bar diagram showing daily rainfall. the first heavy rainfall the soil loosened considerably. This un- doubtedly made the excavation of new nests by the desert-grassland dwelling colonies much easier. The same reasoning can be applied to explain the positive correlation between emigration frequency and daily rainfall in both habitats. However, in the oak-juniper wood- land, the greater amount of vegetation, the rockier soil and the generally moister conditions probably account for the relatively higher emigration activity before the beginning of the rainy season in this habitat.
The need to perform a colony emigration is a contingency almost all species of ants can be expected to face (Wilson 1971). However some species emigrate more than others. Among the legionary ants, particularly the Ecitoninae and Dorylinae, colony emigrations are an integral part of the foraging ecology (Wilson 1971). Oppor- tunistic nesters such as Tapinoma melanocephalum, T. sessile, Paratrechina bourbonica and P. longicornis occupy ready-made nests such as the tufts of dead grass and hollow plant stems which



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19811
Droual & Topoff- Genus Pheidole
P. DESERTORUM
(OAK -JUNIPER WOODLAND HABITAT )
6/15 6/20 6/25 6/30 7/5 7/10 7/15 7/20 7/25 7/30 8/5 8/10 8/15 DATE
Figure 4.
Daily occurrence of emigrations for Pheidole desertorum in oak- juniper woodland (See Fig. 3).
P. DESERTORUM
( DESERT- GRASSLAND HABITAT 1
DATE
Figure 5.
Daily occurrence of emigrations for Pheidole desertorum in desert- grassland (see Fig. 3).




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Psyche [Vol. 88
H- Jnl4-2
/ N
-
I METER
-------
H- Jnl4-3
f N
-
I METER
NEST 7
NEST
DATE
NEST
MOVEMENT
1-2
2- 3
3- 4
4- 5
5- 1
.I -6
6- 7
7 4
DATE
6/18
H- Jnl8- l NEST
MOVEMENT DATE
- -
1-2 7/I
NEST ?
NEST 3* 2-3 7 /5
NEST 6 3-4 7 /II
4-2 7 /I7
/ N 2 -3 7 /21
3-5 7 /29
5-2 a 14
2 - 1 8 /5
-
I METER
Figure 6.
Patterns of emigrations for three colonies of Pheidole hyatii. Date's of nest movements are shown on the right. *This nest had two entrances. are short-lived. When these nests are disturbed, the colonies quickly organize emigrations to other such nests (Holldobler and Wilson 1977). Leptothorax acervorum in oak-juniper woodland, construct delicate nests under stones which can be easily dislodged by la x-ge vertebrates, and are prone to emigrate when their nest is disturbed (Moglich 1979).
Most species build or choose nest sites which are longer-lived and less easily disturbed and are thought to emigrate infrequently. Among these species emigrations can be due to a local factor such as



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19811
Droual & Topoff- Genus Pheidole
D- Jn 25-1
-
I METER
-
I METER
-
I METER
NEST 3q>-NFST 4
NEST I
NEST
MOVEMENT
- 2
2 -3
3-4
4- I
1-4
4 -2
2 -3
DATE
-
7/12
7/15
7 / I6
8/5
8/6
8/15
8/17
NEST
MOVEMENT DATE
- 2
7/ 10
2- 1 7/31
1-3
8/ 1
3-4 8 /4
4-2 815
2 -4
8/ 14
NEST 2
NEST
MOVEMENT DATE
- 2
7/31
2-3 8/3
3- 1
8/7
- 3 8/11
3-4 8/15
4-1 8/16
å å
NEST 2 NEST 3
Figure 7.
Patterns of emigrations for three colonies of Pheidole deferiorum. Dates of nest movements are shown on the right. shading (Brian 1956, Carlson and Gentry 1973), or climatic ad- versity such as drought or frost (Brian 1952). A colony may also be forced to emigrate because of some biotic factor such as inter- and intra-specific competition (Holldobler 1976, Waloff and Blackith 1962, Brian 1952, Brian et. al. 1965) and predation (Gentry 1974). The view that emigrations occur infrequently among most ants



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146 Psyche [v e l . 88
was recently challenged by Smallwood and Culver (1979). T liese investigators conducted a study in which they found that Tapirx ^>ma sessile and Aphaenogasier rudis emigrated frequently. Their s -fcudy differs from ours in that colonies were marked and rechecked <^ nly after intervals of 11-21 days and no attempt was made to follow the behavior of individual colonies. Because T. sessile and A. m d i s choose different nesting sites and have different life styles -t hese investigators deduced that emigrations occur more frequ& ntly