Table Of ContentHYM. RES.
J.
Vol. 9(2), 2000, pp. 377-384
Reproductive Biology of the Seed-harvester Ants Messor julianus
(Pergande) and Messor pergandei (Mayr) (Hymenoptera: Formicidae)
in Baja California, Mexico
Robert A. Johnson
Department of Biology, Arizona State University, Tempe, Arizona 85287-1501, USA
—
Abstract. The seed-harvester ant Messor julianus (Pergande) exhibits a parapatric distribution
pattern with the ecologically equivalent congener M. pergandei (Mayr) in the Baja California pen-
insula of Mexico; M. pergandei replaces M. julianus in drier soil microhabitats within the contact
zone between these two species. This paper describes the reproductive biology ofM. julianus and
M. pergandei to provide a first step in understanding factors involved in causing this replacement
pattern. Mating flights of M. julianus were observed over a several week period from early Feb-
ruary to early March, and thus appear similar to those ofM. pergandei. Likewise, starting nests of
both species contained one foundress. Moreover, the similar ecology and mating flights of M.
julianus and M. pergandei suggest that the replacement pattern exhibited by these two species is
associated with patterns of foundress survival. In regard to mating flights, both M. julianus and
M. pergandei are postulated to have diverged from the putative ancestral condition of summer
mating flights that occur in other Nearctic congeners. This seasonal difference in timing of the
mating flight for these two species correlates with their being the only Nearctic species ofMessor
that are restricted to hot desert habitats. Alate females for both M. julianus and M. pergandei have
poor tolerance to high temperature relative to desert ants in the genera Aphaenogaster and Pogon-
omyrmex.
The seed-harvesting ant genus Messor California Floristic province in the north-
(Hymenoptera: Myrmicinae) is common west portion of the peninsula (R. Johnson
throughout the southwestern deserts of and P. Ward, unpubl. data). The range of
the United States and northwestern Mex- M. stoddardi extends to central portions of
ico. Four species of Messor, M. andrei the peninsula, but this species rarely co-
(Mayr), M. julianus (Pergande), M. pergan- exists with M. julianus or M. pergandei (R.
dei (Mayr), and M. stoddardi (Emery), oc- Snelling, unpubl. data; R. Johnson, pers.
cur in the Baja California peninsula of obs.) (Fig. 1).
Mexico; M. julianus is endemic to the pen- Messorjulianus and M. pergandeiare eco-
insula (Johnson 2000a; R. Johnson and P. logically similar species. Colonies of both
Ward, unpubl. data) (Fig. 1). Two of these species consist of many thousands of
species, M. julianus and M. pergandei, are workers that forage in long columns
common in most low elevation habitats (< (Johnson 2000a), and these two species are
—
1000 m) with their combined geographic the only NearcticMessor that are restricted
distributions encompassing all but the to occurring in hot desert habitats (Wheel-
northwest portion of the peninsula. The er and Wheeler 1973). Geographically, M.
other two species, M. andrei and M. stod- julianus is mostly restricted to central and
dardi, are largely restricted to coastal and southern portions of the peninsula, while
adjacent inland areas along the Pacific M. pergandei occurs in eastern portions of
Coast. In the Baja California peninsula, M. the peninsula to as far south as northeast-
andrei is restricted to the relatively mesic ern BCS (the state of Baja California Sur)
378 Journal of Hymenoptera Research
5
California
Baja California
Messor
A andrei
o
julianus
• pergandei
a stoddardi
Baja California Sur
70 70 140 Kilometers
Fig. 1. Geographic distribution ofspecies ofMt'ssor that occur in the Baja California peninsula, Mexico. The
full geographic distributionofeach species is given in Johnson (2000a).
(Fig. 1). In northern and central BC (the fraction of individuals in each encounters
state of Baja California), M. julianus inhab- the other [Futuyma and Mayer 1980]) with
its a narrow range along the cool Pacific ranges of the two species overlapping
Coast, while M. pergandei is restricted to across a contact zone in the central pen-
xeric desert areas along the eastern coast. insula (Fig. 1). While the two species are
Moreover, these two species exhibit a par- often sympatric within the contact zone,
apatric distribution pattern (herein de- the pattern across the contact zone is one
fined as species that occupy separate but in which the two species replace one an-
adjoining areas, such that only a small other along local gradients that correlate
Volume 9, Number 2, 2000 379
with abiotic habitat features (Johnson out the flight period each day, alates fly-
2000a). Across the contact zone, M. pergan- ing from nests were counted during se-
dei inhabits the drier microhabitats, i.e., quential 2 minute visits to each nest. Am-
those that are lower in elevation or in bient temperature was measured periodi-
which the soils have a higher percentage cally about 5 cm above ground using a
composition of sand (drier soils) (R. John- thermocouple thermometer,
son, pers. obs.). The number of foundresses per starting
Colony founding is the most vulnerable colony was determined by excavating
stage in the life history of ants (Tschinkel founding nests of M. julianus and M. per-
1992, Herbers 1993, Johnson 1998), and it gandei. Data for M. julianus were collected
is this stage that likely determines micro- 17-18 km west of La Purisima, BCS,
and macro-distribution of adult colonies. (26°09'N, 112°13'W) in March 1992, and
Thus, comparative data on reproductive near Punta San Hipolito, BCS, (27°00'N,
biology of M. julianus and M. pergandei 114°00'W) in February 1998. These same
provide a first step in understanding fac- data were collected for M. pergandei near
tors involved in causing the replacement Highway 1 at 17.5 km west ofBahia de los
pattern exhibited by these two species. Angeles, BC, (28°59'N, 113°44'W) and
Mating flights of M. pergandei are well along Highway 1 at 6 km south of the pa-
known and typically occur between late ved turnoff to Bahia de los Angeles, BC,
January and mid-March (Pollock and Riss- (29°00'N, 114°10'W) in February 1995.
ing 1985, Ryti 1988, Cahan et al. 1998), and I assessed relative tolerance tohigh tem-
thus deviate seasonally from the summer perature by comparing survival for alate
mating flights of other Nearctic Messor females of M. julianus and M. pergandei
(Creighton 1953, Cole 1963, McCluskey with that of two species of Aphaenogaster
1963, Wheeler and Wheeler 1973, Snelling (A. albisetosa and A. cockerelli) and four
and George 1979, Brown 1999, R. Snelling, species ofPogonomyrmex (P. barbatus, P. oc-
unpubl. data, M. Bennett, pers. comm.). In cidentalis, P. rugosus, and P. salinus) (see
contrast, the mating flights and the female Table 1 for collection data); mating flights
sexuals of M. julianus are undescribed in of both species of Aphaenogaster and all
the literature. Based on the fact thatM. ju- four species of Pogonomyrmex are trig-
lianus and M. pergandei are the only Ne- gered by summer rains (Johnson 2000a).
arctic Messor that are restricted to hot de- Trials used test tubes that were partially
sert habitats, I hypothesized that the sea- filled with water trapped by cotton plugs,
sonal timing of mating flights was similar Alate females were placed into the tubes
for these two species. I assessed potential and the openings were plugged with
physiological constraints on mating flight moist cotton, thus providing ad libitum
season by comparing high temperature water at both ends. Trials at each temper-
tolerance for alate females of Messor, ature used one tube containing 25 individ-
Aphaenogaster, and Pogonomyrmex. uals of one species that had been collected
Mb HODS from at least four colonies. A separate set
1 Qf individuais was used at each tempera-
I observed ten colonies of M. julianus ture. Each species was tested over 1° C in-
over nine days in mid-February 1993, near crements that resulted in mortality rang-
Highway 1 at 26 km northwest of Santa ing from 0-100%. The tubes were placed
Rosalia, BCS (27°23'N, 112°28'W). All ten in a darkened incubator for 2 hours at the
colonies had large nests and an active for- appropriate temperature; individuals un-
aging column. The site was a sandy plain able to right themselves after that time
dominated by Larrea tridentata, Opuntia were considered dead. Mortality data
cholla, and Pachycereus pringlei. Through- were compared among congeners and
380 Journal of Hymenoptera Research
Table 1. Collection data for alate females used in
Volume9, Number 2, 2000 381
10
382 Journal of Hymenoptera Research
Table 2. Number of foundresses in starting nests ofMessor julianus and M. pergandei in the Baja California
peninsula, Mexico. For location, BC = Baja Californa; BCS = Baja California Sur.
Volume 9, Number 2, 2000 383
queen regardless of the initial founding sumably having invaded North America
strategy (Rissing and Pollock 1987; S. Riss- from Asia via Beringia (R. Snelling, pers.
ing and J. Parker, unpubl. data). These comm.). Alternatively, some recent evi-
data extend the distribution of haplome- dence suggests that the Aphaenogaster spe-
trosis for M. pergandei from southeastern cies belonging to the former Novomessor
California to its southern range limit in the (including A. albisetosa and A. cockerelli)
Baja California peninsula such that both are the sister group to Nearctic Messor
M. pergandei and M. julianus are haplo- (Bennett 2000). Moreover, evaluating the
metrotic where the two species are sym- ancestral mating flight condition depends
patric. Mating occurs in the air for M. per- upon determining the appropriate out-
gandei (S. Rissing, pers. comm.) and M. an- group. The relationship between M. juli-
drei (R. Johnson, pers. obs.), with in copulo anus and M. pergandei is also poorly re-
pairs of both species sometimes falling to solved (Bennett 2000) because the clade
the ground; mating in M. julianus is prob- containing these two species also contains
ably similar. M. lariversi, which is a Great Basin species
The replacement pattern exhibited by that has summer mating flights.
M. julianus and M. pergandei is common One possible evolutionary scenario is
among seed-harvester ants in western the late winter to early spring flights
North America (Johnson 2000a). For ex- evolved one time and are common by de-
ample, the ecologically equivalent species scent in M. pergandei and M. julianus. If
Pogonomyrmex barbatus and P. rugosus seg- this is the case, the shift from a summer
regate microhabitats along gradients of to a late winter to early spring flight sea-
soil texture. Foundresses of P. rugosus son may have been necessitated by phys-
have a higher tolerance to desiccating con- iological constraints related to tempera-
ditions, which correlates with this species ture tolerance, and prerequisite to these
occurring in drier soil microhabitats (John- two species invading hot desert habitats,
son 2000b). The similar ecology and mat- Moreover, alate females of both species
ing flights of M. julianus and M. pergandei have poor heat tolerance compared to spe-
suggest that the micro- and macro-habitat cies of Aphaenogaster and Pogonomyrmex,
differences exhibited by these two species suggesting Messor foundresses could not
are also associated with patterns of foun- survive soil temperatures present during
dress survival. Given that M. pergandei in- summer. A similar temperature constraint
habits the more xeric micro- and macro- occurs in the desert leaf-cutter ant Acro-
habitats, it is predicted that foundresses of myrmex versicolor, whose mating flights
this species are more desiccation tolerant are triggered by late summer rains. Fe-
than are foundresses of M. julianus (John- males of A. versicolor also have poor tol-
son 2000a). erance to high temperature, and the foun-
The late winter to early spring mating dresses survive by selectively initiating
flights of M. julianus and M. pergandei are nests in shaded, cooler microhabitats
correlated with these being the only two (Rissing et al. 1986).
Nearctic Messor that are restricted to hot ACKNOWLEDGMENTS
desert habitats (Wheeler and Wheeler
1973). However, it is difficult to determine Tnismanuscriptbenefited from commentsand dis-
the sequence of character-state change in cussions uith S.W. Rissing and R.R. Snelling, and by
timing of,t.•,he mat,i.ng fnl.ig,ht, wi.t,,hi.n xNtearct.i•c commentsfromtwoanonymousreviewers.
Messor because the phylogenetic relation- LITERATURE CITED
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