Insectes Sociaux, Paris
1984, Volume 31, n ~ 2, pp. 142-154
9 Masson, Paris, 1984
NECTAR THIEVERY BY ANTS FROM SOUTHERN SPANISH
INSECT-POLLINATED FLOWERS
C.M. HERRERA (1), J. HERRERA (2) and X. ESPADALER (3)
(1) Estaci6n Biol6gica de Dohana, Sevilla-12, Spain
(2) Departamento de BoMnica, Facultad de Biologia, Sevilla-12, Spain
(3) Departamento de Zoologia, Universidad Aut6noma de Barcelona, Bellaterra, Barce-
lona, Spain
ReFu le 26 mai 1983. Accept~ le 23 d6cembre 1983.
SUMMARY
Correlates of ant nectarivory have been investigated in a sample of 75 insect-
pollinated, nectar-producing southern Spanish plant species. Ants exploit floral nectar
opportunistically. Variation among plant species in sugar secretion rates and flower
location relative to ground level do not influence either the chances of ant visitation
or the identity of ant species involved.-Mechanical restrictions on nectar accessibility
(tubular corollas, occlusive structures) decrease nectar thievery. Some evolutionary trends
in floral morphology usually related to a decrease in the range of effective pollinators
alone may simultaneously be interpreted in terms of increased plant adaptedness to
exclude non-pollinating insect nectarivores.
RESUME
Consommation de nectar par les fourmis sur les fleurs pollinis6es
par des insectes du sud de I'Espagne
On a dtudid quelques facteurs lids h la consommation du nectar chez les fourmis
dans un dchantillon de 75 esp~ces de plantes du sud de l'Espagne qui sont pollinisdes
par des insectes et qui produisent du nectar. Les fourmis exploitent le nectar de fa~on
opportuniste. La variation entre les esp~ces de plantes en ce qui concerne le taux de
sdcrdtion de nectar et la localisation des fleurs par rapport au sol n'a pas d'incidence,
pas plus que la possibilit6 de visite par les fourmis ni l'identitd des esp~ces enregistrdes.
Les restrictions mdcaniques h l'accessibilit6 du nectar (corolles tubulaires, structures
fermdes) diminuent la consommation de nectar par les fourmis. Quelques tendances
dvolutives dans la morphologie florale, habituellement lides ~t une augmentation de la
spdcificitd des pollinisateurs, peuvent 6tre interprdtdes simultandment en termes de meil-
leure adaptation des plantes pour exclure des insectes non pollinisateurs qui profitent
du nectar.
NECTAR THIEVERY BY ANTS
143
INTRODUCTION
Although they have been occasionally reported as pollinators for some
plants (e.g., HICKMAN, 1974 ; WYATT, 1981 ; BRANTJES, 1981), ants most commonly
are "undesirable" flower visitors which do not perform pollination (PRocTOR
and YEO, 1973 ; FAEGRI and VAN OER PIJL, 1979). Presence of nectar-feeding ants
in flowers has been found associated with decreased seed set and pollinator
visitation rates (WYATT, 1980; FRITZ and MORSE, 1981). In the terminology
of INOUYE (1980), ants behave as " nectar thieves"; while they do not make
holes to extract nectar and they enter the openings used by pollinators, a mis-
match of morphologies and behaviours precludes pollen movement both
among flowers and plants. We present here the results of a regional survey
of the occurrence of nectar-feeding ants in flowers of southern Spanish, nec-
tar-producing insect-pollinated plants, documenting the incidence of this ubi-
quitous group of nectar thieves in a regional flora and analyzing some
correlates of ant nectarivory.
The susceptibility of the plant reward to being taken by detrimental
organisms has presumably played an important role in shaping the evolution
of plant-animal food-mediated mutualisms (e.g., THOMPSON and WILSON, 1978 ;
THOMPSON ; MCDADE and KINSMAN, 1980 ; C.M. HERRERA, 1982 a ; STEPHEN-
SON, 1982). Despite this, disproportionately little work has been done on
the actual incidence of " parasites ", on mutualisms (sensu JANZEN, 1975), in
comparison with the extensive attention received by the interaction of
mutualistic organisms (but see JANZEN, 1977 ; WYATT, 1980 ; YOUNG, 1980 ; FRITZ
and MORSE, 1981 ; MCDADE and KINSMAN, 1980 ; GILL et al., 1982 ; ROUBIK, 1982).
The identification of some ant or plant features associated with nectar thievery
may contribute to the understanding of some general aspects of plant-
pollinator evolutionary interactions.
STUDY AREA AND METHODS
Information presented in this paper ,was gathered during 1981-82 as part of more
extensive regional studies on the floral biology (J. HERRERA, 1982) and reproductive
ecology (C.M. HERRERA, unpubl.) of southern Spanish plants. Data ~vere mostly collected
in 12 localities broadly distributed across Andalusia, the southernmost Spanish region
(table I). They encompass a broad variety of habitat types, ranging from coastal sclero-
phyllous scrublands to montane pine forests (1 700 m elevation), although most sites are
vegetated by various modalities of mediterranean scrub. We are confident that all
major habitat types in the region have been sampled. Sites fall into two contrasting
elevational groups, lying either in the lowlands of the lower Guadalquivir River valley
or at various elevations on any of the ~wo parallel mountain ranges bordering it.
Occasional observations and collections ~were done away from these principal study
sites.
Flowers from 116 species belonging to 28 plant families 'were examined for nectar
production following the methods described by J. HERRERA (1982). This involved both
144
C.M. HERRERA, ,1. HERRERA and X. ESPADALER
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NECTAR THIEVERY BY ANTS
145
the examination of flowers in the field and the collection of flo~vering stems, ~vhich
were ikept in closed plastic bags at room temperature and examined for nectar production
after 24 h. This procedure permitted ascertaining nectar production in a total of 75
species from 26 families, and this smaller sample (see Appendix for a list of species)
will be used here to evaluate differential ant incidence among plant species. Species
~vhose fl(ywers did not secrete nectar have been excluded because, as ants ~vere not
observed in nectarless flowers, its inclusion 'would have artificially decreased ant incidence
figures. Herbs constitute 26.7 %, shrubs 62.7 '%, vines 5.3 %, and trees 5.3 ~ of the
75 nectar-producing plant species considered. They mostly flower in spring and early
summer, in accordance 'with general phenological trends in the region (J. HERRERA, 1982;
C.M. HERRERA, in press).
As an indirect estimate of flower size, average dry ,weight of flowers ~was obtained
for the 75 species considered. Samples of 20-100 individual newly-opened fiowers ~vere
air-dried to constant ,weight, weighed to the nearest 0.1 mg, and an average figure
computed. These data ,were subsequently used to determine any possible relation
bet~veen size of flowers and either frequency of ant nectarivory or characteristics of
foraging ants observed.
Nectar-feeding ants ,were collected from flowers ~vhenever observed in the course
of field ,work involving observations and censuses of flower visitors (3. HERRERA, 1982)
and/or 'while collecting plant stems for the survey of nectar production or other purposes.
In a single instance, ants remained unnoticed until they emerged from flo~vers held
in plastic bags. This case refers to flowers ~vith concealed nectar in ~vhich it ~r
impractical to observe actual nectar feeding by ants in the field, and ~r be discussed
below. Maximum head width has been taken as an indirect estimator of ant size.
Individual ants collected in flowers 'were measured and average figures obtained for
every species recorded. These data have been used to investigate a possible relation
between ant size and floral characteristics. Voucher specimens of ant are at the
Departamento de Zoologla, Universidad Aut6noma de Barcelona, under EN 1 to EN21,
ordered as in COLLINGWOOD (1978).
RESULTS
A summary o[ ant nectar-feeding records, all localities combined, is
presented in table II. Ants belong to the subfamilies Myrmicinae (2 genera,
7 species), Dolichoderinae (1, 2) and Formicinae (6, 12), and were found
feeding on nectar in the flowers of 21 (28.0 %) of the 75 species ,examined,
and 13 (50.0 %) of the 26 families. This represents a conservative estimate
of the actual frequency of nectar thievery by ants in the sample of plant
species examined. Absence of ants from the flowers of a particular species
may simply mean that its use by ants was too infrequent as to have been
detected 'with our sampling scheme. In the case of plant species with
extended flowering seasons, restricted seasonal activity of ants at flowers
(MERLE, 1982) may perhaps have produced some negative records.
That ants found in flowers were actually feeding on nectar and taking
a significant amount of it, is supported by the following observations.
Typically, ants were immobile, with mouthparts in contact with nectar-
secreting structures, for periods of up to several minutes. This indicates
that they were not collecting pollen or catching small flower-dwelling insects.
Often they showed ostensibly distended abdomens. Almost invariably, flowers
146
C.M. HERRERA, Y. HERRERA and X. ESPADALER
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