On the Adaptive Value of Nest Features in the Grass cutting Ant

Biotropica, 17, 347-348, 1985
On the Adaptive Value of Nest Features in the Grass-cutting Ant Acromyrmex landolti
Acromymex landolti F. is one of the most abundant animals in the savannas of the
western Venezuelan low Llanos. These savannas are characterized by marked dry and wet
seasons with annual rainfall (900-1100 mm) concentrated in two periods, between May and
July and between November and December. The average annual temperature is 25oC, and
average wind speed is 9 km/h coming from NNE, NE, and E (De Camino & Irausquin 1977).
We studied some aspects of the nest-building behavior of A. landolti.
This fungus-growing ant collects grasses, the most abundant food resource in this
ecosystem. A nest consists of a single, underground vertical gallery leading to various
chambers where fungus is grown and the brood is kept. Chambers of similar size are
constructed at depths of 15 cm to 5 m along the vertical gallery. The nest entrance is covered
by a characteristic turret of dry grass blades and earth particles-, waste from the nest is
deposited in a heap not far from the nest entrance (Rubio & Timaure 1977).
We observed A. landolti in the field at Chaguaramas, Estado Monagas, Venezuela
during one year. We excavated eleven nests in the dry season and eight in the wet season, The
location of the fungus and the brood in the nest chambers changed throughout the year. During
the dry season (December-May) only chambers below 3 m were used for fungus and brood,
whereas during the rainy season, larger nests filled all chambers with fungus and brood, and
smaller nests filled only the superficial and intermediate chambers. Laboratory colonies (N =
6), were offered the opportunity to build their nests in a terrarium (40 cm wide x 50 cm high x
10 cm deep) filled with earth and humidified continuously at only one corner on the bottom.
The ants built chambers in many different places within the terrarium, but filled with fungus
only those chambers near the humidity source (within approximately 4-9 cm). Such
observations indicate that these ants do not regulate the humidity of the nest chambers through
changes in the nest architecture as Atta spp. do (Eidmann 1935, Stahel & Geijskes 1939), but
apparently seek out the optimum conditions for their fungus within the humidity gradient in
the soil.
The nests of A. landolti usually are located at the base of grass plants. Nest-founding
queens in the laboratory always start digging their nests at the bases of the grass plants offered
in an artificial arena; even if only a single grass blade is offered, digging always starts at the
base of the blade. The queen excavates her first chamber (some 15 cm deep) within the root
system of the grass plant. Nest-founding activity occurs at the beginning of the rainy season
when some humidity has accumulated among the roots of the plants in the otherwise dry sandy
savanna. The water holding capacity of the savanna soil is very low (De Camino & Irausquin
1977), and the most humid place available to the founding queens is among roots. We could
speculate that this behavior also reduces predation on the nest founding queens and on the first
batches of workers, since the grass plants are likely to provide some protection against birds.
The turret at the nest entrance of A. landolti can reach a height of 6 cm but averages 2.4
± 1.3 cm (N = 38). In order to determine the function of these turrets, we performed the
following experiments. The turrets of two groups of 20 colonies each were removed, and the
time needed for the ants to build a new turret 2 cm high was measured. The experiment was
performed with one group in the dry season (January) and with the other in the wet season
June). In the dry season, at least two days passed before the ants rebuilt the turrets, and in 12
cases they did not rebuild the turret during the dry season. In the wet season, all 20 colonies
rebuilt their turrets within no more than two hours. The difference between the two groups is
statistically highly significant (P < 0.001, Median test).
The turrets have one 2.0- X 0.5 ~cm opening and in some cases more than one opening
(mean number of openings is 1.2 ± 0.2, range 1-4, N = 137). The diameter of the nest entrance
at the soil surface below the turret is the same or less than the opening of the turret. Thus, the
amount of rain falling direct1y from above into the nest should be the same with or without a
turret. Therefore we tested the permeability to water of the walls of these turrets. Turrets (N =
12) were removed from field colonies, both ends sealed, the turrets submerged in water, and
removed every 15 min to be examined for infiltration. Turrets resisted infiltration for 15-90
min, and even after infiltration was observed, turrets did not collapse during the eight-hour
duration of the test. Duration of rainfalls in this area very rarely exceed 60 min.
The flat savannas receive heavy rainfalls, and brief periods of sheet flooding may occur
before the soil absorbs the water. Out observations suggest that the turret prevents surface
water from entering the nest, thus protecting against inundation. The possibility that the turrets
are used for starting the nuptial flight may be eliminated, as we observed alate sexuals walking
up grass blades before taking flight and never observed attempts to take off direct1y from the
turret.
Another feature of the nests of A. landolti is the location of a waste heap near the nest
entrance. The size of this heap varies from nest to nest; it consists mainly of excavated earth,
dead fungus, dry leaves, and dead ants. The mean heap-turret distance was 3 1.0 ± 6.8 cm (N =
13 7). In 42 percent of cases, the refuse heaps were located in the quadrant southwest to
northwest from the nest entrance. Refuse heaps in 22, 16, and 20 percent of cases occurred
within the quadrants NW-NE, NE-SE, and SE-SW, respectively, The distribution of refuse
piles around nest entrances deviates significantly from uniform (P < 0.02, XI test, N = 137),
with the greatest fraction falling in the SW-NW quadrant. This orientation was uncorrelated
with the inclination of the soil surface (generally less than 0. 1 %) or the radiation of the son
(from ESE to WSW in winter and from ENE to WNW in summer), unless we assume that the
morning sunshine or afternoon sunshine has a specific effect on the colony. It is more probably
related to wind direction. Prevailing winds in the area come from the E to NE (De Camino &
Irausquin 1977), which is opposite to the position of the heap, and visibly reach the surface of
the scarcely covered savanna. In many cases we saw leaf-carrying workers blown 10-30 cm
downwind.
The laboratory colonies initially placed their refuse at random. After a small refuse pile
eventually had been established in one of the corners, all refuse was carried to this pile. We
speculate that, in the field, the first workers carrying refuse will tend to the
northwest-southwest due to the wind (and/or the waste will be blown in this direction). Once
the waste heap is established, it will not be relocated. Thus, its placement will minimize the
effort required, as the laden ants will walk downwind and the unladen ants will walk upwind.
We thank Dr. C. Bosque for critically reading the manuscript. T*his work was partially
supported by the Compania Nacional de Reforestación and by grant S 1- 13 3 6 of CONICIT,
Venezuela.
RESUMEN
Esta hormiga cultivadora de hongo excava de 1 a 11 cámaras, una debajo de la otra,
hasta profundidades de 5 m, y parece utilizar el gradiente de humedad del suelo para conseguir
las condiciones adecuadas para el hongo. Reinas fundadoras comienzan a excavar la primera
cámara en la base de macollas de pasto, probablemente buscando humedad y escapando de
depredadores. Estas hormigas construyen torrecillas, hechas con hojas secas y tierra, a la
entrada de sus nidos. Estas torrecillas son impermeables al agua y con ellas previenen la
inundación del nido por agua superficial acumulada durante los aguaceros. El montículo de los
desperdicios está localizado en la mayoría de los nidos de NO a SO de la entrada del nido, en
dirección opuesta a los vientos predominantes. Se propone que de esta forma, las obreras
minimizan sus esfuerzos en el transporte de los desechos, ya que las obreras cargadas camina
viento abajo y las obreras sin carga caminan viento arriba.
CAMINO, R., AND J. IRAUSQUIN. 1977. Proyecto para el establecimiento de una plantación
de Pinus caribea al sur del Estado Monagas. Proyecto Chaguaramas, CONARE, Venezuela.
EIDMANN, H, 1935, Zur Kenntnis der Blatschneiderameise Atta sexdens L., insbesondere
il-irer Oecologie, Teil 1. Z. Angew, Entomol. 22: 185-241.
RUBIO, E., AND A. TIMAURE. 1977. Características de los nidos de AcromyrMex landolti
(Forel) en el Oeste de Venezuela. Rev. Facultad de Agronomía, Universidad del Zulia 4:
53-62.
STAHEL, G., AND D. C. GEIJSKES. 1939. Uber den Bau des Nestes von Atta cephalotes L.
und Atta sexdens L. (Hym. Formicidae). Rev, Entomol. (Rio de Janeiro) 10. 27-78.
Jose Gregorio Navarro and Klaus Jaffe
Departamento de Biología de Organísmos,
Universidad Simon Bolivar, Apartado 80659, Caracas 1080, Venezuela