Moving Larvae around nest is fine tuning by Atta

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Moving Larvae around nest is fine tuning by Atta

Post by Acromyrmexbob » Wed Jun 01, 2016 7:06 am

Humidity preference for fungus culturing by workers of the
leaf-cutting ant Atta sexdens rubropilosa
F. Roces and C. Kleineidam
Zoologie II, Biozentrum der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, e-mail:
First published in Insectes sociatus; 47 (2000), 4. - pp. 348-350

In ant societies, the selection of suitable microclimatic conditions
for brood rearing is an ubiquitous and significant
activity. This is accomplished by nurse workers that, following
their preferences, select appropriate places within the
nest to which the immobile brood is transported (Roces and
Núñez, 1995). In several ant species, the nest architecture
helps to stabilize fluctuations of temperature and humidity,
thus preventing a more or less continuous brood transport as
these environmental variables change. Hence, brood transport
inside the nest represents a fine-tuning response to cope
with excessive deviations from the expected values.
Leaf-cutting ants of the genus Atta construct very large
nests that house up to five million individuals and may contain
more than 1000 chambers, one half of them filled with
fungus gardens (Autuori, 1942). The symbiotic fungus has
strict demands of high humidity and temperatures between
25 and 30°C for proper growth (Quinlan and Cherrett, 1978;
Powell and Stradling, 1986). The fungus garden represents
the sole food source for the developing larvae. Consequently,
workers are expected to maximize fungus growth not only
by the selection of appropriate vegetal substrate, but also by
an active choice of suitable places for culturing according to
the prevailing microclimatic conditions in the nest. Beside
brood transport, workers may therefore be able to relocate
the fungus gardens in order to provide them with the most
appropriate environment inside the nest.
It is worth mentioning that the conditions offered in the
experimental arrangement, i.e., stable humidity values inside
a given test chamber, may contrast with the situation observed
in large Atta-nests in the field. Inflow of air as a consequence
of nest ventilation may cause variations in the
humidity conditions of the fungus chambers. Measurements
in mature Atta-nests of two different species, however,
indicate that the relative humidity in the area of the fungus
chambers was always above 90%, so that the ventilation
demands of the colony do not compromise the humidity conditions
inside the nest (Atta sexdens, Stahel and Geijskes
(1940); Atta vollenweideri, Kleineidam and Roces (2000)).
Previous measurements by Eidmann (1935), showing that the
relative humidity in the A. sexdens fungus chambers ranges
from 53 to 57%, are probably erroneous, since gardens maintained
in the lab at such low humidities desiccate very rapidly
(Stahel and Geijskes, 1940).
The observation that the fungus chambers in different
field nests of several leaf-cutting ant species are located
deeper in dry soils than in more humid ones (Escherich,
1929; Eidmann, 1935; Weber, 1957) emphasizes the importance
of humidity requirements for fungal growth, even for
large colonies with elaborated nest architecture and therefore
more stable microclimatic conditions. The relocation responses
following the humidity demands of the fungus are
expected to be more significant for young colonies, which
Figure 1. Hygropreference of workers for fungus culturing and for
themselves in test chambers under different humidities. Data correspond
to means and standard deviation of 10 independent assays. The
horizontal lines indicate values expected under random distribution
(25% of the responses)
have been observed to locate their first fungus chambers up
to 3 m below the surface (Eidmann, 1935), and also for attine
ant species constructing more simple or exposed nests
(Navarro and Jaffé, 1985; Mueller and Wcislo, 1998). In fact,
colonies of the grass-cutting ant Acromyrmex landolti were
observed to excavate new chambers during the dry season to
depths where soil moisture asymptotically approached a
maximum level (Lapointe et al., 1998).
Our results indicate that gardening workers, even though
indifferent in their hygropreference for themselves, select
appropriate nest environments for their fungus, based solely
on its humidity demands. Relocation of fungus gardens to
promote their growth, like the repotting of flowering plants
by humans, clearly illustrates the skill of leaf-cutting ants as
true “gardeners”. Beside temperature and carbon dioxide
concentration (Kleineidam and Roces, 2000), humidity therefore
seems to represent one of the relevant variables that
underlay the evolution of regulatory responses for the control
of fungus growth in leaf-cutting ants.

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