Behavioural interactions: Ants and larvae

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Behavioural interactions: Ants and larvae

Post by Acromyrmexbob » Tue May 31, 2016 6:39 am

You should definitely read the following. There are some really interesting findings here!

Larval isolation and brood care in Acromyrmex leaf-cutting ants
J. F. S. Lopes, W. O. H. Hughes, R. S. Camargo and L. C. Forti
Received 8 November 2004; revised 31 March 2005; accepted 22 April 2005. ... f&site=398

Six behaviours by workers towards larvae were recorded and
quantified: licking larvae, transporting them around the piece
of fungus garden, feeding larvae, ingesting their faecal fluid,
planting hyphae on their bodies and scraping their mouthparts.
In addition, workers frequently antennated larvae and
larvae occasionally pouted. Licking was the most frequent of
the recorded behaviours while feeding and transporting were
relatively rare. The frequencies of all six behaviours were
greater when workers were interacting with larvae that had
been isolated for a period of time than with the control larvae,
but there appeared to be no effect of the length of isolation.
For isolated larvae, the frequencies of licking, ingesting
faecal fluid and scraping mouthparts all decreased over the
course of the observations while the frequency of planting
increased. Transporting and ingesting changed less markedly
over the three observation periods.
Of the six behaviours recorded by workers towards the
larvae, licking was by far the most frequent. It is possible that
this behaviour may be a means by which a worker gains information
about the larva, for example of its state of hunger.
It is also likely that the behaviour plays a role in preparing
larvae for incorporation in the fungus garden and in protecting
larvae against disease. Important individual mechanisms
of defence are grooming and the antibiotic secretions of the
metapleural gland (Hughes et al., 2002; Poulsen et al., 2002).
Larvae are unable to groom, have no metapleural gland and
will therefore be exceptionally vulnerable to parasites. The
frequent licking of larvae by workers may both ensure any
microorganisms on their cuticle are rapidly removed and
may cover the cuticle of larvae in compounds from the labial
and mandibular glands, both of which have antibiotic properties
in leaf-cutting ants (Kermarrec et al., 1986; North et al.,
1997). The scraping of larval mouthparts by workers with
their mandibles seems likely to also serve a hygienic function,
as well as preventing the mouthparts of larvae being
hindered by an accumulation of material. The frequencies of
both licking and scraping decreased over the three observation
periods. Larvae were not licked or scraped after they had
had hyphae planted on them. The decreases in licking and
scraping may therefore simply reflect the decreasing number
of larvae without hyphae on them and/or the increasing
cleanliness of larvae over the course of the observations.
The ingestion of larval faecal fluid (proctodeal trophallaxis)
has previously been observed in the leaf-cutting ant
Atta sexdens (Schneider et al., 2000), as well as a number of
other ant species (Hölldobler and Wilson, 1990), and appears
to be a relatively frequent behaviour. Trophallaxis between
adult workers, in contrast, occurs only rarely in attine ants
(Murakami and Higashi, 1997; Schneider et al., 1999). The
ingestion of larval faecal fluid may simply be for hygienic
reasons, keeping both the larvae and fungus garden clean of
excreta. However, it seems likely that the workers may gain
some nutritive benefit from the fluid. The contents of the
larval hind gut originate from the Malpighian tubules and
in Monomorium have been found to be rich in amino acids
(Hölldobler and Wilson, 1990). Adult leaf-cutting ant workers
require the products of the hydrolysis of plant polysaccharides,
particularly glucose, to survive (Silva et al., 2003),
but are lacking in the requisite enzymes to obtain these
(D’Ettorre et al., 2002). Although the mutualist fungus does
contain a certain amount of such compounds, Atta sexdens
workers survive almost twice as long when provided with a
diet of glucose as opposed to one of fungal material (Silva
et al., 2003). Larvae have much larger quantities of digestive
enzymes than adult workers (D’Ettorre et al., 2002), and
will thus be better able than adult workers to digest fungal
material. The fluid from their hind gut may therefore be both
a source of digestive enzymes and of nutrients for the adult
workers. It is notable that if colonies of leaf-cutting ants lack
larvae then the workers become sluggish and the colony rapidly
dies (Weber, 1972). Although this has been assumed to
be due to larvae producing pheromones that unify the colony,
it is also possible that larval faecal fluid is in fact a critical
source of nutrients and/or enzymes for the adult workers.
It has previously been noted that attine larvae are generally
located within the fungus garden and that they are often
covered in a blanket of hyphae (Weber, 1972; Schultz and
Insect. Soc. Vol. 52, 2005 Research article 337
Meier, 1995; Adams et al., 2000). In this study we observed
workers actively placing fungal hyphae on the bodies of larvae
and then pressing down the hyphae with their forelegs
and mouthparts in a manner reminiscent of the way they
plant hyphae in the fungus garden. This suggests that the occurrence
of hyphae on larvae is not simply a consequence of
their living within the fungus garden, but that it serves some
purpose. It may be that it protects larvae against parasites or
that it acts as camouflage against predators such as army ants
(Weber, 1972; Swartz, 1998; LaPolla et al., 2002). Alternatively,
it may be that either cuticular compounds produced by
the larvae or saliva applied to their cuticle by workers during
the act of licking, may promote fungal growth (Weber, 1956;
1972). Hyphae were only planted on larvae after they had
been thoroughly licked and the frequency of planting was
observed to increase over the three observations presumably
because of the need for larvae to be licked first. It may be that
larvae need to be clean before hyphae can be planted on them
or that the saliva is necessary for fungal growth. Regardless,
the fact that workers actively apply hyphae to larvae suggests
that covering larvae with hyphae is in some way important to
one or other party.
The frequencies of all six behaviours recorded were
greater towards larvae that had been maintained without
workers than towards control larvae. The difference was
significant for licking larvae, transporting larvae, ingesting
their faecal fluid and scraping their mouthparts, and was only
marginally nonsignificant for feeding larvae. This difference
suggests both that the isolation of larvae causes them to be
associated with greater levels of whatever cues stimulate
workers to care for them, and that workers are able to assess
larval needs to at least some extent. Although attine larvae are
immobile they can ‘pout’, or extrude their mouthparts, which
can stimulate workers to feed them (Weber, 1972). It is also
possible that they may emit pheromonal cues that could be
volatile or nonvolatile, and that may perhaps be sensed by
workers during the act of licking. The most obvious reason
for workers to give more attention to larvae that have been
isolated is because they are hungrier. Solenopsis workers, for
example, adjust feeding rate according to the hunger-level of
larvae (Cassill and Tschinkel, 1995). However, the fact the
feeding rate was increased only slightly, and nonsignificantly,
by larval isolation suggests that this was at most only part
of the reason for the increased frequency of behaviour shown
by workers towards isolated larvae. Hygiene, the removal of
excreta and the preparation of larvae for incorporation in the
fungus garden would seem to be more important reasons.
The fact that no differences were found between the
different lengths of isolation, suggests that the cue(s) may
be binary rather than providing more detailed information.
Alternatively, it may be that all of the time periods for which
larvae were isolated were at the extreme of what would naturally
occur and that workers could not distinguish between
such extreme levels. Several of the behaviours also became
less frequent the more time workers had spent interacting
with the larvae. This could relate to decreasing numbers of
larvae giving off the cue(s) because more and more of them
had been dealt with by workers or had been planted with
hyphae, or to individual larvae emitting reduced levels of the
cue(s) once they had been given the attention they required.
Further work is needed to elucidate the mechanism and reason
for the observed increase in worker attention to isolated
larvae. However the results demonstrate that Acromyrmex
workers are to at least some extent able to assess and respond
to the relative needs of larvae.

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