Volume 117, July 2016, Pages 179–186
- a Centre for Social Evolution, Department of Biology, University of Copenhagen, Copenhagen, Denmark
- b Smithsonian Tropical Research Institute, Ancon, Panama
- Received 8 January 2016, Revised 5 February 2016, Accepted 7 April 2016, Available online 9 June 2016
Highlights
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- Leafcutter ant foragers perform energetically costly tasks outside the nest.
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- Many foragers return unladen, but they do not tend to harvest liquid resources.
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- Foragers often leave nests with a ‘lunchbox’, digestive organs full of liquids.
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- Foragers deplete lunchbox liquids when carrying leaves back to the nest.
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- Lunchbox liquids foster symbiotic stability, fungal crops fuel their own production.
Optimal
foraging theory makes clear predictions about the benefits of
maximizing energetic returns per unit of foraging effort. However,
predictions become less clear when animals belong to symbioses that
would be destabilized by such foraging decisions. For instance,
leafcutter ants are dominant herbivores in Neotropical ecosystems that
harvest fresh vegetation and convert it into compost used to cultivate
specialized fungus for food. Individual foragers have long been assumed
to supplement their fungal diets by harvesting liquid nectar outside the
symbiosis, although this has not been demonstrated in the field, and
would probably destabilize the fine-tuned farming systems. By dissecting
liquid storage organs in foragers of four sympatric Panamanian
leafcutter ant species we found that liquid foraging is not a general
strategy in the field. Moreover, while over 40% of these foragers
returned to their nests without leaf fragments, these unladen ants were
not more likely to carry liquids. Instead, we found support for a newly
formulated ‘lunchbox hypothesis’ because most workers exited nests for
foraging trips with midguts full of liquids that were depleted
(assimilated and transferred to hindguts) if workers returned with a
leaf fragment in the field or transported a load in laboratory
experiments. Thus, in contrast to the destabilizing effects of external
nectar foraging, these results provide a novel mechanism promoting
symbiotic stability, as fungi provide fuel for foragers to harvest more
substrate for fungal crop production.
Keywords
- ant colony;
- attine;
- optimal foraging theory;
- proventriculus;
- social stomach
© 2016 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
