We studied in the field the load transport behavior of workers of the polymorphic Mediterranean seed harvester ant Messor barbarus. Individual ants used two different methods to transport food items: carrying and dragging. The probability of dragging instead of carrying varied significantly with both the mass of the item transported and its linear dimension. Moreover, the values of item mass and length at which dragging began to occur increased with increasing size of the workers. However, larger ants began dragging at decreasing values of the relative mass represented by the items transported, which reflects different biomechanical constraints resulting from allometric relationships between the different parts of their body. Transport rate was significantly higher in large ants but varied in the same way for workers of different sizes with the relative mass of the item transported. Nevertheless, although large ants were individually more efficient than small ants in transporting food items, the relative transport rate, defined as the ratio of transport rate to the mass of the ant, was higher for small ants than for large ants. Colonies should thus have a greater benefit in investing in small ants than in large ants for the transport of food items. This may explain why the proportion of large ants is so small on the foraging columns of M. barbarus and why large ants are most often employed in colonies for tasks other than transporting food items.
The authors declare no competing or financial interests.
A.B. and V.F. conceived, designed and conducted field experiments and interpreted their results; V.F., P.M. and A.F. conceived the morphological study of the ants and A.F. performed the morphological measurements and analyses; M.M. developed the R script to determine the position of the center of mass of the ants; V.F. and A.B. did the statistical analyses of the data; V.F. and A.B. wrote the paper and A.F. and P.M. made essential contributions to the text.
A.B. was financed by a doctoral grant from the Fundació Crèdit Andorra and an ATUPS grant from Université Toulouse III – Paul Sabatier. Travel and accommodation expenses for V.F. were supported by a grant ‘Action Ponctuelle de Coopération’ from Université Toulouse III – Paul Sabatier.
Morphometric and transportation datasets are available from the Zenodo repository http:/dx.doi.org/10.5281/zenodo.60347
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.141556.supplemental
- Received April 20, 2016.
- Accepted July 10, 2016.
- © 2016. Published by The Company of Biologists Ltd