A Saturday special from the Gawler Ranges National park, South Australia
Archive for December, 2012
Ants active at dim light face a very interesting problem: they have to do everything that ants active in bright light do, but at very low light levels. It is hence that nocturnal ants have piqued my interest over the last few years. A recent addition to the growing list of nocturnal ants that I have had a chance to watch and study is the Dinosaur ant, Nothomyrmecia macrops. These ants similar to nocturnal Myrmecia, have a strong association with the local Eucalypt trees. They climb up trees, stay up the tree for a significant part of the night and return before sunrise. We always use headlamps with red filters to watch ants at night. It works fairly well and gives one a good opportunity to watch natural behaviour with relatively little intrusion. In Poochera, we knew roughly the type of trees that N. macrops prefer and we started checking such trees for foragers in the evening twilight.
Day 1, we started checking for the presence of these ants from just before sunset and continued till about 2am and had absolutely no luck. We appeased ourselves with odd looking Camponotus, some nice Podomyrma, and even a couple of Myrmecia, but not a single Nothomyrmecia. It was incidentally a new moon day and quite overcast giving us enough reason to console ourselves that perhaps the extremely low ambient light levels which did not support the activity of Nothomyrmecia. The next night, just 20 minutes after sunset, we found one forager on a tree and subsequently we found several on other trees. Each tree that we found ants on got a unique ID and was tagged with retro-reflectors. These reflectors are great as they shine only when light hits them directly. This is very handy to have when working at night and in fact all our fine equipment, especially forceps, magnifying lens are always tagged with these reflectors. And once we had these reflectors on the trees, we developed a routine of checking trees giving us the best chance of finding the ants. Every night, from then onwards we continued to see several foragers of Nothomyrmecia and identified 10 trees within a 15 sqm area on which we found foragers. We even found a dealate queen foraging on the tree – that doesn’t happen to often in the ant world!
We tracked a few workers returning with prey (mostly Dipterans and Hompoterans) to help us find the nests. But finding nests was quite a task as ants often played dead (the record was 25 minutes for one ant!) at the slightest of disturbances and hid well underneath the leaf litter. Add to it the highly inconspicuous nests that were hidden amazingly well within the leaf-litter. After a couple of nights, we eventually found 4 nests all located within 1m from the tree they foraged on. Most surprising was the structure of the nest entrance itself, which collapsed quite easily by the slightest of movements of a small twig or a gum nut. Nest density was quite high and on one particular occasion we found 3 nests within 30 cm of each other, foraging on the same tree. Even more surprising was that ants from multiple nests got along well, exhibiting no signs of aggression when we kept them together in small containers.
While working with the Central Australian desert ant, Melophorus bagoti, about seven years ago I discovered a very neat navigational strategy these ants used. Successfully returning foragers when passively displaced to unfamiliar locations continued to walk in the direction towards where the nest should be, oblivious to the passive displacement. The puzzling find was that ants only walked halfway toward their fictive nest and then began searching. The behaviour was neat and I wrote up a short article reporting this (1), and hypothesised that travelling halfway home is perhaps an optimal strategy for solitary foraging ants that occupy cluttered landmark-rich habitats.
After joining ANU, I met Allen Cheung who was wrapping up his PhD on mathematical modelling on insect navigation. We had brief discussions of how we could tackle this but our discussions didn’t get too far at that point. Subsequently during one of my visits to Bangalore, I was talking about this weird behaviour that I had seen to few of my friends at Koshy’s restaurant over a cup of coffee. A person whom I had never met before joined our table and conversation and after 10 minutes of my long drawn explanations, grabbed a serviette and wrote up an equation with an illustration to explain why this might be the optimal strategy! That was my introduction to Lex Hiby, who I learnt later was instrumental in developing the photo-id software to distinguish individual tigers by their stripes.
Over the next four years, we (mostly Allen) built and expanded on the Lex’s idea to now convincingly show that the chance discovery of ants travelling a specific fraction of the home vector is an optimal strategy and is dependent on the extent of familiar route for animals (2).
1.Narendra A. 2007. Homing strategies of the Australian desert ant Melophorus bagoti I. Proportional path-integration takes the ant half-way home. Journal of Experimental Biology 210: 1798-1803.
2. Cheung A, Hiby L & Narendra A. 2012. Ant navigation: fractional use of the home vector. PLoS ONE 7(11): e50451