Does size matter when using celestial cues to navigate towards home?

A blog post highlighting the article by R. Palavalli-Nettimi and A. Narendra in Insectes Sociaux

By Ravindra Palavalli-Nettimi and Ajay Narendra

Imagine finding a location in a new city without any map. How would you navigate toward your destination?

If you were an ant, you could use celestial cues such as the position of the sun or the polarised skylight pattern (Wehner and Strasser 1985; Zeil et al. 2014) as a compass to navigate in the direction of your destination (e.g., nest). The compound eye of an ant has a few special ommatidia that are sensitive to polarised skylight (light waves oscillating in one orientation). However, the eye size and also the total number of ommatidia in the ants’ eyes decrease with their body size. Some ants have close to 4,100 ommatidia (Gigantiops destructor) in their eyes while a miniature ant has a mere 20 ommatidia (Pheidole sp.). However, it is not clear how this variation affects their ability to navigate.

 

Picture1

Size variation in ant heads.

 

In this study, we investigated how size variation affects ants’ ability to use celestial cues to navigate towards their nest.

To test this, we captured ants on their way to their nest and displaced them to a circular platform. The displacement site was at least 500-1,000 m from the ants’ nest and was surrounded by a creek. Thus, the ants had never foraged there and could not use landmark cues to navigate, but instead, they had to rely on celestial compass cues to walk towards their nest. We filmed the paths taken by the ants using a video camera and later digitized their head position frame by frame.

We found that having fewer ommatidia does not affect the ants’ ability to use celestial cues. The ants’ heading direction on the platform did not significantly differ from the fictive next direction. Since larger ants have greater strides and thus travel more distance for the same number of strides, we also analyzed their heading direction at a distance on the platform scaled to the body size of the ants.

We also found that the smaller ants were slower and had less-straight paths than the larger ants, even after controlling for differences in leg size (correlated with body size and head width) and stride length. This finding means that a reduced ability of the smaller ants to access celestial compass information results in a less straight path and reduced walking speed. However, the overall ability to initially orient towards the nest using a celestial compass is retained in miniature ants. Thus, while miniaturization in ants can affect their behavioral precision, it may not always lead to a loss of vital behavioral capability such as using celestial cues to navigate.

 

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Paths and heading directions of various ants that differed in head width and ommatidia count.

 

In conclusion, finding a destination in a new city might be a lot easier if we were ants—of any size—and could use celestial cues!

References

Wehner R, Strasser S (1985) The POL area of the honey bee’s eye: behavioural evidence. Physiol Entomol10:337–349.

Zeil J, Ribi WA, Narendra A (2014) Polarisation vision in ants, bees, and wasps. In: G Horváth (ed) Polarized light and polarization vision in animal sciences, Springer, Heidelberg, pp 41–60.

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