Implications of miniaturisation in ants on their vision and visual navigation
thesisposted on 29.03.2022, 00:57 authored by Ravindra Palavalli-Nettimi
Miniaturisation is the evolution of extremely small body size compared to their ancestors, reduction beyond which is constrained due to design limitations. Miniaturisation is commonly observed in many animals, including arthropods. It affects morphological features, nervous system, physiology and ecology of the animal. Typically, miniaturisation leads to simplification, modification or loss of organs. As a consequence of their smaller sensory organs and absolute brain size, miniaturisation is hypothesised to result in inferior behavioural capabilities. However, unifying behavioural measures testing the hypothesis are lacking. Ants present an ideal study system to test this, since irrespective of their size, individuals face similar challenges, especially during visual navigation. In this thesis, I aim to empirically test whether miniaturisation in ants affects their vision and visual navigation behaviours. I begin with a systematic review and a meta-analysis to identify the implications of miniaturisation on vision, brain neuropils and visual navigation in insects (Chapter 1). Using a comparative approach, I studied ants of different sizes to determine how a reduction in the number of ommatidia and the size of the lens affects spatial resolution and contrast sensitivity. I show that miniaturisation decreases contrast sensitivity and spatial resolution in ants (Chapter 2). Spatial resolution is important in detecting obstacles/landmarks during navigation. I demonstrate that the reduced spatial resolution affects obstacle detection behaviour in homing ants (Chapter3). Once ants are close to their home, they pinpoint its entrance using landmarks. In an Australian bull ant Myrmecia pyriformis, I tested whether reduced body/eye size affects this behaviour. I show that a reduced size within-colony does not affect their ability to learn and use a nest-associated landmark (Chapter 4). A fundamental form of navigation involves following a compass. Using a behavioural experiment, I illustrate that this ability is not affected by a reduced number of ommatidia, suggesting that high spatial resolution may not be necessary for this behaviour (Chapter 5). Thus both physiological and behavioural studies show that there is a clear effect of miniaturisation on the animal's spatial vision and obstacle avoidance behaviour. However, miniaturisation did not affect the ability to pinpoint landmarks or to use a compass. These findings add to our understanding of the evolution of miniature ants which may be capable of various visual navigation behaviours in spite of limited sensory abilities.