Assessing colour preference in Aldabra giant tortoises (Geochelone gigantea)
Introduction
Information on the strategies adopted by chelonians to detect food is important to improve ex-situ conservation, helping to develop environmental enrichment and training programmes, providing the animals with choice and control over the environment. Most reptiles evolved high visual capabilities, in particular colour vision. Visual cues such as colours and brightness might be useful for herbivore reptiles to detect distant and important food sources (Blazquez and Rodriguez-Estrella, 2007, Costantini et al., 2005). Similarly to other reptile groups, most of the chelonians have a complex eye structure that allows a good vision and are characterized by a well-developed chemosensory system (Chelazzi and Delfino, 1986, Galeotti et al., 2007). However, no previous research has focused on the interactions between chemosensory and visual systems on stimuli (e.g., food) detection and discrimination. In general, few studies have been done on colour preference in chelonians. One previous study considered the colour preference in Hermann’s tortoises (Testudo hermanni) using natural and artificial stimuli. Results of the study highlighted a clear preference for yellow and red stimuli; in particular, each of these colour was preferred by the tortoises when in combination with colours such as white, blue and violet (Pellitteri-Rosa et al., 2010). It has been reported that yellow and red colour in flowers and plants might imply a high presence of carotenoids (Yuan et al., 2015) which are important in the diet of herbivore reptiles; therefore, red and yellow colour preference of tortoises might be explained with the selection of carotenoid-rich food. On the other hand, another study considered the effect of different colours (yellow, red and blue) on biting behaviour in loggerhead sea turtles (Caretta caretta). In this case, individual but not group-level colour preference was found (Piovano et al., 2012).
The olfactory system is also important in food detection in herbivore reptiles, such as chelonians (King, 1996). Indeed, nasal olfactory and vomeronasal systems are highly developed in tortoises and smell is largely used by these species to search for food (Avanzi, 2011, King, 1996, Poschadel et al., 2006). Therefore, both olfactory and visual systems are important in food detection and discrimination in chelonians; however, little research investigated the interaction between the two systems in foraging contexts in these species (Pellitteri-Rosa et al., 2010). This sort of research is important in advancing our understanding of chelonians, their natural history and the adaptive value of some characteristics, to improve their ex-situ and in-situ conservation. Therefore, further similar studies involving a wider array of both turtle and tortoise species are needed.
Aldabra giant tortoises (Geochelone gigantea) are endemic species of the Aldabra Atoll but have also been introduced on the Seychelles islands (Karanth et al., 2005). They have a thick and domed carapace, a long neck, rough and short legs. They could live solitarily or aggregate in herds and have a promiscuous mating system (Grubb, 1971).
Aldabra giant tortoises are mainly herbivores and eat mostly grass, leaves, woody plants, herbs and sedges (Grubb, 1971, Stoddart, 1969, Gerlach, 2006). The aim of the current study was to determine whether Aldabra giant tortoises (Geochelone gigantea) in a social context show a specific colour preference at both the group- and individual-level, between red and yellow stimuli. Moreover, we would like to verify the influence of both the olfactory and visual systems on feeding strategy, particularly looking for food. For this reason, red and yellow stimuli were scattered around the enclosure with a different food item hidden behind. Carrots vs. apples were used as food items in two different periods, to investigate the effect of food typology on the colour preference and to evaluate the interaction between visual and olfactory systems, improving our knowledge on the use of the chemosensory system in the Aldabra giant tortoise, a species which is underrepresented in the literature. According to previous literature, we expect a colour preference in our tortoises, at least at the individual level. Moreover, an influence of food items could be found, showing an effect of the olfactory system when tortoises search for food. In addition, assessing group and individual colour preference might be particularly useful in captive animal husbandry. A preference for red and yellow stimuli has been previously reported in Hermann’s tortoises (Pellitteri-Rosa et al., 2010); therefore, we used these colours for the stimuli in our study to find out which of them could be more suitable for the captive husbandry of a land tortoise as the Aldabra giant tortoise. Identifying individual differences in animal preferences might be valuable in the development of diets and enrichment programmes as choices are added into the environment (Fernandez et al., 2004, Passos et al., 2014, Mehrkam and Dorey, 2014, Mehrkam and Dorey, 2015). In particular, data collected in the current study will be considered in a positive reinforcement training programme with a target of different colour for each individual, according to the colour preference, motivating all subjects to get involved in the training programme. This will improve the husbandry of this group of Aldabra giant tortoises and will be useful to move the animals between different enclosures or in specific positions.
Section snippets
Subjects and area
The study was carried out with a group of eight Aldabra giant tortoises, 3 females and 5 males, housed at Parco Natura Viva, Bussolengo (VR), an Italian zoological garden. All tortoises were nine-ten years old. The tortoise enclosure was made of an outdoor and an indoor exhibit. The outdoor exhibit was a 515 m2 grassy semi-elliptical area with branches, trees, a mud pot and a water pool. The indoor exhibit was a 40 m2 glasshouse in which humidity and temperature were kept at optimal levels for
Overall colour preferences
The tortoise overall colour preference (regardless to the food behind the balls) was assessed by pulling together data from the two periods. At the individual-level, one out of eight subjects displayed a significant colour preference and was red-preferent (z > 1.96); the other subjects showed no preference (−1.96 < z < 1.96) (Table 1). At the group-level, the median PI ± SD was −0.07 ± 0.21. One-sample sign-test revealed no bias in the distribution of colour choices (N = 8, P = 0.855).
First and second period
In the first period,
Discussion and conclusion
The main findings of the study are (a) a lack of group-level yellow-red colour preference in Aldabra giant tortoises; at the individual level only one tortoise displayed a significant preference and was red-preferent (b) an influence of food typology on the colour preference. At the group-level the symmetry of the PI distribution suggests no overall yellow-red colour preference. Similar results were found for both the first and second period in which no yellow-red colour preferences were
Acknowledgments
We would like to thank Dr. Cesare Avesani Zaborra and Camillo Sandri for allowing this study to take place in Parco Natura Viva. Special thanks should be given to Luigi Sala for his help in the project design and accomplishment. Furthermore, we would like to acknowledge Raffaele Grisa and Kim Tran Thi for their collaboration and patience during the study period. Finally, we are grateful to the anonimous reviewers and to the editor Dr Olga Lazareva for the helpful comments and revisions which
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