Facets of brood theft in a tropical ant Diacamma indicum

Paul, Bishwarup (2019) Facets of brood theft in a tropical ant Diacamma indicum. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Bishwarup Paul (11IP05)) 11IP05.pdf - Submitted Version Restricted to Repository staff only Download (7MB)

Abstract

Brood is an important part of a social insect colony. Not only does brood grow into the future workforce and reproductives of a colony, but also acts as a food reserve and is cannibalized to obtain nutrition during scarcity of food. Therefore, brood is essential for the survival of a colony and behaviours increasing brood items in a colony with minimum investment is expected to be favoured by natural selection. One such behaviour is brood theft. Thievery is ubiquitous across the animal kingdom, and theft of brood has been reported in social insects, especially ants. Brood theft has been well documented in ants that are found in the temperate region and those that belong to the subfamilies Myrmicinae and Formicinae. In this thesis we report brood theft for the first time in a primitively eusocial ant Diacamma indicum (subfamily Ponerinae) found in the tropical Indian subcontinent. We begin by assessing the relative importance of brood in this species by giving colonies the opportunity to procure brood from an unguarded nest while they were under the emergency of emigrating from their nest. After observing colonies’ preference towards procuring self as well as conspecific brood without hindering the process of relocation, we proceeded to investigate theft of brood from guarded nests. We observed occurrence of intraspecific brood theft in this species under laboratory conditions as well as in the natural habitat. Exposed nests during relocation was found to increase a colony’s vulnerability towards brood theft by conspecifics. Theft in this species was observed to be opportunistic unlike other species where colonies conduct organized raids. The stolen conspecific brood was not consumed or treated differently from self-brood, rather the stolen brood and the resulting adults were integrated into the colony. Brood theft appeared to be beneficial for thieves as they gained significant increase in their future workforce with minimum risk. While investigating the mechanism of theft in this species, we observed that aggressive interactions were key to defending a colony’s brood from thieves. Thieves were persistent in their attempts despite facing aggression during their visits. Though majority of theft attempts were prevented, thieves were able to ensure success in the rest of the attempts by acting quickly, being furtive and stealing unguarded brood items. Thieves also reduced the time required for orienting the brood to handle it better and increased their speed for egressing from the victim’s nest to avoid getting caught. Direct threat posed by aggression from non-nestmates during theft attempts was the key factor inducing thieves to modify their behaviour. Reward in terms of gain in brood was found to increase the drive of colonies towards procuring additional brood and helped form memory of the location of the brood source. Further investigation of memory formation in this species regarding location of target nest during relocation revealed that both brood transporters and tandem leaders who transport adults form reliable working and long-term memory and use it to navigate towards the nests efficiently. Occurrence of brood theft in a tropical species is intriguing as the cost of rearing brood is considered to be low in tropics due to abundant resource and relatively conducive temperatures; still we have found conspecific brood theft to occur in D. indicum while colonies are under an emergency such as nest relocation. We propose a new dimension to the risks of relocation in the form of brood theft by conspecific neighbours. The opportunistic way of brood theft in this species is a rudimentary method of procuring resources compared to the organized raids observed in highly eusocial species; thereby leading us to speculate that examination of thieving behaviour in other primitively eusocial species may help shed light on the origin and reasons of brood theft, as well as several proximate questions.

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