For most of the animals, if not all, finding their way to a particular place is crucial for survival. To address this challenge of way-finding, different animals have evolved with different homing strategies. Social hymenopterans like honey bees, ants and wasps are of special interest – foragers of these insects show excellent homing capabilities while having simple neural resources. In this study field, honey bees and ants (desert ants, in particular) are among the most studied animals. Compared to these insects, our understanding on the homing mechanisms of social wasp is rather poor. For my thesis, I have studied homing behaviour of the tropical social wasp Ropalidia marginata, a predator in their foraging habit. To begin with, first I had to know their typical foraging range, which I found to be within about 500 m from their nest. Forager wasps possess a surprisingly well-developed familiarity with their foraging landscape, apparently more intricate than honey bees and desert ants. They acquire this spatial familiarity through flying around the landscape before starting foraging for food. Compared to honey bees and desert ants, this learning period in wasps appears to be much longer – this can be attributed to the much higher density of the tropical landscape in which they have evolved. I have also found that, if needed, they can fly to a distance of about 1.5 km for foraging and can return to their nest even if passively displaced to familiar and unfamiliar places. To return from unfamiliar places, they probably use some sort of searching mechanisms – a skill that they improve with their age. Such searching behaviour is prevalent throughout other hymenopteran insects. I conclude that capability and mechanisms of spatial orientation, navigation and homing in animals are much influenced by their evolutionary origin and the environment in which they have evolved.

All living organisms require food for growth and survival. Heterotrophs depend on autotrophs such as green plants which can synthesize their own food unlike heterotrophic animals. Among heterotrophs, only humans and some insects have the remarkable ability to cultivate crops for food. While humans cultivate plants, three insect lineages—ants, termites, and beetles—cultivate fungi inside their nests in obligate mutualistic exo-symbioses. Interestingly, just like human agriculture, insect fungus farms are also threatened by weeds and pests, e.g. the farms of fungus-growing termites which cultivate Termitomyces fungi can be overgrown by weeds such as the parasitic fungus Pseudoxylaria. Studies on ant and beetle fungus-farming systems have uncovered the important role of chemicals and behaviour in helping these insects to protect their crops from parasitic fungi. On the other hand, studies on the termite system till now, have only revealed the presence of antifungal compounds and actinobacteria which are largely non-specific and inhibitory to the mutualistic crop fungi. Antifungal behavioural mechanisms, if present, are yet to be discovered. Therefore, this thesis focuses on different anti-Pseudoxylaria mechanisms employed by fungus-growing termites, viz. role of nest abiotic factors, mechanism of fungal recognition by termite hosts, behavioural response of termite to Pseudoxylaria presence and coupling of this behaviour to anti-Pseudoxylaria activity. The present thesis has been divided into six chapters. CHAPTER 1 gives a brief literature review on fungus-farming insects and the different mechanisms which insects employ in order to keep their fungal farms safe from growth of parasitic fungi with specific reference to fungus-growing termites. The obligate mutualistic interaction between termites and the Termitomyces fungus is 19–49 My-old and is, therefore, a very ancient agriculture system. The mutualistic fungus is cultivated on partially digested plant matter called fungus comb inside the nest and harvested by termites for nutrition. At the same time, the weedy fungal parasite Pseudoxylaria can compete with the mutualistic fungus for nutrition leading to negative effects on the fungal farms. Termite hosts are believed to use abiotic factors, antibiotics and hygienic behaviours to keep their fungal gardens free from parasitic fungi such as Pseudoxylaria. However, the actual mechanisms used by termites against parasitic fungi are unclear. Unravelling the proximate mechanisms used in fungal cultivar protection is central to understanding the evolutionary stability of these farming mutualisms. CHAPTER 2 examines the diversity and population genetic structure of Termitomyces and Pseudoxylaria strains associated with the fungus-growing termite Odontotermes obesus. Genetic diversity of cultivar and parasite could have important implications for the stability of the mutualistic interaction, e.g. genetic clonality arising from monoculture is generally thought to make populations more prone to infection by parasites. Using molecular phylogenetic tools, within-nest genetic homogeneity was found in Termitomyces species but not in Pseudoxylaria species. Lower OTU but higher genotypic diversity (within the most abundant OTU) was found in the genus Termitomyces compared to Pseudoxylaria. Additionally, population genetics methods suggested a sexual population structure for Termitomyces and clonal propagation for Pseudoxylaria species. This is the first study to investigate the population genetics of the symbiotic fungi associated with the termite genus Odontotermes or any other termite species from India. In CHAPTER 3, the effect of nest micro-environment alone on the growth of the parasitic fungus Pseudoxylaria was examined. For this, seasonal changes in nest xiii temperature and CO2 were recorded and in situ and ex situ growth experiments were performed on Pseudoxylaria. The monthly pattern of mound temperatures was found to be similar to the outside—cycling from highs in summer to lows in winter—but characterised by dampened variation compared to high daily fluctuations outside. Moreover, the mound CO2 levels were found to be orders of magnitude above atmospheric levels and, unlike the outside, were characterised by daily and monthly fluctuations. With in situ experiments during summer and winter, the effect of these dissimilar conditions—inside and outside mounds—was examined on Pseudoxylaria growth. The growth of the parasite was found to be greater inside than outside the mound. Following this, the growth of different parasite isolates under controlled ex situ conditions was examined—spanning the variation in environmental conditions that mounds exhibit daily and seasonally. High CO2 levels decreased parasitic fungal growth in general but temperature had an isolate-dependent effect. Taken together, these results suggested that the parasite is adapted to survive in the mound. However, mound environmental conditions still seemed to exert a negative effect on parasite growth, even if they cannot inhibit Pseudoxylaria completely. These results shed light on the possible new role of termite-engineered structures in impacting parasitic fungus ecology, independent of any direct role of termites in suppressing parasite growth. This is the first study to investigate the effect of abiotic factors on Pseudoxylaria growth. In CHAPTER 4, whether termites can differentiate between Termitomyces and Pseudoxylaria was investigated. In a novel, laboratory-based choice assay, termites displayed a differential response towards the two fungi by burying the Pseudoxylaria with agar. Also, termites were found to be able to differentiate between the fungi using olfactory cues, i.e. smell, alone, for this task. The mutualistic and parasitic fungi were found to emit unique volatile bouquets which could help termites to distinguish between them. This is important because, whether termites use antifungal compounds or hygienic behaviours, it is crucial that they are able to differentiate between the parasitic and mutualistic fungi so that they can selectively use antifungal mechanisms—whether chemical or behavioural—against Pseudoxylaria. This is of special significance because, many actinobacteria and anti-Pseudoxylaria compounds isolated from this system till now, lack specificity and inhibit the mutualistic Termitomyces as well. Also, fungal grooming and weeding behaviours as displayed by fungus-growing ants have not yet been reported in termites. This is the first study to show that termites have the behavioural capacity to differentiate between the mutualistic and parasitic fungi in an ecologically relevant setting. In CHAPTER 5, whether the burying of Pseudoxylaria could affect its growth was investigated. It was found that termites can utilise agar, glass beads and soil for deposition over the offered fungal plugs but the use of agar and glass beads did not inhibit Pseudoxylaria growth effectively. On the other hand, soil deposition was found to decrease growth of both Pseudoxylaria and Termitomyces fungi post-burial. However, Pseudoxylaria was found to be affected more strongly than Termitomyces. Further, hypoxia acting alone seemed to decrease only Pseudoxylaria survival without any apparent effect on Termitomyces. Therefore, hypoxia induced by soil deposition may be the reason behind the decrease in Pseudoxylaria survival. However, presence of antifungal compounds can not be ruled out and they may be selectively applied in larger quantities on Pseudoxylaria with soil deposition. This study demonstrates an anti-Pseudoxylaria activity of this insect behaviour, unique to termites among fungus-farming insects, to the presence of the parasitic fungus. CHAPTER 6 concludes the findings of this thesis and suggests a working model for the mechanism of growth suppression of Pseudoxylaria inside a termite nest. In particular, focus is on the important role of abiotic factors when combined with termite behaviour in the apparent absence of Pseudoxylaria from termite nests. These results not only shed new light on how the ecology of these fungi is affected by their termite host but also reveal the mechanistic bases that may contribute fundamentally to the evolutionary stability of this ancient mutualism.

Tropical forests are storehouses of more thanhalf of the world‘s biodiversity and play a key role in global carbon, water and energy cycles. However, as a consequence of rapid anthropogenic climate change, biodiversity and climate functions of these forests are under a threat. Climate is changing not only in mean state but its variability is increasing, with extreme events such as droughts, heat waves and storms also rising. Water is fundamental to plants‘ existence, and in the tropics, is a key determinant of plant species‘richness, composition, growth and survival. There is thus an increasing interest in understanding how changing rainfall may cause functional changes in forests or change their species composition. Therefore, the overarching goal of thisdissertation was to understand the impact of water variability on tropical forest tree growth and vulnerability to drought.Forest tree growth along spatial and temporal rainfall gradientsObservational studies that measure whole forest tree growth along spatial or temporal gradients of rainfall are the most common way of formulating forest growth response curves to water availability, when manipulative experiments are cost-prohibitive or impractical (fire or large mammal disturbance). In the tropics, since very few species show anatomically distinct tree rings, estimating tree growth from trunk diameter is the standard practice to obtain growth patterns across species. However, this method—of equating woody growth to diameter change--is susceptible to bias from water-induced stem flexing. In the absence of bias correction, temporal variability in growth is likely to be overestimated and incorrectly attributed to fluctuations in resource availability, especially in forests with high seasonal and inter-annual variability in water. This problem has been largely ignored in the absence of any corrective measure and due to under-appreciation of the magnitude of error. While diameter re-censuses in permanent sampling plots (PSPs) have been most commonly done at 3-5 year scale (using a graduate tape), increasingly they are done at seasonal and annual scales (using band dendrometers) to closely match variation in rainfall, the scales at which hydrostatic bias may be greater in magnitude relative to woody growth. Besides, along a spatial rainfall gradient, inter-annual variability in water may vary, causing systematic differences in the hydrostatic bias for forests along the gradient. Therefore, one broad objective of this thesis was to evaluate the problem of hydrostatic bias in whole forest growth-rainfall relationship at annual and supra-annual scales, for temporal as well as spatial rainfall gradients and propose and test a novel corrective solution.Further, it also examines if growth-diameter relationship vary along the spatial gradient, which it may arise due to differences in light environments and/or disturbance history and species composition. The missing link of Eco-hydrology Differential responses of tree species in terms of growth and survival to variation in water that they can access, the proximate cause is likely shaped through their life-history strategies, the ultimate cause. However, we neither know the depths at which the diverse tree species in a forest draw water from and its dynamics, nor variation in water at those depths vis-à-vis rainfall patterns—for lack of appropriate methods. This has been a key missing link in understanding how water shapes trees‘ life-history strategies, their demographic trade-offs and co-existence, and also our predictive ability to determine species-specific responses to changing rainfall patterns, especially droughts. Since droughts are highly stochastic events and trees‘ responses to their drought ―experiences‖ may be revealed at decadal scales, long-term evaluations are key. Therefore, the second broad objective of this thesis was to develop a framework to determine trees’ water uptake depths, variation in water availability at those depths and trees’ demographic responses over multiple decades. From this, to understand how belowground hydrology shapes drought-vulnerability, demographic trade-offs and coexistence of forest tree species. This thesis titled—Eco-Hydrology of a Seasonally Dry Tropical Forest: Tree Growth, Belowground Water Dynamics and Drought-Vulnerability—is organized as follows: Chapter 1 lays down an introduction to the thesis, followed by a description of the study site and datasets used in the thesis in Chapter 2. This thesis uses a variety of methods and multiple datasets, all of which are from the protected Seasonally Dry Tropical Forests of the Western Ghats in southern India in the Mudumalai and Bandipur National Parks. It is then followed by three data chapters: Chapter 3 describes the seasonal fluctuations in a five year long (1980-1985) tree diameter time series (using dendrometers) of a Seasonally Dry Tropical Forest in Bandipur National Park to illustrate the issue of hydrostatic stem-flexing. It investigates the possibility that band dendrometers may themselves underestimate stem shrinkage at diurnal or seasonal scale. It also evaluates if there could be a best season and time of the day for undertaking forest diameter censuses that can minimize hydrostatic bias. Chapter 4(published in Forest Ecology and Management)measures the hydrostatic bias in a sample of trees in a 50 ha PSP of a Seasonally Dry Tropical Forest in Mudumalai National Park, and proposes a novel way to correct this bias at the whole community level in the 20 year long 4-year interval growth time series. Chapter 5 (in review with Environmental Research Letters) investigates and presents two new confounding factors in growth-rainfall relationships along a spatial rainfall gradient: hydrostatic bias and size-dependency in growth rates. For this it evaluates forest tree growth estimates in seven 1-ha PSPs (~800 trees, 3-year annual time series 9using dendrometers) along a 1000 mm rainfall gradient spanning a mesic savanna-moist forest transition in Mudumalai National Park. Using the period for which seasonal diameter time series was available (2 yrs), it evaluates if the extent of seasonal fluctuations systematically vary along the gradient—most likely due to hydrostatic stem flexing. It also describes the presence of an anomalous size-diameter relationship in the mesic savanna from a large plots (50 ha PSP, diameter records using graduated tape). These observations are then used to draw insights for ―space for time‖ substitution modeling. Chapter 6 (in prep for Nature Plants) analyses belowground water environments of trees over two decades (1992-2012), a period that includes a prolonged and intense drought, in the 50 ha PSP of a Seasonally Dry Tropical Forest in Mudumalai. It uses a locally parametarised dynamic hydrological model in which site rainfall is also a forcing variable. It then develops a novel dynamic growth model and inversely estimates water uptake depths for adult trees of all common species (include ~9000 trees) in the PSP from their above-ground growth patterns over two decades vis-à-vis belowground water availability at multiple depths. It then examines if species‘ water uptake depth obtained thus is a predictor of their drought-driven mortality. Finally, this is used to evaluate the hydrological niche partitioning tree species operate under and how that drives their water uptake strategies, demographic trade-offs, and drought-vulnerability. Summarizes the thesis and suggests future directions

The thesis presents observations on the CO2 concentration and carbon isotopes in air CO2 (δ13C) to constrain the inter-annual variability of carbon inventory over the Southern Ocean between the years 2011-2013. Based on the observation, the region of CO2 venting was identified over the Southern Ocean. Further, isotopic characterization allowed inferring about the possible sources of CO2 degassing and contribution from the dissolved inorganic carbon (DIC) that exsolved to generate CO2. It is concluded that the origin CO2 is mainly from the degassing of CO2 available from the dissociation of DIC or organic degradation. Live Foraminiferal samples of Globigerina bulloides from towing were captured, separated and analysed for δ18O and δ13C from various locations across the Southern Ocean between 10°N−60°S. A large similarities in the estimated values (deduced from simultaneous composition of ocean water 18O, δ13C in DIC and temperature i.e. SST under equilibrium condition) and measured δ18O and δ13C values were observed until 40°S from the equator, and hence it was concluded that the calcification depth of G. bulloides is confined to a depth of ~75-200m till 40°S latitude. However, further south (>40oS) disequilibrium from the estimates was detected. A number of possible reasons were cited for the observed disequilibrium such as (1) Deeper depth habitat (2) Partial dissolution (3) Non-equilibrium calcification (4) Oceanic Suess Effect and (5) Genetic Variability. A box model of isotopic mass balance was presented in this study to explain the pattern of enrichment in the 13C values of sea water DIC with latitude (up to about 43°S). The model shows that a steady state of the carbon isotope ratio of water is achieved in a relatively short time of ~5000 days. Rainwater isotope in the open marine condition across the latitudinal transects over Southern Ocean marking zone of precipitation and evaporation is another element of this thesis. A variation with excess lighter isotopes in rainwater was observed in high latitude rain in this study. Observed isotopic depletion is attributed to rainout process over the ocean. The average rainout fraction over the Southern Ocean in the region of zone of precipitation is ~44%, while it drops to ~25% in the zone of evaporation. Second part of the thesis presents a novel method of isotope thermometry which is called “clumped isotope (13C18O16O16O-2 in the calcite structure) thermometry”. A revision in the thermometry equation relating 47 vs T in synthetic carbonates precipitates and otoliths was proposed. The revised calibration was used on fish otoliths from the modern and past environment to estimate the temperatures. Together with the clumped isotope, conventional stable isotopes in the shell carbonates were measured to effectively reconstruct the seasonal fresh water fraction at seasonal time scales.

Social insects are the most dominant terrestrial fauna for the last 50 million years. This tremendous ecological success is accompanied by the fact that sociality has evolved multiple times independently and achieved highest degree of complexity in insect lineages. The remarkable social organization found in insect societies is the result of finely balanced cooperation and conflict among the colony members. A typical hymenopteran colony is characterised by one or a few queens monopolizing reproduction and several sterile workers co-operatively raising brood and performing colony activities. The colonies are often conceptualized as superorganisms where groups of cooperative workers are compared with organs in the body, each of which accomplish a particular task like brood care, foraging and defence. The choice of tasks is often regulated by a systematic age polyethism. As the queens monopolize reproduction, they serve as the sole suppliers of eggs in the colony. Therefore, loss or death of the queen creates a crucial void which exposes the colony to potential reproductive conflict for the position of egg-layer. This crisis is expected to be extreme in monogynous colonies. The situation is rescued only after a new queen is established, and the whole process is known as queen succession. I am interested in this crisis management, and my thesis deals with potential and realized conflicts associated with queen succession and behavioural strategies involved in resolution of these conflicts. The queen can be replaced in two ways - either by a newly eclosed specialized reproductive individual, which happens in highly eusocial hymenopterans, or by an existing member of the colony (worker), as it happens in primitively eusocial hymenopterans. Unlike in highly eusocial species, the workers of primitively eusocial species retain their ancestral capability of mating and activating ovaries to produce both sons and daughters, which makes them suitable for taking up the role of queen in their lifetime. Hence, primitively eusocial species provide a unique situation where loss or death of the queen might result in severe reproductive conflict as the queen can be replaced by any one of the existing workers. Strictly monogynous colonies of the tropical primitively eusocial wasp Ropalidia marginata provide ideal opportunities to study the reproductive conflict and its resolution associated with queen succession because the queen is frequently replaced by one of her nestmates resulting in a serial polygyny. These queens have highly variable tenures of queenship ranging from seven to over 200 days, which, together with the fact that they are replaced by a variety of relatives such as daughters, niece and cousins, suggests a potential reproductive conflict with variable degrees of complexity. I have divided my thesis in three parts which are as follows -Natural queen turnover: Previous works from this lab have tried to characterize the queen succession in R. marginata colonies by experimentally removing the queen from the colony. As this design involves the experimenter intervening at a random point of the colony cycle, the colony might not respond in the similar way as it might have done to a natural succession necessitated by loss or death of the queen. But rarity and unpredictability of natural queen turnovers made them difficult to study. Therefore, in this section, we gathered a dataset of long-term and opportunistic quantitative behavioural observations on eleven natural queen turnovers and compared them with available data on queen removal experiments. All our queen removal experiments resulted in a hyper-aggressive potential queen who gradually reduced her aggression, activated her ovaries and went on to become the unanimously accepted new queen of the colony if the original queen was not returned. Here we found a similar phenomenon in natural colonies where a single un-challenged potential queen took over the colony as new queen after the old queen was lost, died or was driven out of the colony. In some of the natural colonies, the transition was preceded by aggression shown by the potential queens towards their nestmates including the queens, which indicates that they might have pre-empted the transition. The potential queens in natural colonies started laying eggs much faster than in experimental colonies suggesting their physiological readiness for the transition. How does a colony respond to a declining queen?: As we could show that some of the potential queens might perceive the upcoming queen turnover, a fair prediction would be that they sense it through the declining fertility status of the queens. Therefore, we tried to ex-perimentally induce situations where the queen appears to be declining, expecting that it might lead to a queen turnover. The growing evidence suggests that R. marginata queen maintains her status by applying a pheromone on the nest surface by rubbing the tip of her abdomen. We knocked down the nest to deny the queen the surface for applying her pheromone, and argued that the queen would be overthrown as the workers would sense her as infertile. To our surprise, the queen maintained her status in six out of seven colonies by applying her pheromone on the entire surface of the cage. However, the effectively insufficient concentration of pheromone elicited aggression from workers towards the queen, and the queen retaliated back with aggression. These results suggest that the pheromone, being an honest signal of fertility, is extremely important for the queen for maintaining her reproductive monopoly, and the workers are able to perceive the decline of the queen from her pheromone. Queen-successor conflict over access to reproduction: Here we more explicitly looked at the potential reproductive conflict between the queen and her successor over access to direct reproduction. We used the theory of parent-offspring conflict proposed by Robert Trivers (1974) as the conceptual framework and adapted it to unravel the pat-tern of queen-successor conflict in R. marginata colonies. According to this idea, we expected that there should be a pre-conflict zone where the queen and the successor both would agree that the queen should continue to reproduce, followed by a conflict zone where the successor would try to takeover but the queen would hang on, finally followed by post-conflict zone where they both would agree that the successor should reproduce. To test this expectation, we maintained the queen and the potential queen on either side of a wire-mesh partition, each with randomly chosen half of the workers. It allowed the potential queen (successor) to establish herself and then we reintroduced the queen to her side of the mesh daily till the queen gave up. We could behaviourally characterise all three zones which always appeared in the expected sequence. The pre and post-conflict zones had no aggressive interaction between the queen and the potential queen, whereas the conflict zone was characterized by aggressive falling fight between them. This is our first success in experimentally creating overt conflict between the queen and her successor. Overall we can say, that the queens and the potential queens of R. marginata show great behvioural plasticity which might have been shaped by natural selection as an adaptation for conflict resolution. We could show that the potential queens sometimes can predict the upcoming transition and pre-pare themselves accordingly, whereas they can also react to an experimentally created sudden loss of queen by hugely elevating their aggression. The docile queens, on the other hand, maintain their reproductive monopoly by a pheromone, which is essentially a feature of highly eusocial species. But these docile queens have not lost their capability to show aggression and can use that to complement the insufficient concentration of her pheromone. This and the behaviour of potential queens in their establishment phase are strongly reminiscent of typical primitively eusocial species. We conclude that Ropalidia marginata is, perhaps, a particularly advanced primitively eusocial hymenopteran situated on an evolutionary continuum from primitive to highly eusocial species.