Multiple traits, including antipredator responses, foraging behaviour, development rate and fecundity, contribute towards an organism's fitness. These diverse traits interact through the shared resources allocated to maximise fitness. Ecological conditions can affect these interactions by driving increased investment in a particular trait at the cost of other traits (inter-trait trade-off). How are these interactions affected when an individual goes through different development stages, which serve different functional roles? What are the consequences for an individual's fitness in such a complex life-cycle?
To understand the trade-offs that operate at multiple levels in a complex life cycle, I investigated the role of early predation risk conditions across the life cycle of the holometabolous insect– Aedes aegypti. Aedes aegypti has four major stages: egg, larva, pupa, and adult. Previous work suggests that ecological conditions experienced by the larval stage affect adult traits. However, we lack knowledge of how early larval conditions affect the pupal stage and the cumulative effects of both stages on adult traits.
To understand multistage, inter-trait trade-offs, I exposed the immature stages to a key selection pressure, predation risk. Leading to our aim of understanding the combined and individual roles of larval and pupal stages in managing trade-offs, I first unravelled the relationship between larval and pupal stages. I adopted behavioural approaches to examine (1) the carryover of larval predation-risk experience on the pupal stage to understand if a pupa independently responds to risk or whether the larval experience influences its response to risk conditions. I discovered that a pupa that has experienced predation risk as a larva modulates its response to predation cues, showing that the larval experience affects pupal traits. This experiment showed that a behaviour or experience with an adaptive value can overcome the barrier of metamorphosis. Since Aedes aegypti larvae and pupae are found in group settings, I also examined (2) the behavioural manifestation of predation experience in a group setting. This allowed me to understand the abilities of the pupal stage in responding to risk conditions under different contexts. I found that experience does not influence the behaviour of an individual pupa if it is in a group. This is probably because being in a group is an antipredator response itself. My first two chapters highlight the need to include the pupal stages in life history studies because of their ability to process different cues while responding to their environment.
After discovering the context-dependent antipredator response of the pupal stage, I examined (3) the multistage trade-offs, driven by early predation risk conditions, between larval-adult, pupal-adult and larval-pupal-adult stages. I performed lab-based controlled experiments where I followed all the life stages under risk and no-risk conditions. On analysing diverse morphological, biochemical and life-history traits of risk-experienced and naive individuals, I demonstrated that the fitness consequences differ for males and females, and it may start from larval-pupal trade-offs and accumulate as the risk persists. I also found that the pupal stage, like the larval stage, can respond to risk conditions both behaviourally and physiologically. However, it is less well-equipped than the larval stage to manage the trade-offs. Fitness consequences are worse when the pupal stage alone experiences risk. Hence, different stages can contribute to trade-offs that lead to various fitness consequences.
My thesis yields novel insights into life history evolution by displaying the ability of individual life stages to manage trade-offs. It highlights the importance of a poorly understood pupal stage, which can respond to different environmental cues, behaviourally and physiologically. It also explains how the abilities of individual stages to manage trade-offs independently and cumulatively can change the consequences for adult fitness.