Publications

2026

1. Stochastic failure accumulation as a foundation for exponential mortality and selective disappearance

   Ananda Shikhara Bhat and Hanna Kokko

   2026

   BioRxiv

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   Most organisms become increasingly likely to die as they grow older, a phenomenon known as demographic senescence. Despite this statement saying nothing about the particular shape of a mortality curve, the Gompertz–Makeham law remains remarkably accurate in a broad range of species. We develop a general mathematical framework in which individuals are modelled as comprising a fixed number of interacting intra-organismal sub-systems, each characterised by stochastic failure and repair rates, such that the number of failed sub-systems follows a birth–death process. In many organisms, ‘failure begets failure’ because sub-systems are typically interdependent. We use diffusion approximations to demonstrate that this interdependence generically produces Gompertz-Makeham mortality curves. Since individuals who die can no longer age, observed cohorts become increasingly composed of ‘lucky’ individuals that avoided death by (stochastically) taking paths in failure space associated with lower mortality despite no intrinsic differences in their quality. Selective disappearance of ‘unlucky’ individuals generates deviations from Gompertz-Makeham predictions at advanced ages, producing a late-life mortality plateau. We show that while these deviations must always exist, they may often be too small to detect, either because the failure accumulation process is stereotyped or because detection requires unreasonably large cohort sizes. Our work establishes Gompertz-Makeham curves arising from stochastic failure accumulation as a null expectation in organisms with many interdependent sub-systems.

2. Cooperation destabilizes communities, but competition pays the price

   Ananda Shikhara Bhat, Suryadeepto Nag, and Sutirth Dey

   Journal of Biosciences, Jan 2026

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   A classic result in theoretical ecology states that an increase in the proportion of cooperative interactions in unstructured ecological communities leads to a loss of stability to external perturbations. However, the fate and composition of the species that constitute an unstable ecological community following such perturbations remains relatively unexplored. Here, we used an individual-based model to study the population dynamics of unstructured communities following external perturbations to species abundances. We found that while increasing the number of cooperative interactions does indeed increase the probability that a community will experience an extinction following a perturbation, the entire community is rarely wiped out following a perturbation. Instead, only a subset of the ecological community is driven to extinction, and the species that become extinct are more likely to be those engaged in a greater number of competitive interactions. Thus, the resultant community formed after a perturbation has a higher proportion of cooperative interactions than the original community. We showed that this result could be explained by studying the dynamics of the species engaged in the highest number of competitive interactions: After an external perturbation, those species that compete with such a ‘top competitor’ are more likely to become extinct than expected by chance alone, whereas those that are engaged in cooperative interactions with such a species are less likely to become extinct than expected by chance alone. Our results provide a potential explanation for the ubiquity of cooperative interactions in nature despite the known negative effects of cooperation on community stability.

2025

1. Eco-evolutionary dynamics for finite populations and the noise-induced reversal of selection

   Ananda Shikhara Bhat and Vishwesha Guttal

   The American Naturalist, 2025

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   Theoretical studies from diverse areas of population biology have shown that demographic stochasticity can substantially impact evolutionary dynamics in finite populations, including scenarios where traits that are disfavored by natural selection can nevertheless increase in frequency through the course of evolution. Here, we analytically describe the eco-evolutionary dynamics of finite populations from demographic first principles. We investigate how noise-induced effects can alter the evolutionary fate of populations in which total population size may vary stochastically over time. Starting from a generic birth-death process, we derive a set of stochastic differential equations (SDEs) that describe the eco-evolutionary dynamics of a finite population of individuals bearing discrete traits. Our equations recover well-known descriptions of evolutionary dynamics, such as the replicator-mutator equation, the Price equation, and Fisher’s fundamental theorem in the infinite population limit. For finite populations, our SDEs reveal how stochasticity can predictably bias evolutionary trajectories to favor certain traits, a phenomenon we call "noise-induced biasing." We show that noise-induced biasing acts through two distinct mechanisms, which we call the "direct" and "indirect" mechanisms. While the direct mechanism can be identified with classic bet-hedging theory, the indirect mechanism is a more subtle consequence of frequency- and density-dependent demographic stochasticity. Our equations reveal that noise-induced biasing may lead to evolution proceeding in a direction opposite to that predicted by natural selection in the infinite population limit. By extending and generalizing some standard equations of population genetics, we thus describe how demographic stochasticity appears alongside, and interacts with, the more well-understood forces of natural selection and neutral drift to determine the eco-evolutionary dynamics of finite populations of nonconstant size.

2. A stochastic field theory for the evolution of quantitative traits in finite populations

   Ananda Shikhara Bhat

   Theoretical Population Biology, 2025

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   Infinitely many distinct trait values may arise in populations bearing quantitative traits, and modeling their population dynamics is thus a formidable task. While classical models assume fixed or infinite population size, models in which the total population size fluctuates due to demographic noise in births and deaths can behave qualitatively differently from constant or infinite population models due to density-dependent dynamics. In this paper, I present a stochastic field theory for the eco-evolutionary dynamics of finite populations bearing one-dimensional quantitative traits. I derive stochastic field equations that describe the evolution of population densities, trait frequencies, and the mean value of any trait in the population. These equations recover well-known results such as the replicator-mutator equation, Price equation, and gradient dynamics in the infinite population limit. For finite populations, the equations describe the intricate interplay between natural selection, noise-induced selection, eco-evolutionary feedback, and neutral genetic drift in determining evolutionary trajectories. My work uses ideas from statistical physics, calculus of variations, and SPDEs, providing alternative methods that complement the measure-theoretic martingale approach that is more common in the literature.

2024

1. Spatial structure could explain the maintenance of alternative reproductive tactics in tree cricket males

   Mohammed Aamir Sadiq, Ananda Shikhara Bhat, Vishwesha Guttal, and Rohini Balakrishnan

   Biology Open, 2024

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   Trait polymorphisms are widespread in nature, and explaining their stable co-existence is a central problem in ecology and evolution. Alternative reproductive tactics, in which individuals of one or more sex exhibit discrete, discontinuous traits in response to reproductive competition, represent a special case of trait polymorphism in which the traits are often complex, behavioural, and dynamic. Thus, studying how alternative reproductive tactics are maintained may provide general insights into how complex trait polymorphisms are maintained in populations. We construct a spatially explicit individual-based model inspired from extensively collected empirical data to address the mechanisms behind the co-existence of three behavioural alternative reproductive tactics in males of a tree cricket (Oecanthus henryi). Our results show that the co-existence of these tactics over ecological time scales is facilitated by the spatial structure of the landscape they inhabit, which serves to equalize the otherwise unequal mating benefits of the three tactics. We also show that this co-existence is unlikely if spatial aspects of the system are not considered. Our findings highlight the importance of spatial dynamics in understanding ecological and evolutionary processes and underscore the power of integrative approaches that combine models inspired from empirical data.

2023

1. Allopatric montane wren-babblers exhibit similar song notes but divergent vocal sequences

   Abhinava Jagan Madabhushi, Ananda Shikhara Bhat, and Anand Krishnan

   Behavioral Ecology and Sociobiology, 2023

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   The songs of many passerine birds consist of notes temporally arranged into vocal sequences following syntactic structures and function both in courtship and territorial defense. Geographic barriers are important drivers of avian speciation and also influence the divergence of song. However, there is relatively little quantitative study of the relationship between geographic barriers and the syntactic structure of vocal sequences. Here, we investigate interspecific divergence in song notes and syntax within the allopatric montane Asian wren-babblers (Spelaeornis). Employing a quantitative analysis of note transitions and co-occurrence using song recordings from publicly accessible databases, we find that Spelaeornis appears to have undergone diversification in song syntax without divergence in note parameters. Broadly, we find three different syntactic structures across the eight species in the genus, each occurring in a different geographic region in Asia, with two species apparently exhibiting intermediate syntax. Species within the genus appear to possess similar song notes, but subgroups confined to different geographic regions (e.g., hills south of the Brahmaputra river) arrange these notes according to different syntactic rules to construct songs. Our computational framework to examine the signal structure and diversification across multiple scales of signal organization may help further our understanding of speciation, signal evolution, and, more broadly, fields such as linguistic diversification.

2022

1. Behavioural context shapes vocal sequences in two anuran species with different repertoire sizes

   Ananda Shikhara Bhat, Varun Aniruddha Sane, K.S. Seshadri, and Anand Krishnan

   Animal Behaviour, 2022

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   Acoustic signals in animals serve to convey context-dependent information to receivers. Birds and mammals combine diverse sounds into complex sequences to communicate, but the role of temporal sequencing of signals remains understudied in other taxa. Anuran vocalizations are a prominent feature of their life history, and function in defence of territories and to attract mates. However, there are few data on whether anurans pattern their calls into sequences, and whether temporal sequences convey information about context. Here, we investigated the context-dependent vocal repertoire and the use of vocal sequences by two anuran species belonging to different lineages, comparing frogs vocalizing alone and in the presence of a territorial rival. Using a robust analytical framework, we present evidence that both species modify their vocal sequence structure according to context. Specifically, one species (with a smaller repertoire, from a more basal lineage) appends notes to generate more complex sequences, whereas the other (more recently diverged and with a larger repertoire) shifts to different note types, resulting in different sequences for different contexts. Thus, despite differences in repertoire size, both frog species are capable of adjusting the temporal sequence of vocalizations to communicate in different contexts. Vocal sequences and context-dependent ‘syntax’ may be more common in anurans than previously thought, and our methodology presents a paradigm to study the evolution and function of these complex vocal patterns.