New Paper by Jitesh Jhawar et al — Noise-induced schooling of fish in Nature Physics

Our new paper in Nature Physics led by the recently minted PhD from our lab – Jitesh Jhawar & with a fabulous collaborative team of Richard Morris, Danny Raj, Amith-Kumar, Harikrishnan & Tim Rogers!

Here is the link to read the paper (FREE): https://rdcu.be/b2pgG

This is the FIRST experimental work of our ‘theory-lab’. Obviously, this is exciting.

This blog is based on the thread I created on twitter. Here is the screenshot of the paper! @iiscbangalore

View image on Twitter

Some background: We have seen collective motion in birds, mammals, fish, insects, microbes, etc – all fascinating patterns.

Each individual has only limited local information about surroundings. Yet they show these fascinating patterns. 2/n

So a question that many of us are interested is: what types of interactions produce these fascinating patterns?

This has been a question of substantial work over the last few decades, and we provide some new insights here. 3/n

To answer this broad question, physicists and computer scientists have built mathematical and computational models since 1980’s.

They show that organisms don’t need complex rules to exhibit collective motion. 4/n

For example, in the classic Vicsek model (PRL 1995), particles follow a simple rule: move in the average direction of their neighbours. This simplistic rule produces a highly-ordered collective motion.

[Pic below is Fig 1 of Vicsek et al 1995 PRL]

View image on Twitter

The first main result from our study is related to the above context — whats the rule that fish follow? We show that in a species called karimeeen (Etroplus suratensis) (i) fish just copy the direction of a (nearby) random fish, or (ii) they turn a bit randomly. 6/n

We call this rule a ‘pairwise copying’ — which is relatively simpler than the Vicsek-class of models which assume that organisms average the direction of neighbours and turn towards them. 7/n

Ours is not the first paper to show that real organisms behave differently compared to Vicsek-like models. In fact, fish school studies by @JHerbertread, @GTheraulaz etc also show a similiar simpl rule of interactions in other fish species. 8/n

That brings me to the second main result- also the title of the paper: We show that schooling in this fish is a rare empirical example of a phenomenon well studied in non-equilibrium stat physics: ‘noise-induced phase transitions’. But what is this? Let’s dig in a bit 9/n

So this is what we do in terms of experiments and analyses to make the connection with the physics theory. We use karimeen (Etroplus suratensis) — a popular edible fish in western coast of southern India and put them in a fish tank.111

So this is what we do in terms of experiments and analyses to make the connection with the physics theory.

We use karimeen (Etroplus suratensis) — a popular edible fish in western coast of southern India and put them in a fish tank.

View image on Twitter

We maintain shallow water, so that fish are effectively in a two-dimensional system. This makes tracking of fish movement relatively easy.

[This is from fig 1 of our paper.]

View image on Twitter

Here is a short video showing fish in the tank. Our experiments has 15 and 60 fish.

From video tracks of how fish, we calculate “polarisation M” – which measures how well aligned are fish with each other.

We then plot this quantity as a function of time. Crucially, we retain all information — not just mean but also how fluctuations are occurring over time.

View image on Twitter

From such a time series of the polarisation, we construct a stochastic differential equation!!!

I think this is one of the coolest part of the paper – because unlike most papers that intuitively derive a model or equation, here we let the data talk!

View image on Twitter

 

In simple words, this equation tells that i. when fish are ordered, even random things that they do, like copy one other, doesn’t change the overall behaviour very much ii. when the fish are moving in a misaligned state, the fluctuations are actually high.214

Therefore, when the fish are not well aligned with group members, the noise grows larger, eventually ‘kicking’ the group from one state — random swimming — to a different state — schooling!!! 

This video (based on simulation of the model) makes the point on how noise is high in the disordered state, and pushes the group towards higher alignment.

With generous support by @DBTIndia @serbonline @IndiaDST.

I will stop here!! There is quite a bit of technical stuff in Methods and Supplementary Materials.

 


Jitesh Jhawar begins his postdoctoral position at the Max Planck Institute of Animal Behavior

IMG-20200306-WA0001Jitesh Jhawara recently minted PhD from our lab — has just begun a postdoctoral research position at the Department of Collective Behavior, Max Planck Institute of Animal Behavior and the Centre for the Advanced Study of Collective Behavior at University of Konstanz, at Konstanz, Germany.

In his new position, he will work on an interdisciplinary project examining ‘collective sensing and group behaviour in epithelial cells and honey bees’. 

During his PhD, Jitesh made some fundamental contributions to the lab — publishing our first experimental work which was also hugely collaborative and investigated the role of stochasticity in collective behaviour, leading to following several important publications (see publications page). He will be badly missed in the lab.

We wish him all the best in his endeavours!


One more PhD from the lab! Jitesh Jhawar defends his PhD thesis investigating Collective behaviour in schooling fish

Congratulations to Jitesh Jhawar, the newly minted PhD from our lab!!!! He defended his thesis in December 2019.

Jitesh defended his thesis titled “Intrinsic noise in collective dynamics” – where he theoretically and empirically investigated how stochasticity plays a very counter-intuitive role and creates order!

The thesis was reviewed by Prof Martin Evans (Dept of Physics, Edinburgh, UK) and Prof Malay Banerjee (Dept of Math, IIT Kanpur)! We thank them for their very constructive comments and their valuable time.

As you can see, although this work is from CES and we look at fish schools, it was reviewed by physicists/mathematicians! This is because there are lots of links between physics and schooling fish.

You can read the synopsis of his thesis here.

Here are three papers arising out his thesis (and two more are expected):

1) Jitesh Jhawar, Richard G. Morris, U. R. Amith-Kumar, M. Danny Raj, Harikrishnan R., Vishwesha Guttal, Noise-Induced Schooling of Fish, arXiv:1903.12132

2) Jitesh Jhawar and Vishwesha Guttal, 2019, Noise-induced Effects in Collective Dynamics and Inferring Local Interactions from Data, https://arxiv.org/abs/1911.09376

3) Jitesh Jhawar, Richard Morris, and Vishwesha Guttal, 2019, Deriving mesoscopic models of collective behaviour for finite populations, In Handbook of Statistics Vol 40: Integrated Population Biology and Modeling  (edited by Arni Srini Rao and C R Rao), Part B, 551-594. DOI: https://doi.org/10.1016/bs.host.2018.10.002;  Pre-print from Arxiv;  Codes and data on GitHub.  Download PDF

Jitesh is continuing as a short-term post-doctoral fellow in the lab. He will soon join Max Planck Institute for Collective Behaviour in Konstanz for his postdoctoral work on honey bees and collective cell migration.

 

 

 


Sabiha Majumder – a former PhD from the lab – joins banking/finance sector

Hearty congratulations to Dr. Sabiha Majumder who is all set to join ING in Amsterdam on a new exciting career path of banking/finance.

Sabiha got a PhD from the Physics department of IISc in 2017/18, working jointly with me and Sriram Ramaswamy. Her thesis applied ideas from nonequilibrium statistical physics and stochastic systems to analyse risks of ecological collapse. She published a number of excellent papers on this topic, in prestigious ecology journals. After her PhD, she joined ETH-Zurich for a postdoc, where she continued for two years.

In her new job, Sabiha will be employing her interdisciplinary skills as a Risk analyst in the banking/finance sector!

This shows how skills learnt in PhD are highly transferable. One need not continue in the traditional academic set up after PhD — something that a lot of PhD aspirants are unaware of. There are a large number of opportunities outside the academic set up where skills of PhD are going to be extraordinarily useful.

 


No summer internships in 2020

I receive a very large number of applications for summer internships from all over India (and occasionally one or two from abroad). I usually take 2-4 students every year.

Unfortunately, this year (2020), I have decided not to take any summer interns due to a rather hectic schedule I have in summer.

I had informed many students to contact me in early February to know about my decision on interns this year. If you see this message, I know this will be disappointing, but I hope we will find ways to work together later. I wish you all the best in finding alternative internships.


Visit by Prof Marlene Zuk to KVPY and CES

I am on the core-committee of KVPY from July this year and got an opportunity invite speakers for the KVPY Vijyoshi camp.

I (and everyone else involved) was thrilled that Prof Marlene Zuk accepted the invitation! She visited India to deliver spectacular talks at KVPY-2019 on sexual selection. She also attended and gave a talk at a mini-symposium/discussion on animal signals in CES.

Here is a picture of her with our lab:

photo_2019-12-25_14-53-39.jpg

L to R: Archana Prakash, Priya Iyer, Shuaib, Tanveen, Vishwesha, Marlene Zuk, Ashwin, Arun and Jitesh. 

 



New paper by Sumithra Sankaran et al: Inferring resilience from spatial patterns in ecosystems

This blog post was created based on my twitter thread, attempting to explain some aspects of the new paper by former students Sumithra Sankaran, Sabiha Majumder and Ashwin – published in Methods in Ecology and Evolution.

It looks really pretty in the formal formatted version 🙂

Before I go further, we are so happy that there is Kannada Abstract to this paper! I will do a Kannada thread as well later.

Thanks to Kolleagala Sharma @kollegala for the help with Kannada abstract. Incidentally, the paper came out on Nov 1st!

Some background: Many ecosystems can ‘suddenly’ switch states, also called regime shifts or tipping points. This can happen in semi-arid vegetation, mussel beds, lakes, corals, etc. Therefore, we want to know which ecosystems are prone to sudden tipping.

An ideal way to find this is to perturb the ecosystem & measure how it returns. But this is difficult & in many cases, such a perturbation may cause the tipping! So this is not even desirable.  

A paper in Nature 2007 proposed that we may infer stability by measuring properties of spatial patterns. Specifically, they focused on semi-arid vegetation. Here is an image from Google Earth, in Rajasthan. Note that not all clusters of plants are of same size.

Basically, they argued that the resilient (or stable) ecosystems do not have any ‘typical size’ of clusters. Broadly, they claimed that cluster-size distribution and its properties can inform us about ecosystem resilience. 

Mathematically, this means that the frequency distribution of cluster sizes is a power-law. Power-laws are fascinating because their mean & variance are infinitely large!

This is in quite a contrast to distributions we regularly use – like Gaussian/normal or exponential. 

To make this clear, we show a graph in the paper tries to explain how power-laws are fundamentally different from normal or exponential decay functions.

Power-laws have a large tail, and hence you are likely to find very large-sized patches in such systems.

These are not just mathematical fantasies! Many empirical systems do show power-law distribution of clusters.

Here is Figure 1 from our paper with empirical examples of power-law clustering.

Does it mean they are highly stable ecosystems? There were several follow up studies, that found mixed evidence to this overall claim.

That’s the background to Sumithra’s work. 

The main result from Sumithra’s work is that the above-proposed link between resilience and cluster-size distribution is NOT robust. So it’s a NEGATIVE result!

To show this, she used a simple computational model of ecosystems  

Here is a pictorial representation of the spatial-model.

The model itself is directly taken from a statistical physics paper (Lubeck, J Stat Phys 2006) but with ecological interpretation thrown in!

Sumithra showed that power-law cluster-size distribution can occur even when systems are very close to tipping points. Hence, power-laws are not indicators of ecosystem resilience. Here is a conceptual diagram and result that explains the results.

Power-law cluster-sizes are also studied extensively in the context of ‘percolation’ in the physics literature. We showed that power-law cluster-size distribution in our ecology models relates to percolation of physical systems! 

We also talk about what else can be measured to infer resilience. There are more technical aspects! Because this paper uses ideas from many areas – ecology, physics, math, computer simulations and statistics of fitting distributions – we explain many technical aspects. 

Finally, I must say that handling Editor Dr Hao Ye at Methods in Ecology and Evolution gave extensive comments that really helped the clarity and presentation of the manuscript.

If you came this far, thanks!! 


Collective behaviour symposium during the visit of Guy Theraulaz

During the visir of Guy Theraylaz, our lab members (Pritha, Jitesh, Akanksha) and collaborator Dr Danny Raj organized an interdisciplinary one-day symposium titled Collective behaviour at CES, IISc. Here is the full schedule:

Symposium_Poster_Final (1)

Around 10 faculty members from IISc and other institutions gave presentations. Prof Theraulaz also gave a fascinating special seminar at the Collective behaviour symposium on self-organized nest architecture.

There was also a very lively poster session where a number of students presented their work.

The symposium was very well attended, including two IT folks who saw the notice on Twitter and joined the entire day of talks!


Workshop at Valparai with Guy Theraulaz

During the visit of Prof Theraulaz, we organized an intense workshop on collective behaviour at Valparai, Tamil Nadu from 08.09.2019 to 10.09.2019.

Many students working on collective behaviour from our lab and our collaborator Dr Danny Raj attended the workshop. The workshop focused on the discussion of existing research in the field of collective behaviour and potential future advances in the same.

The team also did a number of field visits to observe wildlife, assisted by extraordinary scientists and staff from Nature Conservation Foundation.