Animal Behaviour Assignment - 2

Observation of Water striders

Initially the aim of our project was to study the behavior of Water striders (Family – Gerridae) in Pashan lake, but the lake going through rapid eutrophication and as a result of which, the populations of Water Striders in the lake have disappeared. Hence, we have instead worked on a smaller population of them in a puddle adjacent to the lake. This observation itself gave us an idea of the habitat, ecology and behavior of the Water striders. However the timescale of change was too rapid for us to make any noteworthy observations on the Water striders response to the phenomenon.

At the first glance, the Water Striders seem to be skittering randomly across the surface of the water. However, on prolonged observation, they seem to start moving in response to certain cues. Hence, we observed the population of Water Striders to qualitativelydiscern the cues that may cause a response in the Water Striders. These are some of the naive observation we made:

Not territorial – 2 out of 14 were observed to stay at one location and occasional bumping into other water striders was observed with no aggressive behavior

There is increase in movement of the Water Striders with the presence of dragonflies howering around.

They don't move in large groups. (n=14) Stationary Water Striders tend to start moving when there is

some wind. Mostly situated in calm waters and found nearer to the bank

of the lake They stay in open waters away from vegetal growth

Due to their ‘walking on water’ ability, a lot of the available literature is focused on the physiology and the physics behind their remarkable ability. However, some literature also focuses on the behavioral aspects of these organisms. Here are some of the things we learned from the literature:

Water striders prey on dead or alive insects on the surface, of which injured dragon flies are a common prey.

They are predated by birds mostly only and occasionally by fish and frogs.

They are territorial during the mating season where both males and females establish territories.

We wanted to see whether any correlation exists between the patterns of movements and the cues that might be responsible for the movement of the Water Striders.

Starting with observing the water striders to categorize the patterns of movement, we recorded 30-second video clips of the Water Striders. We then tracked each Water Strider individually and marked their trajectories. A few of the trajectories are shown below.These trajectories helped us identify the parameters that could be used to categorize the movement (for example, directionality, number of strides, length of strides etc.)

Starting point

Ending point

Direction of windif it was possibleto identify

From the observations, we have gathered that there are few primarycues that could lead to the behavior we observe, which are yet to betested for quantitatively. The cues we guess might contribute to the movement in the striders are:

Food availability (prey) Presence of predators Following other individuals (social behavior) Physical cues such as wind, flow and ripples

We next plan to conduct a controlled study to decipher which of the above cues contributes most to the movement patterns and if possible try to come up with thresholds associated with each form ofmovement.

Group members : Abhilesh, Abinand, Lokesh