Sexual selection in night singing insects

Sexual selection in night
singing insects
Harsha Kumar, Satish Bodakuntla, Harpreet Singh Kalsi , Akilandeswari B,
Rohit Krishnan H.
IISER PUNE
Animal behavior report 2 • 27 March 14
1
Night singing insects are fascinating to study at a whole new level. This is not only because
they look nice. But also biologically interesting to study as they have to depend on cues other
than vision for sexual selection. And the elaborate species specific songs that have developed
in these insects, must be for sexual selection to act on. The species shows no parental care.
Even if this is the case, sexual selection acts at the level of females (Females get to choose
which male to mate with). This is probably because of the higher cost of production of
nutrition filled eggs and the laborious oviposition into thick woody stems . What songs do
females like? Should they be louder, higher in frequency, faster with lesser silences in
between songs or longer ? This opens up a new open field for research.
Although, what is even more interesting is the fact that temperature can modulate some of
these aspects in insect songs. For example, the most obvious one we see is increase in silences
with decrease in temperature. What we haven’t been able to figure out, is if this relationship is
linear. We will try to attempt that question during the project.
Model organism : Tree crickets ( Oecanthus indicus)
Our question : Songs of males at what temperature do females at a particular temperature
prefer? If a female is at 23 degrees, does it like songs of a male also at 23 degrees or songs of a
male at higher temperatures.
Motivation for asking the question : These crickets sing from dense bushes (ones with red
leaves) which are all over the campus. But on one night, you can hear male crickets all singing
from one particular place ; definitely more than a few individuals. So, why are they sitting
together? Is it because there is food there? Is it because there are more soft leaves to chew on
to make holes (They use this like a tunnel to amplify their sounds. Field crickets also do this
by singing into their burrows which amplifies the sound in the direction opposite to the
singing)? Or would they prefer to sing from warmer or colder places so as to increase their
mating chances?
The last one is our hypothesis we want to test.
Experimental setup : Record one male individual at different temperatures overnight in an AC
room keeping tabs on temperature changes during the whole night. Characterize the songs.
Play them back to females at a particular temperature and see which song it prefers by the time
spent next to speakers.
Status of the project : We have recordings from one male from four different temperatures (
18.4, 23.3, 26.8 and 28.9). This has taken a lot of time and multiple recordings at the same
temperature especially for 18.4 and 23.3 . The unit is degree Celsius. We have characterized
the songs and this is what we present as a part of this report.
2
Results
Effect of temperature on Oecanthus indicus
Frequency profile at different temperatures
Figure 1a : 18.4 degrees
Figure 1b : 23.3 degrees
Figure 1c : 28.9 degrees
Sample amplitude profile song at 23.3 degrees
Figure 2
Parameters chosen :
a)
Length of the song - The time taken to complete one single song. The figure 2 above
indicates the boundaries of one sample song.
b)
Length of silence - The time interval between two successive songs. This is indicated
by a green triangle in Figure 1a.
c)
Number of trills - The number of peaks seen in the amplitude profile in the regime of
one song.
Note: The temperatures shown here are the mean temperatures. They varied by 2 degrees
on either side during the night. We would like to correct for this in our future experiments.
Graphs and interpretations
The R squared value for the graph of Length of song vs Length of silence was low. This shows
that the two are not co-related (A long song does not imply a long silence before the next
song). The same result was obtained at all temperatures.
The R squared value for the graph of Length of song vs number of trills was high; indicating
the strong co-relation between the two ( Length of song invariably varies with the number of
trills sung).
What we explored is whether this trend of strong correlation between length of song and
number of trills is general across temperatures or if this holds true at a fixed temperature
alone.
The table above shows the variation of parameters chosen across different temperatures.
We see that the average number of trills increases with increasing temperature while average
length of silence decreases even when there is no substantial change in the actual length of the
song.
This matches well with the conduction theory of electrical circuits. This shows that the
distance between each trill decreases with increase in temperature.
Future Plans
As we see that there exist differences in parameters studied with temperature, we would like
to understand the role of these changes in sexual selection by setting up playback experiments
on females of the same species. We will be happy to look at other parameters and anything
interesting that comes our way.
3.3