Baffling behavior in the tree cricket Oecanthus henryi: how is leaf area measured? Harsha K Kumar Registration Number : 20111063 Institution: IISER, Pune Institution in which project was carried out: Indian Institute of Science, Bangalore Department: Centre for Ecological Sciences Thesis Supervisors: Rohini Balakrishnan (IISc) and Deepak Barua (IISER) 1 CERTIFICATE This is to certify that this dissertation entitled “Baffling behaviour in the tree cricket Oecanthus henryi: how is leaf area measured?” towards the partial fulfillment of the BS-MS dual degree program at the Indian Institute of Science Education and Research, Pune represents the research carried out by Harsha K Kumar at IISc, Bangalore under the supervision of Professor Rohini Balakrishnan, Centre for Ecological Sciences (CES), IISc and Dr. Deepak Barua, IISER, Pune during the academic year 2015-2016. Dr. Rohini Balakrishnan Professor Centre for Ecological Sciences Indian Institute of Science Bangalore 04.04.2016 2 Declaration I hereby declare that the matter embodied in the report entitled ““Baffling behavior in the tree cricket Oecanthus henryi: how is leaf area measured?”” are the results of the investigations carried out by me at the Centre for Ecological Sciences (CES), IISc Bangalore, under the supervision of Prof. Rohini Balakrishnan and the same has not been submitted elsewhere for any other degree. 3 Abstract Acoustic communication in animals generally serves many purposes including mate attraction and aggression. Males of tree crickets use acoustic communication to attract females. It has been observed that males of the tree cricket species Oecanthus henryi sing from leaf edges to attract females from a distance. Males have also been observed making holes in leaves and singing through them to amplify their song. Previous work has shown that O. henryi can measure leaf sizes and that they always choose relatively large leaves to make holes on, and sing through them to amplify their sound. Song amplitude gain is known to be much higher when a cricket sings from a hole on a bigger leaf compared to that on a smaller one. With females selectively choosing loud males in the environment, there would be immense pressure on such a behavior to evolve. This thesis attempts to elucidate the mechanisms underlying the leaf area-measuring ability of O. henryi. Experiments conducted, indicated that the behavior of individuals on different sized leaves are different. Insects were found not to be using petiole features or hypothetical walks which were correlated well with area. The mean free path of individuals on small leaves was significantly lower than that on large leaves suggesting that it could potentially be used for estimation of leaf areas. List of Figures : Fig 1.1 a,b: Host plants of O. henryi 8 Fig 1.2 a,b : File and Plectrum of O. henryi 9 Fig 1.3: Compression and relaxation of air caused by vibrating 10 Wing membrane Fig 1.4: Illustration of a dipole 10 Fig 1.5: Individuals singing at leaf edge and through baffles 12 Fig 2.1: Boxplots showing time period of observation on small and large leaves 18 Fig 2.2: Q-Q plot showing deviation from multivariate normality 19 Fig 2.3: Figure showing all hypothetical paths 22 Fig 3.1: Average activity budgets 25 Fig 3.2 : Comparison of proportion if time spent SALE on big and small leaves 26 Fig 3.3 a,b : How many factors to choose? 27 4 Fig 3.4 a,b : 3d plot showing clusters from clustering analysis 28 Fig 3.5: Box plot of proportion of time spent on different behaviors 29 Fig 3.6 a,b: Box plot of absolute of time spent on different behaviors (Large leaves) 31 Fig 3.7 a,b,c: Box plot of absolute of time spent on different behaviors (Small leaves) 32 Chart 1: Transition probability chart of Individuals on small leaves 33 Chart 2: Transition probability chart of individuals on large leaves 34 Fig 3.8: Graph showing time spent by different individuals 35 in “decision making’ and ‘baffle making’ (large leaves) Fig 3.9 a, b: Box plots testing sub-hypotheses on cost of making 35 , 36 baffles and more walking being done on large leaves Fig 3.10a ,b, c: Association between petiole features and leaf area 37 Fig 3.11: Association between different walking paths and area 38 Fig 3.12: Example walking path of an individual on a large leaf 39 Fig 3.13: Is mean free path different on small and large leaves? 41 Table 1: Test of sufficiency of number of factors and cumulative variance explained 27 Table 2: Number and proportion of hypothetical walks traced by individuals 40 In experiments 5 ACKNOWLEDGMENTS This thesis was a result of the hard work of many others besides me. It is only just to thank them for their contributions to this thesis. I personally have never liked writing. And if not for people nudging me and keeping on my feet, this thesis would have never come together. First, I would like to thank Rohini Balakrishnan, my advisor for teaching me to ask the relevant questions in science. She taught me to constantly keep looking at the bigger picture during any study. She taught me to logically connect all the dots to see this bigger picture. The enthusiasm and spirit she has for pursuing science is something that I admire about her. Those are things I will take along with me for the rest of my life. During my stay in her lab, I came to the realization that the entire field of animal behavior and cognition like many others has no immediate goal or purpose to aid or support human life. It is simply for the joy of learning how tiny ‘insignificant’ creatures in today’s concrete jungle lead their lives. I would like to thank Dr.Deepak Barua for taking interest in my project and suggesting experiments and ways to analyze data though he doesn’t particularly work on anything closely related to animal cognition. I would like to thank Rittik Deb for the solid foundation he had laid for my project. I also thank him for all the help he provided when I first joined the lab. I would like to thank Diptarup Nandi for teaching me how to write a thesis and for the numerous discussions we have had together on various ways to analyze data. I would like to thank Manju for collecting individuals of Oecanthus henryi for experiments, and for the non-stop comedy in the lab. I would like to thank Viraj, Ashwathy,Monisha, Rochishnu, Sambita, Angarika and Harish from the lab for their support and discussions we have had on various topics. 6 I would like to thank my mother, father, brother, my pets and various music artists (mostly Sattyananda) for keeping me happy through the duration of the study and for creating a good environment for research. Last, I would like to thank KVPY for providing good facilities and funding for completion of the study. 7 Introduction Oecanthus henryi is a small tree cricket of the subfamily Oecanthinae (Family Gryllidae). Individuals of this species are 12.4 ± 0.73 mm in length (Metrani and Balakrishnan, 2005) and are green-white in color and have transparent large wings. They are usually found on their host plants Hyptis suaveolens and Muntingia calabura ; both of which are exotic species (Fig 1.1 a,b) (Deb, 2015). Figure 1.1a: Hyptis suaveolens Figure 1.1b: Muntingia calabura Photo Credits : Figure 1.1a: https://species.wikimedia.org/wiki/Hyptis_suaveolens Figure 1.1b: http://foodnsport.com/blog/muntingia-jamfruit.html The acoustic communication system in this species involves males being stationary and calling on different bushes. Females move from bush to bush and are known to use male calls for three different purposes: a) Identification of con-specific males in the environment; b) For mate choice; c) For localization of calling males. Calling loudly would increase the chance of a female hearing a male and therefore would increase his chances of mating (If sound is louder at source, sound travels further in space when compared to a fainter sound at source). Therefore, there is evolutionary pressure on males to call loudly. 8 Adult males of most cricket species have forewings with a stridulatory apparatus and are thus equipped to produce sounds (Forrest, 1982). Forewings have a sclerotized plectrum and a file each (a row of fine teeth which are engaged by the plectrum) (Fig 1.2a, b) (Forrest, 1982; Bennet-Clark, 1989, 1998, 1999). Sound is produced when the plectrum of one wing rubs over the file of the other wing (Forrest, 1982). This mechanical activity causes vibrations of the membranous wings and sound is produced (Forrest, 1982). Figure 1.2a. File of O. henryi Figure 1.2b: Plectrum of O. henryi (Photo Credits- Sapna Metrani) The vibrating wing membrane (which vibrates to and fro from its natural position) sends out waves of sound which are exactly opposite in phase in opposite directions (away from the head and in the direction of the head) (Forrest, 1991, Bennet-Clark 1970). The vibrating wing membrane compresses the air molecules on one side and relaxes the air on the other side (Fig 1.3). This sets up a small pressure difference across the vibrating wing membrane. The natural tendency of air is to rush back to equalize this pressure difference. The wavelengths of the calls of Oecanthus henryi are larger than their wing sizes (Oecanthus henryi are 12 mm in length (Metrani and Balakrishnan, 2005) and wavelength of song is 11.5 cm at 25 degrees Celsius) (Forrest, 1991; Bennet-Clark 1970; Deb, 2015).