Do predatory adult odonates estimate their adult prey odonates’ body size and dispersal ability to proceed with a successful attack?

Predator-prey encounters are one of the most challenging behaviors that animals engage in and play a key role in structuring trophic linkages within food webs. Empirical studies suggest that predators (except pathogens, parasites, and parasitoids) tend to be larger in body size and have better dispersal ability than their prey items; however, when predators prey upon members of the same taxonomic group, it is unclear whether such relationships exist between predators and their prey items since both groups may have similar body sizes and dispersal abilities. Adult odonates can be used to test this as they prey upon other odonates within the same suborder, family, genus or species, although such records are uncommon. Using a dataset collected from Sri Lanka and India from 2012 to 2020, this study identified three types of predation behaviors between two suborders of Odonata, i.e., (i) Anisoptera (dragonflies) prey upon Anisoptera, (ii) Anisoptera prey upon Zygoptera (damselflies), and (iii) Zygoptera prey upon Zygoptera. There was no evidence of Anisoptera predation by Zygoptera. Study results showed strong evidence that there is a significant difference in body size (i.e., body length) and dispersal ability (i.e., hind-wing length) between adult predatory odonates and adult prey odonates for all three types of predations. This may indicate that predatory odonates estimate other odonate prey precisely to proceed with a successful attack.


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The average body size and dispersal ability of a species significantly depends on its taxonomic order (Siemann et al. 1999). Indeed, there are significant body size and dispersal ability differences between predatory odonates and their typical prey items such as gnats, mayflies, flies, mosquitoes, and other small-sized flying insects. During one of my field visits in Sri Lanka in 2015, I observed an adult dragonfly (Orthetrum sabina) eating another species of dragonfly (O. luzonicum) (Image 1), and their average body sizes and dispersal abilities were similar. Similar observations were being circulated on Odonate-specialists' Facebook (FB) groups, suggesting that adult odonates feed on other species of odonates or even the same species (see Image 2). When predators prey upon members of the same taxonomic group, it is difficult to predict whether the predators still estimate the size and dispersal ability of their potential prey items to proceed with a successful attack (Woodward & Hildrew 2002). This, however, can be measured by using a robust statistical analysis and a precise dataset.
Even though adult odonates feed upon adult odonates, such records are uncommon. To build the dataset, I surveyed two private FB specialists' groups for such potential records. I manually checked every single post of the "DragonflySouthAsia" (https:// www.facebook.com/groups/dragonflyindia) FB group between 2020 to 2016 and posts of the "Dragonfly Interest Group of Sri Lanka" (https://www.facebook. com/groups/256874097746055) FB group between 2020 to 2012. I also searched the "Odonata of India" (https://www.indianodonata.org/) website for more potential records. For most of those records, predator and prey species had been identified by experts within those groups. Prey odonates that could not be identified to species level due to predation were excluded from the final dataset. The records of mature predators preying upon juveniles were also excluded because that might result in some biases in the dataset as those individuals are immature. The final dataset included 67 records of adult predatory and prey odonate encounters from Sri Lanka (24) and India (43) -nine species of predators and 27 species of prey (see Table 1).
Morphometric trait measurement data related to body size and dispersal ability for each predator and prey odonate was extracted from the "Odonate Phenotypic Database" (OPD) at http://www. odonatephenotypicdatabase.org/ (Waller et al. 2019). When the data was not available in the OPD (only for eight species), the data was extracted from other published literature (see the Supplementary data for J TT

Table 2. Differences in body size (average body length in mm) and dispersal ability (hind-wing length in mm) between predator and prey odonates when both groups belong to Anisoptera (dragonflies) suborder (n= 40). SD indicates standard deviations, and L-95% and U-95% indicate 95% credible interval (lower and upper, respectively).
Mean SD L-95% U-95%  Table 3. Differences in body size (average body length in mm) and dispersal ability (hind-wing length in mm) between predator and prey odonates when predators belong to Anisoptera (dragonflies) and prey belong to Zygoptera (damselflies) suborder (n= 16). SD indicates standard deviations, and L-95% and U-95% indicate 95% credible interval (lower and upper, respectively).  The final dataset showed three types of predation behaviors between the two suborders of Odonata, i.e., (i) Anisoptera (dragonflies) prey upon Anisoptera (60 %, n= 40), (ii) Anisoptera prey upon Zygoptera (damselflies) (24 % of n= 16), and (iii) Zygoptera prey upon Zygoptera (16 %, n= 11), but there was no record of Zygoptera preying upon Anisoptera. Therefore, three separate analyses were performed for each type of predation to estimate the body size and dispersal ability differences between adult predatory and prey odonates. Since each suborder was separately analyzed, the hind-wing length measurements were not scaled relative to body length.

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The results of the analysis showed strong evidence that the predatory odonates performing the attack had larger body size and greater hind-wing length than their prey odonates across all three predation types (see Table 2-4). This indicates that predatory adult odonates may estimate the body size and dispersal ability of the adult prey odonates to execute a successful attack even when both groups belong to the same taxonomic group. Orthetrum sabina had the highest percentage with 70 % (n= 47) of attacks on both Anisoptera and Zygoptera species, including O. sabina-O. sabina attacks (Image 2). It is also important to note that the attacks of the predatory odonates were mostly on the head or thorax of their prey odonates.