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Sexy Signals? | A Tree Of Love | Sexual Selection in Sepsis dissimilis |
Sexy Signals?!
Animal sex has fascinated scientists for many decades. Males often have to work very hard to impress females with fancy ornaments or complex behavioural repertoires before they are even allowed to approach the females and get ‘lucky’. However, in sepsid flies, males have a slightly different approach:
First, jump on the female!
Hang onto her wings like in a rodeo!
Then proceed to convince her …
As shown in the clip above, females often shake vigorously when mounted. Hence, the males use their strongly modified fore legs to hang onto the female wing base for dear life, much like a rodeo cowboy. We are interested in studying how these fore leg ornamentations interact with female wings which are highly similar across species.
We also study the mating behaviours of species across the sepsid family. One might expect fly sex to be boring but we find that sepsid mating behaviour is amazingly diverse. We see males flicking their wings, curling their mid leg tarsi and even repeatedly stroking various parts of the female fly.
In fact, we observe more than 40 different behavioural elements based on studies of sixteen species across the family. Interestingly, each species has its own repertoire of ‘moves’ used by males to impress the females.
A Tree Of Love
In order to study the evolution of mating behaviour in Sepsidae, we code the behavioral elements as characters and trace them on the phylogenetic tree for the family. Most characters show much homoplasy thus indicating that courtship behaviour in Sepsidae is evolutionarily labile.
To investigate if morphological structures change in response to changes in mating behaviours, we also conduct a detailed morphological study using SEM. Most of the structures involved in male behaviours (like the mid and hind legs) are not sexually dimorphic and at times are even unmodified between species.
Our results demonstrate that studying behaviour in line with morphology provides valuable additional insights and adds meaning to reconstructing the tree of life for Sepsidae.
Sexual Selection in Sepsis dissimilis
Sepsid flies are also ideal models to study sexual selection. We have conducted a detailed study on one such species, Sepsis dissmilis. We describe the male courtship behaviour and use mating trials to investigate which male and female features determine mating success.
The mating behaviour is characterized by seven very distinct male behavioural elements (e.g., ‘hind leg curl’, ‘mid leg wave’, ‘surstylus stimulation’, ‘wing flick’) and two female behaviours (‘female shake’ and ‘abdomen lowering’). Interestingly, most of the male behaviours are stimulatory in nature and often non-contact. These can be studied in light of sexual selection hypotheses such as female choice and sexual conflict.
In order to test which morphological structures influence male mating success, we measured 13 morphological traits and use a principal component analysis (PCA) for analyzing the data. We find that mating success is positively correlated with larger male-to-female size ratio rather than actual male size.
None of the sexually dimorphic features had a significant effect on mating success. The current literature emphasizes male traits, but our results suggest a need for considering female variables.