Saturday, 19 November 2016

Gender representation in hoverfly records

In theory, the numbers of males and females in a population are relatively evenly matched; or are they? I've seen nothing to suggest that the numbers of hoverfly larvae that reach adulthood are in any way skewed one way or the other. But, in the course of fieldwork, one sometimes gets the feeling that one is only seeing one gender. I therefore wondered about which hoverfly species were represented by disproportionately large numbers of males or females in the photographic records? Obviously a few, such as Sphaerophoria scripta and Syrphus ribesii will only be represented by males or females respectively because we can only put names to one gender. I suspect that in a few genera, such as Melanostoma, the data are skewed because we have more difficulty making a firm diagnosis where photos are from awkward angles. But there are plenty of examples where a firm diagnosis is not dictated by gender or the angle for either male or female characters.

From a biological perspective, if one gender is disproportionately represented, then it seems probable that the other is  not seen because it is doing something that keeps it away from the camera's prying eye. To test this idea I ran a pivot table and then extracted the data for the 50 most frequently seen species in 2016. The results were interesting (figure 1), not least because they helped me detect a number of transcription errors!

To simplify the statistics, I have worked on the basis that a 60/40 split in the genders is indicative of some sort of behavioural separation, such that we don't see one or other gender. What emerges is that in 70% (35) of the examples evaluated (disregarding Syrphus ribesii and Eupeodes luniger), males are outnumbered by females in the dataset and that in 40% of cases they are outnumbered by at least 60:40. Conversely, just 14% of males (7) outnumber females by the same ratio (disregarding Sphaerophoria scripta).

My thesis starting is therefore that the smaller the proportion of males represented in the dataset, the more likely it is that they are doing something that makes them invisible to the recorder. Logically, this suggests that they have mate-finding behaviours that we don't see, Conversely, in a few species, such as Cheilosia variabilis and Xylota segnis, males are so dominant in the dataset that either the females are confined to places where we don't look, or male mate-finding strategies make them very obvious! Alternatively, perhaps the males of some species are considerably shorter-lived than females?

Some of the results are relatively un-surprising, but others are a bit of an eye-opener, especially the relatively low numbers of male Epistrophe grossulariae and Leucozona glaucia! What is going on there? Perhaps it is simply that males require less nectar and pollen and are therefore elsewhere? Is there a temporal difference in the degree to which males and females are visible to the recorder?

The big question, therefore, is where are the missing males/females? and, also, if one gender can be significantly under-represented in the dataset what does this tell us about the data? I have long suspected that at least a proportion of supposed 'rarities' are not rare but are simply un-recorded because they are doing something that makes them largely invisible to the recorder. There is therefore plenty of scope for those with an inquisitive mind to track down what missing males/females are doing.

Figure 1. The relative abundance of males and females in the 50 most frequently recorded hoverfly species in the 2016 photographic dataset.

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