by Isabel Smallengange
Adult male bulb mites (Rhizoglyphus robini) show one of two distinct morphs: fighters, which have thickened legs with sharp ends that they can use to kill other mites, and scramblers (see photo below).
These two types of males occur in single populations and one of the key questions that biologists want to answer is why these two morph coexist. Do we expect them to coexist? For example, if fighters can kill rival males when competing to mate with females, they would always outcompete scramblers. So why do we nearly always see both types in single populations ?
This question has been puzzling scientists for many years. Bulb mites themselves have been studied for nearly a century now , and many evolutionary biologists use bulb mites as a model system to answer key evolutionary questions like “Why do different traits exist?”, and “Why is there variation in trait values?”.
Back in 2008, during my Post Doc at Imperial College London, I started my bulb mite research lab with the aim to answer why the two male morphs coexist in single populations. As I was starting to learn this new system, I noticed that some males looked very different to either fighters or scramblers. They were very large, much larger than other adult males; almost as large as adult females (adult female bulb mites are much larger than adult males). This was very exciting! Was this a new, third morph? Should I publish this as soon as possible? The answer soon arrived in a paper published by Mark Rowland and Douglas Emlen in one of the most prestigious journals, Science, entitled “Two thresholds, three male forms result in facultative male trimorphism in beetles” . In this paper, the authors compared different male morphs within many different beetle species and also uncovered the mechanisms resulting in which morph a male develops into. It is a very thorough study into male trimorphisms. There was no way I could compete!
So, the whole issue of the mega-scramblers, which I had started to call them, drifted to the back of my mind. Until Kat Stewart arrived in my lab last year to do a Post Doc on the male morphs (and also on the dispersal morph that bulb mites display – but that is another story). She heard stories about these elusive mega-scramblers and suggested to make them official by publishing a Natural History Note to present the three morphs to the scientific audience.
And she succeeded! The paper entitled “Evidence for a third male type in a male-dimorphic model species” is now published in Ecology . Below is a photo of all three male morphs, with arrows indicating the leg pair that differs between fighters and (mega-)scramblers:
How exciting! As one of the reviewers of the paper put it “this paper would set the stage for perhaps an explosion of work on this fascinating new system”: because now, we have to figure out how three morphs can coexist in single populations – ouch!
 Radwan J (2009) Alternative mating tactics in acarid mites. Adv Stud Behav 39:185–208
 Foa, A.1919. Studio del polimorfismo unisessuale del Rhizolyphus echinopus. Memoria Accad Pontificia Nuovi Lincei Roma Ser. V, 12, 3–109.
 Rowland JM & Emlen DJ (2009) Two thresholds, three male forms result in facultative male trimorphism in beetles. Science 323: 773-776.
 Stewart, KA, van den Beuken TPG, Rhebergen FT, Deere JA, Smallegange IM (2018) Evidence for a third male type in a male-dimorphic model species. Ecology 99: 1685-1687
*A similar version of this post was posted on isabelsmallegange.com.
Author biography: Isabel Smallegange is an Associate Professor of Population Biology at the University of Amsterdam. Follow her @I_Smallegange, or check out her blogs at isabelsmallegange.com, where she also discusses issues regarding the work-life and gender balance in academia.
Image caption and credit: Photos by Jan van Arkel. Descriptions are given in the text above.