A friend in need is a friend indeed

New research pub­lished today in Evol­u­tion Let­ters reveals that some indi­vidu­als are so sys­tem­at­ic­ally dis­ad­vant­aged by com­pet­i­tion with mem­bers of their own spe­cies that they enter into an alli­ance with anoth­er spe­cies to become more com­pet­it­ive. Lead author Syuan-Jyun Sun explains what they found.

Mutu­al­isms between spe­cies are ubi­quit­ous, and are also key to main­tain­ing biod­iversity and eco­sys­tem func­tion. Yet many mutu­al­ist­ic inter­ac­tions between spe­cies are in fact highly vari­able. So why do mutu­al­isms some­times per­sist, and some­times degrade into more ant­ag­on­ist­ic relationships?

Mutu­al­isms degrade when the costs of asso­ci­at­ing with a part­ner spe­cies start to out­weigh the bene­fits. Our idea is that com­pet­i­tion with­in spe­cies, for a lim­ited resource or a mate, could explain some of this vari­ation. Social inter­ac­tions with­in spe­cies routinely dis­ad­vant­age some indi­vidu­als because they con­sist­ently lose to a rival. We sug­gest that these ‘losers’ stand to gain sub­stan­tial fit­ness bene­fits from enter­ing into part­ner­ship with a second spe­cies. The bene­fits they gain from this alli­ance are so high, that the out­come of the inter­ac­tion is always tipped firmly towards mutu­al­ism. On the oth­er hand, the bal­ance is more likely to be tipped away from mutu­al­ism towards ant­ag­on­ism for the indi­vidu­als that more routinely win against rivals. ‘Win­ners’ don’t need help to win battles with their own kind, so the costs of asso­ci­at­ing with anoth­er spe­cies are more likely to exceed any bene­fits they might gain.

We tested this idea with exper­i­ments on bury­ing beetles and their mites. The mites Poe­cilochirus car­abi are benign pas­sen­gers on their host bury­ing beetles Nicro­phor­us vespilloides. Mites rely on the beetles to trans­port them to dead bod­ies, upon which both spe­cies repro­duce. How­ever, beetles face fierce com­pet­i­tion for the own­er­ship of a car­cass, like a dead mouse. They can’t breed unless they acquire a dead body. Yet smal­ler beetles fre­quently lose con­tests against lar­ger con­spe­cif­ic rivals. So how smal­ler beetles ever breed?

Striking image
A bury­ing beetle car­ry­ing mites about to 
take a flight. Cred­it: Syuan-Jyun Sun

We wondered wheth­er mites could give these ‘losers’ a help­ing hand in fights over a car­cass. In the Kil­ner Lab at the Uni­ver­sity of Cam­bridge, we began by sta­ging con­tests over a dead mouse between two beetles that were matched in size. One car­ried mites, while the oth­er was mite-free. We filmed the fights with infrared ther­mo­graphy (see video here), and found that beetles with mites were hot­ter and more aggress­ive, and were more likely to win. Mites can indeed help beetles to secure a carcass.

To invest­ig­ate how such thermal bene­fits arose, we built a gym for beetles, and exer­cised them on bespoke tread­mills (see video here). Beetles either car­ried mites, or they car­ried a weight that was equi­val­ent to the bur­den of the mites, or they car­ried noth­ing. We found that car­ry­ing extra weight caused beetles to gen­er­ate extra heat as they exer­cised. We also dis­covered that this heat was trapped by the mites, because the mites form an insu­lat­ing lay­er when trav­el­ling on beetles. Import­antly, these effects were most pro­nounced for smal­ler beetles because mites covered a rel­at­ively lar­ger sur­face area than on large beetles. This sug­ges­ted to us that mites are likely to be dis­pro­por­tion­ally bene­fi­cial to smal­ler beetles.

exercise
Bury­ing beetle being exer­cised on a bespoke tread­mill – for a video see https://onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1002%2Fevl3.109&file=evl3109-sup-0010-videoS2.mp4

To test this idea dir­ectly, we again staged fights between two beetles over a dead mouse. This time, the two rivals differed in body size. We also let beetles breed on a mouse, with and without mites. We found that small beetles were much more likely to win a fight for a car­cass when they were asso­ci­ated with mites. The mites then slightly reduced beetle repro­duct­ive suc­cess, because they com­peted with beetle lar­vae for car­ri­on. Nev­er­the­less, the huge bene­fits of acquir­ing a car­cass for repro­duc­tion out­weighed these small costs. For smal­ler ‘loser’ beetles, mites are mutu­al­ists because they increase beetle fitness.

The find­ings were dif­fer­ent for lar­ger beetles. They needed no help to win a car­cass to breed upon. So they gained noth­ing from asso­ci­at­ing with mites. To make mat­ters worse, they then lost fit­ness to the mites when they bred along­side each togeth­er on the car­cass. For lar­ger ‘win­ner’ beetles, mites are ant­ag­on­ist­ic rather than mutu­al­ist­ic because they reduce beetle fitness.

In gen­er­al, our work shows the bal­ance is tipped from mutu­al­ism to ant­ag­on­ism with the increas­ing like­li­hood that indi­vidu­als will single­han­dedly win con­flicts with mem­bers of their own species.

 

Syuan-Jyun Sun is a PhD stu­dent in the Kil­ner Lab at the Uni­ver­sity of Cam­bridge. The ori­gin­al art­icle is freely avail­able to read and down­load from Evol­u­tion Letters.