Guppies change their eye color to deter rivals

June 4, 2018, University of Exeter

Tiny fish called Trinidadian guppies turn their eyes black to warn other fish when they are feeling aggressive, new research shows.

A study led by the University of Exeter, in collaboration with the University of the West Indies, found that when facing a rival, guppiesrapidly turn their irises from silver to black before attacking their adversary.

This makes their eyes more conspicuous and is an "honest" signal of aggression—larger guppies do it to smaller ones whom they can beat in a fight, but smaller ones do not return the gesture.

As part of the study, the researchers made visually realistic robotic guppies to see what would happen if smaller fish displayed their aggressive motivation—and larger fish flocked in to compete with the small imposters for food.

Dr. Robert Heathcote, lead author of the study and from the University of Exeter, said: "Trinidadian guppies can change their iris colour within a few seconds, and our research shows they do this to honestly communicate their aggressive motivation to other guppies.

"Experimentally showing that animals use their eye colouration to communicate with each other can be very difficult, so we made realistic-looking robotic fish with differing eye colours and observed the reaction of real fish."

Guppies rapidly turn their irises from silver to black before attacking smaller rivals. Credit: Robert Heathcote, University of Exeter

Professor Darren Croft, also from the University of Exeter and an author of the study, added: "Eyes are one of the most easily recognised structures in the natural world and many species go to great lengths to conceal and camouflage their eyes to avoid unwanted attention from predators or rivals.

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The Boys Are Bad: Older Male Ants Single out Younger Rivals for Death Squad

ScienceDaily (June 15, 2012) — Male Cardiocondyla obscurior ants are diphenic (either winged or wingless). New research published in BioMed Central's open access journal BMC Ecology demonstrates that the dominant wingless (ergatoid) male is able to identify potential rivals before they emerge from their pupae. Constant patrolling of the nest ensures that this male is able to bite or chemically tag rivals as soon as they emerge from their pupae. Chemically tagged ants are quickly destroyed by workers.

When ants emerge from their pupae (eclosion) they are vulnerable to attack because their exoskeleton has not yet hardened to protect them and their mandibles are also too weak to bite an attacker. Researchers from University of Regensburg and IST Austria investigated the behaviour of male ergatoid C. obscurior ants in response to newly pupated males and females as well as their interaction with developing pupae.

It would be advantageous for newly emerged males to hide from attackers. However, while winged males use chemical female mimicry to avoid being detected, young ergatoid males are easily detected and attacked by older males. If the fight occurs during the first day after eclosion the older male will always win but once the younger male's exoskeleton hardens his chances of surviving rises to 14%. Nearly 50% of fights between older and two-day old males results in the death of both combatants.

In addition to biting the males also smeared each other with secretions from their anuses which encouraged worker aggression. The older males were more likely to chemically tag two-day olds than younger rivals (presumably recognising that they needed help to win the fight) however since both males end up being tagged they both suffer worker attacks. The winners of these fights smear more and are attacked less by workers. So although this tactic overall may provide an 'edge' it is a risky strategy.

Read more:  http://www.sciencedaily.com/releases/2012/06/120615104506.htm

Scared of a Younger Rival? Not for Some Male Songbirds

ScienceDaily (Feb. 7, 2012) — When mature male white-crowned sparrows duel to win a mate or a nesting territory, a young bird just doesn't get much respect.

Researchers found that older male white-crowned sparrows don't put much of a fight when they hear a young male singing in their territory -- probably because the older bird doesn't consider the young rival much of a threat.

But a male sparrow will act much more aggressively if it hears a bird of the same age singing in a territory it claims as its own.

"These male sparrows assess an opponent's fighting ability based on age. And for a mature sparrow, a young male is just not going to scare them," said Angelika Poesel, lead author of the study and curator of Ohio State University's Borror Laboratory of Bioacoustics and the tetrapod division.

Read on:  http://www.sciencedaily.com/releases/2012/02/120207202302.htm