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Popular Science: The bigger lizard attacks

Meeting of two monitor lizards might be quite an aggressive business when they want to settle which one is the boss. The conflict is not only dangerous, but also energetically demanding. It would be advantageous to be able to evaluate one’s chances beforehand. In the first ritual phase of monitor lizards’ conflict males, show off to one another. Can they estimate their chances based on their rivals’ size, and choose the right strategy accordingly?

Body size plays a crucial role in the life of an animal. It is formed by both natural and sexual selection. Bigger males therefore typically win over smaller ones, because the body size is tightly connected to physical strength, and the same goes for lizards too. Based on the size of males, we can estimate which one wins in the conflict and gains not only bigger popularity with the lady lizards, but also better access to resources. For lizards, it would be simply very handy if they could evaluate their possibilities based on their size while facing another monitor.

Varanus indicus. Source: Petra Frýdlová

Scientists from our Department of Zoology had a chance to watch the ritual behaviour in mangrove-dwelling monitor lizards (Varanus indicus) from Prague ZOO. They observed 22 males and 13 females of monitor lizards from hatching until 4 years of age. They always put a pair of lizards (male-male, male-female or female-female) into an enclosed arena with a non-transparent barrier, which they removed after 5 minutes of acclimation. Monitors were left and filmed in the arena for 10 minutes and the videotaped record was later evaluated by the scientists.

No lizards were harmed during the experiment, as any contact aggressive behaviour was immediately stopped. The encounter of female-female or male-female was usually peaceful with minimal signs of aggression. The interaction of two males, however, usually involved initial phases of ritual fight. In individuals after sexual maturation, which occurs between 1 – 1.5 year of age, the first signs of aggressive interaction began to show. The males hissed, smelled each other and stood in elevated postures to demonstrate their size. Aggressive behaviour also involved tail slapping, indirect assaults and biting. The dominant male was easily recognisable by quickly approaching and biting its opponent.

In 79% of cases, the aggressor was the bigger one of the competing males. It suggests the ability to compare one’s own size to the size of the opponent. When a monitor lizard sees that his posing rival is bigger, he may choose to flee without a single physical contact. The bigger male, on the other hand, learns his superiority right away and starts the attack much more often. Detailed analysis of the initial phases of the ritual conflict also suggests that small males sometimes bluff. They stand in elevated positions, trying to appear bigger and prevent the rival from attacking. This strategy was often successful.

Previous studies focused mainly on the results of conflicts. This one is unique by looking at what precedes the fight, and offers the first indirect proof of the ability of mutual assessment of body size in mangrove-dwelling monitor lizards.

Frýdlová P, Šimková O, Janovská V, Velenský P, Frynta D. 2017. Offenders tend to be heavier: experimental encounters in mangrove-dwelling monitor lizards (Varanus indicus). Acta ethol 20:37–45.

Gabriela Suchanová

Published: May 03, 2017 11:00 AM

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