Neale Monks has some advice for a reader who's concerned about possible tension between his male guppies.
Q. I have a 65 l./14 gal fully cycled aquarium, containing four male Blue guppies. I recently added another four male guppies, this time Red snakeskin, and they seem to be pestering each other a lot and I can't tell if they are fighting.
I've read that male guppies will generally refrain from pestering each other. Why is this not the case where my guppies are concerned? Is this behaviour likely to calm down once the guppies get used to each other? And if not, are they likely to cause each other any serious harm?
Sharn Cooper, email
A. I suspect people have different experiences with guppies, and a good deal depends on the numbers kept. In very large groups (like you see in pet shops) the males seem to do little harm to each other, perhaps because no one individual can bully, or be bullied, all the time. But in smaller groups, especially twos and threes, you find one male becomes the aquarium 'boss' and spends a lot of his time harassing any other males kept with him.
An interesting observation has been made with another livebearing species, Ameca splendens, which is practically extinct in the wild but has been kept by aquarists for decades.
Observations of wild specimens and those descended from recently caught specimens indicates males spend most of the time feeding on algae and relatively little time fighting. But the farmed specimens in the hobby are descended from specimens collected in the 60s and 70s, and after generations of maintenance in aquaria, the males we keep as pets spend most of the time fighting and very little time feeding.
Why? One hypothesis is that because we feed them good quality food for just a few minutes per day, these fish have more time to spend on other things, like fighting and breeding. Males that spent time fighting were favoured because the alternative, spending their time feeding, wasn't helpful.
Indeed, the pushier the male, the more likely he got to breed, and the more likely his aggressive genes got passed on into the next generation of Ameca splendens.
But the wild specimens don't have this luxury as the food they eat, algae, is not nutrient rich and they need to graze for hours to take in the calories they require. Wild males will be very careful about wasting energy on chasing away another male unless it's absolutely necessary. If you're an aggressive male you could well end up starving to death long before you had a chance to mate, whereas a more cautious male who built up his energy reserves carefully would be in a good position to impress a passing female and father her offspring.
It's very probable that something similar has happened with guppies, and given how we've bred guppies into countless varieties, their behavioural genes are likely very mixed up too. Some batches may well be more aggressive than others, even within the same basic variety.
Add to this things like the size of the tank, the number of male guppies, the availability of females, and the presence or otherwise of bigger, potentially threatening tankmates, and you can easily see how the behaviour of male guppies could be a very difficult thing to predict.
Anyway, to cut a long story short, yes, male guppies fight, and yes, if their fins are repeatedly damaged then problems like finrot can occur. They may calm down once a pecking order is established, but don't bank on it.
Adding more males can reduce tension, as will providing suitable hiding places at the top of the tank where the guppies live, particularly floating plants (Amazon Frogbit and Indian Fern are the two easiest options here).
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