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Fish that keep their young in their mouths sometimes eat them

Some female cichlid fish keep their offspring in their mouth for protection, but they commonly eat up to half of their brood

Life 9 November 2022

mouthbrooding female Astatotilapia burtoni

A mouth-brooding female Astatotilapia burtoni

Adrian Indermaur

Some fish protect their offspring by keeping them in their mouths, but these animals aren’t the models of parental devotion they might seem to be. New research reveals that they commonly eat up to half of their brood, and doing so may alleviate some of the physiological stress of parenting.

Female Astatotilapia burtoni fish, a species in the cichlid family, don’t hunt or forage during the two weeks in which they keep their eggs and hatchlings in their mouths – leading to weight loss, reduced immunity, faster ageing and lower chances of reproducing again. But by consuming up to half their brood, the mother fish acquire the nutrients needed to overcome some of these hardships, says Peter Dijkstra at Central Michigan University.

“The females are gaining something from it, not just in terms of body condition, but even something that could potentially boost their health,” he says.

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In an earlier study, Dijkstra and his colleagues found that A. burtoni mothers have higher levels of oxidative stress – the production of damaging chemicals inside cells – after laying eggs and keeping offspring in their mouths. They also found that the number of baby fish in the mothers’ mouths often dropped over their two-week mouth-brooding period, with up to 60 per cent of the young disappearing.

In their new study, Dijkstra and Jake Sawecki at Michigan State University observed 31 mouth-brooding female A. burtoni fish and 32 females whose eggs they removed.

All but two of the mouth-brooding females had fewer hatchlings by the end of the two-week study period than they had at the beginning, suggesting they were cannibalising their young, says Sawecki. The number of missing fry varied considerably from one female to another.

“They could have been dropping them, but I observed them every single day for hours and never saw that happen,” says Sawecki, adding that the mothers keep their mouths clamped tightly shut throughout brooding. “So the only really logical explanation was that they were consuming some of them.”

To determine levels of oxidative stress, the researchers took blood and liver samples from each mother fish two days, six days and two weeks after spawning. At the two-day mark, they found 23.7 per cent more DNA damage in the livers of females that were harbouring young in their mouths compared with those whose eggs had been removed, they say.

At six days and two weeks, there wasn’t much difference in liver damage between the brooding and non-brooding females, says Sawecki. The more young that had gone missing, the higher the levels of antioxidants – chemicals that counteract oxidative stress – in the mother’s liver. This suggests the mothers were sacrificing their offspring to boost their own health and reduce the stress on their bodies caused by the parenting burden, he says.

“In the grand scheme of things, it’s probably more beneficial to eat some of those young and be able to reproduce again in the future, rather than to die after that reproductive cycle and only have produced X number of young,” says Sawecki. “So it’s more of an investment in future reproduction.”

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2022.0319

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