By Stephen Beech
Pregnant fish can also get "baby brain" - just like human moms-to-be, according to a study.
Pregnancy-related brain impairment is present in live-bearing fish, say scientists.
Their research shows that instead of affecting learning and memory, as expected from similar research on mammals, it appears to have a stronger impact on decision-making and sensory reception.
Several previous studies into the detrimental impact of pregnancy on mammalian brains - sometimes called "baby brain" or "momnesia" in humans - revealed how the disruption of neurological processes such as neurogenesis, or the creation of new neurons, can affect learning and memory.
But the new study is the first to examine the phenomenon in fish.
While most fish reproduce by laying eggs, some species are live-bearing, or viviparous, and carry their offspring internally before producing live young.
Viviparous species include popular aquarium fish such as guppy, molly and platy.
Study author Tiffany Ernst said: "We wanted to see if these pregnancy-related changes in cognition and neurogenesis occur in other live-bearing evolutionary lineages, particularly in a species that lacks a placenta.
"In mammals, the placenta is thought to help regulate pregnancy-related changes to the mother's brain.
"The species of fish that I work with, Poeciliopsis gracilis, is especially interesting as they are superfetatious, meaning they can be pregnant with multiple temporally overlapping broods of offspring simultaneously.
"Essentially, my fish are almost constantly pregnant in adulthood - so any detrimental impact on cognition might be more evident in this species.
"We hoped to learn how pregnancy might induce changes to the maternal brain in a non-mammalian live-bearing species.
"This could help us to understand whether the evolution of a live-bearing reproductive strategy requires neurological trade-offs to adequately adapt for a healthy pregnancy."
To examine the impact of pregnancy on cognition, pregnant and virgin fish were trained to associate a green disk with the location of food, and then perform a spatial learning task when presented with two seemingly identical disks in different locations.
The research team then tested their cognitive flexibility by swapping the locations of the rewarded and non-rewarded disks to see how the fish adjusted to the new location of the food reward.
To assess the effect on fish neurology, Ernst and her colleagues removed the brains of the pregnant and virgin females and used cell staining to identify areas of new cell proliferation - an indicator of which areas of the brain were producing new cells.
The study is the first to map the areas of brain cell proliferation in this species in a "brain atlas".
Ernst, a PhD candidate in the field of developmental and reproductive biology at Wageningen University in The Netherlands, and her team found that while both pregnant and virgin fish were equally successful in both cognitive tasks, pregnant fish were much more hesitant when choosing which disk to approach.
She said: "We also found no difference in cell proliferation in the regions of the brain most responsible for learning and memory.
"However, we were surprised to see that pregnant females exhibited decreased cell proliferation in the regions which contribute to maternal olfactory reception."
The surprising findings, presented at the Society for Experimental Biology conference in Prague, suggest that pregnancy compromises the fish's ability to interpret scent in the water, which may contribute to their choice aversion in the cognitive tests.
Ernst said: "For P. gracilis, reduced choice-propensity during pregnancy might be an adaptive strategy wherein females do not take the risk of foraging for food when the reward is not guaranteed, thus conserving energy for reproduction.
"Our research indicates that pregnancy may impact maternal cognition and alters brain cell proliferation, but not in the same ways as we would expect from mammals."
She added: "This implies that pregnancy across different evolutionary lineages has an impact on the maternal brain which in turn, affects how mothers cognitively and physiologically adapt to the burdens of live-bearing reproduction."
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