In an earlier post, I wrote about a study which used carbon-dating to show that our brains are mostly stuck with the same neurons they are born with. After birth, neurogenesis – the manufacture of new neurons – is completely absent in most of the brain.
There are only two exceptions, where new neurons are made. The first is the olfactory bulb, which governs our sense of smell. The second, the hippocampus, is involved in spatial awareness and memory. Why these regions alone should produce fresh neurons is unclear.
For the hippocampus at least, scientists thought they had an answer – the fresh neurons play a role in spatial learning and memory. They could allow mammals to learn about new places, routes and directions.
But Imgard Amrein and colleagues from the University of Zurich have found evidence that disputes this idea. When he looked at the hippocampuses of some of the most accomplished mammal navigators, the bats, he found a startling lack of neurogenesis.
Bats need superb spatial awareness to effortlessly fly in three dimensions. Those that feed on fruit and nectar need especially good spatial memories, and indeed, their hippocampuses are relatively large compared to other mammals.
Their memories allow them to remember where the tastiest or ripest food sources are. And they also remember the locations of plants they have recently visited so that they don’t arrive at restaurants with no stock.
Amrein searched for signs of new neurons in 12 species of bats using special antibodies. Some detected proteins that only appear when new cells are born. Others homed in on proteins used by newborn neurons when they migrate to new places.
As expected, these molecular trackers picked up new neurons in the olfactory bulb. But they found no neurogenesis at all in the hippocampus of 9 species, and only the faintest traces in the other three. Clearly, the bats don’t need new hippocampal neurons to learn where things are or to remember how to find them.
Flexibility vs consistency
While Amrein’s bats were few in number, they were also a diverse bunch. They hailed form different evolutionary groups and had diverse diets, territory sizes and ages. This makes it unlikely that these variations in these factors were secretly responsible the trends that Amrein saw.
Instead, he believes that the dearth of new neurons in bats reflects their relatively long lifespans. Humans, apes and monkeys are similarly long-lived, and we too have low levels of neurogenesis as adults.
In contrast, rats and other rodents have short and brutal lives. In order to avoid becoming food for a predator, their behaviour must be as flexible as possible. When threatened, their stream of new hippocampal neurons could allow them to rapidly plan an escape route or find new hiding places.
Bats, and certainly humans, have far fewer predators, and can afford to take things easier. In our long lives, fixed long-term mental maps are very useful and to produce them, we can sacrifice some flexibility in our spatial memories.
This may explain why people tend to rely on the same routes more and more as they age. Fortunately for us, bats show a similar trend. Their reliance on the same flight paths allows canny researchers to catch them in well-placed nets and study how their brains work.
More about neurogenesis:
No new brain cells for you – settling the neurogenesis debate
More about neurons:
Simple sponges provide clues to origin of nervous systems
Monkeys (and their neurons) are calculating statisticians
Non-coding DNA drove brain evolution by making nerve cells stickier
Maternal hormone shuts down babies’ brain cells during birth
Reference: Amrein, Dechmann, Winter & Lipp. 2007. Absent or Low Rate of Adult Neurogenesis in the Hippocampus of Bats (Chiroptera) PLoS ONE.