Bats create spatial memories without making new brain cells

Neurogenesis in the hippocampus is not necessarily a source of new spatial memories.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.

Bat-brains

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.

Bats are some of the best navigators among the 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 bats: 
Moths mimic each others’ sounds to fool hungry bats
Bats: internal compasses and record-breaking tongues

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.

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6 Responses

  1. This is very interesting – but I am skeptical. because some computational models are very clear about the utility of hippocampal neurogenesis.

    Do we know for sure that bats actually have good spatial memories? I know that some species of bat migrate thousands of miles to mate, but this could be driven not by memory but by seasonal preference for a particular “direction.”

    Some birds have a deposit of magnetite in their brains which helps them to identify directionality when in flight. Perhaps something similar happens in bats, permitting them adequate spatial memory without hippocampal neurogenesis?

    Finally, it seems that the spatial memory of bats would be wildly different from that of sighted animals, given that they use echolocation for navigation. So there are too many counfouding differences between spatial processing in bats and humans to start drawing inferences about neurogenesis..

  2. Really good points Chris – I would agree that the bat study doesn’t necessarily mean anything conclusive for human neurogenesis, but as you pointed out, it raises some really interesting questions about spatial memory in these animals.

    Oh and I like your blog :-)

  3. I just discovered your blog today – it’s fantastic! I want to get into more comparative stuff over at DI, but have had difficulty finding rigorous work. I’d be curious how you find the studies you post on, which seem very sound.

  4. Chris – Thanks for the kind words; the blog’s still quite small so the encouragement goes a long way.

    There’s not really any secret to my picking of studies. I only really look at a few select journals like Nature, Science, PNAS and a couple of the PLOS ones and pick interesting papers from there. I write this blog in my spare time which means I don’t have very much time to scour more obscure journals, but it has the happy side effect of ensuring that (on average) most of the papers are fairly solid.

    Not sure there’s anything more than that. One thing that probably helps is that my job partly involves me scanning lots of different papers, separating out interesting papers from dubious ones and writing reports that are carefully worded. I try to apply the same things here, and I do try very hard not to oversell new research.

  5. Bats have a curious talent for unsettling basic assumptions in neuroscience. You miight enjoy the first chapter of this blog: http://nine-radical.blogspot.com/2006/11/preview-of-blog-in-early-1990s-our.html. Thanks for your excellent work. Regards, John

  6. in a few months time i might be starting some research on neurogenesis in adult bats and ive found your blog to be pretty interesting.what i would want to know is:could it be possible that bats have seperate functional cell clusters subserving spatial memory and navigational ability and that neurogenesis occurs in one but not both of these functional clusters thus explaining their relatively low rates in neurogenesis?

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