Biology Reference
In-Depth Information
(Balda & Kamil, 1992). But is it any more remarkable than the memory of related
species that do not store food, or do food storers simply deploy their memory in this
particular way?
Alan Kamil, Russ Balda and colleagues have investigated this question by studying
four species of North American corvids that rely on food storing to different extents:
the Clark's nutcracker, pinyon jay, Mexican jay and Western scrub jay (Balda & Kamil,
2006). They used two laboratory tests of spatial memory and found that the Clark's
nutcracker, which is the most reliant on stored food, performs better than the other
species. On the other hand, when the task involved remembering colour instead of
location, the nutcrackers were no better than the other species. These results, along
with others (Shettleworth, 2010a) seem to show that food storing species do, indeed,
have an especially good spatial memory, perhaps both in terms of the amount of
information stored and the duration of memory, or the extent to which spatial cues, as
opposed to other cues such as colour, are used.
Food storing species also have a specialized brain. In mammals, a special region of
the cortex, called the hippocampus (so-called because in the eyes of some
neuroanatomists it recalls the shape of a seahorse of the genus
Hippocampus
), is
crucial in the formation of spatial and, perhaps, some other memories (Squire,
2004). Birds have an homologous structure that is essential for the recovery of
stored food (Sherry & Vaccarino, 1989). Measurements of the brains of many
different species of birds, including those that store food and those that do not, have
shown that food storers have a larger hippocampus, relative to the rest of the brain,
than do non-storers (Roth
et al
., 2010) (Fig. 3.9a). Within one species, the black-
capped chickadee (
Poecile atricapillus
), populations living in harsher winter
conditions, and therefore more dependent on food storing for winter survival, have
a larger relative hippocampus with more neurons than do birds living in less harsh
conditions (Roth
et al
., 2011; Fig. 3.9b).
The relative volume of the hippocampus, and/or the generation of new neurons, also
varies with season in the black-capped chickadee, although the way in which this
variation is linked to seasonal variations in food storing behaviour is not yet clear
(Sherry & Hoshooley, 2010). One possibility is that new neurons are added in
anticipation of the seasonal onset of storing, another is that the cause-effect arrow
goes the other way round. Nicky Clayton and John Krebs (1994) found that in young
marsh tits, experience of storing, or another spatial memory task, was necessary for
growth of the hippocampus, a case of 'use it or lose it'! Either way, the reduction in the
hippocampus during periods of little or no food hoarding suggests that brain tissue is
costly to maintain.
Brain scans of humans also reveal plastic change in hippocampus volume in response
to environmental demands. London taxi drivers face similar challenges of spatial
memory to those of food hoarding birds. They have to undergo extensive training,
learning to navigate between thousands of places in the city. This training, known
colloquially as 'being on The Knowledge' takes about two years for a full licence.
Magnetic resonance imaging showed that the taxi drivers had a larger posterior
hippocampus (and a smaller anterior hippocampus) compared to control subjects, and
the most experienced taxi drivers had the largest posterior hippocampus volume
(Maguire
et al
., 2000).
Brain
specializations in
food hoarders
and taxi drivers
