Meeting Abstract
The evolutionary reduction of body size can present animals with some major challenges. Animals with smaller bodies tend to have absolutely smaller brains. As a result, tiny animals are expected to face cognitive limitations. I address the hypothesis that brain size limits the amount of time a memory remains functionally active (i.e., the retention interval). I tested predictions of this hypothesis with a behavioral assay in long-bodied cellar spiders, Pholcus phalangioides (Araneae: Pholcidae), which search for captured prey that goes missing. I altered the amount of time between memory formation (prey capture) and memory use (prey searching) to determine the length of time over which memory influences behavior in large and small spiders (spanning a two-fold difference in brain mass). I found that as I increased the delay between memory formation and the opportunity for memory use, search time in small spiders sharply dropped off, and with long enough delays, they did not search at all. Spiders simply rested during delays, so these results cannot be attributed to exhaustion. The same delays decreased search time in large spiders as well, but had a much weaker effect, and even after delays of 16 minutes, large spiders consistently searched for lost prey. Thus, smaller spiders tended to have shorter retention intervals. These results suggest that at small scales, brain size may be important for the persistence of memories and animal’s ability to respond to past events. This may impose important selective pressures on species evolving miniaturization.
