Exposure to dietary mercury alters cognition and behavior of zebra finches

Abstract

Environmental stressors can negatively affect avian cognitive abilities, potentially reducing fitness, for example by altering response to predators, display to mates, or memory of locations of food. We expand on current knowledge by investigating the effects of dietary mercury, a ubiquitous environmental pollutant and known neurotoxin, on avian cognition. Zebra finches Taeniopygia guttata were dosed for their entire lives with sub-lethal levels of mercury, at the environmentally relevant dose of 1.2 parts per million. In our first study, we compared the dosed birds with controls of the same age using tests of three cognitive abilities: spatial memory, inhibitory control, and color association. In the spatial memory assay, birds were tested on their ability to learn and remember the location of hidden food in their cage. The inhibitory control assay measured their ability to ignore visible but inaccessible food in favor of a learned behavior that provided the same reward. Finally, the color association task tested each bird's ability to associate a specific color with the presence of hidden food. Dietary mercury negatively affected spatial memory ability but not inhibitory control or color association. Our second study focused on three behavioral assays not tied to a specific skill or problem-solving: activity level, neophobia, and social dominance. Zebra finches exposed to dietary mercury throughout their lives were subordinate to, and more active than, control birds. We found no evidence that mercury exposure influenced our metric of neophobia. Together, these results suggest that sub-lethal exposure to environmental mercury selectively harms neurological pathways that control different cognitive abilities, with complex effects on behavior and fitness.

Keywords: animal behavior; cognition; ecotoxicology; mercury; spatial memory; zebra finch..

Figure 1

Summary of cognitive assays for zebra finches exposed to mercury and control diets. (A) The number of trials that were passed (hollow bars) or failed (filled bars) by birds in the final spatial memory test. (B) Mean (±95% confidence intervals) percentage of trials passed in the inhibitory control test. (C) Mean (±95% confidence intervals) number of errors made by birds during the color association learning trials.

Figure 2

Mean (±95% confidence intervals) of (A) activity score and (B) agonistic rank for zebra finches from control and mercury-exposed treatments.