Getting a head start: diet, sub-adult growth, and associative learning in a seed-eating passerine

Abstract

Developmental stress, and individual variation in response to it, can have important fitness consequences. Here we investigated the consequences of variable dietary protein on the duration of growth and associative learning abilities of zebra finches, Taeniopygia guttata, which are obligate graminivores. The high-protein conditions that zebra finches would experience in nature when half-ripe seed is available were mimicked by the use of egg protein to supplement mature seed, which is low in protein content. Growth rates and relative body proportions of males reared either on a low-protein diet (mature seed only) or a high-protein diet (seed plus egg) were determined from body size traits (mass, head width, and tarsus) measured at three developmental stages. Birds reared on the high-protein diet were larger in all size traits at all ages, but growth rates of size traits showed no treatment effects. Relative head size of birds reared on the two diets differed from age day 95 onward, with high-diet birds having larger heads in proportion to both tarsus length and body mass. High-diet birds mastered an associative learning task in fewer bouts than those reared on the low-protein diet. In both diet treatments, amount of sub-adult head growth varied directly, and sub-adult mass change varied inversely, with performance on the learning task. Results indicate that small differences in head growth during the sub-adult period can be associated with substantial differences in adult cognitive performance. Contrary to a previous report, we found no evidence for growth compensation among birds on the low-protein diet. These results have implications for the study of vertebrate cognition, developmental stress, and growth compensation.

Figure 1. Relative head widths (mean±S.E.) of birds reared on HI vs LO diets.

A: residuals of head width on tarsus; B: residuals of head width³ on mass. White circles – LO diet birds; black circles – HI diet birds. (See also Table 2.)

Figure 2. Distribution of number of bouts needed to pass the learning task as a function of diet.

(White boxes–LO diet birds(mean±S.E.:20.32±1.57); black boxes–HI diet birds (mean±S.E.: 12.03±1.06). (For statistical effect, see Tables 3 and 4.)

Figure 3. Relationship between residuals of the regression of head width on tarsus (day 175) and bouts taken to pass the learning task.

(White circles–LO diet birds; black circles – HI diet birds. For statistical effect, see Table 3. )

Figure 4. Relationship between early growth (day55 to day 95) and bouts required to pass the learning task.

a) head width; b) mass change. (White circles – LO diet birds; black circles – HI diet birds. For statistical effects, see Table 4.)

Figure 5. Relationship between bouts required to complete the learning task when yellow versus black curtains were associated with a food reward.

(Open circles = LO diet birds; closed circles = HI diet birds. Means±S.E. for black reward: LO –16.67±1.47 (N = 9); HI—11.14 ±1.45 (N = 7).)