"Artificial Selection on Relative Brain Size in the Guppy Reveals Costs and Benefits of Evolving a Larger Brain", Kotrschal et al 2013; fascinating little experiment - it's odd the males showed no intelligence gain, and selecting on volume isn't exactly what you want to select on, but I'd love to see what a larger experiment ran for more generations showed.
Excerpts:
"Here we provide experimental evidence for costs and benefits of increased brain size. We used artificial selection for large and small brain size relative to body size in a live-bearing fish, the guppy (Poecilia reticulata), and found that relative brain size evolved rapidly in response to divergent selection in both sexes. Large-brained females outperformed small-brained females in a numerical learning assay designed to test cognitive ability. Moreover, large-brained lines, especially males, developed smaller guts, as predicted by the expensive-tissue hypothesis [1], and produced fewer offspring. We propose that the evolution of brain size is mediated by a functional trade-off between increased cognitive ability and reproductive performance and discuss the implications of these findings for vertebrate brain evolution.
The original ‘‘expensive-tissue hypothesis’’ [1] attempted to explain variation in primate brain size through a trade-off between brain tissue and gut tissue. However, recent comparative analyses have not supported this hypothesis [3] and have instead suggested that the trade-off occurs between brain size and other costly aspects of an organism’s biology, such as investment in muscle tissue [11], gonads [4], fat storage [3], or reproductive effort [19].
We selected for large and small brain size (brain mass) relative to body size (body length; see Supplemental Experimental Procedures available online for details) in replicated lines and found that brain size responded rapidly to divergent selection (Figure 1; Table S1; Figure S1). Relative brain size was already 9% larger in the upward- compared to the downward-selected lines after two generations of selection (estimated difference across ‘‘down’’ and ‘‘up’’ selection lines for adults [henceforth b, presented with 95% credible intervals (CI)]: b = 0.071 [0.06; 0.08] log (mg)/log (mm), p < 0.001; Figure 1; Table S1). This difference was already apparent in newborn fish, as indicated by a greater optic tectum width measured from digital microscopic images (b = 0.041 [0.019; 0.061], p < 0.001; Table S1). We used optic tectum width, an accurate predictor of overall brain size [24, 25], as a proxy for brain size of neonates because brains of neonates were too small to be removed and weighed. There were no significant main effects of brain size selection on body size in newborns or in adults (neonates: b = 0.06 [20.17; 0.27], p = 0.60; Table S2; adults: b = 20.058 [20.26; 0.16] mm, p = 0.59; Table S2). The realized heritability of relative brain size was substantial and congruent between sexes: 0.48 (0.38; 0.63) in females and 0.45 (0.33, 0.59) in males.
Depending on replicate, second-generation large- and small-brained females (left panel) differ by 8.0%–9.3% (p < 0.001) in relative brain size, while second-generation large- and small-brained males (right panel) differ by 5.0%–8.3% (p < 0.001).
Offspring number was thus 19% lower in the large-brained lines as compared to the small-brained lines, which shows that the evolution of a larger brain has a strong negative effect on an important reproductive trait. ...An alternative mechanism is that neural development of the gut is traded off against neural development of the brain. The gut forms a highly conserved, neuron-rich control center of the enteric nervous system that controls digestion [32] and is sometimes referred to as the ‘‘second brain’’ [33]. This is an important additional aspect of the function of the gut, which we suggest future research should target to fully understand the trade-off between the brain and the digestive system. Regardless of mechanism, in the controlled environment of our experimental setup, diet was kept constant. Therefore, in the absence of any cognitive benefits related to increased brain size, the genetic trade-off between investment in brain size and other expensive tissues, such as the gut, might have caused the reduction in reproductive performance that we observed."
Excerpts:
"Here we provide experimental evidence for costs and benefits of increased brain size. We used artificial selection for large and small brain size relative to body size in a live-bearing fish, the guppy (Poecilia reticulata), and found that relative brain size evolved rapidly in response to divergent selection in both sexes. Large-brained females outperformed small-brained females in a numerical learning assay designed to test cognitive ability. Moreover, large-brained lines, especially males, developed smaller guts, as predicted by the expensive-tissue hypothesis [1], and produced fewer offspring. We propose that the evolution of brain size is mediated by a functional trade-off between increased cognitive ability and reproductive performance and discuss the implications of these findings for vertebrate brain evolution.
The original ‘‘expensive-tissue hypothesis’’ [1] attempted to explain variation in primate brain size through a trade-off between brain tissue and gut tissue. However, recent comparative analyses have not supported this hypothesis [3] and have instead suggested that the trade-off occurs between brain size and other costly aspects of an organism’s biology, such as investment in muscle tissue [11], gonads [4], fat storage [3], or reproductive effort [19].
We selected for large and small brain size (brain mass) relative to body size (body length; see Supplemental Experimental Procedures available online for details) in replicated lines and found that brain size responded rapidly to divergent selection (Figure 1; Table S1; Figure S1). Relative brain size was already 9% larger in the upward- compared to the downward-selected lines after two generations of selection (estimated difference across ‘‘down’’ and ‘‘up’’ selection lines for adults [henceforth b, presented with 95% credible intervals (CI)]: b = 0.071 [0.06; 0.08] log (mg)/log (mm), p < 0.001; Figure 1; Table S1). This difference was already apparent in newborn fish, as indicated by a greater optic tectum width measured from digital microscopic images (b = 0.041 [0.019; 0.061], p < 0.001; Table S1). We used optic tectum width, an accurate predictor of overall brain size [24, 25], as a proxy for brain size of neonates because brains of neonates were too small to be removed and weighed. There were no significant main effects of brain size selection on body size in newborns or in adults (neonates: b = 0.06 [20.17; 0.27], p = 0.60; Table S2; adults: b = 20.058 [20.26; 0.16] mm, p = 0.59; Table S2). The realized heritability of relative brain size was substantial and congruent between sexes: 0.48 (0.38; 0.63) in females and 0.45 (0.33, 0.59) in males.
Depending on replicate, second-generation large- and small-brained females (left panel) differ by 8.0%–9.3% (p < 0.001) in relative brain size, while second-generation large- and small-brained males (right panel) differ by 5.0%–8.3% (p < 0.001).
Offspring number was thus 19% lower in the large-brained lines as compared to the small-brained lines, which shows that the evolution of a larger brain has a strong negative effect on an important reproductive trait. ...An alternative mechanism is that neural development of the gut is traded off against neural development of the brain. The gut forms a highly conserved, neuron-rich control center of the enteric nervous system that controls digestion [32] and is sometimes referred to as the ‘‘second brain’’ [33]. This is an important additional aspect of the function of the gut, which we suggest future research should target to fully understand the trade-off between the brain and the digestive system. Regardless of mechanism, in the controlled environment of our experimental setup, diet was kept constant. Therefore, in the absence of any cognitive benefits related to increased brain size, the genetic trade-off between investment in brain size and other expensive tissues, such as the gut, might have caused the reduction in reproductive performance that we observed."