The Allometry of Brain Miniaturization in Ants

Abstract

Extensive studies of vertebrates have shown that brain size scales to body size following power law functions. Most animals are substantially smaller than vertebrates, and extremely small animals face significant challenges relating to nervous system design and function, yet little is known about their brain allometry. Within a well-defined monophyletic taxon, Formicidae (ants), we analyzed how brain size scales to body size. An analysis of brain allometry for individuals of a highly polymorphic leaf-cutter ant, Atta colombica, shows that allometric coefficients differ significantly for small (<1.4 mg body mass) versus large individuals (b = 0.6003 and 0.2919, respectively). Interspecifically, allometric patterns differ for small (<0.9 mg body mass) versus large species (n = 70 species). Using mean values for species, the allometric coefficient for smaller species (b = 0.7961) is significantly greater than that for larger ones (b = 0.669). The smallest ants had brains that constitute ∼15% of their body mass, yet their brains were relatively smaller than predicted by an overall allometric coefficient of brain to body size. Our comparative and intraspecific studies show the extent to which nervous systems can be miniaturized in taxa exhibiting behavior that is apparently comparable to that of larger species or individuals.

Miniaturization in Otothyris Myers, 1927 (Loricariidae: Hypoptopomatinae)

“…According to Haller's Rule, it is expected that the brain of smaller animals must be larger relative to their body size than are the brains of larger animals. This pattern is common for both vertebrates and invertebrates (Hanken, 1983;Quesada et al, 2011;Seid et al, 2011), and is related to a negative allometric growth between brain and body. Our results show that this pattern also occurs in loricariid catfishes, primarily when we consider the relative widths of the brain and head.…”

Lifespan behavioural and neural resilience in a social insect

“…The lack of functional senescence in P. dentata minor workers, underscored by the maintenance of neuroanatomical and neurochemical substrates that activate and support task performance, suggests that the ant nervous system has evolved robust functionality throughout the relatively short sterile worker lifespan. This resilience may be associated with energy savings resulting from the absence of reproductive costs of workers, reduced neuron and neural circuit size, and lower requirements for redundancy and information storage that could minimize neurometabolic costs in individual worker brains [76][77][78][79]. Additionally, the benefits of living in a highly integrated homeostatic social system capable of collective information processing and emergent cognition may allocate cost reductions in brain rspb.royalsocietypublishing.org Proc.…”

Breaking Haller's Rule: Brain-Body Size Isometry in a Minute Parasitic Wasp

“…We hypothesized that T. evanescens has reached the limits of brain scaling so that smaller wasps cannot further reduce brain size without compromising brain per- [Wehner et al, 2007;Riveros and Gronenberg, 2010;Seid et al, 2011].…”