Last updated: February 14, 2019
Topic: Family
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Mating System and Brain Size in Bats


What is the ‘Social Brain Hypothesis’ and what does it predict with regard to sexual dimorphism in brain size?

The social brain hypothesis suggests that an increase in social difficulty improves cognitive arms races.  The suggestion that individuals with relatively large brains are superior in influencing the behavior of others to favor the manipulator’s own desires has been contended.

However, the social brain hypothesis has been supported and predicts that there is a positive relationship between brain size and social group size (Pitnick, 2006). What is the ‘Sexual Conflict Hypothesis’ and what does it predict with regard to sexual dimorphism in brain size?


More recent hypothetical and experimental studies in sexual selection theory put forward the sexual conflict hypothesis (Pitnick, 2006). This hypothesis implies an existence of sexually opposed coevolution for cognition; whereby females and males are mutually under selection to challenge the reproductive investment and resist being undermined by their sexual partners. It predicts that species reproducing promiscuously will have moderately larger brains than species with genetic monogamy (Pitnick, 2006).


What is the ‘Expensive Tissue Hypothesis’ and what does it predict with regard to sexual dimorphism in brain size?


An additional hypothesis in sexual selection theory, is the expensive tissue hypothesis. Pitnick (2006) entails that changes in brain size may accompany compensatory changes in other expensive tissue for the reason that moderately large brains are metabolically expensive to sustain and develop. This hypothesis has not been measured in any taxon considering ornaments, weapons or sexual organs performing in reproductive antagonism (Pitnick 2006).


How does the ‘Expensive Sexual Tissue Hypothesis’ differ from the ‘Expensive Tissue Hypothesis’?


Noting that sexually selected traits can also be expensive, the expensive sexual tissue hypothesis differs from the expensive tissue hypothesis. The latter is condition–dependent and predicts that more concentrated sexual selection will restrict the evolution of improved brain size as a result of vigorous compromise with costly sexual organs, ornaments or armaments (Pitnick 2006).


What kinds of data do the authors use in this study and what specific hypotheses do they propose to test with regard to brain evolution in bats?


The researchers, Pitnick (2006), analyze comparative data on overall brain and neocortex dimension, testis mass, and social and mating systems for 334 Chiroptera species to assess predictions of the social brain, sexual conflict and expensive sexual tissue hypothesis for brain evolution in bats.


Do the authors generate the data needed to adequately address all of the proposed hypotheses?


Yes, the authors, Pitnick (2006) use data on both brain mass and volume of the neocortex to decide if brain size has been influenced by social complexity or sexual conflict by investigating the association between both the social system and mating system and brain size evolution in bats. After calculating for allometric disparity related with body size, they found highly significant relationships between both brain traits and female promiscuity and mating system (sexual conflict hypothesis) and marginally non-significant relationships with roosting association (social brain hypothesis). With regards to expensive tissue hypothesis, the data of the distributed residual brain mass reflected that of residual testis mass, noting that the bat species bearing relatively largest testes, for which brain dimension data was also available.


Finally the authors addressed the expensive sexual tissue hypothesis by comparatively probing the connection between relative dimension of brain and testis; taking into account phylogeny, allometry and ecological grades associated with diet and echolocation. A considerably negative relationship was observed between independent contrasts in residual brain mass and independent contrasts in residual testis mass of all species (Pitnick 2006).


Based on the introduction to this paper, is ornamentation or testes investment a more reasonable response of male bats to sexual selection?


Testicular tissue can represent a substantive energetic investment and is regarded as a more reasonable response of male bats to sexual selection (Pitnick 2006).

What is a critical assumption of testing of the Expensive Sexual Tissue Hypothesis by comparatively examining the relationship between relative dimension of brains and testes?


The researchers (Pitnick 2006) identify that both brain dimension and breeding structure may display phylogenetic inertia and that the breeding system is predicted to covary with foraging ecology, which is non-randomly distributed across bat clade and known to select on relative brain size in bats. Therefore it is critical to scrutinize the connection between constant variation in brain and testis mass across species while adjusting for phylogeny, allometry and ecological grades connected with diet and echolocation (Pitnick 2006).

Why do the authors have to control for body size allometry before comparing the relationships between social structure and brain/testes sizes among bat species?


To control for allometry in the species-level analyses, analysis of covariate (ANCOVA) was used with body size treated as a covariate (Pitnick 2006).


Explain completely the meaning and interpretation of Figure 1.


The main purpose of figure 1 is to produce residuals from regression of 1n brain mass or 1n testis mass on 1n body mass, with regressions executed discretely by family as partial phylogenetic control (Pitnick 2006).  .

In figure 1. residuals are used to illustrate trait differences among treatment groups.

Figures (1a-c) show mean residual in brain mass whereas figures (d-f) show mean residual in testis mass. Findings show that there was no evolutionary influence on relative brain dimension by comparing monogamy and polygyny in male promiscuity figure (1b). Figure (1c) opposes the social brain hypothesis with findings of an apparent trend towards brain size decreasing as intricacy in roosting group organization increases. However when we take into account that brain evolution may be more important than composition or colony size, we observe a rejection of the sexual conflict hypothesis as seen in figure (1a) since findings show that females inseminated by a single male within a breeding cycle have moderately large brains (Pitnick



Nonetheless, all relationships taken into account show a strong association between sexual selection and brain dimension between bat species. Similarly, it is important to note that the allocation of residual brain mass with treatment groups (figure 1a-c) reflects that of residual testis mass (figure1d-f), with regard to the expensive tissue hypothesis. In addition, figure (1d, f) illustrates that female promiscuity (not male prospect to inseminate multiple females) is linked with the rise in relative testis mass.


Given that echo-locating bats must be very good/efficient fliers relative to fruit-eating bats, can you think of an alternative hypothesis that explains why there would be a trade-off between brain and testes size in echo-locating vs. non- echo-locating bats?


One alternative hypothesis demonstrated in (Pitnick 2006) is genetic constraint (e.g antagonistic pleitropy) as a potential clarification for converse evolutionary brain-testes association. A functionally varied display of genes show evidence of co expression in brain and testis. Another alternative, in my opinion, could be the influence of stress during predation in echo-location bats.



Pitnick, S., Jones, K.E., Wilkinson, G.S. (2006). Mating system and brain size in bats. Proc. R. Soc 273:719-724. [Electronic version]