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Open Access Article << Previous     |     Next >>   Contents Vol 62(6)

Does more maternal investment mean a larger brain? Evolutionary relationships between reproductive mode and brain size in chondrichthyans

Christopher G. Mull A C, Kara E. Yopak B and Nicholas K. Dulvy A

A Earth to Ocean research group, Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada.
B Center for Scientific Imaging, University of California, San Diego, La Jolla, CA, USA.
C Corresponding author. Email: cmull@sfu.ca

Marine and Freshwater Research 62(6) 567-575 http://dx.doi.org/10.1071/MF10145
Submitted: 17 June 2010  Accepted: 25 February 2011   Published: 24 June 2011

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Chondrichthyans have the most diverse array of reproductive strategies of any vertebrate group, ranging from egg-laying to live-bearing with placental matrotrophy. Matrotrophy is defined as additional maternal provisioning beyond the yolk to the developing neonate; in chondrichthyans, this occurs through a range of mechanisms including uterine milk, oophagy, uterine cannibalism and placentotrophy. Chondrichthyans also exhibit a wide range of relative brain sizes and highly diverse patterns of brain organisation. Brains are energetically expensive to produce and maintain, and represent a major energetic constraint during early life in vertebrates. In mammals, more direct maternal–fetal placental connections have been associated with larger brains (steeper brain–body allometric scaling relationships). We test for a relationship between reproductive mode and relative brain size across 85 species from six major orders of chondrichthyans by using several phylogenetic comparative analyses. Ordinary least-squares (OLS) and reduced major axis (RMA) regression of body mass versus brain mass suggest that increased maternal investment results in a larger relative brain size. Our findings were supported by phylogenetic generalised least-squares models (pGLS), which also highlighted that these results vary with evolutionary tempo, as described by different branch-length assumptions. Across all analyses, maximum body size had a significant influence on the relative brain size, with large-bodied species (body mass >100 kg) having relatively smaller brains. The present study suggests that there may be a link between reproductive investment and relative brain size in chondrichthyans; however, a more definitive test requires a better-resolved phylogeny and a more nuanced categorisation of the level of maternal investment in chondrichthyans.

Additional keywords: allometry, encephalisation, pGLS, relative brain size, reproductive mode.


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