Smart as a Possum

What led to the evolution of relatively larger brains in mammals? For that matter, what led to the evolution of relatively larger brains in placental mammals compared to metatherian mammals (marsupials)? These questions remain largely unanswered despite the concerted effort of many scientists.

mouseopossum
Figure 1. A photograph of a mouse opossum from the Neotropics (from icelight/Flickr)

Marsupials and placental mammals have very different reproductive strategies (see Vaughan et al. 2010). Marsupials invest relatively little in
gestation and much more in lactation. Conversely, placental mammals invest far more in gestation than in lactation (thanks to a more elaborate placental connection between mother and offspring). The consequences are that placental mammals are born at a more advanced stage of embryonic development compared to most marsupials of similar size. Could the differences between gestation versus lactation times in these mammals explain the dichotomy in brain size in marsupials and placentals?

brainsize
Figure 2. Regression plots of log brain weight, BMR, and Gestation length against body size in marsupials and placental mammals (From Weisbecker and Goswami 2010)


A recent reanalysis of mammalian brain sizes and life history parameters by Weisbecker and Goswani (2010) reveal that we may be asking the wrong question. Their analysis shows that marsupials and placentals have very similar brain sizes when Primates, a group with relatively large brains compared to other mammals, are excluded from the comparison. In fact, small marsupials appear to have slightly larger brains relative to similar-sized placentals (Figure 2.). Similarly, there is broad overlap in encephalization quotients (EQ), a brain-to-body-weight ratio often used as a surrogate for intelligence, between these major mammalian groups. For example, more ancient placental groups (
Xenarthra –armadillos, anteaters, and sloths and Afrotheria -aardvarks, elephants, hyraxes, sirenians, and tenrecs) have relatively low EQs. Evidently, large brains evolved independently several times from small-brained placental ancestors.

Interestingly, basal metabolic rates (BMR) and reproductive traits also correlate in different ways in the two groups. Marsupials lack a correlation between BMR and brain size, while a positive BMR-brain size correlation appears to be a placental trait. One explanation for this pattern is that marsupials “grow” their brains during an extensive lactation period after birth. In contrast, much of the brain growth in placental mammals occurs during relatively long
in utero gestation periods. High BMR, long gestations, and more intimate placental contact, permit increased energy transfer from the mother to offspring per unit time. The maternal energy is then used to “grow” a relatively larger prenatal brain. Similarly-sized marsupials eventually catch up with placentals at the end of their prolonged lactation periods.


References

Vaughan, T., Ryan, J., and N. Czaplewski. 2010.
Mammalogy, 5th edition, Jones & Bartlett, Sudbury, MA.

Weisbecker, V., and A. Goswami. 2010. Brain size, life history, and metabolism at the marsupial/placental dichotomy.
Proceedings of the National Academy of Sciences, 107:16216-16221.