Body size evolution is widely studied in the scientific community, but remains poorly understood. A variety of organisms seem to display a phenomenon known as Cope’s Rule, which is a general tendency of body size to increase over time. Many different mechanisms behind this evolutionary trend have been proposed.

Though researchers have investigated Cope’s Rule, few studies have tracked body size evolution within lineages over long periods of time.

This project traces body size changes in multiple lineages of deep-sea ostracodes, a taxon previously suggested to exhibit Cope’s Rule.

Methods

The study specimens were obtained from cores from two sites on the Ninetyeast Ridge of the Indian Ocean.

The approximate age of each sample of ostracodes was determined by biostratigraphic methods, and the specimens, spanning 40 million years, were sorted by species.

Relevant specimens were digitally photographed, and the software Image J was used to measure the length, height, and area of the ostracodes.

Individuals were assigned to instars (=growth stages) based on size clustering (Image 3) and the adult-specific characters of prominent hinges and a thick inner lamella. Body size measurements were only compared within instar groupings.

Average body size (measured by log-area) was plotted over time for the most abundant instar of each species from both sites. Body size increases and decreases were counted between adjacent samples, and entire sequences were tested for directional trends using random walk models.

Results

Although ostracode body size showed intervals of both increase and decrease over time, there is evidence of an overall trend toward larger size.

For lineages with a long range, body size generally increased from the earliest to most recent sample.

Including both sites and all lineages, there were 38 body size increases between adjacent time periods, and 28 decreases. The number of significant increases (13) is nearly double the number of significant decreases (7).

Testing for directionality finds that most lineages exhibited increasing body size. Out of the sixteen sequences, an overall size increase was found for thirteen. It is unlikely to get such a skewed result by chance (binomial test, p=0.02), even though in only one lineage, Poseidonamicus major from site 254, is the increase in size significant.

The exact patterns of body size evolution appear to differ between species and sites. However, there were no intervals between adjacent samples with coincident significant increases and significant decreases in body size.

Concordant increases in body size emerge during two time intervals. The most obvious period of concordant size increase occurred between the late Oligocene and early Miocene (approximately 29.2 to 19.9 mya) at site 253, where six out of six ostracode populations showed a size increase (three are significant). During the Pliocene at site 254 (approximately 5.2 to 1.8 mya), six ostracode populations increased in body size (four are significant), while only one decreased (not significant).

Lineages with long ranges increased in body size over time. Bars represent 95% confidence intervals.

Comparison across sites shows differences in body size evolution for shorter time intervals. Bars represent 95% confidence intervals.

Discussion

The results of this research are similar to previous studies demonstrating a tendency for body size to increase within lineages in ostracodes (1), mammals (2,3,4), and other taxa.

One possible cause of this general trend toward larger size is the substantial cooling of deep ocean waters during the Cenozoic.

Ostracodes, like many other animals, have a tendency to be bigger in colder climates. Previous work has indicated that increases in body size of the ostracode genus Poseidonamicus were significantly correlated with decreases in deep-sea temperatures during the Cenozoic (1).

This finding relates Cope’s Rule to the equally notable Bergmann’s Rule, which states that modern organisms in colder climates will be larger than those in warmer climates. Cope’s Rule, in this scenario, is then a temporal manifestation of the spatial Bergmann’s Rule.

On shorter time scales, the patterns of body size evolution appear to differ between species and sites. This indicates that although a cooling global climate may be driving the general trend toward increasing body size in deep-sea ostracodes, there are many other local and species-specific factors capable of influencing body size evolution, such as productivity and interactions with other organisms.

References

Hunt, G. & Roy, K. (2006) Proc. Natl. Acad. Sci. USA 103, 1347-1352.

Alroy, J. (1998) Science 280, 731-734.

Smith, F., Betancourt, J.L. & Brown, J.H. (1995) Science 270, 2012-2014.

Millien, V., Lyons, S.K., Olsen, L., Smith, F.A., Wilson, A.B. & Yom-Tov, Y. (2006) Ecol. Lett. 9, 853-869.

Acknowledgements

This research was supported by the William A. and Nancy F. McMinn Honor Scholarship in the Natural Sciences through Vanderbilt University. I would like to thank my advisor, Gene Hunt, for all of his help and advice throughout the project, Scott Wing for allowing me to be a part of his field team, and Mary Sangrey and the RTP students for helping to make the program such a great learning experience.