Evolution of Body Size

Body size at maturity is one of the most fundamental traits of an organism, and is a key component of their life history, behavior, ecology, and physiology.  Size is considered to be an optimal outcome of trade-offs between early maturation, fecundity, and survival.

Large-scale patterns of body size distribution have been of interest for over a hundred years.  An increase in body size with latitude (Bergmann’s rule) is commonly seen in some species, while the reverse pattern (converse Bergmann’s rule) is seen in others.  The major factors thought to influence these patterns are temperature and season length, respectively.  We were fascinated to discover that horseshoe crabs are one of just a few species to show a mid-latitudinal peak in size distribution (i.e., they appear to have both Bergmann and Converse Bergmann clines).  We are exploring how a limited season length and a physiological temperature constraint might act independently and in conjunction to create this rare pattern.

Across taxa, males and females of the same species rarely have the same adult body size.  Rensch’s rule is another pattern that describes large-scale variation in body size.  Across species, sexual size dimorphism (SSD) often decreases with an increasing average body size (when females are the larger sex).  Given that horseshoe crabs have genetically isolated populations with very different body sizes, and that they are sexually size dimorphic, we wondered if this single species might also conform to Rensch’s rule (across populations).

Perhaps more interestingly, what is the ultimate explanation for why females and males are different sizes?  SSD is thought to be a result of disruptive selection on body size between the sexes, due to their differences in reproductive roles.  Many studies focus on just one selection pressure (e.g., fecundity selection on females), but SSD is more likely the product of a suite of conflicting pressures on each sex.  We have been examining multiple hypotheses to explain the ultimate factors affecting both male and female horseshoe crabs.

Related Publications:

Smith, M.D. and H.J. Brockmann.  In Prep.  Patterns of body size distribution in horseshoe crabs: Bergmann’s rule and Rensch’s rule.

Smith, M.D. and H.J. Brockmann. In Prep.  Ultimate factors underlying sexual size dimorphism in horseshoe crabs: an evaluation of 6 hypotheses.

Keegan, L.K., M.D. Smith, and H.J. Brockmann.  In Prep.  Growth rate of juvenile horseshoe crabs, Limulus polyphemus, at Seahorse Key, FL.