Meeting Abstract
In most animal species the male’s role in reproduction ranges from fertilizing eggs to participating with females in parental care. Because females have fewer and larger gametes, males often compete for access to females, and females are often choosier. Darwin recognized that either form of sexual selection can result in the evolution of sexual dimorphism for size or other traits that influence mating success. Pycnogonids (sea spiders) are atypical because males fertilize and collect eggs that they carry and care for on their own bodies. Females are also atypical in that the ovaries are displaced out into the walking legs, possibly in response to the limited volume of the body. These natural selection pressures could have opposing influences on the evolution of large body (male) and leg (female) sizes, in addition to the potential for sexual selection favoring these traits. To understand whether this reversal in parental care influences sexual dimorphism in the pycnogonid Tanystylum orbiculare, we compared size measurements between males and females and between males with and without eggs. Males were significantly smaller than females in leg size but not body size measurements. Mated males were larger than unmated males in body size and in most leg size measurements. Among mated males, body size was positively correlated with the volume of eggs being carried. The larger leg size of females is consistent with natural selection for accommodating larger ovaries. Although larger males appear to be favored in mating, this does not appear to have resulted in sexual dimorphism in body size as predicted by Darwin.
