A wide range of environmental and physiological factors influence the type and extent of sexual dimorphism found in animals. Influential factors include variable climate, competition for resources and mates, mate choice, and parental investment. In addition, some investigators attribute differences in body size to physiological factors influenced by genetic and environmental variation. Since Darwin's time researchers studying the evolution of animal life-history have discussed the interactions that exist between this wide array of influences on sexual dimorphism in the context of natural selection.
Much attention has been paid to the interrelationships of parental size, offspring size and number of offspring per reproductive bout. Spatial and temporal variation among these parameters have been investigated. However, these relationships have not been adequately examined while accounting for differences in offspring gender. In order to characterize the nature and extent of sexual size dimorphism found in adult red-spotted garter snakes, 108 adult females producing 782 female offspring and 790 male offspring were utilized to examine the relationships of the number and size of offspring, clutch mass and maternal size.
The second facet of our investigation involved the hormonal manipulation of growth over the first year of post-natal life. To gain an understanding of which endogenous hormones may influence growth in this species we constructed a hormone profile of estradiol and testosterone from birth to 15 weeks of age. We experimentally tested the effects of exogenous hormone as well as the removal of endogenous hormone on growth with the use of estrogen and androgen antagonists.
We have found that the extent of sexual size dimorphism observed in adult red-spotted garter snakes is not present at birth. The relative number of males or females born varies differentially with maternal size and age such that larger, older females produce predominately more male offspring. There are no differences in endogenous levels of testosterone and estradiol from birth though 15 weeks and at 36 weeks of age between the sexes. However, these sex steroids do differentially affect the growth of males and females. Finally, we suggest that while all of these factors contribute to sexual size dimorphism in this species, embryonic and environmental influences need to be explored. / Graduation date: 1999
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/33353 |
| Date | 15 July 1998 |
| Creators | Lerner, Darren T. |
| Contributors | Mason, Robert T. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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