Research Interests

Maternal manipulation of offspring sex in the sierra dome spider

Historically, an apparent bias in the field of mating behavior regarded the male role as active and the female role as passive. This bias led to a paucity of research on female influences over fertilization (i.e. the joining of egg and sperm). This bias is curious because for species in which fertilization occurs inside the female, the females have far more opportunity than males to influence the outcome and to increase the number of copies of their genes in future generations.

Research concerning the mechanisms whereby females control male fertilization success (how many offspring a particular male will father), and how and why females might attempt to control the sex of their offspring via sperm selection processes, have emerged as important, relatively new aspects of sexual selection research.

I am investigating whether female sierra dome spiders bias the sex ratio (i.e., change the proportion of males and females) of their offspring in response to the quality of their prospective father(s). Previous research on this species indicates that females actively influence male fertilization success according to physical and behavioral markers of male quality. The research will entail experimentally controlled matings of marked females in nature. Matings will be conducted in which I control the size, fighting ability and number of each female’s mates. Egg sacs will be collected and the number of male and female offspring will be determined by karyotyping (chromosomal analysis).

Size and fighting ability are key determinants of the number of offspring that male sierra dome spiders will father. Moreover, there is evidence that the more sexually competitive males of the study population age more rapidly. Evidence from the field and lab also show that females often age too rapidly to enable production of viable eggs. In this species, sons that inherit strong competitive abilities from their father are likely to obtain many matings early in life, producing more offspring over their lifetime than would a daughter. Moreover, if daughters inherit the short lifespan of competitive fathers, they may die before they can produce eggs. Thus it may benefit females to have a preponderance of sons when they are able obtain sexually competitive fathers for their offspring and to have daughters when a female’s mating partners are restricted to moderate or weakly competitive males.

The matings were conducted at Flathead Lake Biological Station, Montana, because (1) the study species does not occur locally, (2) its mating behavior is well understood, greatly facilitating experimentation, and (3) the process of courtship, mating and fertilization involves behaviors that could be altered by laboratory conditions.

This research will (1) add significantly to the paucity of knowledge regarding female control of sperm competition processes and offspring sex ratio; (2) clarify when and how sex ratio adjustment is likely to occur; and (3) as there is evidence that human females can affect their probability of having a son or daughter, this research may lead to a deeper understanding of our own mating strategies.