Effects of Experimentally Increased Testosterone on Movement and the Effects of Sex and Age on Relative Cortical Volumes in the Northern Pacific Rattlesnake (Crotalus O. Oreganus)

Abstract

Hormones have been shown to play a crucial role in vertebrate reproductive behaviors. Androgens such as testosterone (T) stimulate male sexual behaviors such as mate-searching. Increases in movement and sex differences in navigational demands associated with increased movement are positively related to the volumes of cortical brain regions associated with navigational ability. In mammals and birds, the development and morphology of the hippocampal region, a structure within the forebrain, has been shown to play an important role in spatial learning, memory, and navigational ability. The medial cortex (MC), dorsal cortex (DC) and lateral cortex (LC) of non-avian reptiles are putative homologues to the avian hippocampus. This study sought to investigate the role of T in movement patterns displayed by free-ranging Northern Pacific Rattlesnakes (Crotalus o. oreganus) by utilizing a combined observational and experimental approach.Twenty two adult male rattlesnakes were radio-tracked for four months. At the time of radio-transmitter implantation, twelve of the 22 snakes were randomly selected to receive testosterone implants (T-implanted group) and the remaining 10 snakes received blank implants (control group). We also quantified changes in concentrations of corticosterone (CORT) because CORT can suppress the gonadal axis, thereby functioning to inhibit gonadal hormone release and suppressing the stimulatory effects of T. In order to monitor plasma hormone concentrations over time, we collected blood samples at three times intervals throughout the study. Within the control group, T concentration was significantly positively correlated with several of our movement parameters. Conversely, within the T-implanted group, T concentration was not correlated with any of the movement parameters.Additionally, we sought to investigate whether the difference in spatial ecology and associated navigational demands between the sexes is reflected in the relative volume of the MC, DC, and LC, and if sex differences in the relative volume of cortices are present from birth or develop later in life. For comparisons of adult DC, MC, and LC between the sexes, thirteen adult C. o. oreganus (6 females and 7 males) were collected in early May. In addition, two pregnant females were collected in early August of 2009 and held in the laboratory until parturition. A total of ten neonates (5 females and 5 males) were produced from the two litters. No sex differences in relative DC, MC, or LC were detected in adult or in neonatal rattlesnakes. Overall telencephalon volume was affected by sex in adult rattlesnakes (males larger) but not in neonates.By utilizing a combined descriptive and experimental approach, we have shown that naturally occurring elevations in T are positively associated with certain measures of v movement and home range size. Failure of exogenous T in eliciting increases in movement suggests a steroid binding protein or T receptor saturation point. We show a non-significant trend in support of a complex relationship between th...