Figure 41.4 The Mouse Vomeronasal Organ Identifies Gender
The use of gene knockouts is a commonly used tool to understand gene function. In this study, Stowers et al. (2002) examined the function of the mouse vomeronasal organ (VNO). Previous research had investigated the VNO’s function by physically removing the organ, and results of these studies had suggested that the VNO was necessary for initiating mating behavior. Stowers and colleagues studied the function of this organ by creating knockout mutant mice that do not produce TRP2, a protein necessary for the sensory activation of the VNO. They then exposed both the knockout and wild-type male mice to wild-type females. In contrast to previous studies, they found that there was no difference in the mating behavior of the knockout and wild-type mice. However, the researchers then exposed the knockout male mice to wild-type males. What they observed was surprising; instead of the usual aggression, the knockout males exhibited the same mating behavior shown towards female mice. Thus, Stowers et al. hypothesized that in mice the VNO functions not in the initiation of mating behavior, but in gender discrimination. To definitely demonstrate this, the researchers first had to disprove the alternative explanations for their results. Specifically, perhaps the lack of VNO function creates animals that have an overactive sexual drive, resulting in mating behavior towards males in the absence of females. Alternatively, knockout males may have a different sexual preference, only mating with females in the absence of males. To rule-out these alternative explanations, the researchers introduced knockout males to both wild-type males and females simultaneously. In this experiment the knockout males exhibited mating behavior towards males and females at equal frequency. As a result of this additional experiment, the researchers were able to conclude that the mouse VNO does indeed function in gender recognition.
Original Paper
Stowers, L., T. E. Holy, M. Meister, C. Dulac, and G. Koentges. 2002. Loss of sex discrimination and male–male aggression in mice deficient for TRP2. Science 295: 1493–1500.
http://dx.doi.org/10.1126/science.1069259
Links
Beckman, M. 2002. Pheromone reception: When in Doubt, Mice Mate Rather Than Hate. Science 295: 782.
http://dx.doi.org/10.1126/science.295.5556.782a
iBioSeminars: Catherine Dulac Part I: Sex and Smell: Chemosensory Detection
http://www.ibiology.org/ibioseminars/neuroscience/catherine-dulac-part-1.html
Scitable by Nature Education: Scientists Can Analyze Gene Function by Deleting Gene Sequences
http://www.nature.com/scitable/topicpage/scientists-can-analyze-gene-function-by-deleting-6526138
Kimball’s Biology Pages: Pheromones
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pheromones.html
Dulac, C., and A. T. Torello. 2003. Molecular detection of pheromone signals in mammals: from genes to behaviour. Nature Reviews Neuroscience 4: 551–562.
http://dx.doi.org/10.1038/nrn1140
Figure 41.12 The Costs of Defending a Territory
In the late 1980s, researchers Michael Moore and Catherine Marler at Arizona State University studied Yarrow’s spiny lizards to examine the costs incurred by male lizards when defending a territory. Previous studies had demonstrated that these male lizards defend their territories most aggressively during the fall mating season. To test the hypothesis that testosterone-induced aggressiveness is too costly for Yarrow’s spiny lizards to maintain all year round, the researchers inserted testosterone capsules under the skin of males during the summer and observed their behavior and survival rates. Untreated male Yarrow’s spiny lizards served as controls. The results showed that the testosterone-treated males were more active and exhibited more territorial behaviors than controls. Further, the testosterone-treated males exhibited a higher mortality rate as compared to control lizards. The results showed that 50 days after receiving the implant, only 40 percent of the treated males were still living, as compared to greater than 60 percent of the control males. Together, these results suggest that if Yarrow’s male spiny lizards defended their territory as aggressively in the summer as they do in the fall, it would result in a higher mortality rate. The researchers concluded that this increase is likely due to the enormous amount of time and energy the treated males expended through aggressive interactions with other males. Thus, the cost of defending their territory all year round does not outweigh the benefits of such behavior. Just how much more energy did the testosterone-treated males expend as compared to control males? To answer this question, Moore and Marler measured and compared the amount of energy expended by both testosterone-treated and control captive males using the doubly-labeled water (DLW) technique. This technique was developed in 1966 by Lifson and McClintock to measure free-living energy expenditure in animals. It is based upon the measurement of clearance of radioactively-labeled water from the body. Using the DLW technique, Moore and Marler discovered that testosterone-treated males exhibited a 31 percent increase in energy expenditure as compared to control males. The investigators concluded that this increase is the primary contribution to the higher mortality rate associated with increased territorial aggressiveness in the testosterone-treated males.
Original Paper
Marler, C. A., and M. C. Moore. 1988. Evolutionary costs of aggression revealed by testosterone manipulations in free-living male lizards. Behavioral Ecology and Sociobiology 23: 21–26.
http://dx.doi.org/10.1007/BF00303053
Links
Moore, M. C., and C. A. Marler. 1987. Effects of testosterone manipulations on nonbreeding season territorial aggression in free-living male lizards, Sceloporus jarrovi. General and Comparative Endocrinology 65: 225–232.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3817446&dopt=Abstract
Marler, C. A., G. Walsberg, M. L. White, and M. Moore. 1995. Increased energy expenditure due to increased territorial defense in male lizards after phenotypic manipulation. Behavioral Ecology and Sociobiology 37: 225–231.
http://dx.doi.org/10.1007/BF00177401
Steinman, M. Q. and B. C. Trainor. 2010. Rapid Effects of Steroid Hormones on Animal Behavior. Nature Education Knowledge 1(10):1.
http://www.nature.com/scitable/knowledge/library/rapid-effects-of-steroid-hormones-on-animal-15104619
Briffa, M. 2010. Territoriality and Aggression. Nature Education Knowledge 1(8):19.
http://www.nature.com/scitable/knowledge/library/territoriality-and-aggression-13240908
Wildherps.com: Yarrow’s Spiny Lizard (Sceloporus jarrovi)
http://www.wildherps.com/species/S.jarrovi.html