But just as with natural selection individuals may differ in their ability to reproduce, in sexual selection there can be differential reproductive success among individuals of the same sex and species. In order to mate, males need to gain access to females and vice versa, and not all individuals will be equally successful at this task. If there are genetically based differences in the ability of one sex to insure successful mating with the other sex, sexual selection will occur.
The patterns and processes of sexual selection are best understood in the context of parental investment. Sexual selection occurs because there is a correlation between the gender of an individual and its parental investment in each offspring. Parental investment is the investment of resources that increases in the probability offspring will survive a benefit while decreasing the parent's ability to produce more offspring a cost. By investing in the production of current offspring a parent will reduce the likelihood that it will be able to invest in future offspring.
Costs can be the energetic demands of parenting, increased predation risk, etc. One important component of parental investment is the investment in gametes. This serves as one means of distinguishing the sexes. Females are the sex with a large parental investment per gamete. Example are eggs which have the nutrients to promote the development once fertilization occurs. Males have a smaller parental investment per gamete ; generally carries only genetic information.
The difference is less pronounced, but still significant, in mammals. This difference in investment in gametes has important consequences for how these resources are invested and hence how selection might act differently in the two sexes. These data support hypothesis 2. Hypothesis 3 — Does self-sacrifice pay off with paternity? Maydianne also observed and timed matings in the lab, and then determined the paternity of the eggs that the female ultimately produced. Maydianne focused on the second male to mate with a female.
She thought that a male allowing himself to be eaten might pay off in terms of paternity, particularly if he were able to mate for longer if cannibalized. She observed that cannibalized second males mate for much longer a median of 25 minutes than second males that are not eaten and mate for a median of just 11 minutes.
Here are data from 10 matings:. For each mating, she plotted a point black dot representing how long the second mating took x-axis and the fraction of the eggs in the sac fathered by that second male y-axis.
The data are pretty clear: longer mating is associated with more paternity. This is shown by the upward slope of the regression line red arrow. The dotted lines and blue arrows show how much a second male can improve his fitness by fathering more eggs if he is eaten and mates for 25 min, as opposed to surviving and mating just 11 minutes. These data support hypothesis 3. But note the small sample sizes. More data might make us more confident in this interpretation.
Maydianne started doing research as an undergraduate. She got interested in studying invertebrates, since she could mimic their natural environments in the lab. Teachers can use the following resources to build a lesson sequence around sexual selection and redback spiders for the high school or college levels.
For answer keys to the worksheets above, teachers can email UEKeys berkeley. The following Data Nugget activities for middle and high school students offer students opportunities to analyze data on sexual selection in other animal systems:. Nonparametric tests — includes background information on the Mann-Whitney test.
The linear regression test — background information. Andrade, M. Female phalaropes compete for the plain-colored males, and the latter incubate the eggs and tend the young. There is evidence that female birds of some species e. In contrast, there is surprisingly little evidence that females preferentially select males with different degrees of ornamentation.
One of the most interesting studies involved Long-tailed Widowbirds living in a grassland on a plateau in Kenya. Males of this polygynous six-inch weaver a distant relative of the House Sparrow are black with red and buff on their shoulders and have tails about sixteen inches long.
The tails are prominently exhibited as the male flies slowly in aerial display over his territory. This can be seen from more than half a mile away. The females, in contrast, have short tails and are inconspicuous.
Nine matched foursomes of territorial widowbird males were captured and randomly given the following treatments. One of each set had his tail cut about six inches from the base, and the feathers removed were then glued to the corresponding feathers of another male, thus extending that bird's tail by some ten inches.
A small piece of each feather was glued back on the tail of the donor, so that the male whose tail was shortened was subjected to the same series of operations, including gluing, as the male whose tail was lengthened. A third male had his tail cut, but the feathers were then glued back so that the tail was not noticeably shortened.
The fourth bird was only banded. Thus the last two birds served as experimental controls whose appearance had not been changed, but which had been subjected to capture, handling, and in one cutting and gluing. To test whether the manipulations had affected the behavior of the males, numbers of display flights and territorial encounters were counted for periods both before and after capture and release.
No significant differences in rates of flight or encounter were found. The mating success of the males was measured by counting the number of nests containing eggs or young in each male's territory. Before the start of the experiment the males showed no significant differences in mating success.
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