As Coyne describes on pages 121-122, when invading lions displace resident males of a pride, they slaughter all the unweaned cubs so that the females will come back into estrus (since they are not nursing the cubs anymore, they will be ready to conceive again). By the theory of evolution, as Coyne has outlined throughout the book, this act benefits the fitness of the individual, by increasing the invading lions' reproductive output, though at the expense of the species as a whole, thus increasing the likelihood of extinction. For this 'gene causing infanticide' to have been acted upon by natural selection, it must first have resulted from a random, genetic mutation. Speculate as to why this gene causing infanticide may have been selected to recur throughout the population of lions, taking into consideration any issues of resources and energy on the part of the females in nursing their cubs, as well as the advantages for the lions' mode of development; in other words, how much do lions nurse their young, and what limitations or burden does this place on the female lions in their inability to conceive while nursing their unweaned cubs? Why would it not be feasible for female lions to have had natural selection act upon them to shut off the gene that prevents them from going into heat while nursing? (Perhaps it is not in the interest of the invading males individual fitness to allow the old cubs to live, but it might increase the fitness of the females to do so.)
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Coyne said that there could be “’nicer’ genes, which would have the invading males simply babysit the unrelated cubs” (122). Even if this is a nicer picture than infanticide, it wouldn’t have as much evolutionary benefit to either the male lion or the lioness who are trying to pass on their genes. A lioness cannot go into heat after having her cubs and until they mature or die. This is so she can have her full attention on her cubs and increase their chances of survival as they carry her genes. Carrying her cubs for 110 days takes up a lot of time and energy, and even after birth she does not integrate her cubs and herself into her pride until they’re six to eight weeks old (Wikipedia). She has to provide greater protection for them from hyenas and jackals, use more energy to make milk for them until they’re weaned, while still hunting and trying to survive herself. It wouldn’t make sense and make it less likely for her cubs to survive if the gene that prevented a lioness from going into heat while she was still nursing was turned off. Even if it protected her cubs that were unrelated to the current male lion controlling the pride, she’d have double the work on her hands. A lioness wouldn’t be able to keep an eye on all her cubs (the old male lions offspring and now the new male lions offspring) and they would more likely die from starvation. Statistically 80% of cubs die before the age of two, but nonetheless having more offspring wouldn’t increase morality in the situation (Wikipedia). In the end, when infanticide takes place a lioness might try to protect her cubs, but really she still has the opportunity once she loses that litter to create a new one that will have the protection of the new male lion (Smithsonian). These new cubs will still carry her genes, so the situation still benefits her. Infanticide might seem cruel, but when it comes to passing down the genes of the male it increases his chances of his cubs surviving as well as reproducing as soon as possible with the available females. As the earlier lion had been driven out of the pride there’s no telling when it will be the current male lion’s time to leave
ReplyDeleteSources:
http://en.wikipedia.org/wiki/Lion
http://nationalzoo.si.edu/Animals/GreatCats/lionfacts.cfm.
As Coyne said, "adaptations always increase the fitness of the indicitual, not necessarily of the group or the species" (121). This is quite evident in the behavior of male lions that displace the resident males of a pride. The new males compete with the residents for power. When they have gained control of the pride, they also have control over the females. The male lion crontrols the pride. When he wants to mate with the females, he can't because they are distracted with taking care of the previous lion's offspring. Thus, the new reigning male kills the offspring and the females return to heat and are ready to reproduce. This process does not increase the fitness of the species or group as a whole because they young population is killed off. However, evolution does not specifically state that a gene will be selected for solely to benefit the group as a whole. An adaptation will always increase the fitness of the individual though, as he is able to pass his genes to the next generation.
ReplyDeleteIt is an advantage that female lions do not go into heat while nursing because that not only would exert more energy during a time when they must hunt for food to feed their new cubs, but it also allows them to focus more attention on their current offspring to ensure their survival. Rather than risk another pregnancy and therefore, the need to spread the parenting thin towards several rather than the proper nuture for few, it is beneficial to have few cubs to tend to them properly and be out of heat during that time. This would also benefit the dominant male because it would ensure that his offspring carrying his genes would live on to reproduce thus continuing that hereditary line.
http://drbeetle.homestead.com/lion.html
http://www.bio.davidson.edu/people/vecase/Behavior/Spring2004/shelburne/infanticide.html
Why Evolution is True