Xref: utzoo sci.bio:978 soc.men:3015 Path: utzoo!mnetor!uunet!husc6!cca!g-rh From: g-rh@cca.CCA.COM (Richard Harter) Newsgroups: sci.bio,soc.men Subject: Re: sexual selection and investment Message-ID: <25527@cca.CCA.COM> Date: 14 Mar 88 06:38:21 GMT References: <1988Mar13.160941.22096@utzoo.uucp> Reply-To: g-rh@CCA.CCA.COM.UUCP (Richard Harter) Organization: Computer Corp. of America, Cambridge, MA Lines: 81 In article <1988Mar13.160941.22096@utzoo.uucp> jackson@utzoo.uucp (Don Jackson) writes: > >Richard Harter (25442@cca.CCA.COM) writes: > >>NOTE THAT IT IS THE FEMALE THAT SELECTS. Females select, fundamentally, >>because they make the big investment in offspring. > >This is true as a generalization. However, there are countless numbers of >species where the investment by the male is equal to or exceeds the female. >Numerous species of fish reproduce in nests formed by the male and guarded >by males after spawning. The greater costs (e.g. energy) incurred by the male >(in terms of reduced growth and increased mortality) appear to counter >your argument that females have the greater investment. There are two investments that one has to take into account; one is the investment in creating the offspring, and the other is the investment in caring for them. The thing that distinguishes females from males is the females make the larger investment in creating offspring. The two genders have an intrinsic conflict of interest -- it is to the benefit of each that the other care for the young, and it is to the benefit of both that the young prosper. A general rule is that the last gender to make contact with the young is more likely to get stuck with caring for them. In most higher animals this is the female, since the egg is developed after fertilization. In a number of fish and amphibians, the females lays unfertilized egg masses and the male fertilizes them after they are laid. The male, being the last in contact, gets to make the big investment in caring for them. In the more usual case where fertilization occurs a priori rather than a posteriori, the female has a number of strategies available, the principle one being to refuse to mate unless the male makes a suitable investment in caring for the offspring. How well this works depends very much on the species. In some species the female abandons the effort to get the male to contribute to the rearing of the offspring; instead they settle for selecting the 'best' possible father. These species tend to have strong dimorphism and a few males that do most of the breeding. In species with strong pair bonding the leverage of the female is much greater -- instead of picking the 'best' father along with many other females making the same pick, she picks a single 'more useful' one. Species with pair bonding tend to less dimorphism and more equality in rearing the young. There are no hard and fast rules, though, it is very much a case by case thing. Human beings, are, as one might expect, very much a special case. Dimorphism in human beings is quite marked. From this one would expect that humans would have sexual patterns similar to other dimorphic primates, with an alpha male having primary access to a number of females, and other males having very little access. In actual fact, humans are a strong pair bonding species (albeit with surprising variations) and human males make a large investment in rearing the young. >There may be natural selection against distinctive females through >increased risk of predation. An example of this is the Guppy (Poecilia >reticulata) where coloration increases predation. Therefore, dull >females have greater fitness than colorful females. However, sexual >selection may favour brightly colored males provided the benefits derived >from their coloration are greater than the increased costs associated with >predation. This is rather general; in most species the female is less conspicuous than the male. Inconspicuous is rather generally selected for. >Trying to distinguish between different types of selection (e.g. natural >versus sexual) is a matter of semantics. The process implies that, >on average, some trait (e.g. color, larger tail, etc.) results in a greater >fitness (i.e. more progeny) than a similar organism lacking that trait. I disagree that it is simply a matter of semantics. The final criterion of fitness is, as you say, a matter of more progeny. However this is a matter of contingent probabilites. The individual must survive to breeding age. Given that the individual has survived to breeding age, the individual must breed successfully. Given that the individual has bred successfully, its offspring must do the same. Selection pressures apply equally to the genders when it comes to simple survival; however they apply differently when it comes to successful breeding. -- In the fields of Hell where the grass grows high Are the graves of dreams allowed to die. Richard Harter, SMDS Inc.