Re: Multi-age Broods. Ignorance or Apathy?
John Waters (jdwaters@dircon.co.uk)
Thu, 15 Aug 1996 07:25:35 +0100
Paul Crowley wrote:
>
> In article <3210D49C.3CB7@dircon.co.uk>
> jdwaters@dircon.co.uk "John Waters" writes:
>
> > 3. More important from an evolutionary point of view is the L.B.I.
> > multi-age brood. This is the kind of brood found in hunter-gatherer
> > tribes. Generally there is a birth interval of four years in this type
> > of brood. As Apes are gatherer-hunters, it is generally thought that
> > hominid ancestors were either gatherer-hunters or hunter-gatherers.
>
> This reasoning is weak. The differences between hominids and apes
> are too great for the analogy to work. As I see it, the speed of
> hominid evolution was so rapid that any mechanism that helps to
> explain it must be considered seriously. If famines were common
> then those hominids which were able to re-populate most quickly
> would soon predominate. If the ecology permitted it, a population
> would quickly adopt short birth intervals in preference to long ones.
> Behaviours inherited from the apes would rapidly be lost. Hominids
> would switch from "K" to "r" rates of breeding. The "K" pattern
> seen in traditional gatherer-hunters in simply a response to a
> stable ecology. They have little trouble adapting to "r" rates
> when they tap into welfare provided by the industrial state.
> The mechanism is simple: when the food supply is good, infants are
> weaned from the breast more quickly and the female becomes fertile
> again.
JW: The reasoning may appear weak, but this may be in part due to the restricted nature
of correspondence in a newsgroup context. In matters of this kind, where the concepts
are probably new to most readers, it is necessary to start with simple constructs and
then move into more detailed arguments over a period of time.
In the context of paragraph 3., it should be remembered that life is conservative, all
specie behaviour tends to be very conservative; and hunter-gatherer tribes are no
exception to this rule. There is no evidence that hominid evolution was particularly
rapid. Biological evolution is necessarily slow in the case of relatively large, long
lived animals. Of course, the statement that Apes are gatherer hunters is far too
sweeping. This is a definition which really only applies to Pan Troglodytes.
As far as multi-age broods are concerned, it is important to understand the reasons why
hunter gatherer tribes have L.B.I. multi-age broods. The birth interval is a long one
because the nursing females breastfeed their babies on demand. This stimulates the
continual production of milk, which in turn stimulates the production of a hormone
called prolactin in the bloodstream. When prolactin is maintained at a high level, it
prevents re-ovulation and the associated estrus in the female. As a result, the nursing
female cannot become pregnant until the level of prolactin drops. This happens when the
infant switched from milk to solid foods and milk production is reduced.
There is no reason to believe that hunter gatherer tribes in the past would have been
aware of human reproductive mechanisms. They would simply repeat the behaviour of their
ancestors - as present hunter gatherer tribes do.
It should be noted that hunter gatherer tribes are nomadic. In this regard, the
transport of a large S.B.I. brood of children (and their supplies of food and water)
would have created enormous logistical problems for the tribe. In my view, this is the
main reason why the S.B.I. brood did not become a specie's norm until the development of
sedentary agriculture. Of course, the latter is very labour intensive - particularly at
harvest time; and a large and reliable 'kith and kin' labour source would have been
essential. An example of a biological feedback mechanism.
>
> > 5. It should be noted that the multi-age brood characteristic could only
> > be maintained by a specie with pre-adapted advanced infant rearing
> > characteristics. These pre-adaptions were caused by long term biological
> > selection of improved infant rearing characteristics due originally to
> > changes in the head-to-body ratio of the specie. Such changes increased
> > the period of infant helplessness after birth.
>
> I follow all this -- although I disagree with it -- it's the
> standard doctrine. (Read it very slowly, expand each clause, and
> see if it still makes sense.)
>
> > This created the
> > evolutionary pressure for more advanced maternal responses, including
> > (as you have surmised) bipedalism.
>
> But I don't follow this. Bipedalism was there at 4.4 mya; if
> infant "big heads" ever arrived (and I question it - compare newborn
> chimps/gorillas with newborn Hss) they didn't come until about 2 mya.
>
> Paul.
JW: Note the sentance : "Such changes increased
the period of infant helplessness after birth."
Ape infants are helpless after birth, as are human infants. However, Ape infants are
only completely helpless for a maximum of 24 hours. Human infants, by contrast, are
completely helpless for the first three months of their life. This is a big difference,
and one which must have taken a long time time to evolve.
For a more complete argument concerning these aspects of hominid development I would
suggest that you read my 1986 book "Helpless as a Baby". This will be available on the
Net in the near future. ( I will advise you when it is transferred to the Web Server.)
As I understand it, hominids were semi-bipedal 4.4 Mya, but did not become fully bipedal
until 2.0 Mya. It should be noted that the change in the head-to-body ratio does not
necessarily imply any change in the size of the skull. The effect can be achieved just
as easily through a reduction in the size of the body. In my view, I think there was
probably a slight increase in head size due to a requirement for larger teeth/jaws.
However, the most important change was a reduction in body size to meet a requirement
for a improved power to weight ratio. This necessarily reduced the size of the pelvic
canal and created the birthing problems due to the infant's relatively large head. These
birthing problems were resolved by the selection of variable fetal growth rates, such
that the head of the embryo grew at a slightly slower rate that its body. As a result,
the infants head at the time of birth was slightly smaller than usual, and it could pass
through the pelvic canal without restriction. The head completed its fetal stage outside
the womb. However, this process has a side effect. As the head has not completed its
fetal stage at birth, the infant's cognitive development is delayed - and its period of
complete helplessness consequently increased.
John
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