[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: crocs eating hadrosaurs (was Gr. 9 sci proj)



>From:  Tony Canning[SMTP:tonyc@foe.co.uk]
>
>On Jan 7,  8:30am, John Bois wrote:
>
>> "Limiting" here must be used in a strictly technical sense.  Being big
>> for dinos may have been a response to ultimately dooming selective
>> pressures.  Since mammals don't have the same fecundity problems as
>> dinos (after all, they only have to have one offspring to maintain
>> populations), they don't have to resort to such ultimately unfit
>> strategies as gigantism (relatively speaking).

This comment implies that gigantism was bad, and ultimately unfit.   I don't
see the evidence.  Giant dinosaurs dominated the earth for 150 million years
- twice as long as the period of mammilian domination.    Dinosaur did become
extinct, but not because of gigantism - after all, the small dinos (and many
small creatures other than dinos) went extinct at the K/T event as well.

>Unless I've misunderstood what John is saying, this seems to be looking
>at mammalian fecundity the wrong way round - surely mammals don't
>have small numbers of young because that is sufficient to maintain
>the population; this sounds like group selection theory which is impossible
>to justify.   Mammals have small numbers of young because of the obligate
>parental investment required, both before and after birth.
>
>Mammals that have only one or two young at a time do so because they
>cannot raise more than this at any one time.

Yes - for large mammals anyway.  Small mammals like rodents have high
fecundity.  

The cost of mammalian reproduction to the mother does not scale linearly
with size - large mammals spend proportiately more energy and time on
fewer offspring.  The larger a mammal gets, the smaller the litter size
(usually just one for large mammals), the longer the gestation period,
and the longer the period a mother must care for the young afterwards.
 All of these directly reduce fecundity.   

The hypothesis is that this scale dependent fecundity effectively
limited gigantism in terrestrial mammals.  Dinosaurs appeared to have
high fecundity, irrespective of their size.  Even the biggest dinosaurs
seem to have had the fecundity of a rabbit (loosely speaking).  

Note that did not MAKE them giants - it just didn't stop them from
moving in that direction.  Presumably other evolutionary pressures
favored gigantism.

Nathan