Even marsupials living fast and dying young do not match the pace of life of comparable eutherians, part 2

David Macdonald has partly explained semelparity as follows:
"Semelparity only occurs in predictable, highly seasonable environments, and it has its 'raison d'etre' in the excrutiatingly slow reproduction rate of small marsupials...Because the mother may be suckling as many as ten young (in Antechinus stuartii), her metabolic rate in late lactation (weaning is not before 14 weeks old) can be ten to twelve times the basal rate - a mammalian record. Female reproduction is therefore timed to ensure that late lactation coincides with the period of maximum availability of the insect and spider prey on which the species feeds, which falls in late spring or early summer."

This approach may also aid our interpretation of the only marsupial occurring in the United States and Canada, namely Didelphis virginiana (https://en.wikipedia.org/wiki/Virginia_opossum).

Didelphis virginiana is well-known for its limited life span and correspondingly large number of offspring per birth. It can live to a maximum of five years. By contrast, Felis catus (https://en.wikipedia.org/wiki/Cat) can live to a maximum of 34 years. The normal lifespans of adults in the two species respectively are about three years and about 15 years. The apparent fecundity of D. virginiana seems consistent with its early senescence, and suggests a fast pace of life.

However, the marsupial metabolises slowly compared with like-size eutherians (https://lafeber.com/vet/basic-information-sheet-virginia-opossum/). This difference is partly indicated by body temperatures: only 35 degrees Celsius in D. virginiana vs 38.5 degrees Celsius in F. catus.

Taking the broadest possible view, how can we explain the paradox that the mammals that burn themselves out the fastest are marsupials? I offer the following rationale.

In marsupials, metabolic limitation means limitation in rates of growth. Hence no marsupial can match the rates of growth of the fastest-growing eutherians, such as lagomorphs and ruminants.

Certain marsupials compensate for this by having a large number of offspring per birth, which is particularly possible for them because all marsupials by definition have extremely small neonates.

The combination of large litters with a rapid growth by marsupial standards allow some marsupials to rival the overall reproductive rates of fecund rodents such as rats - even though they do not rival eutherians metabolically.

So why do the males of some of these marsupials senesce after one or two years of life, whereas like-size rodents and other fast-growing mammals usually continue in good health until killed by predators? Because:

  • maximum rates of growth in marsupials may only be possible where a disproportionate share of the available resources is allocated to breeding females, and
  • the niches involved are adaptive to seasonal/episodic conditions, in terms of both rainfall and wildfire.
Posted on April 04, 2022 04:49 AM by milewski milewski


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