Comparison of Sarcophilus and Gulo, part 3

(writing in progress)

Rates of metabolism and mortalities:
 
Obviously, Sarcophilus is a marsupial and Gulo is a eutherian. However, this needs interpretation to be meaningful.
 
What I don’t think has been pointed out before as such is that although it is Gulo that has the faster pace of life in the metabolic sense it is Sarcophilus that has the greater turnover in terms of populations. This may seem both obscure and somewhat contradictory, but I’ll begin to elaborate it here.
 
Most marsupials are inferior to comparable eutherians in body temperature and metabolic rate. Although I don’t have the figures to hand, I have no doubt that Sarcophilus and Gulo conform to this trend. I suspect that the body temperature of Gulo is about 38.5 degrees C whereas that of Sarcophilus is about 36 degrees C. The comparison of metabolic rates is complicated by the fact that Sarcophilus has a smaller body than Gulo, and by the fact that the environment of Gulo is far colder, on average, than that of Sarcophilus. However, I have little doubt that, in general, a given 100 g of the body cells of the eutherian convert chemical energy to ‘heat’ more rapidly in Gulo than in Sarcophilus. And this is perhaps the most basic aspect of ‘pace of life’.
 
The great difference in brain size between the two forms is consistent with the difference in metabolic rates, just as the fact that kangaroos have smaller brains than those of like-size ruminants is consistent with the unquestionably lesser rate of metabolism of the ‘roos in that comparison. Marsupial mammals generally tend to run relatively cool and to require relatively little energy; and although there are various exceptions to this trend (e.g. the koala runs hotter than sloths) I see no reason why Sarcophilus would be an exception.
 
I also get the impression, by the way, that Sarcophilus digests its food less thoroughly than does Gulo.
 
Given this ‘running cooler’ on the part of Sarcophilus, it would be easy to expect the marsupial to be the longer-lived of the two forms. However, the opposite is the case and this is where the comparison gets a bit paradoxical.
 
The truth is that Sarcophilus is much shorter-lived than Gulo, in terms of both life expectancy and longevity. In that sense it is the marsupial that has the greater ‘pace of life’, i.e. its populations turn over more rapidly than those of the eutherian in the comparison.
 
Sarcophilus, although about the same body size as Carnivora such as Felis catus that can live to 15 years, is so short-lived that by 7.5 years it has naturally senesced even if it has experienced no injury or mishap. It just ‘burns out’ by an age half that expected for the senescent ‘burnout’ of comparable eutherians. And in keeping with this ‘hard-wired’ pattern of frequent mortality, the natality of Sarcophilus is correpondingly greater, in various ways, than that of Gulo.
 
For example, whereas each adult female of Gulo tends to reproduce only every other year, that of Sarcophilus can reproduce every year. Whereas each litter of Gulo never consists of >4 neonates, that of Sarcophilus can consist of as many as 50. And whereas Gulo seldom weans more than two juveniles, Sarcophilus routinely weans four. Although I have no reason to believe that the actual rate of somatic growth of infants and juveniles is any greater in Sarcophilus than in Gulo, the rate at which individuals turn over in natality/mortality is clearly greater in Sarcophilus than in Gulo. For a start, by far the majority of every litter is doomed in Sarcophilus, because the mother bears >20 neonates but only has four teats and each neonate can survive only if it manages to attach permanently to a teat. Then, because the pouch is so small that the suckling infants hang haphazardly from the groin of their mother, they tend to get ‘beaten up’ during the normal locomotion of their mother. Then each female Sarcophilus can wean several juveniles per year but given that all the habitat tends already to be occupied it is the fate of most of the newly-independent juveniles to die before finding a home for themselves. And finally: even if everything goes well and an individual finds and home and reproduces successfully, the best it can expect is to die ‘of old age’ by about 6 years old, after only about four years of reproductive attempts.
 
Although it is simplistic to put it this way, Sarcophilus is ‘r-selected’ whereas Gulo is ‘K-selected’. This is because Gulo lives relatively long, reproduces relatively infrequently, bears a relatively small number of neonates at each birth, and leaves the infants securely in a carefully hidden den during the suckling period; the mother makes no attempt to carry the infants or juveniles around on her body while foraging, which inevitably exposes them to risk. And even after weaning, there is a long period in which the adolescent Gulo remains with or near her mother, learning from its parents.
 
Do you see that few of these intriguing differences are explained, per se, by the fact that Sarcophilus is a marsupial while Gulo is a eutherian?

(writing in progress)
 

Publicado el junio 7, 2022 10:48 MAÑANA por milewski