Reproductive rate of spotted hyena in the context of the carnivore guild

(writing in progress)
 
The spotted hyena (Crocuta crocuta, https://onlinelibrary.wiley.com/doi/10.1111/j.1365-294X.2011.05240.x), the lion (Panthera leo), and the African hunting dog (Lycaon pictus) represent a demographic axis of variation.

The hyena reproduces slowly and hypothetically has stable populations. The felid reproduces rapidly, but is subject to fluctuations in population. The canid reproduces extremely rapidly, but is extremely subject to fluctuations.

Rate of reproduction in spotted hyena according to Hofer and East (2003):
  
Much seems to hinge on the relatively slow reproduction of the spotted hyena, when it comes to interpreting its life history strategy and extremely peculiar female genitalia. Therefore, let us look at the numbers.
 
According to Hofer and East (2003), the female spotted hyena in the Serengeti ecosystem raises on average one offspring every ca 3.5 years (see screenshot below). Her lifespan can be long enough to give her nearly 15 years of reproductive attempts, but the average is close to 7.5 years because some females die young.

This means that, on average in the study population, the female spotted hyena raises only 2.36 offspring in her life. The maximum recorded in this study was 7 offspring raised in one mother’s lifetime, which is not much considering that some canids can suckle that many altogether as one litter.
 
The spotted hyena reproduces more rapidly than the human species. This makes sense because, after all, it remains much shorter lived than us. Human offspring take at least a decade to reach reproductive age, in comparison to only about two years in the spotted hyena - a difference of more than five-fold).
 
The mean number of offspring raised by each mother in this population in her lifetime, viz 2.36 offspring, makes sense in a stable population that is neither increasing nor decreasing.

It confirms that each female on average replaces herself and the father during the mother’s lifetime investment in reproduction, with a margin to spare (the 0.36 is easily accounted for by early post-pubertal mortality of offspring, e.g. mortality suffered when male offspring leave the natal clan to emigrate into a nearby clan).
 
So the numbers all seem to make sense but the significant fact is that it requires on average more than three years of maternal investment to raise each offspring to independence, notwithstanding the fact that twins (i.e. litters of two) are common in the spotted hyena. Of this 3.5 years about 16 months is devoted to lactation and the rest (nearly two years) is devoted to teaching the offspring how to hunt and compete.

The latter instruction, by the mother for the benefit of the son or daughter, is – as far as I can see – the only thing resembling altruism in the whole life of the spotted hyena after weaning.
  
Please see the additional excerpt below, in which Hofer & East (2003) summarise the importance of age at first birth.
  
The age at which spotted hyena first gives birth:
  
In the item we’re currently writing about first birth in the spotted hyena, I can now specify the age at which this takes place in the Serengeti ecosystem according to Hofer & East (2015), which I attached in an email to you earlier today.
 
(see original email for excerpt from Hofer and East (2003)
 
The spotted hyena female generally reaches sexual maturity at about two years, but it is not until she is about four years old that she usually gives birth for the first time. The range in ages is 2.6-5.9 years, depending partly on how fast she grows, which depends in turn partly on how superior her mother was at the time in the social hierarchy.
 
It is interesting to note that some individual females, born to low-ranking mothers, only experience their first parturition (according to us their first immune test by clitoral rupture) at about six years old. For comparison: the lion is so short-lived that the male of the species is usually entering his prime at about six years old. By the time the male lion is ten years old, if he hasn’t bred successfully he’s unlikely to do so because by that age he’s starting to lose condition. Meanwhile, the female spotted hyena is still going strong at ten years old and I dare say the same is true for the male spotted hyena.
 
To put ourselves in the shoes of an individual female spotted hyena who happens to have been born to an inferior mother, growing slowly and producing her own first offspring at 5.9 years old in contrast to her more fortunate ‘peers’ who manage to produce their first offspring at a mere 2.6 years old:
 
Time taken until first reproduction is perhaps THE single most important parameter of overall reproductive rate in animals, and probably also of likely reproductive success on a lifetime basis in the spotted hyena.

So it is easy to imagine that, if an individual set back in this way manages to pass the test of first birth and gets social kudos for this, it could make a lot of difference to her fortunes in life? How else could she claw some way back towards lifetime success?
 
The following occurs to me:
 
If the immune system is strongest in youth, then it might become progressively harder for females to remain healthy after their first birth the later this birth occurs. If so, then any individual that does succeed in taking this hurdle in her stride could possibly earn particular kudos in the social system, which to some extent could make up for the poor hand of cards dealt to her by her mother’s poor rank. If so, this could perhaps help to explain the evolution of the clitoral bottleneck?

The following incident seems to confirm the difference in population dynamics between the spotted hyena and the lion.
 
In 1962 there was a ‘plague’ of Stomoxys (https://en.wikipedia.org/wiki/Stomoxys) in Ngorongoro Caldera, which killed 80% of the population of the lion. The population fell from 75-100 to 12. As far as I know, the coexisting population of the spotted hyena was unaffected.
 
Stomoxys is a fly superficially similar to the house fly or bush fly (Musca spp.). However, it belongs to a different genus. It seems to mimic Musca in overall appearance, which allows it to approach and settle on its host animals. It is a blood-sucker with a strangely painful ‘bite’.

I know this genus in my own garden in Western Australia. It breeds in rotting vegetable matter, such as the edges of my compost heaps. It is called ‘stable fly’, because the most typical breeding medium for the larvae of Stomoxys is the moist mixture of dung and straw found in horse stables. The genus lives as a larva on the dung of ungulates and then as an adult lives on the blood of ungulates.

Whenever I am bitten in my garden (which happens once or twice every few months, at most, I am struck with

• how Musca-like the stable fly is,
• how the stable fly sneaks in to make its ‘bite’ as a single individual, not attracting attention to itself by either swarming or buzzing, and
• how disproportionately painful the ‘bite’ is.

For a reason probably related to vagaries of weather, the population of Stomoxys increased in the Ngorongoro Caldera in 1962. The lion was particularly hard-hit, which is a somewhat ironic, considering that this fly breeds in the dung of the ungulates that provide the prey of the felid.
 
However, as far as I know there was no corresponding crash in the population of the spotted hyena. If I am right, this seems to show that the latter species, partly through intelligence and partly through extreme immunity to pests and pathogens, resisted this ‘plague’ either by avoiding being ‘bitten’ or by being relatively unaffected by the ‘biting’.
 
I would also like to read up more on the relationship between the spotted hyena and rabies, canine distemper, etc. I suspect that it is relatively immune to many ‘diseases’, with a demographic profile consistent with this sort of persistence, particularly a slow but steady reproduction.
 
The flip side of this is that where the spotted hyena has been exterminated, e.g. by livestock ranchers, it can take a long time to repopulate once conservation measures protect it. I suspect that this was the case in

• Saadani National Park in coastal Tanzania,
• Hluhluwe-iMfolozi and umKhuze Game Reserves in Zululand (check),
• a conservation area in Gabon (https://www.thejakartapost.com/life/2018/01/20/spotted-hyena-returns-to-gabon-park-after-20-years-researchers.html), and
• Gorongosa National Park in Central Mozambique.

In all these areas, the spotted hyena has been slow to recover, despite the rehabilitation of the ecosystem.

It would be interesting to review the history of repopulation of the two species, spotted hyena and lion, in Kruger National Park just after the turn of the century in the time of Stevenson-Hamilton.

When the park was established, initially there was a policy of shooting all larger Carnivora on sight in order to allow the ungulates to repopulate as rapidly as possible. Despite this hunting pressure the lion came back rapidly, which is testimony to its fecundity if prey is available. I suspect that the spotted hyena repopulated Kruger Park far more slowly, and I wonder if Stevenson-Hamilton noticed this or wrote it in his dairies.
 
(writing in progress)

Publicado el julio 17, 2022 03:27 MAÑANA por milewski