Life-history strategy of the pronghorn (Antilocapra americana): short-lived but with long gestation compared to real antelopes

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See https://www.inaturalist.org/journal/milewski/93100-an-index-to-my-posts-about-the-pronghorn-antilocapra-americana#

The maximum lifespan of the pronghorn (Antilocapridae: Antilocapra americana) is said to be 16 years (https://animaldiversity.org/accounts/Antilocapra_americana/ and https://eol.org/pages/328661/articles).

The mean body mass of adult females is about 45 kilograms (https://ielc.libguides.com/sdzg/factsheets/pronghorn).

How does this compare with other ruminants?

The maximum lifespan of *bovids (Bovidae) and cervids (Cervidae) of comparable body mass is as follows, in order of decreasing lifespan:

• Cervus nippon (40 kg) 25.4 years
• Dama dama (45 kg) 25 years
• *Pseudois (40 kg) 24 years
• *Ovis (about 50 kg) 24 years
• *Rupicapra (about 40 kg) 22 years
• *Capra ibex (40 kg) 22 years
• *Hemitragus (36 kg) 22 years
• Ozotoceros bezoarticus (30 kg) 22 years
• *Capricornis crispus (40 kg) 22 years
• *Capra hircus (20-60 kg) 21 years
• Axis axis (40 kg) 21 years
• Axis porcinus (40 kg) 20 years
• Rusa timorensis (about 50 kg) 20 years
• Odocoileus (about 50 kg) 20 years
• *Nanger dama (37.5 kg) 19 years
• *Antidorcas (30 kg) 19 years
• *Redunca (30-48 kg) 18 years
• *Oreamnos (about 65 kg) 18 years
• *Naemorhedus (27.5 kg) 17.6 years
• Muntiacus muntjak (20 kg) 17.6 years
• *Aepyceros (45 kg) 17.4 years
• Capreolus capreolus (25 kg) 17 years
• *Gazella (15-30 kg) 17 years
• Mazama americana (30 kg) 16 years
• *Ourebia (17 kg) 14 years

The above data show that the maximum lifespan of the pronghorn is shorter than expected for similar body mass in

• *bovids (by 1.4-8 years), and
• cervids (by 4-9.4 years).

However, gestation periods ('lifespan in utero') do not necessarily conform to the above trend.

The pronghorn gestates for 8.2 months, http://placentation.ucsd.edu/prong.html).

This is long, relative to certain like-size

• *bovids, e.g. Capra (which gestates for only 5-6 months) and Pseudois (which gestates for only 4-5.3 months), and
• cervids, e.g. the white-tailed deer (Odocoileus virginianus), which gestates for 6.6 months.

In general, most species, similar in body mass to the pronghorn,

• in *bovids gestate for 5-7.5 months, differing from the antilocaprid by 0.7-3.2 months, and
• in cervids gestate for 7-8 months, differing from the antilocaprid by 0.2-1.2 months.

At adult female body mass of about 50 kg, gestation in bovids tends to be about one month less than in cervids. The bovid genus Redunca (three species, varying in adult female body mass from 30 kg to 70 kg) is an exception, with gestation about as long as in the pronghorn.

The above data therefore show, overall, that the pronghorn has an anomalously short lifespan, but an anomalously long gestation.

DISCUSSION

Pace of life is the rate at which a species metabolises, grows, and reproduces, relative to adult body mass.

Organisms with slow pace of life tend to live long, whereas those with rapid pace of life tend to live briefly.

Pace of life varies among species, even within ruminants.

Forms living in environments where resources are scarce, and predators are few, can be expected to have relatively slow pace of life. Such environments include islands, high mountains, and dense forests.

By contrast, forms living in nutrient-rich, low vegetation on plains, able to migrate annually in search of rich resources, can be expected to have relatively rapid pace of life.

Slow pace of life - within the context of ruminants - is exemplified by Bubalus depressicornis (adult female body mass > 90 kg), of Sulawesi in Indonesia. This bovid has maximum lifespan of 31 years (https://animaldiversity.org/accounts/Bubalus_depressicornis/), nearly double that of the pronghorn.

Comparisons must take into account body size, because we can expect large-bodied forms to live and gestate longer than small-bodied forms. For example, Giraffa (adult female body mass 800 kg) has rapid pace of life, but has a maximum lifespan of 27 years (https://animaldiversity.org/accounts/Giraffa_camelopardalis/).

However, there is no simple correlation between lifespan and pace of life on one hand, and gestation periods on the other, in ruminants.

The following species seem noteworthy in having both slow pace of life and long gestation:

• Camelidae: Vicugna vicugna (adult female body mass 45 kg) has a maximum lifespan of 24 years 9 months, and gestation of 11.3 months
  (https://tb.plazi.org/GgServer/html/03928E699A46FFCCD5D0F3E6F6E9FE05 and https://animaldiversity.org/accounts/Vicugna_vicugna/), and

• Giraffidae: Okapia johnstoni (adult female body mass 280kg) has maximum lifespan of 30 years, and gestation of 15 months
  (https://animaldiversity.org/accounts/Okapia_johnstoni/, https://pubmed.ncbi.nlm.nih.gov/22045457/ and https://sat.gstsvs.ch/fileadmin/media/pdf/archive/2011/11/SAT153110515.pdf and https://www.researchgate.net/publication/51763158_A_long_life_among_ruminants_Giraffids_and_other_special_cases).

The above camelid happens to have similar body mass to the pronghorn.

This shows that in the pronghorn relative to Vicugna vicugna, both parameters are much shorter, viz.

• maximum lifespan (by nearly 9 years), and
• gestation period (by 3.3 months).

What emerges for the pronghorn is that

• lifespan is short for the body size, relative to bovids, and
• gestation period, although much shorter than in a like-size camelid, is much longer than in bovids that are like-size with a similarly rapid pace of life.

The rapid pace of life, hence short lifespan, of the pronghorn can possibly be explained by its particularly nutrient-rich - albeit generally dry - habitat, and mainly dicotyledonous diet (https://www.inaturalist.org/journal/milewski/93032-a-new-explanation-for-the-hypercursoriality-of-the-pronghorn-antilocapra-americana-selenium-as-a-crucial-micronutrient#).

The anomalously long gestation of the pronghorn is not necessarily incongruous with rapid pace of life. However, how it fits into the life-history strategy remains to be explained.

Perhaps the first clue to investigate is Pantholops hodgsonii (26 kg), which occupies a comparable niche in Asia. This pantholopin is said to gestate for 7-8 months (https://animaldiversity.org/accounts/Pantholops_hodgsonii/) - which, if true, is anomalously long for a bovid.

Publicado el abril 24, 2024 03:25 TARDE por milewski