Why are infants of the pronghorn (Antilocapra americana) less cursorial than those of the common impala (Aepyceros melampus)?

Everyone knows that the pronghorn (Antilocapridae: Antilocapra americana) is exceptional in its speed and endurance in running.

I refer to this as hypercursoriality ().

Among bovids, the most cursorial species belong to the tribe Alcelaphini, which run with great speed and endurance. However, the pronghorn is more cursorial than any alcelaphin.

However, the pronghorn is anomalous in a certain way: its infants hide immobile for three weeks before they and their mothers resume their normal gregariousness.

For comparison, infants of the alcelaphins Connochaetes and Damaliscus tend to run with a group right from birth. They are 'followers', not 'hiders'. Why do infants of the pronghorn not perform likewise?

Furthermore, even impalas (Bovidae: Aepycerotini) - which bound prodigiously but are not reputed to be particularly enduring runners - have cursorial infants, which join the group after a hiding period of only a few days.

Making the comparison between pronghorn and impala even more meaningful is the fact that both have synchronised breeding. Most offspring are born within as brief a period each year as in the western white-bearded wildebeest (Connochaetes mearnsi), in which the infants are so precocial that they are fully cursorial shortly at only a few days old.

There are three possible explanations for the apparent incongruity between the hypercursoriality of the pronghorn in adulthood and its immobile hiding in infancy, as follows:

  • The pronghorn produces usually two infants (twins) per birth, as opposed to the single infant born in most comparable ruminants; thus, the infants are too small to be cursorial for the first three weeks of life, hiding instead.
  • Infants of the pronghorn are indeed large-bodied and long-legged enough to run fast and far, but are physiologically and behaviourally committed to a tactic of immobility, for various reasons such as the availability of cover in the form of sagebrush (Artemisia tridentata) in the typical habitat of the pronghorn.
  • The real strategy of the pronghorn in infancy is to combine immobility while hiding with extreme speed and endurance once found and attacked by a predator, which would be mean that the species could actually be regarded as hypercursorial at all ages.

In increasing order of maternal body mass:

Eudorcas thomsoni 18.5 13.5%
Procapra gutturosa 13%
Antidorcas marsupialis 30 12.5%
Aepyceros melampus 45 11.1%
Damaliscus pygargus phillipsi 60 10.8%
Damaliscus lunatus 110 10.0%

For the pronghorn: 45 6.7%

Reference for infant hiding period: https://academic.oup.com/jmammal/article/87/2/312/870959

Reference for neonatal body mass: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4667974/





Posted on May 06, 2024 01:59 PM by milewski milewski


One of the 'followers' (as opposed to 'hiders') among bovids is Ovis canadensis, a member if the tribe Caprini.

What is neonatal body mass as percentage of maternal body mass in O. canadensis (https://en.wikipedia.org/wiki/Bighorn_sheep)?

This is complicated, because a) the various subspecies differ considerably in body mass, and b) litter size is usually one but occasionally two.

According to https://ielc.libguides.com/sdzg/factsheets/bighornsheep/reproduction, neonatal body mass is 3.6-4.5 kg, and maternal body mass averages 57.2 kg.

This would indicate a value of 7%, similar to that of the pronghorn, and surprisingly different from the expected value for a bovid, viz. about 10.9%.

The puzzle that arises is that the pronghorn and O. canadensis show converse anomalies: the former runs with extreme speed and endurance but has infants that hide, whereas the latter runs with mediocre speed and endurance (relying on refuge provided by steep slopes) but has infants that follow.

Posted by milewski 3 months ago

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