Contrasting selection pressure on body and weapon size in a polygynous megaherbivore

Abstract

Sexual size dimorphism (SSD) is a common morphological trait in ungulates, with polygyny considered the leading driver of larger male body mass and weapon size. However, not all polygynous species exhibit SSD, while molecular evidence has revealed a more complex relationship between paternity and mating system than originally predicted. SSD is, therefore, likely to be shaped by a range of social, ecological and physiological factors. We present the first definitive analysis of SSD in the common hippopotamus ( Hippopotamus amphibius ) using a unique morphological dataset collected from 2994 aged individuals. The results confirm that hippos exhibit SSD, but the mean body mass differed by only 5% between the sexes, which is rather limited compared with many other polygynous ungulates. However, jaw and canine mass are significantly greater in males than females (44% and 81% heavier, respectively), highlighting the considerable selection pressure for acquiring larger weapons. A predominantly aquatic lifestyle coupled with the physiological limitations of their foregut fermenting morphology likely restricts body size differences between the sexes. Indeed, hippos appear to be a rare example among ungulates whereby sexual selection favours increased weapon size over body mass, underlining the important role that species-specific ecology and physiology have in shaping SSD.

Quantitative evidence for dimorphism suggests sexual selection in the maxillary caniniform process of Placerias hesternus

“…Additional information can be found via comparison with living sexually dimorphic megafauna with similar features and their inferred ecological roles. Many living mammal megafauna, such as hippopotami, elephants, and walruses, have dimorphic tusks and other facial features that can be classified as weaponry [49,50], which are similar to the caniniforms of Placerias. Some of these taxa (e.g., elephants) also represent species with both SSD and proportional differences in features used as weaponry, while others only differ substantially in proportional size of dimorphic weaponry and not in body size (e.g.…”

“…Some of these taxa (e.g., elephants) also represent species with both SSD and proportional differences in features used as weaponry, while others only differ substantially in proportional size of dimorphic weaponry and not in body size (e.g. male hippos have proportionately larger mandibles than females, and use their lower canines for intraspecific combat despite having similar body sizes) [50]. Within the large Placerias morph, the greater range of size in caniniforms may represent a combination of difference in ages of individuals with heterochronic indeterminate caniniform growth and individual variation based on sexual selection.…”

Are hippos Africa's most influential megaherbivore? A review of ecosystem engineering by the semi‐aquatic common hippopotamus

“…Adult hippos are large, routinely exceeding 1000 kg in body mass. During a culling operation of 2994 hippos between 1961 and 1966 in Queen Elizabeth National Park, Uganda, mean body mass for male and female hippos was 1393 and 1332 kg, respectively, with males being only 5% heavier than females on average (Shannon et al ., 2021). In Kruger National Park, South Africa, a culling operation of 104 hippos along the Letaba River yielded a mean body mass for male and female hippos of 1492 and 1325 kg, respectively (Pienaar, van Wyk & Fairall, 1966).…”

“…In Kruger National Park, South Africa, a culling operation of 104 hippos along the Letaba River yielded a mean body mass for male and female hippos of 1492 and 1325 kg, respectively (Pienaar, van Wyk & Fairall, 1966). The maximum mass of individuals from both sexes has been recorded at over 2000 kg (Bere, 1959; Pienaar et al ., 1966; Shannon et al ., 2021).…”