Sexual dimorphism of structures showing indeterminate growth: tusks of American mastodons (Mammut americanum)

Abstract

Documenting sexual dimorphism for structures that exhibit indeterminate growth can be more difficult than for structures exhibiting determinate growth. Most proboscidean tusks are ever-growing structures that change size and shape throughout life. Sexual dimorphism is pronounced in tusks of mature individuals, but the external form of tusks offers no clear evidence of maturation, and it is difficult to distinguish a young male's tusk from that of an older female. Thus, with previous approaches, knowledge of age was often required to assess sex from tusk measurements. This study examines sexual dimorphism of American mastodon (Mammut americanum) tusks through principal components analysis to determine which aspects of tusk form contribute most strongly to the variance among measurements and to explore the relationship between tusk form and individual age and sex. Twenty-one mastodon tusks from the Great Lakes region were evaluated in two analyses, the first focusing on geometrically distinct aspects of tusk form and the second adding measurements that reflect ontogenetic changes in a single aspect of morphology (circumference). Both analyses separated mastodons by sex (PC-I) and sorted them by age (PC-II). The distribution of tusks on the PC-II versus PC-I plane provides better discrimination of sex than univariate or bivariate methods because tusks of similar size and opposite sex appear near opposite ends of an age spectrum. The second analysis enhances sorting by age, thereby clarifying assessment of sex. This work contributes to studies of mastodon paleobiology by presenting a reliable method for assessing the sex of an individual from tusk measurements without requiring independent knowledge of age.

Protracted growth impedes the detection of sexual dimorphism in non‐avian dinosaurs

“…; Lee & Werning ). These frustrations echo those voiced previously by Smith & Fisher () when trying to detect dimorphism in indeterminately growing mammoth tusks. Dimorphism is much easier to detect in the fast growing rhea because random sampling preferentially selects among the fully differentiated adults.…”

Taxonomic overview and tusk growth analyses of Ziegler Reservoir proboscideans

“…The three tusks in between, with circumferences of 14-14.5 cm were identified on the basis of EIV trends discussed below. In short, we see here a pattern that parallels the well-documented dimorphism in premaxillary tusks (Smith and Fisher, 2011).…”

“…The Clay Mammoth molar examined above suggests a Laws' Age Class of XX-XXII, equivalent to an African elephant age of 34-39 yr (Laws, 1966). Tusk circumference and length of this individual (48 cm, 195 cmestimated) indicate a male, based on sexual size dimorphism, which is especially clear in tusks (Fisher, 2008;Smith and Fisher, 2011). Patterns of epiphysis fusion are consistent with an adult male almost, but not yet, fully grown (Haynes, 1991).…”

A woolly mammoth ( Mammuthus primigenius ) carcass from Maly Lyakhovsky Island (New Siberian Islands, Russian Federation)

“…The circumference of each tusk, especially as it varied with distance from the tip (Supplemental material, Table S1) was the first cue supporting this interpretation. The tusks of an adult male would have had greater maximum girth and would have increased girth more rapidly with distance from the tip (Fisher, 2007(Fisher, , 2008Smith and Fisher, 2011), instead of stabilizing girth only a little beyond the first meter from the tip. The left and right tusk of this specimen have slightly different girth profiles, but this is mostly due to the difference of about 20 cm in their lengths, reflecting greater loss of tusk material from the right tusk by tip fracture and abrasion.…”

“…In addition to the girth profile, another trait consistently associated with adult females, especially in their later stages of life, is possession of pulp cavities shorter than those of similar-age males and much shorter than the depth of the alveolus (Smith and Fisher, 2011). For comparison with the Malolyakhovsky mammoth, the Yukagir mammoth, an adult male of about the same age, has a right tusk 316.4 cm long on its outside curve, a maximum girth of 45.6 cm, near the alveolar margin, and a pulp cavity depth of 26 cm (Mol et al, 2007), all values greater than what we report in Results.…”