In a nice illustration of how science works, there had been conflicting explanations for how the Giraffe evolved its long neck, but before creationists get over-excited neither of them were alternatives to evolution; the debate was about the precise drivers of evolution. That the giraffe's neck evolved from a shorter neck of a common ancestor with the related okapi was never in dispute.
One school of thought was the standard Darwinian view that it was the result of an arms race between giraffes and their preferred food plants; the other was slightly more complex, involving female sex selection and the way male giraffes use their necks against rival males with longer neck being a distinct advantage and the winner winning the favours of the females. Since the length of the neck is not strictly sex-linked, meaning it is mostly coded for by genes located on autosomes, rather than on the Y sex chromosome, both males and females benefit from this sex-selection, or so the second school of thought ran.
Now, a team of scientists led by Penn State biologists believe they have resolved this dispute buy, in the time-honoured scientific way, by examining the evidence and letting it speak for itself. What they found was that females actually have proportionately longer necks than males, so sex selection is unlikely to be the driver. However, what they observed was that females are better able to reach leaves in the crowns of trees and so get the more nutritious food, with obvious benefits for their offspring
The team have published their work, another incidental refutation of creationism that comes from revealing scientific facts, open access, in the journal Mammalian Biology. It is also explained in a Penn State University News release:
Why do giraffes have such long necks? A study led by Penn State biologists explores how this trait might have evolved and lends new insight into this iconic question. The reigning hypothesis is that competition among males influenced neck length, but the research team found that female giraffes have proportionally longer necks than males — suggesting that high nutritional needs of females may have driven the evolution of this trait.
The study, which explored body proportions of both wild and captive giraffes, is described in a paper that appeared today (June 3) in the journal Mammalian Biology. The findings, the team said, indicate that neck length may be the result of females foraging deeply into trees for otherwise difficult-to-reach leaves.
In their classic theories of evolution, both Jean Baptiste Lamarck and Charles Darwin suggested that giraffes’ long necks evolved to help them reach leaves high up in a tree, avoiding competition with other herbivores. However, a more recent hypothesis called “necks-for-sex” suggests that the evolution of long necks was driven by competition among males, who swing their necks into each other to assert dominance, called neck sparring. That is, males with longer necks might have been more successful in the competition, leading to reproducing and passing their genes to offspring.
.The necks-for-sex hypothesis predicted that males would have longer necks than females, and technically they do have longer necks, but everything about males is longer; they are 30% to 40% bigger than females. In this study, we analyzed photos of hundreds of wild and captive Masai giraffes to investigate the relative body proportions of each species and how they might change as giraffes grow and mature.
,Professor Doug R. Cavener, senior author
Dorothy Foehr Huck and J. Lloyd Huck Distinguished Chair in Evolutionary Genetics
Professor of biology
Department of Biology
Penn State University, PA, USA.Although male and female giraffes have the same body proportions at birth, they are significantly different as they reach sexual maturity. Females have proportionally longer necks and longer bodies than males, which might help with foraging and child rearing, while males have wider necks and longer front legs, which might help win fights against other males and with mating.Credit: Illustration provided by Penn State researchers / Penn State. Creative Commons
AbstractIncidentally, there is an interesting discussion in this paper which explains why an evolutionary trend doesn't continue indefinity, part of which is shown below. It explains the balancing act between the selected characteristics of male giraffes and the increasing demands these place on their body, resulting in male giraffes having 25% shorter lives than females:
Giraffes exhibit a large sexual dimorphism in body size. Whether sexual dimorphisms also exist in body proportions of the axial and appendicular skeleton has been debated, particularly regarding the giraffe’s iconic long neck. We examined the anatomical proportions of the neck, forelegs, hindlegs, and body trunk of the Masai giraffe (G. tippelskirchi) in captive and wild populations. We found that female Masai giraffes have proportionally longer necks relative to their forelegs than males in contradiction to the original necks-for-sex hypothesis that proposed that the evolution of the giraffe’s long neck was driven by male-male competition. However, male neck width and apparent mass are proportionally larger than females’, supporting a modification of the necks-for-sex hypothesis. Moreover, male foreleg length is proportionally longer whereas female trunk length is proportionally longer. These sexual dimorphisms were found in both captive and wild Masai giraffes. We speculate that the initial evolution of the giraffe’s long neck and legs was driven by interspecific competition and the maternal nutritional demands of gestation and lactation through natural selection to gain a competitive advantage in browsing, and then later the neck mass was further increased as a consequence of male-male competition and sexual selection. Differences in the proportions of major body components define sex phenotypes, but several giraffes display opposite-sex phenotypes with a significantly higher level of discordancy seen in captive males. We speculate that body proportion sexual dimorphisms are maintained in the wild by natural and/or sexual selection, but in captivity selection is relaxed resulting in a higher occurrence of discordances in sexual phenotypes.
Introduction
Body size sexual dimorphism (SSD) exists in most mammalian groups, which may be either male or female biased (Reiss 1989; Abouheif and Fairbairn 1997; Weckerly 1998; Pérez-Barbería et al. 2002). Evolutionary hypotheses for the development of SSD include natural selection based on the two sexes competing for resources or occupying different ecological niches (Slatkin 1984) or SSD may be a non-adaptive byproduct of natural selection for larger size in both sexes (Leutenegger and Cheverud 1982). For example, giraffes (Giraffa camelopardalis), a polygynous megaherbivore, exhibit extreme SSD with adult males 30–40% larger mass than adult females (Hall-Martin 1977; Mitchell 2021; Roylance-Casson 2021.1). The large stature difference between male and female giraffes has been suggested to be an adaptation to reduce foraging competition between the sexes (Mitchell et al. 2009; Wilkinson and Ruxton 2012; Mitchell 2021). SSD could also result from or be increased by sexual selection based on male-male competition for mates (Darwin 1871; Clutton-Brock 1989.1), male-female coercion (Clutton-Brock and Parker 1995), or female choice (Eberhard 1996). SSD might also be the result of some combination of the above natural and sexual selection processes. The degree of SSD differs among mammalian groups, and Darwin (1871) was the first to point out that polygyny was associated with the largest differences (Ralls 1977.1; Weckerly 1998; Cassini 2020). Moreover, the magnitude of SSD in mammals is typically greater in species with high body mass, known as Rensch’s rule (Rensch 1959; Abouheif and Fairbairn 1997).
Less commonly known is sexual dimorphism in body proportions (BpSD), also known as body shape dimorphisms, that are due to allometric growth and developmental differences between sexes with different overall adult sizes (Badyaev et al. 2001). Characterizing BpSD may illuminate the different selective pressures acting on the two sexes in dimorphic species (Tarnawski et al. 2014), such as when comparing the shapes of male and female skeletal components. In this study we explore body proportional sexual dimorphisms in the Masai giraffe (Giraffa tippelskirchi), which was inspired by a purported sexual dimorphism in length of the neck and an alternative theory for the evolution of the giraffe’s iconic long neck. The observation that male giraffes frequently engage in neck sparring as a means to establish dominance, led Simmons and Scheepers (1996.1) to propose the “necks-for-sex” theory that the giraffe’s long neck evolved via male-biased sexual selection, which proposed that longer and more muscular necks provided a mating competition advantage. A prediction of the necks-for-sex hypothesis is that males would have longer necks than females, and indeed they do (Mitchell et al. 2013). However, the longer neck in male giraffes is primarily a consequence of the overall ~ 30% larger size of males; every skeletal component is larger in adult males than adult females. Our question is: do male giraffes have proportionally longer necks than females when accounting for allometric differences between male and female skeletons? Moreover, do other aspects of the giraffe skeleton exhibit BpSD that underlie important sex-specific adaptations? The answers to these questions will identify skeletal components likely undergoing selection and inform future work using genetic pedigrees to determine the mating system of giraffes and the sex-specific body morphology traits that correlate with higher survival and/or reproduction.
Critically assessing body proportion sexual dimorphisms in giraffes requires the comparison of the appendicular and axial skeleton including the fore and hind legs and vertebral segments that contribute to the length of the neck and trunk. Given that the birthdates of wild giraffes are rarely known, previous studies by Simmons (Simmons and Scheepers 1996.1; Simmons and Altwegg 2010) and Mitchell (Mitchell et al. 2009; van Sittert et al. 2010.1; Mitchell 2021) have used body mass and size or tooth-wear as proxies for age. However, age is a confounding factor in assessing relative body proportions in giraffes because (a) their necks are known to grow at a more rapid rate than their legs during neonatal and juvenile development, (b) the body size sexual dimorphism, which is readily apparent in mature adults, develops over an unknown period of time, and (c) growth rates and terminal size are highly variable among individuals.
Our objective here was to quantify body proportions to test the existing hypotheses about BpSD of adult giraffes. Using photographic images of known-age captive Masai giraffes from North American zoos and safari parks, we calculated the relative body proportions of the neck and trunk, comprising the axial skeleton, and the forelegs and hindlegs, comprising the appendicular skeleton. Because the identity, pedigree, and birthdates of these animals were known, we were also able to quantify the postnatal developmental changes in body proportions over a wide span of ages. By utilizing photographs, we were able to obtain a larger sample size than what was possible in previous studies that utilized postmortem samples. To determine if any observed sex differences in captive giraffe body proportions also existed in wild populations, we used the same methods to estimate body proportions from photographic images of adult wild Masai giraffes located in the Tarangire Ecosystem in Tanzania.
Fig 5.
Examples of male and female Masai giraffe body proportion phenotypes. a Prototypical female giraffe displaying a female body proportion phenotype with long neck and trunk and shorter legs. b Female giraffe displaying a male body proportion phenotype with relatively short trunk and neck and longer legs. c Prototypical male giraffe displaying a male body proportion phenotype with shorter neck and trunk and longer legs. d Male giraffe displaying a female body proportion phenotype with longer neck and trunk and shorter legs, but still displaying male secondary sexual characteristics of the head (thick robust ossicones, secondary ossicones, and thick muscular neck)Cavener, D.R., Bond, M.L., Wu-Cavener, L. et al.
Sexual dimorphisms in body proportions of Masai giraffes and the evolution of the giraffe’s neck.
Mamm Biol (2024). https://doi.org/10.1007/s42991-024-00424-4
Copyright: © 2024 The authors.
Published by Springer Nature. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Discussion (part of)Just another casual and unintentional refutation of creationism by this team of biologists in yet another science paper. And to rub salt into creationists wounds, these scientists even cite Charles Darwin (1871 - The Descent of Man and Selection in Relation to Sex. John Murray, London) to give him the credit for first having proposed a mechanism by which giraffes evolved their body proportions. As it turns out, it is probably more to do with natural selection favouring females with longer necks that was the main driver, for which Darwin and Wallace deserve credit for having first proposed the mechanism which causes biodiversity, and, in this particular case, how giraffes and okapis diverged.
We propose that the male-biased BpSDs are evolutionarily tied to the large male-biased body size sexual dimorphism (SSD) seen in giraffes. Male-biased SSD evolved primarily in ungulate species with comparatively large body size and that are polygynous, social, and live in open habitats (McPherson and Chenoweth 2012.1; Pérez-Barbería et al. 2002; Polák and Frynta 2009; Roylance-Casson 2021.1; Cameron and du Toit 2007). The presence of male-biased SSD in giraffes but not in Okapia johnstoni (Roylance-Casson 2021.1), the giraffe’s closest existing relative, is largely consistent with these trends. Giraffes are larger, more social, and live in open habitats, whereas okapis are exclusively found in closed canopy forests (Stanton et al. 2014.1). Male-biased SSD is argued to be the product of sexual selection particularly in polygynous species where all females will be reproductively successful but not all males will be (McPherson and Chenoweth 2012.1). Female giraffes begin reproduction as early as the third year and may continue to produce offspring throughout their lives. By contrast male giraffes have a much shorter window for reproduction because males are not successful in mating until they become large enough to outcompete other males, and their lifespan is approximately 25% shorter than females in captivity and in the wild (Bingaman Lacky and LaRue 1997.1; Berry and Bercovitch 2012.2; Bercovitch and Berry 2017). The much shorter lifespan of male giraffes is almost certainly due to the considerable cost to their skeletal health of having 30–40% greater body mass (Hall-Martin 1977; Mitchell 2021; Roylance-Casson 2021.1). The increase in body mass, with the largest fraction compressed on top of the forelegs, results in foreleg joint and hoof dysfunction as male giraffes reach 15 years of age. In addition to poorer skeletal health, adult males are less likely than females to survive severe droughts because they require proportionally more nutrition to survive (Mitchell et al. 2010.2). Mitchell (2021) has persuasively argued that giraffes, and particularly adult males, have pushed the limits of the skeletal system to withstand the gravitation force exerted by the mass of the anterior trunk, neck, and head stacked on top of their long, spindly forelegs. We speculate that the elevation of the forelegs and wither height, along with a proportionally shorter trunk in adult males as reported herein exacerbate this physical challenge by shifting additional body mass over the forelegs. Differential niche occupation has been proposed as a potential explanation for SSD in giraffes and other large herbivores (Ruckstuhl and Neuhaus 2002.1). Male and female giraffes do tend to browse at different levels and may select different types of browse (Ginnett and Demment 1999; Cameron and du Toit 2007; Mramba et al. 2017.1). However, the substantial cost of male giraffe stature to longevity and resilience argues against this hypothesis as the primary selective pressure that resulted in SSD. We propose that dominance competitions and access to mating are the major drivers of sexual selection for male-biased SSD and BpSD. For male giraffes, having an elevated anterior body trunk and appearing to be bigger is believed to determine the degree of reproductive success (Pratt and Anderson 1985), but it has yet to be supported with genetic evidence of reproductive success differences among male phenotypes.
Cavener, D.R., Bond, M.L., Wu-Cavener, L. et al.
Sexual dimorphisms in body proportions of Masai giraffes and the evolution of the giraffe’s neck.
Mamm Biol (2024). https://doi.org/10.1007/s42991-024-00424-4
Published by Springer Nature. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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