For an updated version of this bit of writing, please see my 2016 blog post at SAPIENS.org: http://www.sapiens.org/blog/animalia/island-dwarfism
On Homo floresiensis and island dwarfism:
In honor of the 10 year anniversary of the publication of a description of what came to be known as the “hobbit” of Flores, I am sharing this paper, written in the fall of 2004.
Specimens of a small bodied hominin species resembling Homo erectus were recently discovered in later Pleistocene cave deposits on the island of Flores in Indonesia. This hominin species is unique because its stature and brain size are comparable to those of Pliocene Australopithecus, yet its skeletal morphology is primarily Homo erectus-like. The species has been depicted as a dwarfed descendent of Homo erectus that evolved in an insular island environment and is classified as a new species, Homo floresiensis, due to its unique combination of primitive and derived features (Brown et al., 2004).
A full grown adult female of the new species was described in October of 2004 (Brown et al., 2004). The bones attributed to this specimen comprise the skull, thigh, leg and patella, a partial pelvis, manual and pedal bones, and vertebral, sacral, scapular, clavicle and rib fragments. The specimen, Liang Bua 1 (LB1), is similar to members of Homo erectus in having small postcanine teeth, reduced facial height and prognathism, relatively thick cranial vault bones, similar cranial shape indices, an occipital torus, similar petrous morphology, a posteriorly oriented infraorbital region, (slight) frontal sagittal keeling, lack of a chin, and double mental foramina. The stature of this individual, however, is estimated to be approximately 106 cm and its endocranial volume is estimated at 380 cc, both well below the accepted ranges for Homo erectus. In addition to its short stature and small cranial capacity, this species differs from Javan Homo erectus by the greater cranial base flexion, arching browridges, a less prominent occipital protuberance, more anterior incisive canal, location of zygomatic over the first molar, overall morphology of the mandibular symphysis, relatively larger femoral medullary canals, more prominent intertrochanteric crests, and the tibial size and shape indices.
Bones from this individual were discovered in deposits that date to approximately 18 kyr (Morwood et al., 2004). Other specimens from different sectors at the Liang Bua cave site, a mandibular left premolar and a partial radius, are thought to be representative of Homo floresiensis and have been dated to 37.7 +/- 0.2 kyr and 74-95 kyr respectively. This implies that H. floresiensis may have inhabited the island of Flores for more than 75,000 years. The earliest evidence of hominin occupation on the island, in the form of stone tools, dates to approximately 880 +/- 70 kyr at the site of Mata Menge (Morwood et al., 1998). There is no hominin skeletal evidence to confirm what species made these stone tools, but it has been suggested that Homo erectus used watercraft to initially reach the island, maybe around this time (Morwood et al., 1998). Due to the lack of evidence we do not know whether this early population arrived on the island “full-size” or in a dwarfed state.
Among the other animals found at Liang Bua is a dwarfed species of Stegodon thought to have survived on the island until approximately 12 kyr when it was forced to extinction by a volcanic eruption (Morwood et al., 2004). Interestingly, the 880 ky old Manta Menge deposits contain fossils of a large Stegodon species. Morwood et al. (1998) and Sondaar (1987) both support the extinction of an earlier pygmy Stegodon species around 900 kyr, possibly as a result of predation by hominins (although the giant tortoises and giant Komodo dragons went extinct as well), and a secondary recolonization of Flores by the large species. They do not indicate, however, whether this large species could have evolved into the later dwarfed species of Stegodon that is found at Liang Bua.
The presence of dwarfed mammals on the island of Flores lends support to the argument that Flores was an insular environment that drove the evolution of a change in body size of mammal species. This follows Foster’s (1964) “island rule.” Foster examined 116 North American and European island taxa and showed that in an insular environment, smaller species (such as rodents) tend to get larger and larger species (such as carnivores and artiodactyls) tend to get smaller. There are exceptions to this rule, however, as pointed out in Case (1978); species that are found on more than one island do not always do the same thing on those islands. Changes in body-size that occur in island populations seem to be dependent on a number of basic factors, essentially tied to the island resources, competition and predation pressures, and energetics.
For a small mammal, an increase in body-size can result from an ability to exploit a larger body-size niche because of reduced competition or predation pressures and an availability of a wide range of resources. This can occur in small-bodied dietary generalists (Lawlor, 1982). A larger body size confers all the benefits of being bigger – dominance over competitors, the ability to eat big and small items, and the ability to have larger offspring that may be better equipped for survival.
In the case of body-size reduction, one may infer a reduction in predation pressures. If an animal does not need to be large for fleeing or defense from predators, this particular selective pressure is relaxed and body size can decrease to reduce per capita energetic requirements. A smaller body-size confers the benefits of reducing the amount of energy needed per capita in an environment where overpopulation and exhaustion of resources is a threat, particularly for dietary specialists. This would allow individuals to allocate more energy to reproductive efforts (Raia et al., 2003).
Although all this seems relatively straightforward, the relationships between all of the factors that may be directly or indirectly related to changes in body-size on islands are quite complex. A desire to better understand these relationships has led to two primary models, which are not necessarily mutually exclusive, for explaining how larger-bodied species may dwarf in an insular environment. Each involves a relaxation of predation and competition pressures, but emphasizes a different driving force behind the reduction in body-size. I will examine each model and then discuss how I would go about testing which model best explains dwarfism on the island of Flores.
Models to explain dwarfism
The first model basically follows the idea that because island environments are structured differently than mainland environments, the ability for island species to exploit resources as effectively as they did on the mainland is diminished. This may be due to a lack of variety or absence of high quality resources on the island, or simply that there is a limited amount of resources to be exploited on an island, particularly for a population of large-bodied animals that exceeds a particular size with limited ability to migrate (due to island geography; Raia et al., 2003). Essentially, this argument says that islands just don’t have “enough good stuff” to support larger body sizes for some mammals. As a result, there are high levels of competition for resources and large animals might experience stunting of growth and lower reproductive rates. If those species begin to produce smaller individuals that require less or lower quality resources, competition decreases and the smaller individuals may be relatively more reproductively successful and pass on their genes more frequently; hence, a reduction in species body-size occurs for the population.
This idea is illustrated in Lawlor’s (1982) study of insular animal populations in Mexico. According to Lawlor (1982:64) “for animals specializing on particulate resources and facing resource deprivation on islands, there may be a selective premium placed on additional measures to conserve energy. Because the metabolic demands for a small mammal are less than those for a large one, small body size should evolve.” Lawlor (1982) makes a point of differentiating between specialist and generalist species by emphasizing that specialists will be most affected by depressed resource levels. Sondaar (1977) also makes the point that when predation pressures are relaxed, and thus the need for large body-size for defense mechanisms is lost, smaller size would be selected for to increase survival in an environment with depleted resources.
Lamolino (1985) assessed degrees of dwarfism or gigantism in numerous island species as an expansion on Foster’s (1964) study. Lamolino (1985:312) supported the idea that there is a “graded trend from gigantism in…smaller species of insular mammals to dwarfism in…larger species” and presented a model for how this occurs that coincides with Lawlor’s (1982) views that resource limitation, and thus the factors promoting dwarfism, become more important with increasing mammal size. It can be noted, though, that other authors have documented some exceptions to this rule, though the use of this model as a possible explanation for body-size changes is still supported (Angerbjorn, 1985).
The other, more recently developed, model attempts to explain island-dwarfism by going beyond the resource based explanation of the first model, citing selection for an optimal body size for energy acquisition and use as the driving force behind dwarfism (Damuth, 1993). According to Damuth (1993:748), “If on an island a species’ usual competitors and predators are absent, it should tend to evolve toward the optimum body size, and the adaptive advantages of doing so would be greatest for populations starting at body-size extremes.” This optimum body size (100 g according to Symonds, 1999 or 1 kg according to Damuth, 1993) is described as the point at which “mammals attain an optimal energetic tradeoff between resource provisioning and offspring production abilities” (Raia et al., 2003: 294). Some authors have argued against this model (Symonds, 1999; Roy et al., 2000) based on observed exceptions to this “rule.”
Raia et al. (2003) conducted a study of the dwarf elephant Elephas falconeri from the island of Sicily to assess the likelihood that this species was dwarfed as a result of selection for a more optimal, smaller body size or due to overcrowding and an ecologically restricted environment (the first model). These small elephants were nearly 150 times smaller than large Elephas species, reached sexual maturity earlier than larger elephants, and had relatively high calf mortality rates. Promislow and Harvey (1990) suggested that such high juvenile mortality rates would lead to selection for production of smaller offspring in higher numbers. If the Elephas species dwarfed rapidly (dwarfing has been estimated to occur as quickly as 3000 years), then the small elephants minimized their individual energetic needs, and could prevent resources on the island from being depleted (note that limited resources is not driving dwarfism in this case, as in the first model). The authors’ primary evidence that resource deprivation was not driving dwarfism is that an earlier species of elephant that dwarfed on the island, but to a lesser degree, was subject to even more constraints than E. falconeri. Thus, Raia et al. (2003) supported the optimal body-size hypothesis.
Raia et al. (2003) thought that by aiming for an optimum body size (100 kg in this case) the energy the elephants were saving in growth could be put toward reproduction, specifically reproducing earlier and at faster rates, increasing intraspecific competitive success and reproductive fitness as a result. Since the authors (Raia et al., 2003) documented a large percentage of juveniles in the assemblage, they assumed that reproductive output was high for the population. Other evidence that they provided to support increased allocation of energy to reproduction includes a lack of tusks in females and paedomorphosis of the adults. They note that organisms with fast growth rates tend toward high juvenile mortality rates, which would explain the estimated mortality rates for juveniles of this species (leading us back to selection for larger numbers of smaller offspring).
It’s clear to see how these two models are connected with one another. Both are dependent on a reduction in predation and competition pressures and both emphasize the importance of maximizing individual reproductive success. The difference is that one is driven by the fact of diminished resources and the other by the selective advantage of maximizing energy allocation for reproduction by balancing it with a reduction in energy utilized for growth.
Homo floresiensis is the first documented example of a hominin “following” the “island rule.” The researchers who discovered the material believe that there is enough anatomical evidence to show that this species is most likely a dwarfed descendent of a physically larger population of Homo erectus that may have used watercraft to reach the island. This could be the case if the dates at the Mata Menge site are accurate and Homo erectus did inhabit the island at approximately 900 kyr. It is feasible that this population was the one that came to the island with stone tool technology, hunted already dwarfed Stegodon, became dwarfed themselves, potentially in as little time as 3000 years, and survived in a dwarfed state for nearly a million years. It is also possible, of course, that it was not this Homo erectus population that evolved into Homo floresiensis, but a later population of Homo erectus (or another species) and that Homo floresiensis only inhabited the island for 75,000 years. Regardless, if one accepts that Homo floresiensis is the product of island dwarfism of a larger hominin species, then these tiny hominins beg one question in particular, among many: Why dwarfism?
Why did a large bodied hominin species become dwarfed? In order to get at the marrow of this question, we can approach it in the context of the two models of dwarfism presented earlier. Therefore I would ask: Did Homo floresiensis evolve because of diminished resources that affected the stature of a population trying to survive in such conditions or was dwarfism a more complex process related to a need to expend energy on reproduction in order to maintain fast growth rates, a young age for sexual maturity and high reproductive output?
In a brief discussion of this issue, Brown et al. (2004) proposed that Flores offered a limited supply of energetic resources for hominins. They proposed that such a low caloric environment would favor individuals that were smaller and required less caloric intake. Essentially, they support the first model presented earlier for why body-size reduction occurs in large mammals on islands. Brown et al. (2004) do not investigate this issue any further, however, and do not supply strong evidence for this explanation, except for mentioning that other authors have shown that modern human foragers in tropical rainforests, in the absence of agriculture, do not have access to an abundant supply of calories.
Therefore, I feel that a more in-depth analysis of the question, “Why dwarfism?,” as it applies to the Flores hominins, is necessary.
As a first step to this investigation, predation pressure would be assessed. Predation affects both models, as they both call for reduced predation pressure as a precursor to dwarfism. This is fairly simple to address. The only potential hominin predators on the island are the Komodo dragon and another varanid (Brown et al., 2004). Today, Komodo dragons on the island of Komodo near Flores eat “any and all of the other large animals on the island, including wild boar, deer, water buffalo, dogs and goats. If hungry, a Komodo will eat snakes, birds, and even smaller Komodos” (PBS Online, 2004). The bite of a Komodo is extremely poisonous and deadly. These large reptiles have been clocked at speeds up to 20 miles per hour. Clearly, a small hominin would be susceptible to being attacked by a Komodo, but potentially the hominins were able to avoid these large predators somehow or were able to defend themselves with weapons. Examining Komodo bones for evidence of cut marks would be worthwhile to determine whether the hominins defended themselves against, or even hunted, these animals. If one could conclude that the hominins were able to defend themselves against these large predators easily, or that veranids seldom attacked hominins, one could assume that predation pressures were low on Flores.
The next step is to assess interspecific competition. Brown et al. (2004) report that the fauna of Flores comprised monkeys, deer, and birds. If the Flores hominins were greatly dependent on fruits, these species may have been competition for them. One way to investigate this would be through dental morphological, microwear and isotope analyses. Microwear and isotope analyses could be informative regarding the composition of the hominin diet. If the evidence did not suggest that hominins were incorporating significant amounts of tropical fruits into their diet, to the degree that they were the primary staple of their diet, then interspecific competition may have been low, as is required in both models.
Another way to assess interspecific competition would be to try and determine whether the hominins were in fact hunting with the stone tools found in the assemblage. If evidence of hunting (cutmarks on mammal bones) were found, then this could also support reduced competition with primates, browsing artiodactyls and bird species. The only interspecific competition then would be with the other predators on the island – the veranids. If the relative population sizes of hominins and veranids could be estimated, this could possibly be used as an indicator of whether they were competing for resources. An analysis of dwarfed Stegodon bones (the species most likely preyed upon by both) for hominin cut marks or reptile damage could be used to assess this, as well.
If established that predation pressures and competition levels were low for the Flores hominins, one could investigate which of the models of dwarfism is more applicable. Brown et al. (2004) presented an argument that modern foragers in tropical forests face low caloric resource availability, but this research is not specific to island foragers. A study of any non-agricultural island forest populations would be a more appropriate analog since it has been documented that island environments tend to differ dramatically from their mainland equivalents. Also, an assessment of growth and development in island tropical forest forager populations would shed light on whether this purported reduction in caloric intake does affect their stature. Such a study may be quite challenging, as any such population may not be accessible (geographically or culturally) to researchers.
Modern pygmy populations live in forests and have reduced stature, but this has been shown to be related to other factors besides diminished resources (Brown et al., 2004). And although their stature is reduced, brain size and cranio-facial proportions in pygmies are not reduced in the way they are in the Flores specimen. As a result, pygmies and other modern human forest foragers may not be an appropriate analog for the Flores hominins.
Therefore, it may be more useful to assess the other Flores island animal that dwarfed to determine if the cause for dwarfism in that species, Stegodon, may have been a result of stunting from resource deprivation. An investigation into the dietary habits of the dwarfed Stegodon, based on dental morphology or isotope analysis, could be informative in this case, in particular determining whether Stegodon was a specialist, or like other elephants, an effective generalist, in which case resource deprivation would not likely have been the cause for reduction in body-size. Notably, most of the Stegodon from Liang Bua are juveniles, a pattern seen in Raia et al.’s (2003) study of Elephas falconeri, possibly indicating a high reproductive output and fast growth rates associated with high juvenile mortality.
It must be addressed here that relatively fast growth rates and a relatively young age at sexual maturity are both characteristics that have been inferred for Homo erectus (Dean et al, 2001). Analyses of growth and development of the Nariokotome skeleton of Homo erectus, from studies of the dentition, have concluded that Homo erectus had a more ape-like (vs. Homo sapiens-like) growth trajectory. This means that Homo erectus grew faster and became sexually mature earlier when compared to modern humans. The Nariokotome skeleton provides an estimated stature of at least six feet for that individual and an age estimate of only 8 years. This is relevant here because if Homo floresiensis did evolve from a Homo erectus ancestor, it is possible that the first population to inhabit this island already had to maintain fast growth and development rates. This is only possible when high quality resources are available, otherwise there will be a trend toward lower reproductive rates. It doesn’t seem reasonable that a species with an already high demand for high quality resources could migrate to a low caloric environment, experience decreased reproductive rates and stunting of growth, and yet be so successful for so long, whether that was 75,000 years or a million years.
It order to determine whether growth and development of Homo floresiensis was similar to that documented for Homo erectus an analysis of dental growth and development similar to that of Schwartz and Godfrey (2003) or the analysis of the Nariokotome skeleton (Dean et al., 2001) should be conducted. This could be used as evidence that Homo floresiensis was phylogentically predisposed to having a fast growth rate and an early age for sexual maturity.
If the resources on Flores were so poor that the original hominin inhabitants were subject to resource deprivation, one could question (though probably not test) whether they would have survived long enough to become dwarfed (even though this is a relatively fast process). A species with such high energetic demands would probably suffer from low reproductive rates and a reduction in growth rates as a result. If an analysis of the dentition showed that Homo floresiensis maintained a growth curve like that of Homo erectus, or even increased the rate an which they reached sexual maturity (as documented in dwarfed Elephas) one might conclude that the resources on Flores were of relatively high quality. Evidence for successful hunting of Stegodon or veranids by the hominins could support this argument.
So if resources were adequate and caloric intake was reasonably high, then why was a reduction in body size selected for in these hominins? We can revisit Raia et al.’s (2003) study of Elephas falconeri. Elephants have a long pregnancy duration, precocial offspring, and only have one offspring at a time. Raia et al. (2003) suggest that if Elephas evolved toward an optimum body size (100 kg in this case) the energy the elephants were saving in growth could be put toward reproduction, and even allow for earlier and faster reproduction, increasing intraspecific competitive success and reproductive fitness in an environment with finite resources (both energetic and reproductive). But, if Homo erectus already had fast growth rates and were producing offspring at a maximally young age, this might imply that a reduction in body size was predominantly a result of reduced predation pressures and interspecific competition. In an environment with adequately high quality resources and relaxed selection pressures such as these, Homo erectus may simply have had no reason to be big. A trend toward a body size that was optimal for survival – one that balanced energy expenditure for resource acquisition and for reproduction may have been inevitable and unavoidable. And once this was achieved, the species was able to thrive in that environment for tens of thousands of years.
In order to test the hypothesis that this Homo population got smaller mostly as a result of relaxed selection pressures – “just because it could” – the relationship between Homo erectus and Homo floresiensis body size needs to be clearly understood. A thorough analysis of the material using allometric formulae needs to be performed. If the size and shape of Homo floresiensis’ skeletal features do not correspond with it being a “scaled down” version of Homo erectus other avenues of explanation of how and from what taxon Homo floresiensis evolved would need to be explored. In addition, as assessment of whether any of Homo floresiensis’ features can be considered paedomorphic is important. The dwarf species of Elephas appears to exhibit paedomorphism, and this is evidence used to support the reallocation of energy away from growth and development and toward reproduction. If Homo floresiensis is thought to be peadomorphic in any way, this could support the second model.
Homo floresiensis provides evidence for the first identified case of island dwarfism in a hominin population. Some authors (Brown et al., 2004) have suggested that the dwarfism was a result of an initial population (on Flores or a nearby island) having to survive in a low caloric environment. Calorie restriction, however, has been associated with delayed sexual maturity, decreased reproductive efforts, and a “shift toward a greater investment in somatic maintenance” (Raia et al., 2003:302). This trend would not be consistent with the fact that this hominin population could have thrived on the island for anywhere between 75,000 and one million years. A more likely explanation might be that, given a phylogenetic predisposition for fast growth and reproductive rates, respectable resources, and, most importantly, reduced pressure from predation and interspecific competition, this hominin species evolved toward an optimal body size “just because it could.”
A detailed paleoecological analysis of the flora and fauna in deposits associated with Homo floresiensis skeletal remains will be very informative, particularly with regard to estimating predation and interspecific competition levels. This information, combined with dental morphological, microwear and isotopic analyses of the Homo floresiensis specimens, as well as zooarchaeological analyses of the skeletal remains of the Stegodon and the veranid species, will shed light on the diet of these diminutive hominins. The combination of dietary information with knowledge of the growth and development of this species, allometric relationships between Homo erectus and Homo floresiensis and any evidence of paedomorphism could elucidate the driving mechanisms for this incidence of island dwarfism.
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 It has been suggested that the term “nanism” is more appropriate for describing evolution of reduced body-size than dwarfism (Gould and McFadden, 2004). Despite this, I will use dwarf and dwarfism to reflect terminology used by the authors that published the description of the Flores material.
 This term was actually coined by Van Valen (1973; Lomolino, 1985).
 Lawlor (1982) is specifically referring to seed specialists here, but remarks, “food deprivation…must occur commonly in large mammals on islands [and]…selection should favor small body size as the metabolically most expeditious way to contend with prolonged food shortages” (67).
[A writing assignment submitted for credit at Arizona State University in December, 2004.]
Why Dwarfism? (.pdf download)