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If fertility decreases but mortality remains the same, a population will shrink because it is no longer able to replace all deceased individuals. Since fewer babies are bom per adult during each reproductive cycle, an adult bias develops in the overall age structure. Figure 8.5 depicts the age structure of a typical shrinking population, created by reducing the productivity of female hares in a stable HGM population. The living structure is characterized by a shift to the right end of the age axis, a more gradual slope, and an increase in the average and maximum ages of the population in comparison to stable populations.
Changes in mortality and fertility may be caused by a variety of external and internal factors including, but not limited to, predation, disease, environmental degradation, interspecific competition, and climatic change. These factors can cause a
confound seemingly straightforward archaeological interpretations. Fortunately, living structures tend to depart from the ideal in predictable ways. Thus, if we know something about the basic ecological characteristics of the species, and have access to information on the archaeofaunal assemblages under investigation, it is possible to rule out some explanations.
The Effect of Human Hunting Pressure on Living Age Structures of Prey Human hunting pressure can be a major source of prey mortality. Increased hunting pressure by humans can push prey populations into growth mode, increasing the proportion of juveniles in the population, just like certain other mortality factors. The living structure of a population exposed to heavy hunting pressure is thus the same as depicted for a growing population (Figure 8.4). The power of hunting pressure to restructure prey living structures was demonstrated by the gazelle simulations in Chapter 6 (see Figures 6.6 and 6.7). The simulations showed gradual increases in the proportions of juveniles in the living age structures of the gazelle population with increased hunting pressure, and associated decreases in the average age of the population. In the gazelle simulation, adults and juveniles were hunted indiscriminately. If adults were preferentially selected, the bias toward juvenile animals under conditions of heavy exploitation would only be more pronounced.
Hunting Pressure and Rate of Population Turnover Although all populations exhibit the same basic responses to predator pressure, the intensity of a species response varies most closely with the rate of its population's
was shown in Chapters 6 and 7, populations with low rates of turnover (e.g., tortoises and gazelles) are easily depressed by hunting. These populations are much more sensitive indicators of hunting pressure than their more resilient counterparts. Populations with high rates of population turnover grow quickly and recover rapidly from bouts of even intensive hunting (Stiner et al. 1999, 2000). The living structures of these populations are not expected to change substantially except under extreme, prolonged exploitation.
Examples of pressured populations of fast-reproducing animals, such as European rabbits, do exist in the wildlife literature; some populations experience astronomical armual death tolls from the shooting of thousands of individuals each year (e.g., Ferreira and Guimaraes 1996). On the other hand, it is remarkable that these fast-growing populations can withstand such heavy hunting. Their tremendous resilience precludes them from effectively monitoring the scale of hunting intensity that might reasonably be expected to occur in the Natufian or any other Paleolithic period. For this reason, high-turnover species (e.g., partridges and hares) do not figure in the remaining discussion on prey age structures.
SEPARATING HUNTING PRESSURE FROM OTHER FACTORSHuman hunting pressure, seasonality and other sources of prey mortality play influential, yet predictable roles in determining the age composition of animal populations. As noted previously, however, some of these processes may have similar results. Factors most likely to be confounded with signs of hunting pressure include the
habitual hunting strategy, or a change in the mean size of the hunting group, may alter the age composition of archaeological death assemblage.
Seasonal Hunting The impact of seasonal hunting on living age structures is dependent on the reproductive cycle of a prey species. If reproduction is seasonally restricted, the wildlife literature can be consulted to determine the timing of its reproductive cycle in the region of interest and to estimate its living structure in different seasons. When possible, the season of site occupation, or at least the relative duration of occupation should also be established using independent measures — not an easy task. The expected age profiles for the season of occupation can then be compared against the observed mortality profiles to determine whether seasonality played an influential role in assemblage formation.
Multi-season occupation at a site may diminish the influence of seasonal biases by time averaging archaeological deposits, evening out biases introduced by seasonal factors.
This may be most true for Natufian among Paleolithic periods, but most specifically the Early Natufian.
It should also be noted that seasonal explanations are ineffective in the face of large-scale synchronic or diachronic shifts in prey age structures. In most landscapes, human settlement patterns must incorporate occupations from the full spectrum of seasons, but some localities may be used only in certain seasons. If seasonality is a primary factor in assemblage formation, then some intersite variation is expected in the
Human Hunting Preferences The strategies and preferences of human foragers also shape prey mortality profiles. Human hunters often intentionally target prey sub-groups based on age or sex, but they may also favor particular age groups as an unintended byproduct of a hunting strategy. Some hunting strategies (i.e., communal or encounter ambush techniques) are indiscriminate in their selection of prey age groups and may thus produce age structures broadly representative of the living population (Caughley 1977; Lyman 1987; Klein 1978, 1982; Stiner 1990, 1994 ).
Despite their influence over the formation of mortality profiles, human hunting strategies are not expected to confound the signature of hunting intensity significantly.
Hunting intensity will result in the inflation of juvenile individuals, resulting in juvenilebiased or U-shaped age structures (a.k.a. attritional, Caughley 1966; 1977). U-shaped age profiles display an inflated proportion of senile individuals and an underrepresentation of prime aged animals. When predation is the mortality factor, U-shaped patterns are created by hunting the weakest members of the population (the young and the old). This strategy is generally typical of cursorial predators; those that run down their prey.
Human foragers, however, only produce U-shaped or juvenile-biased age profiles under rare and limited circumstances (i.e., Nunamiut skin-hunting in spring, Binford 1978;
Stiner 1994). Following a thorough review of archaeological and anthropological data sets, Stiner (1991, 1994) concluded that hominids rarely if ever create U-shaped mortality patterns in ungulates. Scavenging, which is unknown as a dominant human foraging
produce U-shaped mortality profiles, but is not considered a reasonable option here.
Instead, human hunters almost universally ambush their prey using communal or solitary techniques, and a tendency to selectively target prime-aged animals is evidenced in many cultures with diverse technologies (Stiner 1990, 1994). Ambush strategies can be indiscriminate hunting techniques that rely on the element of surprise, in which case the resulting age structure (catastrophic or living structure) reflects the living structure of the original prey population. Ambush techniques can produce prime adult or juvenile biases, but this is most likely to occur if the prey live in age-sex segregated groups and/or form temporary calving herds. In sum, the effects of human hunting strategies differ from the anticipated effects of human hunting pressure, in that the latter only produces juvenilebiased or U-shaped living structures. In other words, the bias is highly directional in the case of hunting pressure. Prime-dominated mortality profiles for high-ranked prey are largely erased under conditions of heavy hunting pressure. Given the early evolution of human's tendency to favor prime adult artiodactyl prey (Stiner 1990, 1994), if hunting pressure is great enough to inflate the proportions of juveniles in the living structure during the Natufian, then prime-aged animals should not be sufficiently abundant to meet human demands. In the face of intense hunting pressure during the Natufian, humans are expected to shift their emphasis to lower-ranked juvenile animals. Two species are of interest here, because of their substantive role in Natufian diets; tortoises and gazelles.
Both reproduce slowly but they have very different ecological characteristics. Gazelles and tortoises are two of the highest ranked species in the Natufian diet and should have
Hunting Pressure and Gazelle Living Age Structures Gazelle populations have low rates of population turnover and slow population growth. These factors magnify the impact of hunting pressure on their populations. As simulated in Chapter 6 and demonstrated above, the proportion of juvenile gazelles should increase with the intensity of hunting pressure. Reconstructions of stable gazelle living structures carmot be directly applied to the archaeological record. The results of the gazelle simulations presented in Chapter 6 do, however, define the magnitude of change expected in the relative proportion of juveniles when their populations are subjected to increasing hunting intensity; the population is composed of up to two times as many juveniles when subjected to hunting, than when subject to only non-human agents of mortality. An intensification in gazelle procurement is thus expected to be recorded in the archaeological record as an increase in the relative proportion of juvenile gazelles.
Hunting Pressure and Tortoise Living Age Structures Unlike mammalian species, tortoises grow throughout much of their adult lives.
They invest more energy into growth during their first twenty years, but they continue to grow slowly thereafter (Blasco et al. 1986; Lambert 1982; Shine and Iverson 1995).
Male and female tortoises have different otonogenic growth patterns. During the first ten years of life females devote much of their energy to growth, while males invest somewhat more in early reproduction. Due to their head start, females are consistently larger than males from the same age cohort, and males almost never reach the full body size of
energetic returns, large adult females were likely preferentially removed firom tortoise populations by hunters. In fact they are more visible than young tortoises in the environments they inhabit.
Previously it was shown that tortoise populations are exceptionally sensitive to changes in human hunting pressure. Armual recruitment through natural growth and migration from adjacent populations can slow depletion, but it remains true that tortoise populations can withstand only low levels of hunting pressure, due to low productivity.
As tortoise populations are pushed into growth mode, inflation in the proportion of juveniles results. This process may be accelerated by the selective removal of large adults, most often females, which form the population's long-lived reproductive core.
Body size here serves as a proxy for individual age. Hunting pressure is expected to result in body-size diminution in tortoise populations due to the combined effects of inflated proportions of juveniles, selective removal of large females, and very slow individual growth/development rates (Klein and Cruz-Uribe 2000; Stiner et al. 2000).
Although tortoises reproduce seasonally, their age structures show surprising stability, because of low rates of population turnover, low adult mortality; and high rates of juvenile mortality. Females lay several eggs each year, but most hatchlings do not survive beyond their first month (Doak et al. 1994, for Gophenis populations). Low population turnover and long life spans translate into high proportions of adults in stable populations. Annual recruitment therefore comprises only a small proportion of the total
of the remaining population. Seasonal birth patterns in tortoises are thus not expected to cause significant fluctuations in tortoise body size.
Recently, Speth and Tchemov (n.d) have suggested that seasonality may influence tortoise living structures in a different way. They cite a study of a Spanish T. gracea population (Diaz-Paniagua et al. 1995, 1996) that displays seasonal variation in the activity rates of males and females; females are more active than males in the early summer, and males are more active in late vi^inter and spring. If human hunters are more likely to capture active tortoises, then the sex ratio of the hunted population will differ by season. Because males and females differ significantly in size, assemblages captured in different seasons may also differ in average size (of course the differences in activity cycles cannot be too great, or males and females will never meet). Though no good evidence exists for differential activity patterns in Levantine tortoises (Speth and Tchemov n.d.), the potential for seasonal biases will be evaluated following the presentation of results. Seasonal biases can be largely ruled out in the face of universal or long-term unidirectional trends in tortoise body size that make very little sense as products of seasonal change.