North American Tree Rings, Climatic Extremes, and Social Disasters - Dendroclimatology - page 317

by unusual wetness, has been witnessed during other infectious disease events
(Epstein 2002 ) , including the hantavirus outbreaks on the Colorado Plateau in 1993
and 1998 (Hjelle and Glass 2000 ) . In the case of hantavirus, the incidence of infec-
tion in the rodent host is believed to have been magnified by a population bottleneck
during drought. During the subsequent wet conditions, rodent populations expanded
and infection was spread to human populations through rodent excreta. The agent
responsible for the cocoliztli epidemics of sixteenth-century Mexico has not been
identified. However, the tree-ring data suggest that extreme climate conditions may
have magnified the impact of these disease catastrophes.
10.4 Suspected Social Impacts of Drought Extremes During
the Precolonial Era
The native populations of Mesoamerica developed calendrical and hieroglyphic
writing systems centuries before the arrival of Cortez. The Aztec calendar was based
on the combination of a 260-day religious calendar and a 360-day solar calendar.
The Aztec year was divided into 18 'months,' each 20 days long, leaving 5 days
each year that were not included in the formal calendar and were considered bad
luck by the superstitious Aztecs (Caso 1971 ; Keber 1995 ) . The religious and solar
calendars rotated through all 18,980 unique daily combinations, resulting in one
complete cycle of the two counting systems every 52 years. Each year of the 52-year
cycle was identified by one of four possible iconic symbols, which were rabbit,
reed, flint knife, and house (Keber 1995 ) . The individual years were then numbered
consecutively as follows: the year One Rabbit, Two Reed, Three Flint Knife, Four
House, Five Rabbit, Six Reed, Seven Flint Knife, Eight House, Nine Rabbit, etc.,
until the 52-year cycle was completed with the year Thirteen House. The sequential
order of each unique 52-year cycle is not obvious from the Aztec calendar alone,
but the sequence of cycles was specified by the Aztec scribes according to royal
succession and major political events. Each cycle was then related to the Julian cal-
endar by Jesuits and surviving Aztec scribes during the mid-sixteenth century, so
that every year of Aztec traditional history can be tentatively linked to a specific
year in the Western calendar, especially during the 14 th ,15 th , and 16 th centuries
preceding Conquest.
The Aztecs recorded notable political, celestial, and environmental events with
pictorial images linked to specific calendar year signs in ancient diaries known as
codices. Codices were prepared by scribes for each city-state of the Aztec empire,
but they were considered blasphemous by the Spaniards, and most were destroyed
soon after the conquest (Keber 1995 ) . Nevertheless, a few important codices survive
and with them fragments of recorded Aztec history. Therrell et al. ( 2004 ) noted
13 events specifically identified in the codices as dry years and used independent
tree-ring data from Mexico to substantiate most of these Aztec droughts.
Perhaps the most extreme drought of the prehispanic Aztec era occurred in the
year of One Rabbit in 1454, for which the codices indicate parched fields, wilted
crops, and human corpses littering the ground (Therrell et al. 2004 ) . The tree-ring