Long‐term effects of tetanus toxoid inoculation on the demography and life expectancy of the Cayo Santiago rhesus macaques

Abstract

Tetanus was a major cause of mortality in the free-ranging population of rhesus monkeys on Cayo Santiago prior to 1985 when the entire colony was given its first dose of tetanus toxoid. The immediate reduction in mortality that followed tetanus toxoid inoculation (TTI) has been documented, but the long-term demographic effects of eliminating tetanus infections have not. This study uses the Cayo Santiago demographic database to construct comparative life tables 12 years before, and 12 years after, TTI. Life tables and matrix projection models are used to test for differences in: (i) survival among all individuals as well as among social groups, (ii) long-term fitness of the population, (iii) age distribution, (iv) reproductive value, and (v) life expectancy. A retrospective life table response experiment (LTRE) was performed to determine which life cycle transition contributed most to observed changes in long-term fitness of the population post-TTI. Elimination of clinical tetanus infections through mass inoculation improved the health and well-being of the monkeys. It also profoundly affected the population by increasing survivorship and long-term fitness, decreasing the differences in survival rates among social groups, shifting the population’s age distribution towards older individuals, and increasing reproductive value and life expectancy. These findings are significant because they demonstrate the long-term effects of eradicating a major cause of mortality at a single point in time on survival, reproduction, and overall demography of a naturalistic population of primates.

Drivers of Diversification in Individual Life Courses

“…Changes in density are clearly identifiable because it is a closed island population. More importantly, this primate population exhibits a high annual population growth rate (λ > 1:10; Rawlins and Kessler 1986; Blomquist et al 2011; Kessler et al 2015) and exists frequently at relatively high densities where population dynamics are regulated by reduced reproduction (Hernández-Pacheco et al 2013, 2016a). …”

A 75‐year pictorial history of the Cayo Santiago rhesus monkey colony

“…This was one of the first times that a disease was eradicated from any primate population, including human [Southwick, 1989]. Thirty years later, it was shown that the tetanus vaccination program caused significant long-term effects on the demographics of the population not only affecting mortality, but life expectancy and fecundity as well [Kessler et al, 2014]. …”

Managing the Cayo Santiago rhesus macaque population: The role of density

“…Animals are provisioned with 0.23/kg/animal/day of commercial monkey chow and water is available ad libitum. Veterinary intervention is minimal except during the annual trapping period when yearlings are captured, marked for identification, physical samples are collected (including blood samples for paternity testing), and tetanus inoculation is given [Kessler et al, 2014]. Severely injured animals are either removed from the island for treatment or euthanized in accordance with guidelines of the American Veterinary Medical Association.…”

“…It also establishes the importance of addressing density effects to optimize management strategies for the population. Given the expected reproductive value post-tetanus toxoid inoculation of the colony [Kessler et al, 2014] and density-dependence in fertility among adult females [Hernández-Pacheco et al, 2013], we hypothesize that the annual removal of adult females, and not immature individuals, will have the highest impact on the population growth and that the exponential growth following past removal events is the result of increased female fertility rates.…”