Research Article |
Corresponding author: Agustin M. Abba ( abbaam@yahoo.com.ar ) Academic editor: Diego Astúa
© 2017 Agustin M. Abba, Veronica V. Benitez, Santiago R. Doyle.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Abba AM, Benitez VV, Doyle SR (2017) Population ecology of Chaetophractus vellerosus: the first report for an armadillo in South America. Zoologia 34: 1-7. https://doi.org/10.3897/zoologia.34.e20785
|
The aim of this work was to obtain the first estimates of survival rates (S), capture probability (p) and life expectancy for armadillos in South America by analyzing capture-mark-recapture data obtained from a population of Chaetophractus vellerosus (Gray, 1865) located in Magdalena, Buenos Aires, Argentina. From June 2006 to June 2011, we conducted 16 field surveys that resulted in 365 capture events of 152 adult C. vellerosus. For the survival analysis we used a Cormack-Jolly-Seber (CJS) modelling framework. Interannual variation in S made an important contribution to overall variation in the survival rate of C. vellerosus. The average life expectancy for females and males after attaining sexual maturity was estimated at 1.70 and 1.65 years respectively. The period of lowest survival probability was associated with dry seasons that might have affected the availability of food. This study provides the first estimates of demographic parameters for xenarthrans in South America.
Argentina, capture probability, CMR, screaming hairy armadillo, survival rates
Armadillos are one of the most distinctive mammals of South America, and the only group that originated in this continent. Although these singular animals have attracted the attention of travelers and naturalists since the 15th century, the development of scientific studies on their ecology and other topics is very scarce (
Chaetophractus vellerosus (Gray, 1865) is an armadillo that has an considerable amount of data available on its diet, home range, thermoregulation, activity pattern, distribution, genetics, and physiology (
The aim of this work was to obtain the first estimates of survival rates for armadillos in South America by analyzing capture-mark-recapture (CMR) data obtained between 2006 and 2011 for adult individuals from a portion of the isolated population of C. vellerosus that was located in Magdalena, Buenos Aires province (Argentina), and to establish the effect of sex and interannual variation on adult survival rates.
(1) Geographical range of Chaetophractus vellerosus and location of the relict population in Magdalena, Buenos Aires Province. Map was extracted from IUCN SSC Anteater, Sloth and Armadillo Specialist Group, C. vellerosus. The IUCN Red List of Threatened Species. (2) Satellite image of the fields where the armadillos were captured. The dotted line represents the boundaries of the study area. Scale bar: 250 m.
We sought to capture armadillos within a 100 ha area located in an agricultural field (Figs
For each survey, three field observers walked 30-m-wide transects until the entire sampling area was covered; total sampling effort for all surveys was 61 days (4–5 days per survey) and 1500 hours of field time. During surveys, we attempted to capture and process (i.e., measured and marked) all armadillos that were detected during a survey. All burrows with signs of recent activity (e.g., accumulation of grass, soil that had been removed, etc.) were sampled to check for the presence of armadillos; this check consisted of a visual inspection of the initial 50 cm of the burrow. Animals were captured by hand or in a net and burrows were checked by hand or, on occasion, by opening the first 50 cm with a shovel. During the first two years of the study the ears of the animals were marked with numbered ear tags (National Band and Tag Company, Newport, KY, #1005-1); subsequently animals were marked using a passive transponder system (Trovan ID-100). For temporary identification we used a sticker affixed to the carapace, which allowed us to follow the animals after release and avoid recapturing them again on the same day. Age was estimated using body length and weight (
We used a Cormack-Jolly-Seber (CJS) modelling framework (
The CJS model was chosen because its assumptions best fit some important features of our study system. CJS models do not take into account migration, i.e., they assume a geographically closed population (
The set of candidate models was constructed as follows. We included the sex of individuals and the year in which a survey was conducted as potential effects to account for variation in survival rate. Capture effort was not constant among field surveys, as previously noted. Thus, to account for variation in capture probability, in addition to the sex of individuals as a potential effect, we included an indicator of the campaign for each field survey. Candidate models were constructed that included independent effects on survival rates and capture probability, and that considered both first-order effects and interactions between all effects. In order to explore the effect of interannual variation on survival rates, we considered time periods of one year that started and ended in winter (i.e., June of one year to July of the following year). However, this was problematic for the two-year period between June 2007 to June 2009. We therefore decided to retain the 2008 data, but to make only one estimate of survival probability for the entire 2007–2009 period. We felt this was reasonable because the low number of captures in the 2008 field survey, where only seven adult individuals were caught, provided insufficient data to make reliable estimates of survival probability for a one-year period.
CJS models are based on the binomial distribution, and as such do not independently model mean and variance. When fitting a capture-recapture model, it is not uncommon that observed variance is greater than expected, a phenomenon known as overdispersion (
Model selection was performed using an information-theoretic approach following
The CJS models were fitted using program MARK (
A total of 152 adult individuals were caught, 82 females and 70 males, in a total of 365 capture events (Table
In order to examine the effect of interannual variation on survival probability, a multi-model inference scheme was followed. The estimated annual survival probabilities were similar between sexes, but strongly varied between study periods (Fig.
The projected survivorship curves for C. vellerosus adult individuals over the entire study period showed a similar pattern for both sexes (Fig.
Descriptive summary of annual field surveys for Chaetophractus vellerosus in Magdalena, Buenos Aires Province, Argentina.
2006–2007 | 2007–2009 | 2009–2010 | 2010–2011 | |
---|---|---|---|---|
Adults individuals captured | 74 | 76 | 29 | 34 |
Sex | 43♀–31♂ | 40♀–36♂ | 16♀–13♂ | 17♀–17♂ |
Capture events | 117 | 131 | 43 | 74 |
Sampling effort (days) | 16 | 21 | 10 | 14 |
Mean values (± SE) of weekly capture probability (p) for adult Chaetophractus vellerosus for each field survey, estimated by multi-model inference based on the set of candidate CJS models. Values are presented for both sexes combined because differences in capture probability between males and females were in all cases < 3%.
CJS models fitted to the capture-recapture data of Chaetophractus vellerosus, ranked according to the Akaike information criterion. Note that only the top 9 out of 25 candidate models are presented, which collectively accounted for >0.99 of the cumulative Akaike weight. Model structure is indicated by listing between parentheses which variables affect survival rates (S) and capture probability (p), using a single point (.) when no variable has an effect.
Model | Parameters | Deviance | AICc | ΔAICc | Model likelihood | AICc Weight | |
---|---|---|---|---|---|---|---|
1 | S (year) p (campaign) | 19 | 289.6 | 636.2 | – | 1.00 | 0.40 |
2 | S (.) p (campaign) | 16 | 298.7 | 638.4 | 2.18 | 0.34 | 0.13 |
3 | S (year) p (sex + campaign) | 20 | 289.4 | 638.5 | 2.21 | 0.33 | 0.13 |
4 | S (sex + year) p (campaign) | 20 | 289.5 | 638.6 | 2.32 | 0.31 | 0.12 |
5 | S (sex x year) p (campaign) | 23 | 284.2 | 640.4 | 4.16 | 0.12 | 0.05 |
6 | S (sex + year) p (sex + campaign) | 21 | 289.2 | 640.6 | 4.35 | 0.11 | 0.05 |
7 | S (.) p (sex + campaign) | 17 | 298.7 | 640.7 | 4.44 | 0.11 | 0.04 |
8 | S (sex) p (campaign) | 17 | 298.7 | 640.7 | 4.46 | 0.11 | 0.04 |
9 | S (sex x year) p (sex + campaign) | 24 | 283.8 | 642.5 | 6.21 | 0.04 | 0.02 |
Projection of a survivorship curve for Chaetophractus vellerosus adult individuals of both sexes, assuming constant survival, from an age of nine months old, when sex maturity is achieved (
This study provides the first estimates of demographic parameters for xenarthrans in South America, and just the second study of the population ecology of armadillos overall (and, importantly, the first for C. vellerosus). Survivorship curves for males and females were presented and are only the second to have been calculated for any species of armadillo.
As previously pointed out, the only study dealing with the population ecology of xenarthrans was performed by
On the other hand, the capture probability (p) of D. novemcinctus (
In the present study the influence of sex on survival probability was not significant compared with temporal variability. This is consistent with a range of studies conducted previously that have found few differences between male and female C. vellerosus in traits such as morphology, diet, home range, etc. (
The survivorship estimate for C. vellerosus presented here can be compared with that provided by
The results of this work suggest that the survival rate is similar for adult individuals of both sexes, and that temporal variability is the main driver of variation in the survival of C. vellerosus. Therefore, it will be important for future studies to both confirm these trends, and to unravel the influences of environmental variables such as precipitation, temperature and atmospheric pressure that seem to be important factors in C. vellerosus biology (
We thank L.G. Pagano and M.C. Ezquiaga for their assistance during fieldwork, and Lic. Ana Teresa Gómez (Jefe Departamento, CIM, SMN) for her support with the climatic data (SMN, Exp_144540). We appreciate the improvements in English usage made by W. J. Loughry and the valuable comments. We also want to thank two anonymous reviewers for valuable input that helped us to improve the manuscript. This work was partially supported by the Agencia Nacional de Promoción Científica y Tecnológica (BID PICT2010-1412) and Universidad Nacional de La Plata (PPID/N004).