Models developed for population conservation and management have historically focused on either population dynamics (Morris and Doak 2002
) or threshold numbers required to avoid genetic problems (Franklin 1980
). The population ecology and population genetics of a species are frequently not integrated in these models. When they are, the demographic effects of inbreeding are often based on data from captive animals and/or program defaults (e.g., Brito 2009
; Maehr et al. 2002
; Nilsson 2004
). An important first step towards developing integrated models is determining the functional relationship between measures of inbreeding and juvenile survival in the wild, especially for managed populations.
Overall, the model-averaged estimate of annual survival probability of Florida panther kittens was 0.323 ± 0.071; this estimate is lower than those reported for western North American populations of pumas (range: 0.44 to 0.72; Lambert et al. 2006
; Laundré et al. 2007
; Logan and Sweanor 2001
; Robinson et al. 2008
). Our estimate of kitten survival also is substantially lower than those used in earlier demographic analyses of the Florida panther (Maehr and Caddick 1995
: 0.84 - 0.87; Maehr et al. 2002
: 0.735 - 1.0; Root 2004
: 0.62). Although our estimates of kitten survival varied slightly depending on the model (0.30 to 0.34; ), they were consistently lower than those used in previous analyses. Furthermore, kittens were not tagged immediately after birth, and we could have missed kittens that had died before they were PIT-tagged at the den site; thus, true survival may be slightly lower. However, since most of our data on kitten survival came from a period of population growth (McBride et al. 2008
), we have no reason to believe that the kitten survival rate for this population is unsustainable. Ongoing work developing updated population models for Florida panthers may shed additional light on the role of kitten survival in determining population dynamics.
Survival rates did not differ between male and female kittens. This result was as expected due to the lack of sexual dimorphism among kittens, although some authors have suggested sex-specific differences in survival of puma kittens (Logan and Sweanor 2001
). There was no evidence that litter size influenced survival of kittens. This may reflect the possibility that females most able to successfully raise large litters are those most likely to produce them.
Perhaps the most dramatic panther conservation measure undertaken to date is the release of 8 female Texas pumas into the Florida panther population in 1995 (Seal 1994
). Genetic introgression has been highly controversial (e.g., Maehr and Caddick 1995
; Maehr et al. 2006
; Pimm et al. 2006
). Pimm et al. (2006)
reviewed the arguments for and against genetic introgression in general, and in the Florida panther population in particular. They reported that admixed kittens are more than three times as likely to survive as purebred Florida panthers. Pimm et al’s (2006)
conclusions were based solely on the proportion of kittens PIT-tagged in the den that were subsequently captured and (generally) radio-collared. These kittens were recaptured at ages ranging from 6.5 months to 3.5 years and detection probability was not taken into account, so it is unclear how well these ratios represent differences in kitten survival.
In the present study, we used multiple sources of field data, a thorough determination of ancestry of nearly all kittens captured since genetic introgression occurred in 1995, and a statistically rigorous live-recapture dead-recovery modeling approach that takes detection probabilities into account to test for the effect of genetic introgression on the survival of Florida panther kittens. As recommended by Creel (2006)
, we limited this portion of the analysis to a period when both canonical and admixed kittens were on the landscape. We also were able to examine the effects of heterozygosity, which differed dramatically between canonical Florida panthers (0.161) and Texas pumas (0.318; Driscoll et al. 2002
; W. Johnson, unpubl. data). Furthermore, we were able to determine ancestry of panther kittens at a much finer scale than Pimm et al. (2006)
. We found that admixed kittens generally survived better than kittens born to canonical Florida panthers. Furthermore, F1
admixed kittens had the highest survival, followed by backcrossed to Texas kittens (). Finally, we found that average heterozygosity positively influenced survival of Florida panther kittens (). These results provide unambiguous evidence for the positive, population-level impact of genetic introgression.
Index of panther abundance negatively influenced kitten survival, which is suggestive of a density dependent effect. This could result from infanticide by sub-adult and adult males during territorial disputes or for mating opportunities (Garrison et al. 2007
; Logan and Sweanor 2001
; Packer et al. 2009
), from deaths of the dependent kittens’ dams due to intraspecific aggression, or from competition for food or other resources. We note, however, that our results are based on an index of abundance, and thus may not be sufficient to conclusively demonstrate density-dependent influence on kitten survival. Interestingly, a substantial proportion of temporal variation in kitten survival was explained by temporal variance in index of abundance.
Analysis of data organized with 3-month time intervals revealed no evidence for seasonal (i.e., wet vs. dry) variation in survival of kittens. However, we found that survival was lowest during the first 3 months of life, which is consistent with other large carnivore populations (Garrison et al. 2007
; Logan and Sweanor 2001
). These results point to the fact that young kittens are particularly vulnerable to various mortality factors.
Estimating survival of young can be challenging in elusive carnivores because they are rarely observed. Additionally, in the case of panthers, kittens are typically too small to be fitted with radio-transmitters when sampled at the den site – a method that would be effective for monitoring their fate during the first year of life. Furthermore, the small population size of endangered species and invariably small sample sizes may not be adequate for rigorous estimates of survival of the young. For example, estimates of kitten survival that did not include litter failure data lacked robustness as well as precision; models that were similarly ranked based on QAICc
provided very different estimates of kitten survival (Appendix C
). We addressed this challenge by using a modeling framework that can utilize multiple data sources. Resulting estimates of kitten survival were more robust and precise, and also allowed us to test various hypotheses regarding factors influencing survival of Florida panther kittens. Similar approaches can be used to estimate survival of young in other elusive species that occur in low numbers.
Our study is the first to provide rigorous estimates of survival of Florida panther kittens. We found that kitten survival in our study population was lower than reported for western cougars, and substantially lower than values used in previous demographic analyses of Florida panthers. These results are consistent with an earlier suggestion that results of a previous PVA (Maehr et al. 2002
) that assumed kitten survival of up to 87% may be unreliable (Beier et al. 2003
). Finally, we found strong evidence that survival of admixed (particularly, F1
) kittens was substantially higher than that of purebred kittens, and that heterozygosity positively influenced survival of Florida panther kittens. These results are consistent with hybrid vigor theory (Crow 1948
; Frankham et al. 2002
), and suggest that genetic introgression had positive population-level effects in our study population. Although there are concerns that genetic introgression can have undesirable or even negative impacts on populations (Edmands 2007
; Maehr and Caddick 1995
; Shields 1993
), our analyses revealed beneficial results of deliberate introgression for this population. A combination of these results with other recently published or on-going assessments of demography pre- and post-introgression should assist in directing management and research priorities that will ensure the continued persistence the critically endangered Florida panther.