Inbreeding in captivity has apparently reduced the body size of Mexican wolves. One wonders if the concomitant reduction in the fitness of individuals has been at least partially responsible for the difficulties faced thusfar in the Mexican wolf reintroduction program. The authors' findings suggest that cross-lineage matings should be encouraged, since this may contribute to the genetically-restored fitness of the subspecies. Obviously, the genetic background of future candidates for release into the wild should be a determinant -- JM
The Mexican gray wolf (Canis lupus baileyi) appears to be extinct in the wild and exists now only in captivity and in a single, small reintroduced population. A recent study of captive animals found no evidence for inbreeding depression in juvenile viability or litter size. We examined the relationship between inbreeding and body weight in captive wolves. We found that captive wolves with little or no known inbreeding had lower body size than wild-caught wolves. In addition, captive wolves with higher inbreeding had lower body size than captive wolves with little or no inbreeding. The captive population was descended from three founders until two other lineages, each descended from two founders, were recently added to the population. Consequently we examined the potential statistical power associated with future comparisons of body weights between inbred wolves and the offspring of cross-lineage matings. In this case it appears likely that in the next few years there will be an adequate sample size to statistically evaluate differences in body size between these groups.
Minimizing inbreeding and the loss of genetic variation are major concerns of captive breeding programs for endangered species. Increases in inbreeding have correlated with decreases in juvenile survival in captivity in a number of different species. In addition, a number of recent studies in wild populations have demonstrated that a reduction in genetic variation resulting from inbreeding and/or genetic drift has been associated with the decay of phenotypic attributes related to fitness in wild populations....
Mexican wolves (Canis lupus baileyi), the southern-most subspecies of gray wolf, currently exist primarily as a captive population numbering about 200 individuals. Other than a small, recently reintroduced population in Arizona and New Mexico, there have been no confirmed sightings of wild Mexican wolves in more than 20 years. The current population of wolves originated from three independent captive lineages with each having only two or three founders captured during the 1960s and 1970s from northern Mexico and southern Arizona. Because of the few founders, each of the lineages has become inbred in captivity. Following a genetic evaluation of the three lineages in which no evidence of coyote (Canis latrans), or dog (Canis familiaris), ancestry was found, the McBride, Ghost Ranch and Aragon lineages were recently combined to form a single population with seven founders.
The most common phenotypic trait used to evaluate the effects of fitness in captive populations of endangered species has been juvenile survival; secondarily, reproductive traits, such as litter size, have been used. Kalinowski et al. (1999), however, recently found no effect of inbreeding on juvenile survival and litter size in captive Mexican wolves. The only other trait for which a substantial amount of data currently exists for captive Mexican wolves is adult body size measured by mass. Some body size data is also available from historical wild-caught wolves. For these reasons and others, body size has been proposed as a phenotypic indicator with which to monitor the effects of merging the three lineages. Because until recently there had been no breeding between the three lineages, offspring from crosses between lineages will be free of inbreeding. If inbreeding has negatively impacted body size or other fitness related traits then individuals free of inbreeding may show changes in these traits. Except for a few offspring that have resulted from recent crosses between the lineages, all Mexican wolves alive today are inbred to some extent.
For wolves, changes in body size resulting from inbreeding in captivity may result in reduced fitnesses of individuals reintroduced into the wild. This may consequently result in a lower viability of wild, reestablished populations because body size is important in successful prey capture....
Here we look for evidence of an effect of inbreeding on adult body size in captive Mexican wolves and evaluate the utility of this trait for detecting potential future changes in size of offspring from cross-lineage matings. Specifically we ask (1) whether historic wild-caught adult wolves were larger than captive adult wolves with little or no inbreeding and (2) whether wolves in the McBride captive lineage with little or no inbreeding were larger than adults later in the lineage with greater amounts of inbreeding....
MATERIALS AND METHODS
For comparisons involving historic, wild wolves we used the masses of 28 wild Mexican wolves killed in Chihuahua and Durango, Mexico during the 1970s as part of control efforts .... For comparisons involving captive wolves, we used 71 masses from 26 male and 24 female adult wolves from the McBride captive lineage. Two of these wolves were wild-caught, and one was a lineage founder .... Of the three lineages only the McBride lineage was managed from its inception as part of a captive breeding program; the weight data used here was taken during the course of these activities. Because the other lineages were not similarly managed until more recently, similar data for the Ghost Ranch and Aragon lineages do not exist.
Because the level of inbreeding in the captive wolves varied we split the wolves into two groups based on the level of inbreeding in individuals. Wolves with no inbreeding in captivity and wolves comprising the first generation of inbred individuals were combined to form a group with little or no inbreeding, and wolves with greater levels of inbreeding were combined to form the second group....
RESULTS AND DISCUSSION
Comparisons using existing data
For wild and captive wolves combined, female body sizes ranged from 20.2 to 37.4 kg, while male sizes ranged from 21.8 to 41.3 kg. Historic wild male and female wolves were significantly larger than wolves in the McBride lineage with little or no inbreeding. Even with the limited data currently available, the mean size of inbred female wolves was significantly smaller than those of females with little or no inbreeding, however, there was not a significant difference between males....
Although inbreeding did not appear to have an effect on viability and litter size in Mexican wolves, our results provide some evidence of change in body size that may be manifestations of inbreeding depression. Conclusions from these small samples, however, may or may not be supported when larger samples from inbred wolves in the McBride lineage become available .... Masses from additional wolves with little or no inbreeding in the McBride lineage will not be available because most of these individuals are dead and no such matings are now possible.
Comparisons using future data
....Differences...between inbred wolves and the offspring from cross-lineage matings will likely be detectable, should they occur, with the sample sizes likely available in the next few years .... [A]ssuming several litters of cross-lineage wolves are produced per year, enough data should soon be available to evaluate the effect of merging the lineages on body size. It will be interesting to determine if crossing to other lineages will genetically restore fitness-related traits in Mexican wolves, as found in recent studies of other species.
Table 1: Sample sizes and mean body sizes (kg) for historic, wild Mexican wolves and captive wolves by level of inbreeding.