W. Thomas Jones

Philopatry, Dispersal, and Habitat Saturation in the Banner-Tailed Kanagaroo Rat, Dipodomys Spectabilis

We report natal dispersal distances for 331 banner—tailed kangaroo rats (Dipodomys spectabilis) at high and low population density. The data were collected in 29 censuses of marked individuals in two populations over 8 yr. One population underwent four—fold variation in density. The other was consistently at high density. Because the number of burrow systems (i.e., breeding sites) remained fairly constant, we interpret changes in density as changes in degree of habitat saturation. Dispersal distances were shorter and the proportion of philopatric young (moving less than a home range diameter) was greater at high density than at low density. This result does not conform to models of dispersal based on studies of cycling small mammals or to a simple competition model of dispersal. All data are consistent with a habitat saturation model, which holds that high—density saturated conditions favor philopatric tendencies in offspring. See full-text article at JSTOR

Sex ratio and host size in a parasitoid wasp

SummaryParasitoid wasps often lay male eggs in small hosts and female eggs in larger hosts. The selective advantage of this strategy can be explained by assuming wasp fitness increases with host size and that this fitness increase is greater in females than in males. I conducted experiments to test a model based on this explanation and found the results generally supported the model with one exception; unlike what the model assumed, these wasps were unable to adjust their offspring sex ratios in each generation to different host size distributions. This finding suggests an alternate view as to how selection might operate in the evolution of parasitoid sex ratios.

Natal philopatry in bannertailed kangaroo rats

SummaryI describe prolonged retention of offspring in natal home ranges, or natal philopatry, in the bannertailed kangaroo rat Dipodomys spectabilis. Though weaning occurs at about one month of age, offspring shared natal burrows with their mothers for three to seven months, and 39% of surviving offspring remained within natal home ranges through reproductive maturity. Males as well as females were philopatric. Data on the availability of burrows and on patterns of resettlement suggest that natal philopatry in this species may be a means of providing juveniles with access to essential resources, in this case food caches and large complex burrow systems, that are not readily available outside natal home ranges.

Survivorship in Philopatric and Dispersing Kangaroo Rats (Dipodomys Spectabilis)

I use long—term mark—recapture data for banner—tailed kangaroo rats (Dipodomys spectabilis) to assess survival of philopatric and dispersing individuals. Results for 147 juveniles indicate that individuals that acquired parental resources, in this case large complex burrow systems and food catches, were significantly more likely to survive to reproductive age than individuals that successfully dispersed but did not acquire parental resources. Survival of juveniles that left their natal burrows and moved to other burrows was independent of the distance moved, which suggests that familiarity with natal areas and proximity to relatives impart no survival advantage in this species. Of the 37 juveniles that acquired natal burrows, 5 males and 7 females did so after their mothers abandoned those burrows. The frequency of burrow abandonment by adult females was independent of maternal age, and survival of abandoning mothers did not differ significantly from that of nonabandoning females See full-text article at ...

Dispersal distance and the range of nightly movements in Merriam's kangaroo rats

Lifetime dispersal distances in Merriams kangaroo rat, Dipodomys merriami , ranged from 0 to 265 m in males and from 0 to 158 m in females. Radiotelemetry data on nightly movements detected males ≤146 m and females ≤123 m from home-range centers. Eighty-nine percent of lifetime dispersal distances and 96% of moves between consecutive censuses were within the radii of familiarity indicated by radiotelemetry. Most effective “dispersal” appears to occur as a series of shifts in centers of activity within familiar areas. Occasional excursions beyond the usual area of activity and frequent moves to new burrows may contribute to the ability of the D. merriami to shift home ranges. The traditional view of dispersal as high-risk movement into unfamiliar areas does not apply to effective dispersal in D. merriami . Neither is dispersal in this species described precisely by models in which investigators assumed that individuals move to the nearest vacant site at a particular age or season.

Survival and reproductive effort in banner-tailed kangaroo rats

Nine years of demographic data for two unmanipulated populations of ban- ner-tailed kangaroo rats (Dipodomys spectabilis) in Arizona provide measures of survival, fecundity, age structure, and density. We use these data to search for costs of reproduction in two important components of fitness: survival and future fecundity. We also examine the interaction effects among reproductive effort, age, sex, and density on survival and future fecundity. None of the analyses reveals a significant negative effect of reproductive effort on survival or future fecundity. This result may be seen as evidence that reproduction has no measureable costs in fitness for bannertails. However, another explanation is that bannertails adjust their reproductive effort to individual circumstances so as to minimize costs.

Heritability of dispersal in banner-tailed kangaroo rats, Dipodomys spectabilis

Abstract In a wild population of banner-tailed kangaroo rats, heritability of dispersal was estimated using two measures of dispersal tendency: distance moved from the natal site and likelihood of leaving the natal home range. Neither of the heritability measures was significantly different from 0. The results indicate that the main causes of variation in dispersal behaviour in this species are environmental, and suggest that there is no class of ‘innate’ dispersers. The possibility cannot be excluded that this population retains modest additive genetic variance for dispersal tendency. The survival consequences of dispersal in kangaroo rats are known to depend on population density and to change significantly between years, so that selection should maintain genetic variation in dispersal tendencies. Modest genetic variation for dispersal tendencies, especially if dispersal is a conditional trait, will be extremely difficult to detect in field studies.

Variation in Sex Ratios of Banner-Tailed Kangaroo Rats in Relation to Population Density

Over a 5-year period, sex ratios of juvenile Dipodomys spectabilis varied significantly at one study site, whereas there was a recurrent and significant male bias at a second site. Population density was variable at the first site, but at the second site it was consistently at high levels. The proportion of males was correlated positively with population density. Demographic and dispersal data revealed no variation in competition for local resources or in differential mortality that ultimately could account for the variation in sex ratio. However, male weights relative to female weights at the end of parental care were lower where sex ratios were male-biased, suggesting a correspondence between male-biased sex ratios and relatively cheaper males. The adjustment to male-biased sex ratios apparently occurred in utero or during lactation. A hypothesis that might explain the variation in sex ratio is that it is an adaptive response either to variation in the relative costs of producing sons and daughters or to variation in the life-history expectations of sons and daughters that occurs with variable population density. Another hypothesis is that competition among siblings results in higher mortality in one sex, and this effect is more pronounced at higher population density.