By: A.B Marr, P. Arcese, W.M. Hochachka, J.M Reid and L.F Keller

Inbreeding depression is the reduced fitness of a given population as a result of breeding in related individuals. - This can affect the size and persistence of a small, relatively isolated population. Environmental stressors may exacerbate inbreeding depressions if genes that affect stress tolerance are also affected by inbreeding depression or if selection against deleterious mutations is more intense under stressful conditions. Study conducted to test for influence of environmental stressors on reproductive strategies known to be affected by inbreeding depression. -“Stressful” environmental conditions are those which reduce mean trait values for fitness components.

Used data from the long-term study of song sparrows Melospiza melodia on Mandarte island. Calculated Wright’s coefficient, F, to describe the probability that two alleles at a randomly chosen locus will be identical to ancestor shared by mother and father. -F=0 completely out bred birds; F=0.06 mating between cousins; F=0.25 mating between full siblings with no previous relationship. Four reproductive traits tested for effect of inbreeding and environmental stress. - Male mating success, fledgling survival, laying date for females, and hatching success. -Paternal F predictor of inbreeding depression in fledgling survival, maternal F predictor of inbreeding depression of hatching success.

Earlier Known Knowledge Females typically produce two to three nests annually. About 40% of females had more than one social mate within a season. Most successful males attracted one female at a time, although 4% had two or more females nesting in their territory at one time. Males without territories or females were also included in data results. Results Males fathered fewer nests annually when there were more males in the population. (Decreased # of males per female) Inbred males fathered fewer nests as well. - Highly inbred males fathered 32% fewer nests than out bred males.

To asses survival, offspring were counted at days 12 and 24 by listening for begging calls and observation. -Fledglings beg more often during rainy periods. Measured average daily rainfall in 2-, 3-, and 4-day intervals. Results Fledgling survival was lower on rainy periods than on dry periods. -The average daily rainfall during the rainiest three day interval showed the most variation in fledgling survival. The rainiest two day interval was supported as well. Highly inbred fathers yielded offspring that survived 28.4% worse in extreme environmental stress than offspring yielded by an out bred male. This shows a strong correlation between inbreeding depression and environmental stress.

Temperature can influence the onset of breeding in birds. -Females which began breeding earlier had more nests, fledglings and recruits than females initiating breeding later in the year. Measured daily average air temperature for six one month time intervals and correlated that to female breeding period. Results Inbred females laid their eggs later in the spring when temperatures were cooler. - When environmental stressors of colder weather were established, there was a smaller difference in inbreeding depression and out bred female fitness. -Evidence that environmental stress has direct interactions with inbreeding depression. Differences in laying date between inbred and out bred birds was greater under less stressful conditions of warmer springs.

Poor hatching success sometimes coincided with rain storms or periods of high rainfall. Nests were omitted from study in later years if parasitic cowbird eggs were found in nests. Results Hatching success was lower in eggs laid by inbred females. Particularly when incubation coincided with periods of rain. - Out bred females reared about.886 hatchlings per egg while inbred females (F=.25) had a hatchling success of.856, both in the absence of rain. - During periods of heavy rain, outbred females had only slightly lower hatching success(.867/egg) and inbred females were affected severely rearing only about a tenth of the eggs produced. (.062/egg)

Found clear evidence that environmental stress affected the fitness traits affected by inbreeding depression In two cases the environmental stressor greatly impacted the effect of inbreeding depression. (fledgling survival and hatching success) The affects on environmental stress and inbreeding depression are interacted. -Increased inbreeding depression during periods of poor environmental conditions marked a clear decrease in inbred birds’ fitness. However, predatory and extra pair fertilization variables were not taken into account. - This may have lead to underestimations of inbred nests.