Fitness variations due to natural variation in the size of the first clutch and its laying date were estimated using Fisher's reproductive value for both the clutch (Vc) and the parent (Vp) in a population of great tits. In order to test the hypothesis that individual birds maximize their reproductive value by the choice of clutch size, artificial variation in brood size was introduced and the consequences in terms of reproductive value estimated. Maximal Vc, computed on the basis of natural variation in clutch size, occurred at a clutch size of 15.2, and increased slightly with laying date (Fig. 1A) Vp increased with natural variation in clutch size and decreased with date (Fig. 1B). The total reproductive value V (= Vc+Vp) was maximal at a clutch size of 15.4 (Fig. 1C), substantially higher than the population mean clutch size (9.2). The components of the reproductive value of the clutch (Vc) that were negatively affected by manipulation were the survival of the nestlings and the recruitment rate. The reproductive value of the parent (Vp) was negatively affected only through the probability of having a second clutch. Maximal Vc computed on basis of artificial variation in clutch size, occurred at a clutch size of 10.0, and also increased with date (Fig. 1D). Vp decreased with artificial variation in clutch size (Fig. 1E causing the clutch size maximising reproductive value V to shift to a value of 9.4 (Figs 1F, 3), very close to the population mean clutch size (9.2). It is concluded that the majority of great tits produces the number of eggs (9-10) that maximizes their individual fitness, even though those individual birds laying 15 eggs have the highest reproductive value in the population. The fact that birds laying very large clutches have the highest reproductive value points in the direction of a selection pressure towards larger clutches. Yet, over the last 30 years clutch sizes have not increased in the study population. This apparent contradiction is discussed. Either no genetic variation in clutch size is involved, or a complex polymorphism exists.
The time in spring when a male kestrel rapidly increases his daily hunting time and his hunting yield, and thereby the amount of food delivered to the female, determines the date when she lays the first egg. Food experiments in free-living and captive kestrels gave a significant advance in laying date. Clutch size, which decreases with progressive laying date, did not change independent of date in response to food manipulation. These effects are in agreement with most other feeding experiments. Photoperiod experiments in kestrels advanced the reproductive cycle in constant long days, and a similar seasonal decline in clutch size was found. It seems that there is an internally preprogrammed decrease in clutch size within an annual "reproductive window". A proximate control model for the seasonal decline of clutch size is proposed, modified from an earlier model by HAFTORN (1985). This incorporates an increasing tendency to incubate the first eggs with progression of the season, an egg contact-incubation positive feedback loop, and the resorption of further follicles in the ovary when the laying female incubates 50% of the time. This follicle resorption fixes the clutch size ca. four days before the last egg is laid. the 50% incubation level is reached earlier in late females and consequently resorption starts earlier and the resulting clutch is smaller than in early females. Experiments in kestrels with removal and addition of eggs, in combination with measurements of incubation behaviour are discussed in relation to the model. Plasma prolactin data of female kestrels show that this hormone is a serious candidate for a physiological component relaying time of year in our model for clutch size regulation.
The theory that individual birds maximize their fitness by the two major decisions in reproduction concerning date (when to start laying eggs) and clutch size (when to stop laying eggs) is empirically approached in the Kestrel by quantifying Fisher's Reproductive Value for both the clutch (Vc = c. Vo/2) and the parents (Vp). The reproductive value of an egg (Vo) was found to decrease monotonically with laying date (d) due to significant associations with d of the components So (probability for an egg to survive till fledging), S1 (probability to survive from fledging till age 1), S2 (survival age 1 till age 2), and P1 (probability of breeding at age 1). Vp declined negligibly with laying date, although there were significant associations between d (laying date) and N (probability of the nest to produce at least one fledging), Pr (probability of a repeat clutch following nest failure), and La (probability of local survival of the parents following breeding). In experiments where brood size at day 10 after hatching was increased or reduced, Vc increasing experimental brood size, while Vp simultaneously decreased. Total reproductive value (V = Vc + Vp) remained unaffected by the experiments. This result suggests that a rather broad range of clutch sizes maximizes total reproductive value, as far as detectable by the data. While the yield of kestrel hunting, and hence the number of young raisable with constant parental effort (and constant Vp), increased with the spring increase in vole population density, reproductive value of the clutches decreased. For any particular food situation (hunting yield) this leads to a unique combination of clutch size and laying date maximizing V. This could be worked out by calculating fitness contours for all combinations and for different yields (Fig. 12). The optimal solutions are on a declining slope, with smaller clutches associated with later dates. 59.4% of all clutches observed obeyed the maximization criteria. Furthermore, there was a reasonable, unbiased association between predicted laying dates and clutch sizes based on individual male hunting yields and observed dates and clutches as laid by the females. Qualitatively, any method predicts a seasonal decrease in the optimal clutch size when the environment improves while reproductive value declines with progressive date. Preliminary results from an experimental approach to test the assumption of a causal effect of date on Vo, using the release of juvenile kestrels reared in captivity under artificial light schedules, are presented.
1. Theories postulating that sexual task differentiation may lead to polygamy such that the sex investing the least effort in raising the offspring, engages in simultaneous matings, contrast with polygyny in raptors where the male provides most of the food for its females and nestlings. A field study was undertaken to describe parental effort and success in marsh harriers of different mating status to elucidate this controversy. 2. Data on clutch size and laying date were collected on 421 nests in two Dutch land reclamations, Flevoland and Lauwersmeer. 156 nests were known to have monogamous parents, 30 males had two females and nests. Bigamous males raised on average twice as many fledglings (5.7) than monogamous males (3.0). However, their primary females had more success (3.5) than secondary females (2.3), related to increased nestling mortality in secondary nests (Table 7). Male fledglings were significantly heavier in primary than in secondary nests (Fig. 16). 3. Nest observations made on 22 nests (5 of monogamous, 17 of polygamous males) revealed that daily prey deliveries by males were fewer in mono- than in bigamous males (Fig. 12). The latter delivered prey by preference to their primary nests (Fig. 14). The prey delivered by a trigamous male were consistently larger than those of a bigamous and monogamous male in the same area (Table 4). 4. Time budget observations revealed that hunting effort was maximal in the nestling phase (ca 8 hrs foraging per day for all three males observed (Fig. 10); at other times of year foraging was reduced in early morning and late afternoon (Fig. 11). Net hunting yield (prey brought to nests per hour of hunting) increased in three males with their number (1, 2, or 3) of mates (Fig. 13). With progress of the breeding season, male hunting ranges extended further outside the breeding territories (Figs. 7, 8, 9) and had a great measure of overlap, suggesting that territory quality was not a major factor in male hunting yield. 5. Secondary females participated in provisioning for the nestlings more than primary or monogamy-females (Fig. 15), thus compensating for reduced male prey deliveries. 6. Classical polygyny theory addresses the question of female choice: which benefits compensate a secondary female for reduced breeding success by mating with an already paired male? Several hypotheses (enhanced offspring survival, offspring genetic quality, parent chances of future reproduction) are discussed, but evidence is nearly completely lacking (ch. 6.1-3). 7. An alternative approach stresses the male's role in the decision process. Males may have more interindividual variation in their capacity to bring food than females in their capacity to lay and incubate eggs. Optimal strategies for males would then range with increasing quality from non-breeding via polyandry and monogamy to polygyny (Fig. 17). In species like harriers, non-breeding may be optimal for yearling males with submaximal hunting skills, thus creating a skewed sex ratio forcing some females to accept secondary status as mate of older, high quality males. Polygyny is then associated with slower male than female maturation. The evolution of polyandrous traits in species living isolated in poor environments is likewise explained by this model.
Rats are known to display a temporary deficit in memory function 6 h after training on a learning task, a phenomenon known as the 'Kamin effect'. Later studies showed that maximal retrieval recurs in 24 h intervals after a single training and implied the role of the circadian clock in the suppression of memory retrieval at non-24 h intervals. This study aimed to investigate this further by analysing retention deficits following passive avoidance training in the Syrian hamster. The availability of hamsters carrying the tau mutation was exploited to address the role of the circadian system in periodic retention deficits. It was expected that tau mutant hamsters with an endogenous circadian period of approximately 20 h would have a high retention score at 20 h after training. Surprisingly, deficits in retention were found at 12, 18, 24, and 36 h after training in wild-type hamsters with best performance at 30 h after training. Tau mutant hamsters had significant deficits in memory retention at 20, 24, and 30 h, and no clear periodicity in retention could be observed. Step-through latency scores for mutant hamsters were low at all times except training-testing intervals of 0.25 and 6 h. These results demonstrate the absence of clear memory deficit oscillations in both wild-type and mutant hamsters, and may suggest in particular a long-term memory deficit in tau mutant hamsters.