Sven Krackow

Design variability in web geometry of an orb-weaving spider.

We studied the effect of several variables (environmental and physiological) on web geometry in the garden cross spider Araneus diadematus. Variables were: web support, wind, temperature, humidity, and silk supply. All had an effect. The spiders generally attempted to fit their webs to the shape of the supporting frame (standard, small, vertical, or horizontal). Windy conditions (0.5 m s-1) during web construction caused spiders to build smaller and rounder webs, laying down fewer capture spirals while increasing the distances between capture-spiral meshes. Decreasing temperature from 24 degrees to 12 degrees C caused the capture spiral to have fewer and wider spaced meshes, which did not change overall capture area but reduced the length of capture-spiral threads laid down. Subsequent increase of temperature to 24 degrees C restored the number of meshes laid down, but the wider mesh was retained, causing the capture area to be increased over initial control values. Decreased humidity (from 70 to 20% rH) had the effect of reducing web and capture-spiral size, the latter by reducing mesh number while keeping mesh spacing constant. Subsequent increase of humidity to control level (70%) restored web and capture area. However, this was achieved by laying down capture meshes at larger distances, rather than returning to initial mesh numbers. Silk supply also had a strong effect. Webs built in unnaturally rapid succession by the same spider (4 in 24 h when 1 is the norm) became sequentially smaller, had fewer radii, shorter capture spirals, and were wider meshed.

Evolving dispersal: Where to go next?

Habitat destruction and global climate change are two major threats to the persistence of ecosystems. The probability that a species survives such changes depends on its ability to track environmental shifts, either by moving between patches of habitat or by rapidly adapting to local conditions. This explains why the evolution of dispersal has become an integrative topic of paramount importance in evolutionary and behavioral ecology, as demonstrated by a recent conference*. A wide panel of researchers, who highlighted the recent major advances and the most promising lines of future research, were present at this meeting.

Timing of Mating, Developmental Asynchrony and the Sex Ratio in Mice

According to the developmental asynchrony hypothesis, changing the time of mating within the estrous cycle could alter the interval between completion of blastocyst development and uterine responsiveness for implantation. This may then lead to sex ratio skews in animals that exhibit sex-differential blastocyst development, because uterine stage may now benefit either slow (female) or fast (male) developing blastocysts. To test this hypothesis, the responses of two strains of mice to altered mating dynamics were compared. In a strain that exhibits higher male than female blastocyst developmental rates, sex ratios became significantly female-biased when mated late during the estrous cycle as opposed to early mating. However, timing of mating did not affect sex ratios in a strain with synchronous development of male and female preimplantation embryos. Hence, it is concluded that developmental asynchrony between male and female blastocysts on the one hand, and blastocysts and uterus on the other, are indeed responsible for the effect of timing of mating on litter sex ratios in mice.

Kin-preferential cooperation, dominance-dependent reproductive skew, and competition for mates in communally nesting female house mice

Little is known about the behavioural mechanisms facilitating kin-preferential communal breeding in wild house mice (Mus domesticus). We evaluated the effect of kinship and male availability on aggression, social structure and reproductive skew in groups of female mice freely interacting and reproducing in semi-natural indoor enclosures. Triplets of either sisters or non-sisters were established in enclosures provided with either one or three littermate males, which were unrelated and unfamiliar to the females. Sisters were more spatially associated and less aggressive than non-sisters, leading to higher incidences of communal breeding and reproduction. This is in agreement with theoretical considerations on kin selection in house mice. Reproductive success was highly skewed in favour of dominant females due to subordinate infertility or complete loss of first litters, which might have been caused by dominant females. In spite of this, subordinates only rarely dispersed from the enclosures, suggesting that perceived dispersal risk generally outweighed relatively reduced reproductive potentials. Aggression levels among females were significantly higher when one male was available, compared to when three males were available. We suggest that this might result from higher female-female competition for mates, due to the risk of missing fertilisation when synchronously oestrous females encounter limited numbers of males in a deme. Our results indicate that, first, communal nursing in house mice might have evolved to ‘make the best out of a bad job’ rather than to enhance offspring fitness; and, second, that female-female mate-competition might play an important role in shaping female social structure in this polygynous mammal.

Effects of spatial and cognitive enrichment on activity pattern and learning performance in three strains of mice in the IntelliMaze.

The IntelliMaze allows automated behavioral analysis of group housed laboratory mice while individually assigned protocols can be applied concomitantly for different operant conditioning components. Here we evaluate the effect of additional component availability (enrichment) on behavioral and cognitive performance of mice in the IntelliCage, by focusing on aspects that had previously been found to consistently differ between three strains, in four European laboratories. Enrichment decreased the activity level in the IntelliCages and enhanced spatial learning performance. However, it did not alter strain differences, except for activity during the initial experimental phase. Our results from non-enriched IntelliCages proved consistent between laboratories, but overall laboratory-consistency for data collected using different IntelliCage set-ups, did not hold for activity levels during the initial adaptation phase. Our results suggest that the multiple conditioning in spatially and cognitively enriched environments are feasible without affecting external validity for a specific task, provided animals have adapted to such an IntelliMaze.

Maternal investment, sex-differential prospects, and the sex ratio in wild house mice

In a population of first-generation offspring from wild-caught house mice (Mus musculus domesticus), previous evidence suggested that male fitness is more strongly affected by an increase in body weight than female fitness. This paper shows that in these mice the young are weaned at heavier weights the smaller the litter and the better the maternal body condition. These effects persisted into adulthood and were less pronounced in female young. However, contrary to expectation from conventional sex ratio theory, maternal condition and litter size had no detectable effect on sex ratios. Also, litter size did not affect sex ratios in two populations of laboratory-kept, wild-caught western (M. m. domesticus) and eastern house mice (M. m. musculus). Wild house mice, therefore, appear not to adaptively manipulate the sex ratio of offspring. It is argued that this absence of sex ratio trends might not be maladaptive, but rather that models currently used to predict sex ratio trends in rodents may not be valid.

Staying put or leaving home: endocrine, neuroendocrine and behavioral consequences in male African striped mice.

Social flexibility occurs when individuals of both sexes can change their social and reproductive tactics, which in turn can influence the social system of an entire population. However, little is known regarding the extent to which individuals of socially flexible species vary in their social behavior and in the underlying physiological mechanisms that support different social tactics. The present study in African striped mice modeled in captivity three male tactics described from the field: (a) philopatric males remaining in the family; (b) solitary roamers; or (c) group-living breeding males. Sixteen pairs and their offspring were kept in captivity, while one male offspring from the family remained as singly housed after he reached 21 days of age. Differences in behavior, morphology, hormone and neuropeptide levels were tested, and physiological measurements were correlated with behavioral measurements. In standardized arena experiments group-living males (philopatrics and breeders) were significantly more aggressive than singly housed males, in agreement with previous data suggesting that group-living, but not roaming males, are territorial. Philopatric males showed signs of reproductive suppression, small testes, lower testosterone and higher corticosterone levels than their singly housed brothers. Higher levels of arginine vasopressin (AVP) were measured in the PVN and BNST of singly housed males compared to group-living males. Based on these findings we hypothesize that roamers are physiologically primed, and capable, if the opportunity to mate arises, to release AVP, form social bonds and become territorial, thus quickly adopting the tactic as breeding male which would yield a higher reproductive success.

Note on falconiforme sex ratios given in Olsen and Cockburn 1991: avian raptors exhibit no unique sex-ratio bias

F r o m a c o m p i l a t i o n o f sex ra t io d a t a given on 12 fa lconifo rme species, Olsen and C o c k b u r n (1991) conc lude tha t 6 species show a s ignif icant ly female -b iased sex ra t io , bu t no species p r o d u c e s s ignif icant ly m o r e males than females. This wou ld be in r e m a r k a b l e con t r a s t to the theore t ica l p red ic t ion o f a ma le -b i a sed sex ra t io in species where females are s ignif icant ly heavier t han males at t e r m i n a t i o n o f p a r e n t a l i nves tmen t (F isher 1930), which is obv ious ly t rue for all f a lcon i formes (Olsen and C o c k b u r n 1991). I t w o u l d also represen t a un ique case o f congruence in such a large t a x o n o m i c g roup o f h igher ve r tebra tes (cf. C l u t t o n B r o c k and I a s o n 1986; Cockbu rn 1990; van Schaik and H r d y 1991). However , s ta t is t ical r e -eva lua t ion o f the d a t a given in Olsen and C o c k b u r n (1991, thei r Table 1) revealed tha t the Z 2 values given were incorrect . I have cor rec ted them as well as ca lcu la ted the two-s ided b inomia l p r o b a bili t ies, which are given in Table 1. Add i t i ona l l y , I have inc luded recent ly pub l i shed d a t a on p o p u l a t i o n s o f Har r is s hawks , E u r o p e a n kestrels and A m e r i c a n kestrels (Table 1). I t appea r s tha t af ter co r rec t ion the sex ra t ios o f only 2 species o f the 12 f rom the or ig ina l f igures in Olsen and C o c k b u r n (1991) s ignif icant ly devia te f rom the expec ted even sex ra t io (Table 1). In teres t ingly , in A m e r i can kestrels , the new d a t a set o f Wiebe and Bor to lo t t i (1992) exhibi ts a bias in the d i rec t ion oppos i t e to the d a t a set cited by Olsen and C o c k b u r n ( ]991; see Table 1), while the c o m b i n e d d a t a are no t s ignif icant ly b iased (639 f e m a l e s : 6 8 5 males, ns). Therefore , only the

Basal blood glucose concentration in free-living striped mice is influenced by food availability, ambient temperature and social tactic

Vertebrates obtain most of their energy through food, which they store mainly as body fat or glycogen, with glucose being the main energy source circulating in the blood. Basal blood glucose concentration (bBGC) is expected to remain in a narrow homeostatic range. We studied the extent to which bBGC in free-living African striped mice (Rhabdomys pumilio) is influenced by ecological factors with a bearing on energy regulation, i.e. food availability, abiotic environmental variation and social tactic. Striped mice typically form extended family groups that huddle together at night, reducing energetic costs of thermoregulation, but solitary individuals also occur in the population. We analysed 2827 blood samples from 1008 individuals of seven different social categories that experienced considerable variation in food supply and abiotic condition. Blood samples were taken from mice in the morning after the overnight fast and before foraging. bBGC increased significantly with food plant abundance and decreased significantly with minimum daily ambient temperature. Solitary striped mice had significantly higher bBGC than group-living striped mice. Our results suggest that adaptive responses of bBGC occur and we found large natural variation, indicating that bBGC spans a far greater homeostatic range than previously thought.

Agonistic onset during development differentiates wild house mouse males (Mus domesticus).

Wild house mouse populations have been suggested to locally adapt to varying dispersal regimes by expressing divergent aggressivity phenotypes. This conjecture implies, first, genetic polymorphism for dispersive strategies which is supported by the finding of heritable variation for male dispersal tendency in feral house mice. Secondly, aggressivity is assumed to translate into dispersal rates. This speculation is reinforced by experimental evidence showing that non-agonistic males display lower dispersal propensity than same-aged males that have established agonistic dominance. However, the actual ontogenetic behavioural pattern and its variability among populations remain unknown. Hence, in this study the timing of agonistic onset is quantified within laboratory-reared fraternal pairs, and compared between descendants from two different feral populations. Males from the two populations (G and Z) differed strongly in agonistic development, as Z fraternal pairs had a 50% risk of agonistic onset before 23.5±2.7 days of age, while this took 57.3±5.4 days in males from population G. This difference coincided with significant genetic differentiation between the males of the two populations as determined by 11 polymorphic microsatellite markers. Furthermore, in population G, males from agonistic and amicable fraternal pairs exhibited significant genetic differentiation. These results corroborate the supposition of genetic variability for dispersive strategies in house mice, and identify the ontogenetic timing of agonistic phenotype development as the potential basis for genetic differentiation. This opens a unique opportunity to study the genetic determination of a complex mammalian behavioural syndrome in a life history context, using a simple laboratory paradigm.