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Paternal Behavior and its Effects on Offspring Parent-offspring interactions in early maturation can influence the development of the offspring’s brain and behavior and, consequently, survival. The majority of studies that have been done have focused on prenatal or maternal effects on offspring, but there is a lack of emphasis on the profound impact that paternal care can have on offspring as well. For example, in humans, fathers who are involved during pregnancy could help reduce the risk of infant mortality during their child’s first year of life. Moreover, babies without fathers were nearly four times as likely to die in their first year than children with two active parents [1]. Male care of young does exist in mammals, most significantly in rodents. This most likely evolved because it sometimes improves offspring survival to such a degree that it outweighs the costs of lost mating opportunity.

Contents
 * 1)  The Effect of the Presence or Absence of the Father
 * 2)  Survival Rate
 * 3)  Paternal Care, Survival Rate and Varying Food Availability
 * 4)  Biological and Physiological Effects
 * 5)  Paternal Deprivation and Changes in Neurons
 * 6)  Paternal Pup Retrievals and Testosterone Levels
 * 7)  Aggression in Offspring
 * 8)   Father’s Retrieval Behavior
 * 9)  Pup Retrieval, Aggression, Vasopressin Expression

Survival Rate
Paternal care most likely evolved because it exerts some influence on offspring survival rate. In the study Adaptive significance of male parental care in a monogamous mammal, field evidence demonstrated that paternal behavior improves offspring survival in the biparental California mouse, Peromyscus californicus. In order to determine fitness effects in P. californicus, male presence was manipulated and offspring survival rate in father-present groups was compared to father-absent groups. Adult females were dusted with a specific fluorescent powder color that transferred to both their young and mates. At capture, males were checked for a female specific pigment, which indicated a pairing with that female. Fathers of the male removal group were removed within 3 days after their first litter was born while all other fathers remained.

There was no difference found in the number of young born to father-present or father-absent groups. Yet, significantly more young appeared from father-present (81%) compared to father-absent families (26%) meaning that more pups emerged from the tracking locations that were used. Thus, the results indicate that male presence in caring for P. californicus pups significantly influences survivorship. The evolution of paternal care in this species seems to be a result of its importance on infant survival rate. Although some females were able to care for offspring on their own, male assistance greatly enhanced pup survival rate [5].

Paternal Care, Survival Rate and Varying Food Availability
Male parental care may be more advantageous in species that evolved in an environment where food resources and the population size were stable. These species emphasize maintaining the current population size and utilizing limited resources to produce viable offspring. Evidence from studying the California mouse suggests that they resemble this type of species (“K-strategist”). The present study, The importance of paternal care on pup survival and pup growth in Peromyscus californicus when required to work for food, tested whether paternal care increased pup survival and pup growth under varying food availability conditions. The three conditions consisted of having a wheel and needing to run to obtain food (Wheel Contingent), having a wheel present but receiving food whether they ran or not (Wheel Noncontingent) and not having a wheel and receiving food (No Wheel). In each of the three conditions fathers were present or fathers were removed before the pups were born.

It was discovered that significantly more pups survived in the male present group compared to the male absent group. Overall, more pups survived in the No Wheel condition than in the Wheel Contingent condition. However, in the Wheel Contingent condition, more pups survived in the male present group than in the male absent group. Thus, the male’s presence increased pup survivorship when mice were required to forage for food (Wheel Contingent) but not when it was readily distributed. These results are consistent with previous studies that demonstrate that paternal investment in offspring is most pronounced in poor quality environments. The male’s presence may indirectly increase pup survival by altering the amount of time their partners had to spend foraging for food as seen in the Wheel Contingent groups where females did not have to expend more energy in addition to running the wheel since males were present. This study suggests that monogamy in P. californicus may be an advantageous reproductive strategy as the male’s presence increased pup survivorship [7].

Biological: Paternal Deprivation and Changes in Neurons
Paternal care is a source of neonatal sensory stimulation and has been shown to be necessary for developing structure and function of sensory cortices (outer layer of another organ). The study Paternal Deprivation Induces Dendritic and Synaptic Changes and Hemispheric Asymmetry of Pyramidal Neurons in the Somatosensory Cortex, examined the biparental rodent Octodon degas to observe the effect of paternal deprivation on dendritic and synaptic development in the somatosensory cortex. The somatosensory cortex is a sensory system composed of the receptors to produce sense experiences such as touch, temperature, body position and pain. They determined the amount of paternal investment (37%) in relation to total parental care and compared dendritic and synaptic nerve development in the somatosensory cortex of rodents raised in a father absent condition and a father present condition. The majority of somatosensory stimulation performed by the father comprised huddling, licking, grooming, and playing. Compared to the offspring raised by both parents (father present condition) they found that father-deprived rodents exhibited significantly reduced spine numbers on the basal dendrite neurons. Moreover, the father deprived animals demonstrated shorter and less complex dendrites in the left somatosensory cortex than in the right hemisphere. These results show that paternal deprivation leads to delayed or altered dendritic and synaptic development in somatosensory circuits in the offspring [6].

Physiological: Paternal Pup Retrievals and Testosterone Levels
Studies have demonstrated that the survivorship of the offspring in the monogamous and biparental California mouse,Peromyscus calfornicus, is affected by paternal care during the early stages of development. More specifically, adult offspring aggression and the development of neuropeptide vasopressin (which is associated with many social behaviors like aggression in mammalian species) has been linked to paternal pup retrievals (grasping the pup by the scruff and carrying it back to the nest). In the study entitled Paternal behavior increases testosterone levels in offspring of the California mouse, prepubertal male P. californicus pups were tested to determine whether paternal pup retrievals increase testosterone levels since testosterone increases arginine vasopressin and aggression. In order to test this hypothesis, the pups were divided into three groups: hormone baseline, nonretrieval control, or retrieval. The pups were then removed from the cage and the focal male pup was put outside of the nest to reflect the retrieval group, or was put in the nest to reflect the nonretrieval group. The influence of paternal behavior (determined by the retrievals) on offspring physiology (measured by the changes in offspring hormone) was observed. The retrieved pups displayed increased testosterone at 45 minutes, whereas the nonretrieved pups did not. There was a positive correlation between pup retrievals and testosterone level in the 45-minute group. This indicated that paternal retrieval may contribute to offspring aggression through a response in testosterone and that fathers have a significant effect on male offspring physiology [2].

Father’s Retrieval Behavior
There are multiple facets to paternal care for offspring. A lack of paternal care can lead to detrimental behavioral effects in offspring. Parents influence offspring aggression, but this can be caused by genetic as well as non-genetic factors. In the study The Association Between Male Offspring Aggression and Paternal and Maternal Behavior of Peromyscus Mice, the aggressive biparental Caliofornica mouse were cross-fostered with the less aggressive, less parental white-footed mouse (two P. californicus pups or four P. leucopus pups from the same litter were raised by parents of the other species) to determine any parental behavioral effects. The duration of various behaviors, such as huddling, nursing, retrieving, grooming, nest-building were measured, and the relationship between the fostered male offspring’s aggression and the foster parents behavior was examined. It was found that resident-intruder (R–I) aggression of offspring was positively correlated with paternal time spent retrieving pups. Furthermore, the interaction between species and retrievals on R-I attack latency was also tested to control for any differences contributed to the significant effect of retrievals. There was a negative correlation between retrieval time and attack latency, which suggests that males that retrieved their pups more frequently raised the most aggressive male pups with the shortest attack latencies. The results that the father’s retrieval behavior was associated with higher R-I aggression their offspring indicate that fathers may influence the development of aggression [3].

Pup Retrieval, Aggression, Vasopressin Expression
As noted, pup retrieval by fathers influences the development of aggression as well as extra-hypothalamic vasopressin systems in offspring. In the study Paternal behavior influences development of aggression and vasopressin expression in male California mouse offspring, paternal retrieval behavior (male grasped a pup with his mouth and lifted it off the ground) between days 15 and 21 postpartum were manipulated to determine whether retrieval affected the systems, and paternal retrieval and grabbing frequencies of pups were observed and recorded. In addition, huddling and grooming behavior was also decreased by castrating one subset of fathers, since licking and grooming behaviors have been known to be associated with behavioral development in rats. It was determined that there was a negative correlation between the frequency of paternal pup retrieving behavior and attack latency in resident-intruder (which was used to estimate territorial aggression in rodents) in male and female adult offspring. However, reducing huddling and grooming did not have an effect. Furthermore, vasopressin immunoreactivity (AVP-ir) in two regions of the posterior bed nucleus of the stria terminal was also analyzed. Increased retrieval was associated with a shift in AVP-ir distribution. This study supports the hypothesis that increased retrieving and grabbing of offspring by fathers is correlated with an increase in offspring aggression, and its results also illustrate that paternal behavior has an impact on the development of the offspring. Moreover, a previous study suggested that paternal retrieving behavior and thereby, aggressive behavior, can actually be passed down between generations, which is a topic of future interest [4].

Although maternal investment has been the primary focus affecting offspring development, paternal care also plays a critical role in the maturation, behavior, and survival of offspring especially in rodents. Paternal pup retrievals can impact aspects of offspring behavior such as aggression and testosterone levels. In studies controlling whether males were present or absent in families, survivorship of young significantly increased in male present conditions. These results demonstrate that paternal care is sufficiently important to outweigh the benefits of alternative mating strategies.

Maternal Bond Parental Investment California Mouse Parenting