HMM of Maternal Behavior Figure 4. Maternal behavior strategy of C57BL/6 mothers. Graphical representation of composition, duration, frequency, and transition probabilities of HMM states for C57BL/6 mothers. Most states are composed of a single dominant behavior and multiple minor behaviors. State frequency and mean duration are indicated in site each circle. The area of each circle is proportional to state frequency. Arrows indicate absolute transition probabilities between states (transitions/minute). doi:10.1371/journal.pone.0014753.g004 frequently interrupted by arched-back nursing (11% vs. 16%) and licking/grooming pups (3% vs. 7%) in BALB/c vs. C57BL/6 mothers. In the ABN state, arched-back nursing was less frequently interspersed with licking/grooming pups (4% vs. 9%) in BALB/c vs. C57BL/6 mothers. Similarly, in the LG state BALB/c mothers showed more self-grooming (32% vs. 10%) and less arched-back nursing (5% vs. 8%) than C57BL/6 mothers. Moreover, within the LG state BALB/c mothers appeared to replace licking/grooming pups (41% vs. 66%) with sniffing pups (5.9% vs. 2.4%) consistent with a less transitive maternal nurturing style. A comparison of state transition probabilities revealed a strikingly different pattern of behavioral sequences in the two strains (Table S10). In BALB/c mothers the ABN state more frequently deteriorated into BLN, contributing to an increased frequency and duration of this state. In BALB/c mice the GRO state replaced the LG state as the intermediate state between nursing and non-nest behavior, with frequent transitions from this state to BLN without intervening LG. This change explained why the LG state was less frequent in BALB/c mothers, despite showing a similar duration. Finally, the EAT state was more frequent and persistent in BALB/c mice due to reduced transitions to ACT. HMM analysis of maternal behavior in B6xC and CxB6 mice Overall the structure of HMM states in reciprocal hybrid mice was more similar to C57BL/6 mothers (Figure S6, S7; for detailed values and statistical significance, see Tables S11, S12, and S13). In both reciprocal hybrid strains the LG state served as the principal pathway in and out of nursing, while GRO typically intervened between in and out of nest behavior. However, even more than in C57BL/6 mice, nursing behavior in reciprocal hybrid mothers was dominated by ABN at the expense of BLN showing a more active nursing style in the hybrid mothers despite intermediate levels of time spent in the nest (Figure 1D, S6, S7). As expected, differences in maternal behavior states between reciprocal hybrid mothers were highly restricted. Consistent with the maternal behavior of their mothers, B6xC mothers spent significantly more time in the LG state and less time in the GRO and SLP states than CxB6 mothers (see Table S12 and Figure S6, S7). CxB6 mothers showed an increase in self-grooming in the LG state compared to B6xC mothers (27% vs. 17%) consistent with the more self-oriented style of the BALB/c pedigree (see Table S11). No significant differences in state transition probabilities between F1 hybrid mothers were observed. Discussion We have provided a detailed description of maternal behavior in C57BL/6 and BALB/c inbred mice and their reciprocal F1 hybrid female offspring. We carried out observations of maternal behavior during the first three weeks of postnatal development using an ethogram of twenty-one behaviors. Analysis of the frequencies of these behaviors confirmed and extended previous findings demonstrating large differences in maternal behavior repertoire between inbred strains and the transmission of a restricted subset of these behaviors from the mothers to their PLoS ONE | www.plosone.org 6 March 2011 | Volume 6 | Issue 3 | e14753