Date of Award


Document Type

Restricted Thesis

Terms of Use

© 1997 Julianna C. Patrick. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts



First Advisor

Sara Hiebert Burch


This study was designed to examine the effect of photoperiod on the aggressive behaviour of deer mice (Peromyscus maniculatus nebrascensis). Aggressiveness was determined by analysis of behaviours of animals observed in same and opposite sex dyads. The individual behaviours scored and combined for a total aggression score were 1/2 Crouch, Full Upright, Lunge and Fight. The first two behaviours were categorized as "threats" and the second two as "attacks." The animals were first tested in a long day photoperiod of 16L:80 (LD) in which they had been kept for several months. They were then switched to a short day photoperiod of 8L:160 (SD) for six to ten weeks and tested again. The reproductive state of the males was assessed by external testis measurements. Testis size of the males was significantly larger in LD (Mean ± SE = 83.4± 3.8 mm²) than in SD (38.0± 5.4 mm²) (p < 0.0001). In the same sex pairings, there was no significant difference between the two photoperiods in the total number of aggressive acts of males or females (F = 0.015, P > 0.05) but the females were significantly more aggressive than the males in both daylengths (F = 8.092, P < 0.01). For individual behaviours, there was a sex dependent effect of daylength on the Lunge behaviour (F = 6.456, P < 0.05) but not on the other behaviours. Males Lunged more in LD than in SD but females Lunged more in SD than LD. For opposite sex pairings, again daylength did not have a significant effect on the total number of aggressive acts (F = 0.186, P > 0.05) and again the females were significantly more aggressive than the males (F = 6.313, P < 0.05). Females in these opposite sex pairings used significantly more Full Uprights (F = 6.379, P < 0.05) and Lunges (F = 9.108, P < 0.01) than the males in both daylengths. There were significantly more fights in LD than SD (F = 3.995, P = 0.05), showing that daylength did have a significant affect on this behaviour. In a comparison of the differences between total number of aggressive acts in same and opposite sex pairings, animals were significantly more aggressive in intra-sex pairings than inter-sex pairings (F = 5.956, P = 0.0163). This combined effect of sex of aggressor and sex of opponent was also seen in the 1/2 Crouch (F = 7.599, P < 0.01) but not in any of the other behaviours. There were more fights in LD than SD (F = 4.646, P < 0.05) but none of the other behaviours were affected by daylength. If we assume that the concentration of gonadal hormones is directly correlated to the size of the testis size of an animal (Desjardin & Lopez, 1983), testosterone concentration does not seem to be directly related to aggression in this study. We did not, however, measure the concentration of testosterone directly. The high aggression in the female is consistent with that of other social systems in which both parents care for the young and the female retains exclusivity of a male by mate guarding. Also, the reduction in "attack" aggression in the short day photoperiod may be related to the increase in the number of mice on one stable home range which creates wintering groups (Banfield, 1974; Nowak, 1991).