Sex differences in parental care for offspring are common among socially monogamous, monomorphic seabirds. Even among such animals, bodily sex differences remain significant. Seabird females bear the energetic cost of laying eggs. Among Little Auk, the female produces one egg that weighs about 20% of the mother’s body mass. Another important, related bodily sex difference is that females know who their offspring are, while males have varying degrees of paternity confidence. Extra-pair paternity shares are often above 10% in socially monogamous animals. Among razorbills, a socially monogamous species of auk closely related to Little Auk, about 50% of females engage in extra-pair copulation. Various theories relate these two fundamental sex differences — female egg-laying cost and male paternity uncertainty — to various sex differences in parental care. The scholarly literature thus far seems to show mainly that sex differences are pervasive and difficult to explain generally.
Sex differences in offspring relations are associated with sex differences in communication. Male and female razorbill parents share duties of incubating the (one) egg and share duties of feeding the chick. The chick leaves the nest when it has a body size about 30% of an adult. The male razorbill parent then alone cares for the young bird at sea, feeding and protecting it. Despite razorbills’ equal parental care at the nest, males are more closely connected to chicks communicatively:
males responded preferentially to their own chick’s calls and chicks responded preferentially to the calls of their male parent. In contrast, the playback experiments were not able to show any evidence of recognition between razorbill female parents and their offspring. Females responded infrequently to their chick’s calls and indifferently to the calls of strange chicks. Finally, we found that females only rarely vocalized to their chicks…. Male parents gave their strongest response to playbacks close to the time of fledging, when they were off the nest attempting to counter-call with their chicks. In contrast, females were never observed counter-calling with their chicks either on or off the nest. 
Why male razorbills, but not female razorbills, care for offspring at sea isn’t easy to explain. Relatively good communication capabilities surely facilitates male parental care for offspring at sea. To identify sex differences in communication, look for sex differences in relation to offspring.
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- anisogamy less important than sex differences in communication
- sex differences in expected human lifespan
- gynocentrism is fundamental feature of primate social life
 Welcher et a. (2009) p. 510. A human newborn typically weighs about 8 pounds. That’s usually under 5% of the mother’s body weight.
 For a summary of sex differences in chick-provisioning among monomorphic birds and scholarly explanations, see Table 1 in Elliott et al. (2010).
 Insley et al. (2003) p. 30.
[image] Razorbill on the Isle of May, Firth of Forth, Fife, Scotland, 2010. Thanks to Steve Garvie and Wikipedia.
Elliott, Kyle Hamish, Anthony J. Gaston, and Douglas Crump. 2010. “Sex-specific behavior by a monomorphic seabird represents risk partitioning.” Behavioral Ecology. 21 (5): 1024-1032.
Insley, Stephen J., Rosana Paredes Vela, and Ian L. Jones. 2003. “Sex differences in razorbill Alca torda parent-offspring vocal recognition.” The Journal of Experimental Biology. 206: 25-31.
Wagner, Richard H. 1992. “Confidence of Paternity and Parental Effort in Razorbills.” The Auk. 109 (3): 556-562.
Welcher, Jorg, Harald Steen, Ann M.A. Harding, and Geir W. Gabrielsen. 2009. “Sex-specific provisioning behaviour in a monomorphic seabird with a bimodal foraging strategy.” Ibis. 151 (3): 502-513.