The seasonal pulsing in size of skulls and penises

The gentle pulsing in size of body parts is one of Nature’s seasonal wonders. Evidence of this — with skulls and penises, respectively — is documented in two (well, three) studies published this year.

Some Weasel Skulls Pulse Bigger and Smaller, Seasonally

Growth overshoot and seasonal size changes in the skulls of two weasel species,” Scott LaPoint, Lara Keicher, Martin Wikelski, Karol Zub, Dina K. N. Dechmann, Royal Society Open Science, vol. 4, 2017, 160947. The authors report:

[We quantified] the effect of location, Julian day, age and sex on the length and depth of 512 and 847 skulls of stoat (Mustela erminea) and weasel (M. nivalis) specimens collected throughout the northern hemisphere…. Standardized braincase depths of both species peak in their first summer, then decrease in their first winter, followed by a remarkable regrowth that peaks again during their second summer.

Parallel detail appears in the later paper “Profound reversible seasonal changes of individual skull size in a mammal,” Javier Lázaro, Javier Lázaro, Dina K.N. Dechmann, Scott LaPoint, Martin Wikelski, and Moritz Hertel, Current Biology, vol. 27, no. 20, 23 October 23, 2017, pp. R1106–R1107. The kind of animal here is a shrew (Sorex araneus). Here’s a bit of detail from that study:

Some Ducks Grow Bigger Penises In Some Social Circumstances

Evidence of phenotypic plasticity of penis morphology and delayed reproductive maturation in response to male competition in waterfowl,” Patricia L.R. Brennan, Ian Gereg, Michele Goodman, Derek Feng, and Richard O. Prum, The Auk, vol. 134, 2017, pp. 882–893. The authors report:

Here we examined whether penis morphology is affected by social environment. We found experimental evidence that in a male-biased social environment, consisting of several males and fewer females, the penis in Lesser Scaup (Aythya affinis) grew longer in 2 separate years, than in males housed in pairs, as predicted if male–male competition influences penis morphology. In Ruddy Ducks (Oxyura jamaicensis), males instead showed evidence of reproductive delays that were explained both by a male’s size and his social environment: most males in social groups exhibited shorter penises, variable onset and duration of genital maturation, and faster penis growth rate.