Paleo breeding: mating in the wild.

I’ve adapted much of this chart from Howell-Skalla (2002)  and Tsubota (1998).

Canadian polar bears: bona fide seasonal breeders.circannual hormones

The light cycle increases until June, then decreases until December.  Melatonin goes in the exact opposite direction. Testosterone peaks around the onset of breeding season (springtime, April/May), coinciding with LH (as expected). There is also a lot of bear-on-bear violence at this time due to: 1) testosterone-induced aggression; and 2) the high female:male ratio –-> females rear their cubs and are thus out of the game for about 3 years, but males like to breed every year.

Females followed a similar pattern, with estrogen peaking around breeding season and prolactin following the light cycle.

The authors mentioned that prolactin levels mirrored day length, and according to Wiley this would be the prolactin peak that normally occurs when you’re sleeping, but has spilled over into the daytime due to short sleep / long light cycle… not total prolactin levels (24h AUC?), which should be highest in winter (see below).

Female bears get pregnant in spring, corresponding with peak estradiol, but apparently the fertilized egg doesn’t implant for a few months, when they enter their dens in the autumn; this corresponds with a peak in progesterone.

The prolactin picture is admittedly the most difficult for me to grasp (circadian AND circannual rhythms), but is at least starting to come together: they also mentioned that, in black bears, preventing the spring seasonal increase in daytime prolactin with a long acting form of bromocriptine resulted in lower serum testosterone and prevented both an upregulation of testicle LH receptors and maximal testicular growth.seasons

Hamsters operate on a similar level: breeding during the waxing light cycle & not so much in winter. Klosen (2013) showed some of this is mediated by TSH.  In short-day adapted hamsters (winter; high melatonin), infusion of TSH switched the gonadotropic axis from winter to summer-breeding mode = increased fertility. Long light cycle induces TSH, which stimulates sex hormones.

The authors showed that melatonin inhibits TSH, which reduces these “RF-peptides” and thus impairs fertility.  Lowering melatonin levels in spring = high “RF-peptides” = increased fertility. “RF-peptides” activate testicular activity [in hamsters].  While TSH & T3 might seem useful to keep us warm in the winter, their local effects are actually rather important in these seasonal breeders to promote sex hormones and fertility.

New conclusion – -> endogenous melatonin proper: good for sleep & health, but low levels in summer still better for fertility.  Prolactin might be more similar to dopamine in at least one regard: for dopamine, it’s all about “location, location, location!” Ie, in which region of the brain is dopamine acting.  For prolactin, it seems to be all about “timing!”prolactin

If we assume the black bars indicate night time in the figure above, and that in spring they should be getting smaller (but aren’t), then I can see how there might be some prolactin spillover into the daylight hours (Haus 2007).  Overall levels still way higher in winter, though.

What about humans?  According Lam & Miron (1994), conception actually follows an opposite pattern.  The figure below is from Louisiana, but similar trends were found for a variety of locations.

seasonal Louisiana


I don’t think this means the model is wrong, but rather that we are.  Bollixed, that is.  This is purely speculative, but the seasonal impact on birth rates has steadily declined over time, which may partially reflect the increased use of artificial lights and all of our attempts to subvert circadia.  Note how the height of the troughs & peaks varies widely 50-60 years ago, indicating strong seasonal effects, but by 1990 is reduced by over half:

Louisiana births

This was indirectly confirmed by Cummings (2007) who showed that in Germany, the peak in birth rates gradually shifted from spring in 1950 to autumn in 1990.  And a complete loss in seasonality occurred in Spain between 1940 & 2000 (Cancho-Candela et al., 2007).

Courtesy of Boone (2003):

and after:

calories proper

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  • George

    True that melatonin is seriously anaphrodisiac, even 5-HTP blunts the urges.

    • Jane Plain (Woo)

      Melatonin and 5ht not only directly suppress dopamine which is important for desire as well as performance but melatonin particularly suppresses sex steroids like testosterone leading to sexual apathy. Prolactin behaves similarly and is closely related to mel in that sense . It is a hormone that helps adapt to stresses ( along with progesterone, the nutrition demand of pregnancy). All stress hormones induct infertility, this is highly adaptive as nothing is more long term expensive than reproducing at an inopportune time.

      Dopamine rules opportunism,, namely seasonal opportunism. All behavior effects like risk taking thrill seeing, aggression disinhibition and reproductive interest are merely effects of seasonally adaptive opportunistic behaviors exaggerated to caricature. This is as polar opposite melatonin induced reverse seasonally adaptive energy conservation, risk& expansion avoidance.

  • Jane Plain (Woo)

    Zomg hot bear action

    • Tess Howell

      Teddy should see this…. 😉

      • Jane Plain (Woo)

        Let’s batsignal cow and have interspecies orgy…again.

        I will wear nurse outfit like I promised teddy.

  • Jane Plain (Woo)

    Improve from d2 support as lower insulin and improved sensitivity is the solution.nteresting that bears respond to bromocriptine prolactin suppression with infertility; prolactin actively suppresses fertility in humans, breast feeding women will not ovulate secondary to elevated prolactin levels to facilitate breast feeding. Similarly, people wImprove from d2 support as lower insulin and improved sensitivity is the solution.ith prolactinoma will become infertile.

    However, if the bromo is suppressing normal insulin fat tissue activity, our insulin in brain, this may be perceived as starvation from an endocrinological standpoint and infertility results.

    In humans, women with starvation amenorrhea will experience more declines in hormone profiles when given bromocriptine, and this is likely secondary to further insulin thus Leptin declines. On the other hand, infertility associated with hyperinsulinemia like pcos will improve with d2 support as pcos follows an opposite pattern and improves from lower insulin via increased sensitivity.

    • William Lagakos

      Tl;dr: Prolactin is breaking my brain.

      IIRC, Wiley didn’t talk about prolactin too much, but when she did it was almost always circadian.

      Perhaps the lower nighttime & 24hr AUC of prolactin in Spring/Summer allows conception (if Haus’ 24hr circadian data in women apply to bears) – the higher daytime peak doesn’t matter because overall levels are lower.

      Nighttime levels weren’t measured in bears, but they are probably similar (highest in winter). The daytime prolactin peak (Tsubota study) in Spring might be there to promote sex hormones, and this is what is inhibited by bromo. Would replacing sex hormones in bromo-treated bears restore fertility? <– the answer to that question will probably not unbreak my brain.

      disclaimer: I've re-written this comment about 6 times, all seemingly complete opposites. And they all seemed rationale at the time.

      • Jane Plain (Woo)

        Another thing to remember wrt prolactin is that the bias of estrogen vs testosterone is reflected in dopamine vs serotonin; as female bears enter estrus estrogen will rise and this suppresses dopamine via serotonin bias of nervous system. In humans at least, increasing estrogen in females (and males, too) relates to suppression of dopamine. The primary regulator of prolactin is dopamine, so higher daytime prolactin in fertile female bears may merely represent the very high estrogen they are making in the morning, which is a result of high GnRH -> LH/FSH at night. Estrogen in fertile women is highest in the morning as this is a reflection of the high LH/FSH made throughout the night.
        In studies of shift workers wrt menstrual cycle and hormone changes for example it is found melatonin levels are lower (obviously), prolactin levels are also lower. This represents an increased bias of dopamine, lower estrogen, lower serotonin, and in women, this means reduced fertility / infertility and poor follicular development.

  • George

    Does the alteration in human birth rhythm also reflect decline of traditional marriages, which were often grouped at one time of year for cultural convenience (food for feasts, or a period of rest between labours)?

    • Jane Plain (Woo)

      that’s a good question George which warrant investigation :)

    • William Lagakos

      Anthropology isn’t my strong suit, but it seems like a strong possibility.

      If they every truly existed, I think you’d have to go back pretty far in the history of mankind to find bona fide paleo-breeders (long before the advent of artificial light).